A text-book of assaying
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Title: A text-book of assaying
for the use of those connected with mines.
Author: C. Beringer
J. J. Beringer
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A TEXT-BOOK OF ASSAYING:
FOR THE USE OF THOSE CONNECTED WITH MINES.
by
C. AND J. J. BERINGER.
Revised by
J. J. BERINGER,
Assoc. of the Royal School of Mines; Fellow of the Chemical Society and
of the Inst. of Chemistry; Principal of the Camborne Mining School; and
Late Public Analyst for the County of Cornwall.
With numerous Diagrams and Tables.
Ninth Edition.
London:
Charles Griffin and Company, Limited,
Exeter Street, Strand.
1904.
[All rights reserved.]
PUBLISHER'S NOTE TO THE NINTH EDITION
The continued popularity of the present work, the last edition of which
was published only a little over a year ago, continues to be a source of
gratification to the publishers, who have much pleasure in issuing the
present edition.
_January 1904._
PREFACE TO THE SIXTH EDITION
The principal changes in this edition are additions to the articles on
Gold, Cyanides, and Nickel, and a much enlarged Index. The additional
matter covers more than forty pages.
J. J. BERINGER.
CAMBORNE,
_January 1900._
PREFACE.
The Text-book now offered to the public has been prepared to meet the
existing want of a practical "handy book" for the Assayer.
To mining men the word "assaying" conveys a sufficiently clear meaning,
but it is difficult to define. Some writers limit it to the
determination of silver and gold, and others imagine that it has only to
do with "furnace-work." These limitations are not recognised in
practice. In fact, assaying is becoming wider in its scope, and the
distinction between "assayers" and "analysts" will in time be difficult
to detect. We have endeavoured rather to give what will be of use to the
assayer than to cover the ground within the limits of a faulty
definition.
At first our intention was to supply a description of those substances
only which have a commercial value, but on consideration we have added
short accounts of the rarer elements, since they are frequently met
with, and occasionally affect the accuracy of an assay.
Under the more important methods we have given the results of a series
of experiments showing the effect of varying conditions on the accuracy
of the process. Such experiments are often made by assayers, but seldom
recorded. Statements like those generally made--that "this or that
substance interferes"--are insufficient. It is necessary to know under
what conditions and to what extent.
Students learning any particular process cannot do better than repeat
such a series of experiments. By this means they will, at the same time,
acquire the skill necessary for performing an assay and a confidence in
their results based upon work under different conditions.
The electrolytic method of copper assaying given under _Copper_ is a
modification of Luckow's; it was introduced by us into the offices of
the Rio Tinto Copper Company, and has been in use for many years with
success. This modification is now employed in copper-works in Spain,
Germany, and England, and is used in place of the dry assay for the
commercial valuation of copper ores.
We have adhered to the gram and the "c.c." as the units of weight and
volume. Those who prefer working with grains and grain-measures can use
the figures given, multiplied by ten. For example:--When 1 gram is
mentioned, 10 grains should be used, and 10 grain-measures take the
place of 1 "c.c." It is not advisable to mix the two systems, as by
using gram weights and grain-measures.
We have intentionally to a large extent omitted to mention the names of
those who have originated or modified the various processes. The
practice of naming a process after its discoverer has developed of late
years, and is becoming objectionable. It is a graceful thing to name a
gas-burner after Bunsen, or a condenser after Liebig; but when the
practice has developed so far that one is directed to "Finkenerise" a
residue, or to use the "Reichert-Meissl-Wollny" process, it is time to
stop.
We are indebted to the standard works of Allen, Crookes, Fresenius,
Lunge, Michell, Percy, and Sutton, and wish to express our sense of
special indebtedness to Mr. Richard Smith, of the Royal School of Mines.
One or two of the illustrations are taken from Mr. Sexton's excellent
little book on _Qualitative Analysis_. Our obligation to some others is
mentioned in the text.
Finally, we have to thank for assistance in the experimental work
Messrs. Bailey, Beswick, Clarke, Grant, Higgins, and Smith.
THE AUTHORS.
CAMBORNE, _Nov. 1889_.
CONTENTS.
PART I.
CHAPTER I.
INTRODUCTORY.
Page
Object of assaying 1
Sampling 1
Drying: determination of moisture 5
Calculation and statement of results 7
Laboratory books and report forms 9
Quantity to be taken for an assay 11
Exercises 14
CHAPTER II.
METHODS OF ASSAYING.--DRY GRAVIMETRIC METHODS.
Methods of assaying 15
Gravimetric methods 15
Mechanical separations 16
Dry assays 16
(a) Fluxes 16
(b) Reducing agents 21
(c) Oxidising agents 22
(c) Apparatus 24
CHAPTER III.
WET GRAVIMETRIC METHODS.
Wet gravimetric methods 27
(a) Solution 29
(b) Precipitation 30
(c) Filtration 31
(c) Drying and igniting 32
CHAPTER IV.
VOLUMETRIC ASSAYS.
Titrometric assays 35
(a) Standard solutions 36
(b) Standardising 37
(c) Methods of working 42
(c) Indirect titration 43
Colorimetric assays 44
Gasometric assays 44
CHAPTER V.
WEIGHING AND MEASURING.
Weighing 47
Measuring liquids 49
(a) Graduated flasks 49
(b) Pipettes 50
(c) Burettes 51
Measuring gases 52
CHAPTER VI.
REAGENTS.
Acids, &c. 54
Bases, salts, &c. 59
CHAPTER VII.
Formulæ, equations, &c. 68
CHAPTER VIII.
SPECIFIC GRAVITY.
Introductory 75
Determination of specific gravity--
(a) Hydrometers 76
(b) Specific gravity bottles 78
Calculations depending on specific gravity 84
PART II.
CHAPTER IX.
SILVER, GOLD, PLATINUM, CYANIDES, MERCURY.
SILVER--Detection 87
Dry assay 87
(1) Scorification 88
(2) Pot assays, average ores 90
" ores with metallic oxides 91
" ores with metallic sulphides 91
Explanatory notes on the fusion 93
The effect of charcoal, flour, &c. 94
The effect of nitre 95
The effect of mineral sulphides 95
(3) Cupellation 98
The loss of silver 101
Condition affecting the loss 102
Methods of correction 103
Lead required for cupellation 105
(4) Calculation of the results in ounces to the
ton of 2240 lbs. Table 107
Ores with metallic particles 108
(5) Explanatory notes 110
(6) Examples of dry silver assays 113
Wet assays 116
Gravimetric method 117
Gay-Lussac's method 119
Volhard's method 121
A modified Gay-Lussac 123
Volhard's method applied to arsenic 124
GOLD--Detection 126
Amalgamation assay 126
Dry assay 127
(1) Size of charges 127
(2) Sampling 127
(3) Assay tons 131
(4) Small buttons, weighing 131
" " measuring 133
(5) Concentration in lead 136
Quartz ores 136
Ores with oxide of iron 138
Ores with metallic sulphides 139
(6) Cyanide charges, residues, &c. 140
(7) Cupellation 142
Cupels 142
Cupellation temperature 143
Cupellation loss 145
(8) Inquartation 146
(9) Flatting 149
(10) Parting, in flasks 151
" in test tubes 152
" in glazed crucibles 153
" Loss, &c. 154
(11) Check assays, surcharge 154
(12) Bullion assays in special apparatus 156
Silver, &c., in gold bullion 157
(13) Sampling of base bullion, &c. 157
CYANIDES--Commercial cyanides 160
Double cyanides 161
Prussic acid 162
Gold-dissolving power of cyanide liquor 162
Assay for cyanide strength 163, 165
Assay of commercial cyanide 167
Alkalinity of cyanides 167
Acidity of ores 168
Metals in cyanide liquors 169
Cyanicides 169
PLATINUM 170
IRIDIUM 171
MERCURY 171
Dry assay 172
Wet method 173
CHAPTER X.
COPPER, LEAD, THALLIUM, BISMUTH, ANTIMONY.
COPPER--Introductory 175
Dry assay 176
Valuation of copper ores 181
Wet methods 183
(1) Electrolytic assay 184
Volumetric methods 194
(1) Cyanide method 194
(2) Iodide method 199
(3) Colorimetric method 203
Examination of commercial copper 205
LEAD 211
Dry assay 211
Wet assay 213
(1) Gravimetric method 213
(2) Volumetric method 214
(3) Colorimetric method 218
THALLIUM 219
BISMUTH 220
Dry assay 221
Wet method 221
(1) Gravimetric determination 222
(2) Colorimetric assay 223
ANTIMONY 225
Dry assay 225
Wet method 227
(1) Gravimetric assay 228
(2) Volumetric method 229
CHAPTER XI.
IRON, NICKEL, COBALT, ZINC, CADMIUM.
IRON 231
Gravimetric determination 233
Permanganate and bichromate methods 234
Stannous chloride method 244
Colorimetric determination 247
NICKEL 251
Dry assay 251
Electrolytic assay 254
Titration by cyanide 255
COBALT 259
ZINC 261
Gravimetric method 262
Volumetric method 263
Gasometric method 266
CADMIUM 269
CHAPTER XII.
TIN, TUNGSTEN, TITANIUM.
TIN 271
Vanning 273
Dry assay 276
Detection, &c. 279
Gravimetric determination 281
Volumetric determination 282
Examples 284
TITANIUM 292
TUNGSTEN 295
NIOBIC AND TANTALIC OXIDES 297
CHAPTER XIII.
MANGANESE, CHROMIUM, ETC.
MANGANESE 298
Gravimetric determination 300
Volumetric determination 300
Ferrous sulphate assay 301
Iodine assay 302
Colorimetric determination 306
CHROMIUM 307
VANADIUM 310
MOLYBDENUM 311
URANIUM 312
CHAPTER XIV.
EARTHS, ALKALINE EARTHS, ALKALIES.
ALUMINA 314
THORIA 317
ZIRCONIA 317
CERIUM 318
LANTHANUM AND DIDYMIUM 319
YTTRIA 319
BERYLLIA 319
LIME 320
STRONTIA 324
BARYTA 326
MAGNESIA 328
THE ALKALIES 330
SODIUM 334
POTASSIUM 336
LITHIUM 338
CÆSIUM 339
RUBIDIUM 340
AMMONIUM 340
PART III.
CHAPTER XV.
OXYGEN AND OXIDES--THE HALOGENS.
OXYGEN 344
OXIDES 345
WATER 350
THE HALOGENS 358
CHLORINE 359
BROMINE 361
IODINE 362
FLUORINE 363
CHAPTER XVI.
SULPHUR AND SULPHATES.
SULPHUR 367
Gravimetric determination 369
Volumetric determination 370
SULPHATES 377
SELENIUM 379
TELLURIUM 379
CHAPTER XVII.
ARSENIC, PHOSPHORUS, NITROGEN.
ARSENIC 381
Gravimetric determination 383
Volumetric method, "iodine" 384
" " "uranic acetate" 389
PHOSPHORUS 394
Gravimetric determination 396
Volumetric determination 397
NITROGEN AND NITRATES 400
CHAPTER XVIII.
SILICON, CARBON, BORON.
SILICON AND SILICATES 405
CARBON AND CARBONATES 414
COALS 418
SHALES 420
CARBONATES 424
BORON AND BORATES 429
APPENDIX A.
Table of atomic weights and other constants 433
Table for converting degrees of the centigrade
thermometer into degrees of Fahrenheit's scale 435
Tables showing strengths of aqueous solutions of nitric
and hydrochloric acids, of ammonia and of sulphuric
acid 436
APPENDIX B.
Estimation of small quantities of gold 440
Practical notes on the iodide process of
copper assaying 441
Method of separating cobalt and nickel 442
APPENDIX C.
A lecture on the theory of sampling 444
INDEX 450
A TEXT-BOOK OF ASSAYING.
CHAPTER I.
INTRODUCTORY.
Assaying has for its object the determination of the quantities of those
constituents of a material which add to or detract from its value in the
arts and manufactures. The methods of assaying are mainly those of
analytical chemistry, and are limited by various practical
considerations to the determination of the constituents of a small
parcel, which is frequently only a few grains, and rarely more than a
few ounces, in weight. From these determinations calculations are made,
which have reference to a mass of material of, perhaps, hundreds of
tons. But in all cases, whether the mass under consideration be large or
small, whether the material be obtained by mining, grown, or
manufactured, the assayer is supposed to receive a small quantity,
called "the sample," which is, or ought to be, the exact counterpart of
the mass of material that is being dealt with. The taking and making of
this sample is termed "sampling"; and the men whose special work it is
to select such samples are "the samplers."
But although "sampling" is thus distinct from "assaying," the assayer
should be familiar with the principles of sampling, and rigorous in the
application of these principles in the selecting, from the sample sent
him, that smaller portion upon which he performs his operations.
~Sampling.~--_In the case of gases_, there is absolutely no trouble in
mixing. The only difficulty is in drawing off a fair sample where, as in
flues, the body of the gas is in motion, and varies a little in
composition from time to time. In this case, care must be taken to draw
off uniformly a sufficient volume of the gas during a prolonged period;
any portion of this larger volume may then be taken for the analytical
operation.
_In the case of liquids_, which mix more or less easily--and this class
includes metals, &c., in the state of fusion--more or less severe
agitation, followed by the immediate withdrawal of a portion, will yield
a fairly representative sample.
_In the case of solids_, the whole mass must be crushed, and, if not
already of fairly uniform quality, mixed, before sampling can take
place. Most of the material which a sampler is called upon to deal with,
is, however, in a more or less divided state and fairly uniform. In
practice it is assumed that 5 per cent. of the whole (= 1/20th), if
taken in portions of equal weight and at frequent and regular intervals,
will represent the mass from which it was taken. Taking a heap of ore,
A, and selecting one out of every twenty spade-, bag-, barrow-, or
wagon-fuls, according to the quantity of stuff in the heap, there is
obtained a second heap, B, containing one-twentieth of the stuff of the
heap A. If we crush the stuff in B until this heap contains
approximately the same number of stones as A did--which means, crushing
every stone in B into about twenty pieces--B will become the counterpart
of A. Selecting in the same manner 5 per cent. of B, there is got a
third heap, C. This alternate reduction and pulverising must be carried
on until a sample of suitable size is obtained. This may be expressed
very clearly thus:--
A = 1000 tons of rocks and lumpy ore.
B = 50 " " rough stones, 1/20th of A.
C = 2.5 " " small stones, 1/20th of B.
D = 0.125 " " coarse powder, 1/20th of C.
[Illustration: FIG. 1.
CONE
PARTLY REDUCED CONE
PLAN OF FRUSTRUM DIVIDED.
ELEVATION OF FRUSTRUM DIVIDED.]
If the material to be sampled is already a dry powder, 5 per cent. of it
should be heaped in a cone; each lot being added on the apex of the
cone already formed, so that it may distribute itself by falling evenly
in all directions. When the cone is completed, convert it into a low
frustrum of a cone by drawing stuff uniformly and in a direct line from
the centre to the circumference. Draw two diameters at right angles to
each other, and reserving any two alternate quarters, reject the others.
Mix; and form another cone, and proceed until a sample is got of the
bulk required.
This is the usual plan, and all samples should be treated in this way
when the stuff is fine enough to fall evenly down the sides of a cone.
Samples as they reach the assay office are seldom in a fit state for the
work of the assayer; they are generally too coarse, and ought always to
be more than he wants for any particular determination. The portion he
requires should never be taken at hap-hazard; the sample must be reduced
systematically to the quantity required.
1. _If the sample is a liquid:_ it is sufficient to shake the bottle,
and take out a measured or weighed quantity for the assay.
2. _If a liquid with a solid in suspension:_ measure the whole of it.
Filter. Make up the filtrate with the wash-water or water to the
original bulk. Assay it. Dry and weigh the residue, and make a separate
assay of it.
3. _If of a creamy consistency, free from heavy particles:_ mix well;
spread out evenly on a glazed tile. Take up equal portions at equal
distances. Mix and assay.
4. _If a mud of coarse and fine particles, or of particles of unequal
density:_ weigh and transfer to a porcelain dish, or weigh in the dish.
Dry at 100° C., weigh. Treat the residue as a solid capable of being
powdered.
5. _If a solid capable of being powdered, or already powdered:_ heap up
into a cone; flatten with a spatula; divide along two diameters at right
angles, and carefully reject the whole of two alternate quarters,
brushing away any fine powder. Mix the other quarters, and repeat (if
necessary). For small quantities a fine state of division is essential.
6. _If a solid with metallic particles:_ powder and pass through a
sieve; the metallic particles will not pass through. Weigh both portions
and assay separately. _Sifting should be followed by a very thorough
mixing._
7. _If a metal or alloy in bar or ingot:_ clean the upper surface of the
bar, and bore through the bar. Use the borings. If the ingot or bar is
small, cut it through and file the section. Filings must be freed from
fragments of the file by means of a magnet; and from oil, if any be
present, by washing with a suitable solvent.[1] Where practicable,
metals and alloys are best sampled by melting and granulating. The
student must carefully avoid any chance of mixing dirt or particles of
other samples with the particular sample which he is preparing. One ore
should be done at a time, and when finished, it should be labelled and
wrapped up, or bottled, before starting on a fresh sample.
When an ore requires to be very finely ground in an agate mortar, it is
often advisable to mix with a little pure alcohol and rub until free
from grit; dry at 100° C. and mix well before weighing.
When an assay is required of a quantity of ore made up of parcels of
different weight and quality, each parcel should be separately sampled
and parts of each sample, bearing to each other the same proportion by
weight as the original parcels, should be taken and mixed. For example,
a lot of ore is made up of one parcel of A, 570 tons, one of B, 180
tons, and another of C, 50 tons; a sample representing the whole may be
got by mixing 57 parts of a sample of A with 18 parts of a sample of B,
and 5 parts of a sample of C.
[Illustration: FIG. 2.]
A bruising plate, like that in fig. 2, is convenient for general office
work. The slab is of cast iron, about an inch thick. It is firmly
supported on a solid block of wood, and pivoted for convenience in
emptying. The bruising-hammer is steel-faced, about 4 inches square, and
1-1/2 inch thick. The block is firmly fixed to a small table or tressel,
so that the slab is about 2 feet 6 inches from the ground. The slab is
cleaned, and the sample collected with the help of a stiff-haired brush.
~Drying: Determination of Moisture.~--In practice, the moisture is
generally determined by the samplers, and the proportion is specified in
grains per pound on the label attached to the sample when it reaches the
assay office. The method adopted is usually to dry 1 lb. = 7000 grs. of
the ore in a frying-pan heated over a gas flame, or in an ordinary oven,
until a cold bright piece of metal or glass is no longer damped when
held over it. The loss of weight in grains = moisture.
Properly, however, this work should be done by the assayer, if only for
the following reason. It is assumed that the dry ore of the sampler and
of the assayer are the same thing; according to the nature of the ore,
this may or may not be the case. The assayer, however, uses the sample
which he has dried for his moisture-determination, as the dry ore on
which he makes his other assays, and no variation in moisture would
influence the other and more important determinations. Some ores are
sent to the smelter with from 5 to 15 per cent. of adherent water. In
these cases it is best to spread out the sample, and taking equal
portions fairly at regular intervals, weigh into a Berlin dish 20 grams.
This should then be dried over a sand-bath, or if the ore is likely to
be injured by excess of heat, over a water-bath until the weight is
constant. The loss of weight multiplied by 5 gives the percentage of
water present.
Example:--
Weight of dish + wolfram 32.67 grms.
" " dish 12.67 "
-----
" " wolfram 20.00 "
" " dish + wolfram 32.67 "
" " " dried 30.15 "
-----
" " water 2.52 "
2.52 × 5 = 12.6 ~12.6%.~
There are other ores which are not apparently wet, but in the state
called "air-dried." It is easier to take fair samples of these, and,
consequently, it is not necessary to use so large a quantity as 20
grams. But with a smaller quantity, extra precautions must be taken. All
dry solids at ordinary temperatures absorb moisture from the air. The
amount varies with the nature of the material and with the quantity of
surface exposed. Light bulky powders absorb more than heavy ones,
because of the greater condensing surface. It is on this account that it
is well to weigh substances, which have been dried, between
close-fitting watch-glasses. The method of determining moisture is to
weigh out into the glasses 5 grams of ore, and dry in the water-oven
until there is no further loss of weight. On taking the glasses out of
the oven, they should be at once closed, the clip put on, and after
cooling in a desiccator weighed. If after a second trial the loss is the
same, or only increased by a milligram, the determination is finished.
Example:--
Weight of glasses + pyrites 31.0470 grms.
" " glasses 26.0470 "
-------
" " pyrites 5.0000 "
" " glasses + pyrites, dried 1 hour 30.8965 "
" " " " dried 1-1/2 " 30.8957 "
" " " " 31.0470 "
" " " " dried 30.8957 "
-------
" " moisture 0.1513 "
0.1513 × 20 = 3.026 ~3.02%.~
[Illustration: FIG. 3.]
Sometimes it may be advisable to dry 10 grams, in which case multiplying
the loss by 10 will give the percentage. The dried ore should be
transferred to a weighing-tube (fig. 3), and reserved for the subsequent
determinations. The weighing-tube with the ore must be marked, and kept
in a desiccator.
Most ores and inorganic substances can be dried, and their moisture
determined by the loss in this way. When, however, the substance
contains another somewhat volatile ingredient, it is exposed over
sulphuric acid in a desiccator for two days (if _in vacuo_, all the
better), and the loss determined. Moisture in dynamite should be
determined in this way.
When water is simply mechanically mixed with a substance it presents but
little difficulty. The combined water is a different matter. Slaked
lime, even when perfectly dry, contains much water; and if the water of
soda crystals were separated and frozen, it would occupy a volume equal
to that of the original crystals. Perfectly dry substances may contain
much water, and this combined water is retained by different materials
with very unequal vigour. Sodium sulphate and sodium phosphate crystals
lose water even when exposed under ordinary conditions to dry air. Soda
crystals when heated melt, and at a moderate temperature give off their
water with ebullition. The temperature at which all the water is given
up varies with each particular salt; the actual determination of the
water in each case will require somewhat different treatment. Such
determinations, however, are seldom required; and from a practical
point of view this combined water causes no trouble.
_In assaying ores_, we term "moisture" all water which is lost by
exposure in a water-oven at 100° C., and the "dry ore" is the ore which
has been dried at this temperature. No advantage, but rather endless
confusion, would be caused by varying the temperature with the object of
estimating the whole of the water which a hydrated salt may contain. The
results of the assay of the other components should be calculated on the
"dry ore." One advantage of this is obvious:--The dry ore has a constant
composition, and the results of all assays of it will be the same, no
matter when made; the moisture, however, may vary from day to day, and
would be influenced by a passing shower of rain. It is well to limit
this variability to the moisture by considering it apart, and thus avoid
having the percentage, say, of copper rising and falling under the
influence of the weather.
In the case of certain salts, however, such as soda crystals and
hydrated sulphate of copper (when these constitute the bulk of the
substance to be assayed), it is as well to perform the assay on the
moist, or at any rate air-dried, substance.[2] It would be equally
convenient to calculate on the substance dried at 100° C.; but in this
case it would be well, in order to avoid a somewhat shallow criticism,
to replace the term "moisture" by the longer but equivalent phrase
"water lost at 100° C."
~Calculation and Statement of Results.~--By far the most generally
convenient method of stating the results of an assay is that of the
percentage or parts in a hundred, and to avoid a needlessly troublesome
calculation it is well to take such a quantity of ore for each assay as
by a simple multiplication will yield the percentage. In these
calculations decimals are freely employed, and students should make
themselves familiar with the methods of using them.
Other methods of statement are in use, and have advantages in certain
special cases. With bullion the parts in a thousand are given, and in
those cases in which the percentage is very small, as in water analysis,
it is convenient to report on parts in 100,000, or even on parts per
1,000,000. These are easily got from the corresponding percentages by
shifting the decimal point one, three, or four places to the right. Thus
92.5 per cent. is 925 per thousand; and 0.0036 per cent. is 3.6 per
100,000, or 36 per million.
With ores of tin, silver, and gold, the result is stated as so many
cwts., lbs., or ozs., in the ton. With dressed tin ores as they are
sent to the smelter, the produce is given in cwts. and quarters to the
ton. The corresponding percentage may be obtained by multiplying by
five; or, inversely, if the percentage is given, the produce may be got
by dividing by five. A produce of 13-1/2 equals a percentage of 13.5×5
= 67.5; and a percentage of 70.0 equals a produce of 70/5 = 14. With
tin ores as raised (in which the percentage is small) the reduction must
be carried to pounds per ton. One per cent. equals 22.4 lbs. to the ton;
consequently, if we multiply the percentage by 22.4, the produce will be
given. Thus, if an ore contains 6.7 per cent. of oxide of tin, the
produce is 6.7×22.4 = 150 lbs. (or 1 cwt., 1 quarter, and 10 lbs.) to
the ton. With gold and silver ores, the proportion of precious metal is
small, and it is necessary to carry the reduction to ozs. and dwts. to
the ton; and since gold and silver are sold by troy weight, whilst the
ton is avoirdupois, it is of importance to remember that the ounces in
the two systems are not the same. A ton contains 15,680,000 grains,
which equal 653,333.3 dwts. or 32,666.6 ozs. (troy). The following rules
are useful:--
To get ozs. (troy) per ton, multiply parts per 100,000 by 0.327;
To get dwts. per ton, multiply parts per 100,000 by 6.53;
To get grains per ton, multiply parts per 100,000 by 156.8.
Where liquids are being assayed, cubic centimetres are held to be
equivalent to grams, and the usual method of statement is, "so many
parts by weight in so many by measure." Where the statement is made as
grams per litre or grains per gallon, there can be no doubt as to what
is meant; and even if it be expressed in parts per 100,000, parts by
weight in a measured volume must be understood unless the contrary is
expressly stated.
In some cases, where the density of the solution differs greatly from
that of water, the percentage by weight may be given; and in others,
mixtures of two or more liquids, the percentages may be given by volume
or by weight; as so many c.c. in 100 c.c., or as so many grams in 100
grams, or even as so many grams in 100 c.c. In such cases it must be
distinctly shown which method of statement is adopted.
One grain per gallon means 1 grain in 70,000 grain-measures, or one part
in 70,000. Dividing by 7 and multiplying by 10 will convert grains per
gallon into parts per 100,000. Inversely, dividing by 10 and multiplying
by 7, will convert parts per 100,000 into grains per gallon.
Grams per litre are parts per 1000; multiplying by 100 will give parts
per 100,000, and multiplying by 70 will give grains per gallon.
Among foreign systems of weights, the French is by far the best.
Kilograms (2.205 lbs.) per quintal (220.5 lbs.) are parts per cent.; and
grams (15.43 grs.) per quintal are parts per 100,000. From the rule
already given, grams per quintal may be converted into ounces to the ton
by multiplying by 0.327.
The German loths per centner (1/2 oz. (avoirdupois) to 100 lbs.) equal
parts per 3200; they are converted into parts per cent. by dividing by
32, or into ounces (troy) per ton by multiplying by 10.208.
In the United States, as a sort of compromise between the avoirdupois
and metric systems, a ton is taken as 2000 lbs. There, too, the custom
is adopted of reporting the gold and silver contents of an ore as so
many dollars and cents to the ton. In the case of gold, an ounce is
considered to be worth 20.6718 dollars. With silver, the _nominal_ value
is 1.2929 dollars per ounce, but frequently in assay reports it is taken
as one dollar. The practice is objectionable. The prices of metals vary
with the fluctuations of the market, and if the assayer fixed the price,
the _date_ of his report would be all important; if, on the other hand,
he takes a fixed price which does not at all times agree with the market
one, it leaves a path open for the deception of those unacquainted with
the custom. American "dollars on the ton of 2000 lbs." may be converted
into "ounces in the ton of 2240 lbs." by dividing by 1.1544 in the case
of silver, and by 18.457 in the case of gold.
~Laboratory Books and Report Forms.~--The record which the assayer makes
of his work must be clear and neat, so that reference, even after an
interval of years, should be certain and easy. One method should be
adopted and adhered to. Where there are a large number of samples, three
books are required.
_Sample Book._--This contains particulars of the samples (marks, &c.),
which are entered by the office-clerk as they arrive. He at the same
time puts on each sample the distinguishing number.
EXAMPLE OF PAGE OF SAMPLE BOOK.
+----------+----------+--------------------------+----------------+
| Date. | Number. | Sample. | Remarks. |
+----------+----------+--------------------------+----------------+
| Feb. 1 | 482 | Tough Copper | For Arsenic. |
| " 2 | X | Piece of Metal | For Ni and Cu. |
| " | 483 | Tough Copper. | |
| " | 73 | Silver Precipitate, | With Letter. |
| | | 4 casks, 24 cwt. 1 qr. | |
| " | 494 | Purple Ore, 200 tons. | |
| " | 1 J.T. | Lead Ore, 1 J.T. | From Corsica. |
| " | 2 J.T. | " 2 J.T. | |
+----------+----------+--------------------------+----------------+
_Laboratory Book._ This is the Assayer's note-book, in which he enters
clearly the particulars of his work--the results obtained, as well as
how these results were arrived at. The calculations should be done on
scrap-paper, and should not be entered, although, of course, detail
enough must be shown to enable the results to be recalculated.
EXAMPLE OF PAGE OF LABORATORY BOOK.
______________________________________________________________
Purple Ore 5 grams
19/10/89 0.0042 grm.
0.0021 "
------
Colorimetric 0.0063 × 20 = 0.13% Copper
______________________________________________________________
482
Tough Copper 10 grams
Feb. 1/89 10.5 c.c. Uranium.
= 0.52% Arsenic
______________________________________________________________
2082
Tough Copper 10 grams
12.7 c.c. Uranium.
= 0.63% Arsenic
______________________________________________________________
491 10 grams
Tough Copper 13.7 c.c. Uranium
Feb. 1/89
= 0.68% Arsenic
______________________________________________________________
Standard of Uranium acetate.
0.150 gram As_{2}O_{3} = 23.3 c.c. Uranium.
.'. 100 cc. Uranium = 0.5 gram As.
______________________________________________________________
10071 5 grams
Tin Ore Cruc. and SnO_{2} 9.6065 grms.
Feb. 3/89 Cruc. and Ash 9.4235 "
------
SnO_{2} = 0.1830 = 2.88% Tin
______________________________________________________________
_The Assay Book._--This is the Official book, and is a combination of
the Sample and Laboratory books. It corresponds with the report-forms.
Without being loaded with detail, it should contain sufficient to
characterise each sample.
Key to following example page of Assay book:
DR = DATE REPORTED.
Not Det. = Not detected
EXAMPLE OF PAGE OF ASSAY BOOK.
-------------------------------------------+----+-------+---------------+----
DESCRIPTION OF SAMPLE. | | Water | Assay on |
------+--------------------+---------------| |Lost at| the Dry |
Date. | Material. | Weight. |No. |100° C.|Material. | DR
------+--------------------+---+---+---+---+----+-------+---------------+----
1889 | |ton|cwt|qrs|lbs| | | |
Feb. 1|Tough cake copper | | | | | 482| |Arsenic, 0.52% | 7
" |Tough cake copper | | | | |2082| |Arsenic, 0.63% | 7
" |Tough cake copper | | | | | 491| |Arsenic, 0.68% | 7
| | | | | | | | |
Feb. 2|Nickel disc for C.R.| | | | | X | |Copper, 73.75 | 7
| | | | | | | |Nickel, 24.34 |
| | | | | | | |Iron, 2.18 |
| | | | | | | | ----- |
| | | | | | | | 100.27 |
| | | | | | | | ------ |
" |Silver precipitate, | | 24| 1| 0| 73| Not | |
| 4 casks | | | | | | det. |Silver, 4.851 | 10
| | | | | | | |Gold, 0.0215|
| | | | | | | |Lead, 19.37 |
| | | | | | | |Zinc, 2.00 |
| | | | | | | |Silver, 1584.7 |
| | | | | | | | ozs. per ton |
| | | | | | | |Gold, 7.0 |
| | | | | | | | ozs. per ton |
" |Purple ore |200| | | | 494| Not |Copper, 0.13% | 11
| | | | | | | det. |Sulphur 0.15% |
------+--------------------+---+---+---+---+----+-------+---------------+----
When the number of samples is small, the Sample Book may be omitted, and
the entries made in the Assay Book as the samples arrive.
_Report-forms._ These should entail as little writing as possible in
making out the report. For general purposes the form given on p. 12 is
useful.
~The quantity of substance~ to be taken for any particular assay depends
largely upon the method of assay adopted. There are, however, some
general considerations which should be remembered, and some devices for
simplifying the calculations which should be discussed.
The smaller the percentage of the substance to be determined, the larger
should be the amount of the ore taken. The following table will give a
general idea as to this:--
Percentage of the substance Amount of ore, &c. to
to be determined. be weighed.
100-10 1 gram.
10-5 2 grams.
5-1 5 "
1-0.1 10 "
0.1-0.01 20 "
[Illustration: ASSAY NOTE]
The rougher the method of assay adopted, the larger should be the
quantity of ore taken. If the degree of accuracy attainable with the
methods and instruments at the assayer's service is known, it is easy to
calculate what quantity should be taken for any particular case. If the
results are good within 0.001 gram, then, taking 1 gram of ore we can
report within 0.1 per cent., or if they are good within 0.0002 gram,
taking 20 grams of ore, we can report within 1 part per 100,000, or very
closely within 6-1/2 dwt. to the ton. If it is wished to be yet more
particular in reporting, larger quantities must be taken. The difficulty
of manipulating very small or very large precipitates, &c., must be
borne in mind. So, too, must the fact that the greater the weight of the
final product of an assay, the less, as a rule, is the percentage error.
The distinction between absolute and percentage error, often overlooked,
is important. If 0.5 gram of silver be cupelled with 20 grams of lead,
there may be obtained a button of 0.495 gram; the absolute loss is 0.005
gram, and this equals 1 per cent. of the silver present. Similarly,
cupelling 0.1 gram, the resulting button may be 0.098; the absolute loss
is only 0.002 gram, but this equals 2 per cent. of the silver present.
In the same way the student should see that the two results, 91.5 per
cent. and 92.0 per cent., are really more concordant than the results
9.1 per cent. and 9.2 per cent.
A device often adopted in practice where a large number of assays of one
kind are made, and the report is given as so many ounces or pounds to
the ton, is that known as the _assay ton_. The assay ton may be any
arbitrary and convenient weight, but its subdivisions must bear to it
the same relations as pounds and ounces bear to the actual ton. On the
other hand, in a laboratory where many kinds of work are performed,
different sets of weights of this kind would only tend to confusion,
even if they were not unnecessary. With a set of gram weights and its
subdivisions anything may be done. If it is desired to report as pounds
to the ton, then, since there are 2240 lbs. to the ton, a weight of
2.240 grams may be taken as the assay ton, and each 0.001 gram yielded
will equal 1 lb., or 22.4 grams may represent the ton, and each 0.01
gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton;
if we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1
oz. to the ton. In some cases it may be convenient to have, in addition
to the usual gram weights, one or other of the "assay tons" mentioned
above, but generally it is better to work on a purely decimal system,
and convert when required into ounces per ton, &c., either by actual
calculation or by reference to a set of tables.
PRACTICAL EXERCISES.
The student should practise such calculations as the following:--
1. Calculate the percentages in the following cases:--
(a) Ore taken, 2 grams; copper found, 0.2155.
(b) " 1.5 gram; iron found, 0.8340.
(c) " 30 grams; lead found, 23.2.
2. Calculate the parts per thousand in the following:--
(a) Bullion taken, 1.1 gram; silver found, 1.017.
(b) " 1.14 gram; silver found, 1.026.
(c) " 0.6 gram; gold found, 0.5500.
3. Calculate parts per 100,000 in the following:--
(a) Ore taken, 20 grams; silver found, 0.0075.
(b) " 50 grams; gold found, 0.0026.
(c) Water taken, 500 c.c.; solids found, 0.1205.
4. Calculate cwts. to the ton in the following:--
(a) Ore taken, 5 grams; tin found, 2.816.
(b) " 5 grams; tin found, 3.128.
(c) An ore with 68.2 per cent. of tin.
5. Calculate lbs. to the ton in the following:--
(a) An ore with 3.28 per cent. oxide of tin.
(b) Ore taken, 20 grams; oxide of tin found, 1.67.
6. Calculate ozs. (troy) to the ton in the following:--
(a) Ore taken, 50 grams; gold found, 0.0035.
(b) " 20 grams; silver found, 0.0287.
(c) " 25 grains; silver found, 0.0164.
7. Calculate in grains per gallon:--
(a) 0.51 gram per litre.
(b) 24.6 parts per 100,000.
(c) Solution taken, 100 c.c.; copper found, 0.0045 gram.
(c) " 50 c.c.; iron found, 0.165 gram.
8. Convert into ozs. (troy) per ton:--
(a) 7 loths per centner.
(b) 30 grams per quintal.
(c) 15 parts per 100,000.
FOOTNOTES:
[1] Ether or carbon bisulphide.
[2] Such substances are best dried by pressing between folds of dry
filter-paper.
CHAPTER II.
METHODS OF ASSAYING.--DRY GRAVIMETRIC METHODS.
The methods of assaying are best classed under two heads, Gravimetric
and Volumetric, in the former of which the final results are weighed,
whilst in the latter they are measured. A commoner and older division is
expressed in the terms much used in practice--wet assays and dry assays.
Wet assays include all those in which solvents, &c. (liquid at the
ordinary temperature), are mainly used; and dry assays, those in which
solid re-agents are almost exclusively employed. Dry assays form a
branch of gravimetric work, and we shall include under this head all
those assays requiring the help of a wind furnace. Wet assays, as
generally understood, would include not only those which we class as wet
gravimetric assays, but also all the volumetric processes.
~Gravimetric Methods~ aim at the separation of the substance from the
other matters present in the ore, so that it may be weighed; and,
therefore, they must yield the _whole_ of the substance in a pure state.
It is not necessary that a metal should be weighed as metal; it may be
weighed in the form of a compound of definite and well known
composition. For example, one part by weight of silver chloride contains
(and, if pure, always contains) 0.7527 part of silver; and a quantity of
this metal can be as exactly determined by weighing it as chloride as by
weighing it in the metallic state. But in either case the metal or its
chloride must be pure.
Exact purity and complete separation are not easily obtained; and
methods are used which are defective in one or both of these respects.
It is well to note that an impure product increases the result, whilst a
loss of the substance decreases it; so that if both defects exist in a
process they tend to neutralise each other. Of dry methods generally, it
may be said that they neither give the whole of the substance nor give
it pure; so that they are only calculated to show the amount of metal
that can be extracted on a manufacturing scale, and not the actual
quantity of it present. Their determinations are generally rough and
always low. The gold and silver determinations, however, will compare
very favourably with any of the other processes for the estimation of
these metals in their ores.
The calculation of the results of a gravimetric assay has already been
referred to. If the result is to be stated as percentage, it may always
be done by the following rule:--_Multiply the weight of the substance
got by the percentage of metal it contains, and divide by the weight of
ore taken._
Gravimetric methods are divided into three groups: (1) mechanical
separations; (2) dry methods; and (3) wet methods.
~Mechanical Separations.~--Under this head are classed the method of
assaying tin ores, known as vanning, and the amalgamation assay for
gold. A set of sieves to determine the relative proportion of powders of
different degrees of fineness is sometimes useful. A set with 10, 20, 40
and 80 meshes to the inch is convenient.
~Dry Assays.~--An important distinction between wet and dry methods of
assaying is, that in the former the substance is got into the liquid
state by solution, whilst in the latter fusion is taken advantage of.
The difference between solution and fusion is easily illustrated: a lump
of sugar heated over a candle-flame melts or fuses; suspended in water
it dissolves. Many substances which are insoluble or infusible of
themselves, become soluble or fusible when mixed with certain others;
thus, in this way, solution is got with the aid of reagents, and fusion
with the help of fluxes. For example, lead is insoluble in water, but if
nitric acid be added, the metal rapidly disappears. It is convenient,
but somewhat inaccurate, to say that the acid dissolves the lead. If the
lead be acted on by nitric acid alone, without water, it is converted
into a white powder, which does not dissolve until water is added; in
this case it is obvious that the water is the solvent. The function of
the acid is to convert the lead into a soluble compound.
~Fluxes~ may act as true solvents. Fused carbonate of soda dissolves
baric carbonate, and perhaps in many slags true solution occurs; but in
the great majority of cases a flux is a solid reagent added for the
purpose of forming a fusible _compound_ with the earthy or stony
minerals of the ore. Few of the minerals which occur in the gangue of an
ore are fusible; and still fewer are sufficiently fusible for the
purposes of the assayer, consequently the subject is one of importance,
and it ought to be treated on chemical principles. An idea of the
composition of some of the more frequently occurring rocks may be
gathered from the following table, which represents rough averages:--
-----------------------------------------------------------------
| | |Oxide|Lime and |
|Silica.|Alumina.| of |Magnesia.|Alkalies.
| | |iron | |
-----------------------------------------------------------------
| % | % | % | % | %
Sandstone, grit, | | | | |
quartzite, &c. |80-100 | -- | -- | -- | --
Granite, gneiss, | | | | |
quartz-porphyry, | | | | |
fire-clay, &c. | 70-75 | 13-20 | 2 | 2 | 5-8
| | | | |Less in
| | | | |fire-clay.
Mica-schist | 65 | 18 | 5 | 3 | 3
Trachyte, syenite | 60 | 17 | 7 | 4-7 | 6-9
Clay-slate | 60 | 18 | 10 | 8 | 3
Diorite | 54 | 17 | 12 | 9 | 3-4
Horneblende-rock | 50 | 18 | 15 | 12 | 3-4
Brick-clay | 50 | 34 | 8 | 6 | --
China-clay | 47 | 39 | -- | -- | --
Basalt, dolerite, &c.| 50 | 15 | 15 | 16 | 3
Serpentine | 44 | -- | -- | 44 | --
Chalk, limestone, | | | | |
dolomite, &c. | -- | -- | -- | 45-55 | --
-----------------------------------------------------------------
Silica itself, and the silicates of alumina, of lime, and of magnesia,
are practically infusible; the silicates of soda, of potash, and of iron
are easily fusible if the base (soda, potash, or oxide of iron) be
present in sufficient quantity, and if, in the case of the iron, it is
present mainly as lower oxide (ferrous silicate). The addition of lime,
oxide of iron, or alkali to silicate of alumina results in the formation
of a double silicate of alumina and lime, or of alumina and iron, &c.,
all of which are easily fusible. Similarly, if to a silicate of lime we
add oxide of iron, or soda, or even alumina, a fusible double silicate
will be formed. Thus lime, soda, oxide of iron, and clay, are _fluxes_
when properly used; but since lime, clay (and oxide of iron if there be
any tendency to form peroxide), are of themselves infusible, any excess
of these fluxes would tend to stiffen and render pasty the resulting
slag. So, too, soda, which is a very strong base, may act prejudicially
if it be in sufficient excess to set free notable quantities of lime and
magnesia, which but for that excess would exist in combination as
complex fusible silicates. There are many minerals which with but little
soda form a glass, but with more yield a lumpy scoriacious mass. There
are many minerals, too, which are already basic (for example, calcite),
and which, when present, demand either a less basic or an acid flux
according to the proportions in which they exist. For purposes of this
kind borax, or glass, or clay with more or less soda may be used, and of
these borax is by far the most generally useful. An objection to too
basic a slag (and a very important one) is the speed with which it
corrodes ordinary crucibles. These crucibles, consisting of quartz and
clay, are rapidly attacked by lime, soda and bases generally.
[Illustration: FIG. 4.]
In considering what is and what is not a good slag, certain chemical
properties are of importance. If a mixture of many substances be fused
and allowed to solidify in a crucible, there will be found some or all
of the following. At the bottom of the crucible (fig. 4) a button of
metal, resting on this a speise; then a regulus, next a slag made up of
silicates and borates and metallic oxides, and lastly, on the top
another layer of slag, mainly made up of fusible chlorides and
sulphates. In assaying operations the object is generally to concentrate
the metal sought for in a button of metal, speise or regulus, and to
leave the earthy and other impurities as far as possible in the slag;
whether there be one or two layers of slag is a matter of
indifference;[3] but the chemical action of the lower layer upon the
speise, or regulus, or metal, is of great importance.
A _regulus_ is a compound of one or more of the metals with sulphur; it
is usually brittle, often crystalline, and of a dull somewhat greasy
lustre. It is essential that the slag, when solid, shall be so much more
brittle than the regulus, that it shall be easy to crumble, and remove
it without breaking the latter; and it must not be basic. The effect of
fusing a regulus with a basic slag is well seen when _sulphide of lead_
is fused with _carbonate of soda_; the result is a button of metal (more
or less pure), and a slag containing sulphides of lead and sodium; and
again, if sulphide of lead be fused with an excess of oxide of lead, a
button of lead will be got, and a slag which is simply oxide of lead
(with whatever it may have taken up from the crucible), or if a
sufficient excess has not been used, oxide of lead mixed with some
sulphide. When (as is most frequently the case) the desire is to prevent
the formation of regulus, these reactions may be taken advantage of, but
otherwise the use of a flux having any such tendency must be avoided. A
good slag (from which a regulus may be easily separated) may be obtained
by fusing, say, 20 grams of ore with borax 15 grams, powdered glass 15
grams, fluor spar, 20 grams, and lime 20 grams; by quenching the slag in
water as soon as it has solidified, it is rendered very brittle.
Sulphide of iron formed during an assay will remain diffused through
the slag, instead of fusing into a button of regulus, if the slag
contain sulphide of sodium. The same is true of other sulphides if not
present in too great a quantity, and if the temperature is not too high.
_Speises_ are compounds of a metal or metals with arsenic. They are
chiefly of interest in the metallurgy of nickel, cobalt, and tin. They
are formed by heating the metal or ore in covered crucibles with arsenic
and, if necessary, a reducing agent. The product is fused with more
arsenic under a slag, consisting mainly of borax. They are very fusible,
brittle compounds. On exposure to the air at a red heat the arsenic and
the metal simultaneously oxidize. When iron, cobalt, nickel, and copper
are present in the same speise, they are eliminated in the order
mentioned.
_Slags_ from which metals are to be separated should not be too acid; at
least, in those cases in which the metal is to be reduced from a
compound, as well as separated from earthy impurities. Where the object
is simply to get a button of metal from a substance in which it is
already in the metallic state, but mixed with dross (made up of metallic
oxides, such as those of zinc or iron), from which it is desired to
separate it, an acid flux like borax is best; or, if the metal is easily
fusible, and there would be danger of loss of metal by oxidation or
volatilising, it may be melted under a layer of resin or fat. Common
salt is sometimes used with a similar object, and is often useful. Under
certain conditions, however, it has a tendency to cause the formation of
volatile chlorides with a consequent loss of metal.
In the great majority of cases, the fusion of the metal is accompanied
by reduction from the state of oxide; in these the slag should be basic.
It is not easy to reduce the whole of a reducible oxide (say oxide of
copper or of iron) from a slag in which it exists as a borate or
silicate; there should be at least enough soda present to liberate it.
When the object is to separate one metal, say copper, without reducing
an unnecessary amount of another (iron) at the same time, a slag with a
good deal of borax is a distinct advantage. The slag then will probably
not be free from copper, so that it will be necessary to powder and mix
the slag with some soda and a reducing agent, and to again fuse the slag
in order to separate this residual metal. In all those cases in which
the slag retains an oxide of a heavy metal, this cleaning of the slag is
advisable, and in the case of rich ores necessary. Slags containing
sulphides are especially apt to retain the more easily reducible metals.
The following are the ordinary and most useful fluxes:--
~Soda.~--The powdered bicarbonate, sold by druggists as "carbonate of
soda," is generally used. It gives off its water and excess of carbonic
acid readily and without fusion. Where the melting down is performed
rapidly, the escaping gas is apt to cause trouble by frothing, and so
causing waste of the material. Ordinary carbonate of soda, when hydrated
(soda crystals), melts easily, and gives off its water with ebullition.
It is unfit for use in assaying, but when dried it can be used instead
of the bicarbonate. One part of the dried carbonate is equivalent to
rather more than one and a half parts of the bicarbonate. From two to
four parts of the flux are amply sufficient to yield a fluid slag with
one part of earthy matter. This statement is also true of the fluxes
which follow.
~Borax~ is a hydrated biborate of soda, containing nearly half its
weight of water. When heated it swells up, loses its water, and fuses
into a glass. The swelling up may become a source of loss in the assay
by pushing some of the contents out of the crucible. To avoid this,
_fused_ or _dried borax_ may be used, in which case a little more than
half the amount of borax indicated will suffice. Borax will flux almost
anything, but it is especially valuable in fluxing lime, &c., and
metallic oxides; as also in those cases in which it is desired to keep
certain of the latter in the slag and out of the button of metal.
~Oxide of Lead~, in the form of red lead or litharge, is a valuable
flux; it easily dissolves those metallic oxides which are either
infusible or difficultly fusible of themselves, such as oxides of iron
or copper. The resulting slag is strongly basic and very corrosive; no
crucible will long withstand the attack of a fused mixture of oxides of
lead and copper. With silicates, also, it forms very fusible double
silicates; but in the absence of silicates and borates it has no action
upon lime or magnesia. Whether the lead be added as litharge or as red
lead, it will exist in the slag as monoxide (litharge); the excess of
oxygen of the red lead is thus available for oxidising purposes. If this
oxidising power is prejudicial, it may be neutralised by mixing the red
lead with 1 per cent. of charcoal.
~Glass~: broken beakers and flasks, cleaned, dried, and powdered will
do. It should be free from lead.
~Fluor~: fluor-spar as free as possible from other minerals, powdered.
It helps to flux phosphate of lime, &c., and infusible silicates.
~Lime~: should be fresh and powdered. It must not be slaked. Powdered
white marble (carbonate of lime) will do; but nearly double the quantity
must be taken. One part of lime produces the same effect as 1.8 parts of
the carbonate of lime.
~Tartar~ and "black flux," are reducing agents as well as fluxes. The
"black flux," which may be obtained by heating tartar, is a mixture of
carbonate of potash and charcoal.
REDUCING AGENTS.--The distinction between reducing agents and fluxes
(too often ignored) is an important one. Fluxes yield slags; reducing
agents give buttons of regulus or of metal. The action of a reducing
agent is the separation of the oxygen or sulphur from the metal with
which it is combined. For example, the mineral anglesite (lead sulphate)
is a compound of lead, sulphur, and oxygen; by carefully heating it with
charcoal the oxygen is taken away by the charcoal, and a regulus of lead
sulphide remains. If the regulus be then fused with metallic iron the
sulphur is removed by the iron, and metallic lead is left. The charcoal
and the iron are reducing agents. But in defining a reducing agent as
one which removes oxygen, or sulphur, from a metallic compound so as to
set the metal free, it must be remembered that sulphur itself will
reduce metallic lead from fused litharge, and that oxygen will similarly
set free the metal in fused lead sulphide. There is no impropriety in
describing sulphur as a reducing agent; but it is absurd to call oxygen
one. Some confusion will be avoided if these substances and those which
are opposite to them in property be classed as oxidising and
de-oxidising, sulphurising, and de-sulphurising agents. Most oxidising
agents also act as de-sulphurisers.
_The de-oxidising agents_ most in use are the following:--
~Charcoal.~--Powdered wood charcoal; it contains more or less
hygroscopic moisture and about 3 or 4 per cent. of ash. The rest may be
considered carbon. Carbon heated with metallic oxides takes the oxygen;
at low temperatures it forms carbon dioxide, and at higher ones, carbon
monoxide. Other conditions besides that of temperature have an influence
in producing these results; and as the quantity of charcoal required to
complete a definite reaction varies with these, it should be calculated
from the results of immediate experience rather than from theoretical
considerations.
~Flour.~--Ordinary wheat flour is convenient in use. On being heated it
gives off inflammable gases which have a certain reducing effect, and a
residue of finely divided carbon is left. It is likely to vary in the
quantity of moisture it contains. Two parts of flour should be used
where one part of charcoal would be otherwise required.
~Tartar.~--This is crude hydric potassic tartrate; the purified salt,
cream of tartar, may be used. On being heated it gives off inflammable
gases, and leaves a residue formed of potassic carbonate mixed with
finely divided carbon. Five parts of tartar should be used in the place
of one of charcoal.
~Anthracite~ or ~Culm~ is a kind of coal containing 90 per cent. or more
of carbon. It gives off no inflammable gas. It is denser, and takes
longer in burning, than charcoal. Its reducing effect is little inferior
to that of charcoal. Almost any organic substance can be used as a
reducing agent, but it is well not to select one which melts, swells up,
or gives off much water and gas when heated in the furnace.
~Potassic Cyanide~ is an easily fusible and somewhat volatile salt,
which, when fused, readily removes oxygen and sulphur from metallic
compounds, and forms potassic cyanate or sulphocyanate as the case may
be. Commercial samples vary much in purity; some contain less than 50
per cent. of the salt. For assaying, only the better qualities should be
used.
~Iron~ is a de-sulphurising rather than a de-oxidising agent. Iron is
used in the form of rods, 1/2-inch in diameter, or of nails, or of hoop
iron. In the last case it should be thin enough to be bent without
difficulty. Wrought iron crucibles are very useful in the processes
required for making galena assays.
_The chief oxidising agents (which are also de-sulphurisers)_ are the
following:--
~Nitre~, or Potassic Nitrate.--This salt fuses very easily to a watery
liquid. It oxidises most combustible substances with deflagration, and
thereby converts sulphides into sulphates, arsenides into arsenates, and
most metals into oxides. In the presence of strong bases, such as soda,
the whole of the sulphur is fully oxidised; but in many cases some
arsenic is apt to escape, and to give rise to a peculiar garlic-like
odour. The sulphates of soda and potash are thus formed, and float as a
watery liquid on the surface of the slag.
~Red lead~ is an oxide of lead. About one-quarter of its oxygen is very
loosely held, and, hence, is available for oxidising purposes, without
any separation of metallic lead. The rest of the oxygen is also
available; but for each part of oxygen given off, about 13 parts of
metallic lead are deposited. In silver assays this power of readily
giving up oxygen is made use of. The residual oxide (litharge) acts as a
flux.
~Hot air~ is the oxidising agent in roasting operations. The sulphur and
arsenic of such minerals as mispickel and pyrites are oxidised by the
hot air and pass off as sulphur dioxide and "white arsenic." The metals
generally remain in the form of oxide, mixed with more or less sulphate
and arsenate. The residue may remain as a powdery substance (a calx), in
which case the process of roasting is termed calcination; or it may be a
pasty mass or liquid. In the calcination of somewhat fusible minerals,
the roasting should be done at a low temperature to avoid clotting;
arsenic and sulphur being with difficulty burnt off from the clotted
mineral. A low temperature, however, favours the formation of sulphates;
and these (if not removed) would reappear in a subsequent reduction as
sulphides. These sulphates may be decomposed by a higher temperature
towards the end of the operation; their removal is rendered more certain
by rubbing up the calx with some culm and re-roasting, or by strongly
heating the calx after the addition of solid ammonic carbonate. In
roasting operations, as large a surface of the substance as possible
should be exposed to the air. If done in a crucible, the crucible should
be of the Cornish type, short and open, not long and narrow. For
calcinations, _roasting dishes_ are useful: these are broad and shallow,
not unlike saucers, but unglazed. In those cases in which the products
of the roasting are liquid at the temperature used, a _scorifier_ (fig.
38) is suitable if it is desired to keep the liquid; but if the liquid
is best drained off as quickly as it is formed, a _cupel_ (fig. 5)
should be used.
[Illustration: FIG. 5.]
A scorifier is essentially a roasting dish sufficiently thick to resist,
for a time, the corrosive action of the fused metallic oxides it is to
contain. The essential property of a cupel is, that it is sufficiently
porous to allow the fused oxide to drain into it as fast as it is
formed. It should be large enough to absorb the whole of the liquid; and
of course must be made of a material upon which the liquid has no
corrosive action. Cupels do not bear transport well; hence the assayer
generally has to make them, or to supervise their making. A quantity of
bone ash is carefully mixed with water so that no lumps are formed, and
the mixture is then worked up by rubbing between the hands. The bone ash
is sufficiently wet when its cohesion is such that it can be pressed
into a lump, and yet be easily crumbled into powder. Cupel moulds should
be purchased. They are generally made of turned iron or brass. They
consist of three parts (1) a hollow cylinder; (2) a disc of metal; and
(3) a piston for compressing the bone ash and shaping the top of the
cupel. The disc forms a false bottom for the cylinder. This is put in
its place, and the cylinder filled (or nearly so) with the moistened
bone ash. The bone ash is then pressed into shape with the piston, and
the cupel finished with the help of three or four smart blows from a
mallet. Before removing the piston, turn it half-way round upon its axis
so as to loosen and smooth the face of the cupel. The cupel is got out
by pressing up the disc of metal forming the false bottom; the removal
is more easily effected if the mould is somewhat conical, instead of
cylindrical, in form. The cupels are put in a warm place to dry for two
or three days. A conveniently sized cupel is 1-1/4 inches in diameter
and about 3/4 inch high. The cavity of the cupel is about 1/4 inch deep,
and something of the shape shown in fig. 5.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
There are two kinds of furnaces required, the "wind" and "muffle"
furnaces. These are built of brick, fire-brick, of course, being used
for the lining. They are connected with a chimney that will provide a
good draught. Figure 6 shows a section of the wind furnace, fig. 7 a
section of the muffle furnace, and fig. 8 a general view of a group
comprising a muffle and two wind furnaces suitable for general work.
When in operation, the furnaces are covered with iron-bound tiles. The
opening under the door of the muffle is closed with a loosely fitting
brick. The floor of the muffle is protected with a layer of bone-ash,
which absorbs any oxide of lead that may be accidentally spilt. The fire
bars should be easily removable.
Few tools are wanted; the most important are some cast-iron moulds,
tongs (fig. 9), stirrers for calcining (fig. 10), and light tongs of a
special form for handling scorifiers and cupels (_see_ SILVER).
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
The coke used should be of good quality; the formation of a fused ash
(clinker), in any quantity, causes ceaseless trouble, and requires
frequent removal. The coke should be broken into lumps of a uniform size
(about 2 in. across) before being brought into the office. The furnace
should be well packed by stirring, raising the coke and not ramming it,
and it should be uniformly heated, not hot below and cold above. In
lighting a furnace, a start is made with wood and charcoal, this readily
ignites and sets fire to the coke, which of itself does not kindle
easily.
In commencing work, add (if necessary) fresh coke, and mix well; make
hollows, and into these put old crucibles; pack around with coke, so
that the surface shall be concave, sloping upwards from the mouths of
the crucibles to the sides of the furnace; close the furnace, and, when
uniformly heated, substitute for the empty crucibles those which contain
the assays. It is rarely advisable to have a very hot fire at first,
because with a gradual heat the gases and steam quietly escape through
the unfused mass, while with too strong a heat these might make some of
the matter in the crucible overflow. Moreover, if the heat should be too
strong at first, the flux might melt and run to the bottom of the
crucible, leaving the quartz, &c., as a pasty mass above; with a gentler
heat combination is completed, and the subsequent fiercer heat simply
melts the fusible compound into homogeneous slag.
The fused material may be left in the crucible and separated from it by
breaking when cold. It is generally more convenient to pour it into
cast-iron moulds. These moulds should be dry and smooth. They act best
when warmed and oiled or black-leaded.
Air entering through the fire-bars of a furnace and coming in contact
with hot coke combines with it, forming a very hot mixture of carbonic
acid and nitrogen; this ascending, comes in contact with more coke, and
the carbonic acid is reduced to carbonic oxide; at the top of the
furnace, or in the flue, the carbonic oxide meeting fresh air, combines
with the oxygen therein and re-forms carbonic acid. In the first and
third of these reactions, much heat is evolved; in the second, the
furnace is cooled a little. It must always be remembered, that the
carbonic oxide of the furnace gases is a reducing agent. When these
gases are likely to exert a prejudicial effect, and a strongly oxidising
atmosphere is required, the work is best done in a _muffle_.
FOOTNOTES:
[3] There is an exception to this, as when the slag is liable to be
acted on when exposed to the air and to the gases of the furnace. In
this case a layer of fused common salt floating on the slag, so as to
protect it from the air and furnace gases, is a distinct advantage.
CHAPTER III.
WET GRAVIMETRIC METHODS.
In _dry assays_ the metal is almost always separated and weighed as
metal; in _wet_ gravimetric assays the metal is more usually weighed in
the form of a definite compound of known composition. The general
methods of working resemble those of ordinary chemical analysis, and
their successful working is greatly helped by a knowledge of, at any
rate, those compounds of the metal which enable it to be separated, and
of those which are the most convenient forms in which it can be weighed.
But the work of the assayer differs from that of the analyst, inasmuch
as the bulk of his estimations are made upon material of practically the
same kind, varying only in richness; consequently in assaying, it is
possible (and necessary) to work on such a definite plan as will involve
the least amount of labour in weighing and calculating.
The assayer connected with mining has generally two classes of material
to deal with: those comparatively rich and those comparatively poor. For
example, silver in bullion and in ores; copper precipitates or regulus,
and copper ores and slags; and "black tin" and tin ores. He is only
occasionally called on to assay the intermediate products. It is
indispensable that he should have an approximate knowledge of the
substance to be determined. With new ores this information is best got
by a qualitative testing. Knowing that only certain bodies are present,
it is evident that the number of separations can be reduced, and that
simple methods can be devised for arriving at the results sought for.
The best method is that which involves the least number of separations.
The reactions must be sharp and complete, and yet not be liable to error
under varying conditions.
To bring the richer and poorer materials under the same conditions for
the assay, a small weight, say 1 gram of the richer, and a larger weight
(5 or 10 grams) of the poorer, substance is weighed up. A method is then
adopted which will concentrate the whole of the metal (either during or
after solution) in a product which need not necessarily be pure. The
work on this product is comparatively easy. In separating small
quantities of a substance from a large bulk of impurities, the group
separations must not as a rule be too much relied on. Very large
precipitates carry down small quantities of bodies not belonging to the
group, more especially when there is a tendency to form weak double
compounds. The re-dissolving and re-precipitating of bulky precipitates
should be avoided.
When a large number of assays of the same kind have to be carried out, a
plan something like the following is adopted:--The samples, after having
been dried, are placed in order on a table at the left hand of the
assayer. He takes the first, marks it with a number, samples and weighs
up the quantity required, and transfers it to a flask, which is
similarly marked. As the weighings are finished, the samples are placed
in the same order on his right hand. The assistant takes the flasks in
batches of four or five at a time to the fume cupboard, where he adds a
measured quantity of acid. When solution has been effected, dilution
with a measured volume is generally necessary. The assayer sees to this
and (whilst the funnels and filters are being prepared) makes any
separation that is necessary. The filters are arranged in order on a
rack (fig. 11), and need not be marked unless the precipitates or
residues have subsequently to be dried. The filters are washed with hot
water, and if the filtrates are wanted flasks are placed beneath, if
not, the solution is drained off down the sink. Precipitation or
reduction (or whatever it may be) is now made; the assistant filters the
prepared samples, one at a time, whilst the assayer is engaged with the
others. The same style of work is continued until the assays are
completed. If one should be spoiled, it is better to allow it to stand
over for assaying along with the next batch. If one filters slowly or is
in any way less forward than the rest, it may lessen the accuracy of the
other assays, owing to oxidation, &c., it should, therefore, be put on
one side. The assays are dealt with in batches of ten or twenty, so that
a large quantity of work can be quickly finished.
[Illustration: FIG. 11.]
When the assays are finished, it is the duty of the assistant to clean
the apparatus (with reagents, if necessary), and to put the vessels in
the place set apart for them. Flasks are best kept inverted on a rack,
so that they may be dry and clean by the next morning. Berlin crucibles
must be cleaned and ignited.
The amount of apparatus employed should be as little as is feasible. The
assay should be carried out as much as possible in the same flask. The
bench must be clean, and altogether free from apparatus not in actual
use. Crucibles and dishes in which weighings are made should be marked
with numbers or letters; and their weights recorded, together with the
date of weighing, in a small ledger, which is kept in the drawer of the
balance. By this means a record of the "wear" of each piece of apparatus
is obtained, and, what is more important, much weighing is saved, and
increased confidence is gained. The weight of each piece of apparatus
need not be taken daily. It will be seen from the record in the book and
a knowledge of the use it has been put to how often a checking of the
weight is necessary. The entries are made in black lead as follows:--
Dish, A. Feb. 3 9.4210 grams.
5 9.4225
6 9.4230
7 9.4200
Platinum vessels and apparatus lose, and porcelain ones slightly gain,
weight with continued use.
The special details of the work is given under each assay; certain
general instructions will be given here.
~Solution.~--It is not always necessary to get the whole of the mineral
in solution, provided the body sought for is either completely dissolved
or altogether left in the residue. It is often only by a qualitative
examination of the solution (or residue, as the case may be) that the
assayer can satisfy himself that it is free from the substance sought.
But previous experience with the same kind of ore will show to what
extent this testing is necessary.
Solution is generally best effected in flasks; but where the resulting
liquid has afterwards to be evaporated to dryness and ignited,
evaporating dishes (fig. 12) are used. With them clock glasses are used
as covers during solution to avoid loss through effervescence.
Evaporating dishes are also best when an insoluble residue has to be
collected, since it is difficult to wash out most residues from a flask.
Bumping occurs less frequently in dishes than in flasks.
[Illustration: FIG. 12.]
After the addition of the acid, and mixing by agitation, the vessel
containing the substance is heated. This is best done on the "hot
plate" (fig. 13). This consists of a slab of cast iron about half or
three-quarters of an inch thick, supported on loose fire bricks, and
heated by two or three ring burners (figs. 14 and 15). The burners are
connected to the gas supply by means of _lead_ tubing, to which they are
soldered. Flasks and dishes after being put on the plate are not further
handled until solution is complete or the evaporation is carried to
dryness. The hot plate is contained in a cupboard so as to be out of the
reach of cold draughts.
[Illustration: FIG. 13.]
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
The action of the acids and other solvents is described in the chapter
on Reagents.
~Precipitation.~--In precipitating add sufficient of the reagent to
complete the reaction. The student must be on his guard against adding a
very large excess, which is the commoner error. In some reactions the
finishing point is obvious enough; either no more precipitate is formed,
or a precipitate is completely dissolved, or some well-marked colour or
odour is developed or removed.
In those cases in which there is no such indication, theoretical
considerations should keep the use of reagents within reasonable limits.
The solutions of the reagents (_see_ REAGENTS) are generally of five or
ten per cent. strength. A small excess over that demanded by theory
should be sufficient.
[Illustration: FIG. 16.]
[Illustration: FIG. 17.]
[Illustration: FIG. 18.]
[Illustration: FIG. 19.]
~Filtration.~--Solutions are best filtered hot whenever the assay allows
of this being so done. The precipitate should be allowed to settle, and
the clear liquid decanted on the filter with the aid of a glass rod if
necessary. The filter-paper must not be too large, but at the same time
it must not be overloaded with the precipitate. There should be ample
room for washing. For general use three sizes of filter-paper are
sufficient. Common quick filtering-paper (English) is best for most work
in assaying. The specially prepared paper (Swedish or Rhenish) is used
for collecting those precipitates which have to be weighed. The papers
are folded as shown in fig. 16, and should not project above the funnel.
The filter-paper works better if damped with hot water. In special cases
filtering is hastened by means of an air-pump. The apparatus used
consists of a water-jet (fig. 17), which is connected with the tap, as
also with a bottle fitted as shown in fig. 18. The pump draws the air
out from the bottle, and atmospheric pressure forces the liquid through
the filter-paper. The bottom of the funnel is provided with a platinum
cone, which supports the filter-paper, and prevents its breaking. The
pump is only used in exceptional cases; nearly all the filtrations
required by the assayer can be made without it. The usual methods of
supporting the funnel during filtration are shown in fig. 19. Where the
filtrate is not wanted, pickle bottles make convenient supports. After
the precipitate has been thrown on the filter, it is washed. In washing,
several washings with a small quantity of water are more effective than
a few with a larger quantity of that fluid. The upper edge of the
filter-paper is specially liable to escape complete washing. Excessive
washing must be avoided; the point at which the washing is complete is
found by collecting a little of the filtrate and testing it. The
precipitate is removed from the filter-paper for further treatment by
opening out the paper and by washing the precipitate with a jet of water
from a wash-bottle into a beaker, or back through the funnel into the
flask. In some cases, when the precipitate has to be dissolved in
anything in which it is readily soluble, solution is effected in the
filter itself allowing the liquid to run through as it is formed.
~Drying and Igniting.~--Precipitates, as a rule, require drying before
being ignited. With small precipitates the filter-paper may be opened
out, and placed on a warm asbestos slab till dry; or the funnel and the
filter with the precipitate is placed in a warm place, and supported by
any convenient means. The heat must never be sufficient to char the
paper. Some precipitates must be dried at a temperature not higher than
100° C. These are placed in the water-oven (fig. 20), and, when
apparently dry, they are taken from the funnel, placed between glasses,
and then left in the oven till they cease to lose weight. Such
precipitates are collected on tared filters. Those precipitates which
will stand a higher temperature are dried in the hot-air oven at a
temperature of from 120° to 150°. The drying is continued until they
appear to be free from moisture, and until the precipitate ceases to
adhere to the filter. In drying sulphides the heat must not be raised to
the melting point of sulphur, since, if there is any free sulphur
present, it fuses and filters through.
[Illustration: FIG. 20.]
The precipitate, having been dried, is transferred to a watch-glass. The
filter-paper is opened out over a sheet of note-paper, and, with a
camel-hair brush, the precipitate is gently brought into the glass. Most
precipitates come away easily, and the transfer can be made without
apparent loss. The watch-glass is covered by the funnel, and the
filter-paper (folded into a quadrant) held by the tweezers and set fire
to with the flame of a Bunsen burner. It is allowed to burn over the
crucible, into which the black bulky ash is allowed to drop, and two or
three drops of nitric acid are then added. The crucible is placed on a
pipe-stem triangle (fig. 21), supported on a tripod. It is at first
heated gently with a Bunsen burner, and afterwards more strongly, until
the residue is free from carbon. It is cooled, and treated with any acid
necessary to convert the small amount of precipitate into the state in
which it is to be weighed; heated again, and cooled. The main
precipitate is transferred to the crucible, and the heating repeated
very gently at first, but more strongly towards the end of the
operation. It is next placed in the muffle, and, after two or three
minutes at a red heat, it is removed and allowed to cool in the
desiccator before weighing. This is for bodies that will bear a red
heat; for those compounds that require a lower temperature the heating
in the muffle is omitted. The muffle used for this purpose must not be
used at the same time for cupelling; a gas muffle (fig. 22), such as one
of Fletcher's, is best. A desiccator (fig. 23) is an air-tight vessel
which prevents access of moisture, &c., to the substance. Usually the
air in it is kept dry by means of a basin containing sulphuric acid.
[Illustration: FIG. 21.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
The crucible is removed from the muffle with the tongs and carried to
the desiccator. It is best, in an office, to have a large desiccator
permanently fixed alongside the balance, into which all substances may
be put before being weighed. The substance is removed from the bench or
muffle in the small hand apparatus generally sold, and carried to the
balance room to be transferred to the large desiccator, where it is
allowed to become thoroughly cold before being weighed. Twenty minutes
is generally the time allowed after ignition before it is advisable to
weigh. Bodies allowed to cool in the air after they have been ignited will
absorb moisture, and hot bodies placed in the balance-pan will disturb
the equilibrium and show false results. Compounds that absorb moisture
must be weighed quickly; they should, therefore, be weighed in covered
vessels. Such compounds are detected by their continually-increasing
weight. They should be ignited and weighed again
in a well-covered dish.
Substances that have been washed with alcohol, ether, or any readily
volatile liquid are dried in the water oven. They quickly dry if there
is no water present, and are generally fit for weighing in less than one
hour. Sometimes drying for a few minutes only will be sufficient.
The weight of the crucible and precipitate having been obtained, the
weight of the crucible and ash is deducted; for example--
Crucible and precipitate 10.183 grams.
Crucible and ash 9.527 "
------
0.656 "
The weight of the ash is best added to that of the crucible. The amount
of ash in filter-papers must not be neglected, although papers are now
made almost free from ash, and the amount to be deducted is found by
taking eight or ten papers and burning them until they become white, and
then weighing the ash. The amount varies from 0.004 to 0.0005 gram for
different papers. Having determined the ash, place in the balance-drawer
three of the filter-papers pinned together, with the weights marked on
them in the way shown in fig. 24, so as to be readily seen when there is
occasion to refer to them.
[Illustration: FIG. 24.]
It must be remembered that the determination of small quantities of
substances generally involves the use of reagents which are often
contaminated, as an impurity, with the body sought for. Thus, in
assaying silver, the oxide of lead or metallic lead used is rarely free
from silver; and in the case of arsenic, the acids, zinc or ferric
chloride are sure to contain arsenic. The same observation applies to
the precipitation of lead by zinc, &c. The errors caused by these
impurities are more marked in the determination of material having small
quantities of metal than in that of ores which contain larger
quantities. Errors of this kind are counteracted or neutralised by
"blank" or "blind" determinations. These consist in carrying out by the
side of and during the assay a duplicate experiment with the reagents
only, which are thereby subjected to the same processes of solution,
evaporation, filtration, &c. The final result thus obtained is deducted
from that given by the assay, the difference gives the corrected result.
In some cases, where it is desired or necessary to have a tangible
residue or precipitate, some _pure_ inert material is added.
CHAPTER IV.
VOLUMETRIC ASSAYS.
These have been already described as those in which the results are got
by measuring, either--(1) the volume of a reagent required to complete
some reaction, or (2) the volume of the resulting product. For example,
if a permanganate of potash solution be added to a solution containing a
weighed amount of iron, dissolved in sulphuric acid, the strong colour
of the permanganate of potash will be removed until a certain quantity
of it has been added. Repeating the experiment, it will be found that
the same amount of iron decolorises the same volume of the permanganate
solution within certain narrow limits of variation, known as "error of
experiment." This error is due to variation in the method of working and
to slight differences in the weighings and measurings; it is present in
all experimental methods, although the limits of variation are wider in
some than in others. Apart from this error of experiment, however, it is
certain that a given volume of the permanganate of potash solution
corresponds to a definite weight of iron, so that if either is known the
other may be calculated. Similarly, if a known weight of zinc (or of
carbonate of lime) be dissolved in hydrochloric acid, a gas will be
given off which can be measured, and so long as the conditions of the
experiment do not vary, the same weight of zinc (or of carbonate of
lime) gives off the same volume of gas. The weight of the one can be
determined from the volume of the other.
Or, again, the quantity of some substances may be measured by the colour
of their solutions, on the principle that, other things being equal, the
colour of a solution depends upon the quantity of colouring matter
present. So that if two solutions of the same substance are equally
coloured they are of equal strength. In this way an unknown may be
compared with a known strength, and a fairly accurate determination may
be made. These three illustrations serve as types of the three chief
classes of volumetric assays--titrometric, gasometric, and colorimetric.
~Titrometric Assays.~--Within the limits of the error of experiment, a
definite volume of a solution or gas represents a certain weight of
metal or other substance, hence the exact weight may be determined by
experiment. The error of experiment may be reduced to insignificant
dimensions by repeating the experiment, and taking the mean of three or
four determinations. This will at the same time show the amount of
variation. Thus, if 0.5 gram of iron were dissolved and found to require
50.3 cubic centimetres of the solution of permanganate of potash, and if
on repeating, 50.4, 50.2, and 50.3 c.c. were required, the experimenter
would be justified in saying that 50.3 c.c. of the permanganate solution
represent 0.5 gram of iron, and that his results were good within 0.2
c.c. of the permanganate solution. So that if in an unknown solution of
iron, 50.5 c.c. of the permanganate solution were used up, he could
state with confidence that it contained a little more than 0.5 gram of
iron. With a larger experience the confidence would increase, and with
practice the experimental error will diminish.
But supposing that the unknown solution required, say, 100.5 instead of
50.5 c.c., he would not be justified in saying that, since 50.3 c.c. are
equivalent to 0.5 gram, 100.6 c.c. are equivalent to twice that amount;
and that, consequently, the unknown solution contained a little less
than 1 gram of iron; or, at least, he could not say it except he (or
some one else) had determined it by experiment. But if on dissolving 1
gram of iron, he found it to require 100.6 c.c. of the solution, and in
another experiment with 0.8 gram of iron that 80.5 c.c. of the solution
were required, he would be justified in stating that _the volume of
solution required is proportional to the quantity of metal present_.
There are a large number of volumetric assays of which this is true, but
that it is true in any particular case can only be proved by experiment.
Even where true it is well not to rest too much weight upon it, and in
all cases the quantity of metal taken, to determine the strength of the
solution used, should not differ widely from that present in the assay.
There are certain terms which should be explained here. When the
solution of a reagent is applied under such conditions that the volume
added can be correctly determined, the operation is called "titrating,"
the solution of the reagent used the "standard solution," and the
process of determining the strength of the standard solution is
"standardising." The "standard" is the quantity of metal equivalent to
100 c.c. of the standard solution.
~Standard Solutions.~--In making these the salt is accurately weighed
and transferred to a litre flask, or to the graduated cylinder, and
dissolved. The method of dissolving it varies in special cases, and
instructions for these will be found under the respective assays.
Generally it is dissolved in a small quantity of liquid, and then
diluted to the mark. For those substances that require the aid of heat,
the solution is made in a pint flask, cooled, and transferred; after
which the flask is well washed out. After dilution, the liquids in the
measuring vessel must be thoroughly mixed by shaking. This is more
easily and better done in the cylinder than in the litre flask. The
solution is next transferred to a dry "Winchester" bottle and labelled.
The label may be rendered permanent by waxing it.
Standard solutions should not be kept in a place exposed to direct
sunlight. Oxidising and reducing solutions, such as those of
permanganate of potash, ferrous sulphate, iodine, hyposulphite of soda,
&c., gradually weaken in strength; the solutions of other salts are more
stable; while those of potassium bichromate and baric chloride are
almost permanent. Solutions of potassium permanganate may be kept for a
month or so without much change. The solutions of hyposulphite of soda
and of iodine should be examined weekly. Ferrous sulphate solutions, if
acidulated with sulphuric acid, may be depended on for two or three
weeks without fresh standardising. Before filling the burette, the
"Winchester" bottle should be well shaken and a portion of about 50 or
100 c.c. poured into a dry beaker or test-glass. Besides the standard
solutions, which are required for titrating an assay, permanent
solutions of the metal or acid of equivalent strength are very useful.
When the finishing point of a titration has been overstepped (_i.e._,
the assay has been "overdone"), a measured volume, say 5 or 10 c.c., of
a solution containing the same metal may be added. The titration can
then be continued, but more cautiously, and the value in "c.c." for the
quantity added be deducted from the final reading.
~Standardising.~--Suppose the object is to standardise a solution of
permanganate similar to that referred to above. A convenient quantity of
iron (say 0.5 gram) would be weighed out, dissolved in dilute sulphuric
acid, and the solution titrated. Suppose 49.6 c.c. of the permanganate
solution are required, then
49.6 : 0.5 :: 100 : _x_
_x_ = 1.008 gram.
This result, 1.008 gram, is the "_standard_." When a gas is measured,
the standard may be calculated in the same way. For example: with 0.224
gram of zinc, 75.8 c.c. of gas were obtained. Then the quantity of zinc
equivalent to 100 c.c. of the gas is got by the proportion.
75.8 : 0.224 :: 100 : _x_
_x_ = 0.2955 gram.
Using the term "standard" in this sense, the following rules hold
good:--
To find the weight of metal in a given substance:--_Multiply the
standard by the number of c.c. used and divide by 100._ For example: a
piece of zinc was dissolved and the gas evolved measured 73.9 c.c. Then
by the rule, 0.2955×73.9/100 should give the weight of the piece of
zinc. This gives 0.2184 gram.
To find the percentage of metal in a given substance:--_Multiply the
standard by the number of c.c. used and divide by the weight of
substance taken._ For example: if 2 grams of a mineral were taken, and
if on titrating with the permanganate solution (standard 1.008) 60.4
c.c. were required, then (1.008×60.4)/2 = 30.44. This is the
percentage.
If the standard is exactly 1 gram, and 1 gram of ore is always taken,
these calculations become very simple. The "c.c." used give at once the
percentage, or divided by 100 give the weight of metal.
If it is desired to have a solution with a standard exactly 1.0 gram, it
is best first to make one rather stronger than this, and then to
standardise carefully. Divide 1000 by the standard thus obtained and the
result will be the number of c.c. which must be taken and be diluted
with water to 1 litre. For example: suppose the standard is 1.008, then
1000/1.008 gives 992, and if 992 c.c. be taken and diluted with water to
1000 c.c. a solution of the desired strength will be obtained. The
standard of this should be confirmed. A simpler calculation for the same
purpose is to multiply the standard by 1000; this will give the number
of c.c. to which 1 litre of the solution should be diluted. In the above
example a litre should be diluted to 1008 c.c.
It has been assumed in these rules that the titration has yielded
proportional results; but these are not always obtained. There can be no
doubt that in any actual re-action the proportion between any two
re-agents is a fixed one, and that if we double one of these then
exactly twice as much of the other will enter into the re-action; but in
the working it may very well be that no re-action at all will take place
until after a certain excess of one or of both of the re-agents is
present. In titrating lead with a chromate of potash solution, for
example, it is possible that at the end of the titration a small
quantity of the lead may remain unacted on; and it is certain that a
small excess of the chromate is present in the solution. So, too, in
precipitating a solution of silver with a standard solution of common
salt, a point is reached at which a small quantity of each remains in
solution; a further addition either of silver or of salt will cause a
precipitate, and a similar phenomenon has been observed in precipitating
a hydrochloric acid solution of a sulphate with baric chloride. The
excess of one or other of the re-agents may be large or small; or, in
some cases, they may neutralise each other. Considerations like these
emphasise the necessity for uniformity in the mode of working. Whether a
process yields proportional results, or not, will be seen from a series
of standardisings. Having obtained these, the results should be arranged
as in the table, placing the quantities of metal used in the order of
weight in the first column, the volumes measured in the second, and the
standards calculated in the third. If the results are proportional,
these standards will vary more or less, according to the delicacy of the
process, but there will be no apparent order in the variation. The
average of the standards should then be taken.
+-------------+---------------+----------+
| Weight. | Volume found. | Standard |
+-------------+---------------+----------+
| 0.2160 gram | 72.9 c.c. | 0.2963 |
| 0.2185 " | 73.9 " | 0.2957 |
| 0.2365 " | 79.9 " | 0.2959 |
| 0.2440 " | 82.3 " | 0.2964 |
| 0.2555 " | 85.9 " | 0.2974 |
+-------------+---------------+----------+
Any inclination that may be felt for obtaining an appearance of greater
accuracy by ignoring the last result must be resisted. For, although it
would make no practical difference whether the mean standard is taken as
0.2961 or 0.2963, it is well not to ignore the possibility that an error
of 0.4 c.c. may arise. A result should only be ignored when the cause of
its variation is known.
In this series the results are proportional, but the range of weights
(0.216-0.2555 gram) is small. All processes yield fairly proportional
results if the quantities vary within narrow limits.
As to results which are not proportional, it is best to take some
imaginary examples, and then to apply the lesson to an actual one. A
series of titrations of a copper solution by means of a solution of
potassic cyanide gave the following results:--
+---------------+---------------+-----------+
| Copper taken. | Cyanide used. | Standard. |
+---------------+---------------+-----------+
| 0.1 gram | 11.9 c.c. | 0.8403 |
| 0.2 " | 23.7 " | 0.8438 |
| 0.3 " | 35.6 " | 0.8426 |
| 0.4 " | 47.6 " | 0.8403 |
+---------------+---------------+-----------+
These are proportional, but by using a larger quantity of acid and
ammonia in the work preliminary to titration, we might have had to use
1 c.c. of cyanide solution more in each case before the finishing point
was reached. The results would then have been:
+---------------+---------------+-----------+
| Copper taken. | Cyanide used. | Standard. |
+---------------+---------------+-----------|
| 0.1 gram | 12.9 c.c. | 0.7752 |
| 0.2 " | 24.7 " | 0.8097 |
| 0.3 " | 36.6 " | 0.8191 |
| 0.4 " | 48.6 " | 0.8230 |
+---------------+---------------+-----------+
It will be noted that the value of the standard increases with the
weight of metal used; and calculations from the mean standard will be
incorrect.
By subtracting the lowest standardising from the highest, a third result
is got free from any error common to the other two; thus:--
0.4 gram = 48.6 c.c. "cyanide."
0.1 " = 12.9 " "
--- ----
0.3 " = 35.7 " "
And the standard calculated from this corrected result is 0.8404.
Further, if 0.3 gram requires 35.7 c.c., then 0.1 gram should require
11.9 c.c., or 1.0 c.c. less than that actually found.
We may therefore use the following rules for working processes which do
not yield proportional results. Make a series of two or three
titrations, using very different quantities of metal in each. Subtract
the lowest of these from the highest, and calculate the standard with
the remainder. Calculate the volume required by this standard in any
case, and find the excess or deficit, as the case may be. If an excess,
subtract it from the result of each titration; if a deficit, add it; and
use the standard in the usual way. The following table shows an actual
example:--
+--------------+---------------+-----------+
| Chalk taken. | Gas obtained. | Standard. |
+--------------+---------------+-----------+
| 0.0873 gram | 17.8 c.c. | 0.4904 |
| 0.1305 " | 27.3 " | 0.4780 |
| 0.1690 " | 35.8 " | 0.4721 |
| 0.1905 " | 40.4 " | 0.4715 |
| 0.2460 " | 52.5 " | 0.4686 |
| 0.3000 " | 64.0 " | 0.4687 |
+--------------+---------------+-----------+
It will be seen that the standard decreases as the quantity of chalk
increases; this points to a deficiency in the quantity of gas evolved.
Then
0.3000 = 64.0 c.c.
0.0873 = 17.8 "
------ = ----
0.2127 = 46.2 "
and 0.2127×100/46.2 = 0.4604. Then, multiplying the weight of chalk
taken by 100, and dividing by 0.4604, we get the calculated results of
the following table:--
+--------------+------------+-----------------+-------------+
| Chalk taken. | Gas found. | Gas calculated. | Difference. |
+--------------+------------+-----------------+-------------+
| 0.0873 gram | 17.8 c.c. | 18.9 c.c. | -1.1 c.c. |
| 0.1305 " | 27.3 " | 28.3 " | -1.0 " |
| 0.1690 " | 35.8 " | 36.7 " | -0.9 " |
| 0.1905 " | 40.4 " | 41.4 " | -1.0 " |
| 0.2460 " | 52.5 " | 53.4 " | -0.9 " |
| 0.3000 " | 64.0 " | 65.1 " | -1.1 " |
+--------------+------------+-----------------+-------------+
By adding 1 c.c. to the quantity of gas obtained, and taking 0.4604 as
the standard, the calculated results will agree with those found with a
variation of 0.1 c.c. When a large number of assays of the same kind are
being made, this method of calculation is convenient; when, however,
only one or two determinations are in question, it is easier to make a
couple of standardisings, taking quantities as nearly as possible the
same as those present in the assays.
Sometimes it is necessary to draw up a table which will show, without
calculation, the weight of substance equivalent to a given volume of gas
or of solution. The substance used for standardising should be, whenever
possible, a pure sample of the substance to be determined--that is, for
copper assays pure copper should be used, for iron assays pure iron, and
so on; but when this cannot be got an impure substance may be used,
provided it contains a known percentage of the metal, and that the
impurities present are not such as will interfere with the accuracy of
the assay. Including compounds with these, the standard may be
calculated by multiplying the standard got in the usual way, by the
percentage of metal in the compound or impure substance, and dividing by
100. If, for example, the standard 1.008 gram was obtained by using a
sample of iron containing 99.7 per cent. of metal, the corrected
standard would be 1.008×99.7/100 = 1.005.
In volumetric analysis the change brought about must be one in which the
end of the reaction is rendered prominent either by a change of colour
or by the presence or absence of a precipitate. If the end of the
reaction or finishing-point is not of itself visible, then it must be
rendered visible by the use of a third reagent called an indicator.
For example, the action of sulphuric acid upon soda results in nothing
which makes the action conspicuous; if, however, litmus or
phenolphthalein be added the change from blue to red in the first case,
or from red to colourless in the second, renders the finishing-point
evident. Some indicators cannot be added to the assay solution without
spoiling the result; in which case portions of the assay solution must
be withdrawn from time to time and tested. This withdrawal of portions
of the assay solution, if rashly done, must result in loss; if, however,
the solution is not concentrated, and if the portions are only withdrawn
towards the end of the titration, the loss is very trifling, and will
not show-up on the result. The usual plan adopted is to have a solution
of the indicator placed in drops at fairly equal intervals distributed
over a clean and dry white porcelain-plate: a drop or two of the
solution to be tested is then brought in contact with one of these and
the effect noted. Another plan is to have thin blotting-paper, moistened
with a solution of the indicator and dried; a drop of the solution to be
tested placed on this shows the characteristic change. When the assay
solution contains a suspended solid which interferes with the test, a
prepared paper covered with an ordinary filter-paper answers very well;
a drop of the solution to be tested is placed on the filter-paper, and,
sinking through, shows its effect on the paper below.
Except when otherwise stated, all titrations should be made at the
ordinary temperature; cooling, if necessary, by holding the flask under
the tap. When a titration is directed to be made in a boiling solution,
it must be remembered that the standard solution is cold, and that every
addition lowers the temperature of the assay.
On running the solution from the burette into the assay, do not let it
run down the side of the flask. If a portion of the assay has to be
withdrawn for testing, shake the flask to ensure mixing, and then take
out a drop with the test-rod; the neglect of these precautions may give
a finishing-point too early. This is generally indicated by a sudden
finish, in which case on shaking the flask and again testing no reaction
is got. Do not remove the drop on the point of the burette with the
test-rod; let it remain where it is or drop it into the solution by
carefully opening the clip.
Generally the methods of working are as follows:--
(1) _When the finishing-point depends on a change of colour in the
solution._--Increase the bulk of the assay up to from 100 to 150 c.c.
with water. Boil or cool, as the case may be. Run in the standard
solution from a burette speedily, until the re-agent appears to have a
slower action, and shake or stir all the time. Then run 1 c.c. or so at
a time, still stirring, and finally add drops until the colour change is
got.
(2) _When an outside-indicator is used._--Pour the standard solution
from a burette into the assay until 5 or 6 c.c. from the
finishing-point; then run in 1 c.c. at a time (stirring and testing on
the plate between each) until the indicator shows the change wanted, and
deduct 0.5 c.c. for excess. When greater accuracy is sought for a
duplicate assay is made. In this case the standard solution is run in
close up to the end, and the operation is finished off with a few drops
at a time.
(3) _Where the finishing-point depends upon the absence of a precipitate
and no outside-indicator is used._--As in the last case, run in the
standard solution up to within a few c.c. of the end, then run in 1 c.c.
at a time until a precipitate is no longer formed, but here 1.5 c.c.
must be deducted for excess, since it is evident that the whole of the
last "c.c." must have been, and a portion of the previous one may have
been, in excess.
~Indirect Titration.~--The action of permanganate of potash upon a
ferrous solution is one of oxidation, hence it is evident that if any
other oxidising agent is present it will count as permanganate. In such
a case the titration can be used (indirectly) to estimate the quantity
of such oxidising agent, by determining how much less of the
permanganate is used. For example, suppose that 1 gram of iron dissolved
in sulphuric acid requires 100 c.c. of standard permanganate to fully
oxidise it, but that the same amount of iron only requires 35.6 c.c. of
the same standard permanganate if it has been previously heated with 0.5
gram of black oxide of manganese. Here it is evident that 0.5 gram of
black oxide does the work of 64.4 c.c.[4] of the permanganate solution,
and that these quantities are equivalent; moreover, if 64.4 c.c.
correspond with 0.5 gram, then 100 c.c. correspond with 0.7764 which is
the standard. On theoretical grounds, and by a method of calculation
which will be explained further on (under the heading "Calculations from
Formulæ"), it can be found that if the standard for iron is 1 gram, that
for the black oxide will be 0.7764 gram.
The principles of these indirect titrations become clearer when
expressed in a condensed form. Thus, in the example selected, and using
the formulæ Fe = Iron, KMnO_{4} = permanganate of potash, and MnO_{2} =
oxide of manganese, we have:--
(1) 1 gram Fe = 100 c.c. KMnO_{4}
(2) 1 gram Fe = 35.6 c.c. KMnO_{4} + 0.5 gram MnO_{2}
.'. 100 c.c. KMnO_{4} = 35.6 c.c. KMnO_{4} + 0.5 gram MnO_{2}
(100-35.6) c.c. KMnO_{4} = 0.5 gram MnO_{2}
64.4 c.c. KMnO_{4} = 0.5 gram MnO_{2}
The iron does not enter into the calculation if the same quantity is
present in the two experiments.
An indirect titration thus requires three determinations, but if more
than one assay is to be carried on, two of these need not be repeated.
The standard is calculated in the usual way.
~Colorimetric Assays.~--These are assays in which the colour imparted to
a solution by some compound of the metal to be determined is taken
advantage of; the depth of colour depending on the quantity of metal
present. They are generally used for the determination of such small
quantities as are too minute to be weighed. The method of working is as
follows:--A measured portion of the assay solution (generally 2/3, 1/2,
1/3, or 1/4 of the whole), coloured by the substance to be estimated, is
placed in a white glass cylinder standing on a sheet of white paper or
glazed porcelain. Into an exactly similar cylinder is placed the same
amount of re-agents, &c., as the portion of the assay solution contains,
and then water is added until the solutions are of nearly equal bulk.
Next, a standard solution of the metal being estimated is run in from a
burette, the mixture being stirred after each addition until the colour
approaches that of the assay. The bulk of the two solutions is equalised
by adding water. Then more standard solution is added until the tints
are very nearly alike. Next, the amount added is read off from the
burette, still more is poured in until the colour is slightly darker
than that of the assay, and the burette read off again. The mean of the
readings is taken, and gives the quantity of metal added. It equals the
quantity of metal in the portion of the assay. If this portion was
one-half of the whole, multiply by two; if one-third, multiply by three,
and so on. When the quantity of metal in very dilute solutions is to be
determined, it is sometimes necessary to concentrate the solutions by
boiling them down before applying the re-agent which produces the
coloured compound. Such concentration does not affect the calculations.
~Gasometric Assays.~--Gasometric methods are not much used by assayers,
and, therefore, those students who wish to study them more fully than
the limits of this work will permit, are recommended to consult Winkler
and Lunge's text-book on the subject. The methods are without doubt
capable of a more extended application. In measuring liquids, ordinary
variations of temperature have but little effect, and variations of
atmospheric pressure have none at all, whereas with gases it is
different. Thus, 100 c.c. of an ordinary aqueous solution would, if
heated from 10° C. to 20° C., expand to about 100.15 c.c. 100 c.c. of a
gas similarly warmed would expand to about 103.5 c.c., and a fall of one
inch in the barometer would have a very similar effect. And in
measuring gases we have not only to take into account variations in
volume due to changes in temperature and atmospheric pressure, but also
that which is observed when a gas is measured wet and dry. Water gives
off vapour at all temperatures, but the amount of vapour is larger as
the temperature increases.
By ignoring these considerations, errors of 3 or 4 per cent. are easily
made; but, fortunately, the corrections are simple, and it is easy to
construct a piece of apparatus by means of which they may be reduced to
a simple calculation by the rule of three.
The volume of a gas is, in practice, usually reduced to that which it
would be at a temperature of 0° C., when the column of mercury in the
barometer is 760 mm. high. But, although convenient, this practice is
not always necessary. The only thing required is some way of checking
the variations in volume, and of calculating what the corrected volume
would be under certain fixed conditions.
Suppose that at the time a series of standardisings is being made, 100
c.c. of air were confined in a graduated tube over moist mercury. These
100 c.c. would vary in volume from day to day, but it would always be
true of them that they would measure 100 c.c. under the same conditions
as those under which the standardisings were made. If, then, in making
an actual assay, 35.4 c.c. of gas were obtained, and the air in the tube
measured 105 c.c., we should be justified in saying, that if the
conditions had been those of the standardising, the 105 c.c. would have
measured 100 c.c., and the 35.4 c.c. would have been 33.7; for 105:
100:: 35.4: 33.7. The rule for using such a piece of apparatus for
correcting volumes is:--_Multiply the c.c. of gas obtained by 100, and
divide by the number of c.c. of air in the apparatus._
If it is desired to calculate the volumes under standard conditions
(that is, the gas dry, at 0° C. and 760 mm. barometric pressure) the
calculations are easily performed, but the temperature and pressure must
be known.
_Correction for Moisture._--The "vapour tension" of water has been
accurately determined for various temperatures, and it may be looked
upon as counteracting the barometric pressure. For example, at 15° C.
the vapour tension equals 12.7 millimetres of mercury; if the barometer
stood at 750 mm., the correction for moisture would be made by
subtracting 12.7 from 750, and taking 737.3 mm. to be the true
barometric pressure.
The vapour tensions for temperatures from 0° C. to 20° C. are as
follows:--
-------+----------++-------+----------++-------+----------
Temp. | Tension. || Temp. | Tension. || Temp. | Tension.
-------+----------++-------+----------++-------+----------
0° | 4.6 mm. || 7° | 7.5 mm. || 14° | 11.9 mm.
1° | 4.9 mm. || 8° | 8.0 mm. || 15° | 12.7 mm.
2° | 5.3 mm. || 9° | 8.6 mm. || 16° | 13.5 mm.
3° | 5.7 mm. || 10° | 9.2 mm. || 17° | 14.4 mm.
4° | 6.1 mm. || 11° | 9.8 mm. || 18° | 15.3 mm.
5° | 6.5 mm. || 12° | 10.5 mm. || 19° | 16.3 mm.
6° | 7.0 mm. || 13° | 11.2 mm. || 20° | 17.4 mm.
-------+----------++-------+----------++-------+-----------
The _correction for pressure_ is:--Multiply the volume by the actual
pressure and divide by 760.
The _correction for temperature_:--Multiply the volume by 273 and divide
by the temperature (in degrees Centigrade) added to 273.
For all three corrections the following rules hold good. _To reduce to
0° C. and 760 mm. dry._
Volume × 0.3592 × (Pressure-tension)
Corrected volume = --------------------------------------
Temperature + 273
To find the volume, which a given volume under standard conditions would
assume, if those conditions are altered.
Volume × 2.784 × (Temperature + 273)
Resulting volume = ------------------------------------
Pressure - tension
As an example, we will suppose that it is desired to enclose in the
apparatus referred to on p. 45, a volume of air, which, when dry (at 0°
C. and 760 mm.), shall measure 100 c.c., whilst the actual temperature
is 15° C., and the pressure 750 mm.
The second formula is the one to be used, and we get 108.7 c.c.
100 c.c.×2.784×288
Required volume = ----------------------
750-12.7
80179.2
= -------
737.3
= 108.7 c.c.
FOOTNOTES:
[4] 100-35.6 = 64.4.
CHAPTER V.
WEIGHING AND MEASURING.
~Weighing.~--The system of weights and measures which we have adopted is
the French or metric system; in this the gram (15.43 grains) is the unit
of weight; the only other weight frequently referred to is the
milligram, which is 0.001, or 1/1000 gram. The unit of volume is the
cubic centimetre, which is approximately the volume of 1 gram of water,
and which thus bears to the gram the same relation as grain-measures
bear to grains. It is usual to write and even pronounce cubic centimetre
shortly as c.c., and the only other denomination of volume we shall have
occasion to use is the "litre," which measures 1000 c.c., and is roughly
1-3/4 pints.
The weights used are kept in boxes in a definite order, so that the
weights on the balance can be counted as well by noting those which are
absent from the box as by counting those present on the scale-pan. The
weights run 50, 20, 10, 10, 5, 2, 1, 1 and 1 grams, and are formed of
brass. The fractions of the gram are generally made of platinum or of
aluminium, and are arranged in the following order:--0.5, 0.2, 0.1, 0.1,
and 0.05, 0.02, 0.01, 0.01. These may be marked in this way, or they may
be marked 500, 200, 100, 100, 50, 20, 10, 10; the 500 meaning 500
milligrams.
Some makers send out weights in the series 50, 20, 20, 10, &c.
Weights of less than 0.01 gram are generally present in a box, but it is
much more convenient to work with a rider. This is a piece of wire which
in the pan weighs 0.01 gram; it is made in such a form that it will ride
on the beam, and its effective weight decreases as it approaches the
centre. If the arm of the beam is divided into tenths, then each tenth
counting from the centre outward equals 0.001 gram or 1 milligram, and
if these tenths be further subdivided the fractions of a milligram are
obtained; and these give figures in the fourth place of decimals. A
fairly good balance should be sensitive to 0.0001 gram. The weights must
never be touched with the fingers, and the forceps for moving them is
used for no other purpose. When not in actual use the box is kept
closed. The weights must not be allowed to remain on the pan of the
balance. The balance-case must not be open without some reason. It must
be fixed level, and, once fixed, must not be needlessly moved. The bench
on which it stands should be used for no other purpose, and no one
should be allowed to lean upon it.
[Illustration: FIG. 25.]
When using a balance sit directly in front of it. Ordinarily the
substance to be weighed is best put on the pan to the user's left; the
weights and the rider are then easily manipulated. Powders, &c., should
not be weighed directly on the balance; a counterpoised watch-glass or
metal scoop (fig. 25) should be used. In some cases it is advisable to
use a weighing-bottle. This is a light, well-stoppered bottle (fig. 3)
containing the powdered ore. It is first filled and weighed; then some
of the substance is carefully poured from it into a beaker or other
vessel, and it is weighed again; the difference in the two weighings
gives the weight of substance taken. A substance must always be cold
when weighed, and large glass vessels should be allowed to stand in the
balance-box a little while before being weighed. Always have the balance
at rest when putting on or taking off anything from the pans. Put the
weights on systematically. In using the rider (except you have a reason
to the contrary), put it on at the 5; if this is too much, then try it
at the 3; if then the weights are too little, try at the 4, if still not
enough, the correct weight must be between the 4 and 5; try half-way
between.
It is best to work with the balance vibrating; equilibrium is
established when the vibration to the left is the mean of the preceding
and succeeding vibrations to the right. For example, if it vibrates 6
divisions to the right on one swing, and 5 divisions on the next, the
intermediate vibration to the left should have been 5-1/2.
Note whether the substance increases in weight whilst on the balance. If
it does it may be because it was put on warm, and is cooling, or it may
be because it is taking up moisture from the air. Substances which take
up moisture rapidly should be weighed in clipped watch-glasses or in
light-weighing bottles or tubes.
Students, in recording the weights, should first read off those missing
from the box, writing down each order of figures as determined; first
tens, then units, and so on. Remember that the first four platinum
weights give the figures of the first place of decimals, the second four
give the second place, and that the third and fourth places are given by
the rider. Having taken down the figures, confirm them by reading off
the weights as you put them back into the box. Do not rest a weight on
the palm of your hand for convenience in reading the mark upon it.
Remember one weight lost from a box spoils the set. Do not take it for
granted that the balance is in equilibrium before you start weighing:
try it.
[Illustration: FIG. 26.]
~Measuring Liquids.~--For coarse work, such as measuring acids for
dissolving ores, graduated glasses similar to those used by druggists
may be used. It is well to have two sizes--a smaller graduated into
divisions of 5 c.c. (fig. 26), and a larger with divisions equal to 10
c.c. No measurement of importance should be made in a vessel of this
kind, as a slight variation in level causes a serious error.
~Graduated flasks~ must be used when anything has to be made up to a
definite bulk, or when a fixed volume has to be collected. If, for
example, a certain weight of substance has to be dissolved and diluted
to a litre, or if the first 50 c.c. of a distillate has to be collected,
a flask should be used. Each flask is graduated for one particular
quantity; the most useful sizes are 1000 c.c., 500 c.c., 200 c.c., 100
c.c., and 50 c.c. The mark should be in the narrowest part of the neck,
and should be tangential to the curved surface of the liquid when the
flask _contains_ the exact volume specified. The level of a curved
surface of liquid is at first somewhat difficult to read: the beginner
is in doubt whether the surface should be taken at A, B, or C (fig. 27).
It is best to take the lowest reading C. In some lights it is difficult
to find this; in such cases a piece of white paper or card held behind
and a little below, so as to throw light up and against the curved
surface, will render it clear. In reading, one should look neither up at
nor down upon the surface, but the eye should be on the same level with
it. It must be kept in mind that flasks _contain_ the quantity
specified, but deliver less than this by the amount remaining in them
and damping the sides. If it is desired to transfer the contents say of
a 100 c.c. flask to a beaker, it will be necessary to complete the
transfer by rinsing out the flask and adding the washings; otherwise
there will be a sensible loss. Graduated cylinders (fig. 28) are
convenient for preparing standard solutions.
[Illustration: FIG. 27.]
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
~Pipettes~ and burettes are graduated to _deliver_ the quantities
specified. The principle of the pipette, and the advantages and
disadvantages of its various forms, may be understood by considering the
first form shown in fig. 29. It is essentially a bulbed tube drawn out
to a jet at its lower end, and having on each side of the bulb a mark so
placed that when the surface of the liquid falls from the upper to the
lower mark the instrument shall deliver exactly 100 c.c. The bore of the
jet should be of such a size as will allow the level of the liquid to
fall at the rate of about one foot in two minutes. If it runs more
quickly than this, an appreciable error arises from the varying amount
of liquid remaining, and damping the sides of the bulb. The flow of
liquid from a pipette must not be hastened by blowing into it. The lower
tube or nose of the pipette should be long enough to reach into the
bottle or flask containing the liquid about to be measured. The pipette
is filled by sucking at the open end with the mouth; this method of
filling renders the use of the instrument dangerous for such liquids as
strong acids, ammonia, and such poisonous solutions as that of potassic
cyanide. One attempt with a fairly strong solution of ammonia will teach
the beginner a very useful lesson. As soon as the liquid rises above the
upper mark in the pipette, the mouth is withdrawn, and the pipette
quickly closed by pressing the upper aperture with the index finger of
the right hand; it is well to have the finger slightly moist, but not
damp. The neck of the pipette should be long enough to allow its being
firmly grasped by the fingers and thumb of the right hand without
inconvenience. The pipette is first held in a vertical position long
enough to allow any moisture outside the tube to run down, and then the
liquid is allowed to run out to the level of the upper mark; this is
easily effected by lessening the pressure. If the finger is wet, the
flow will be jerky, and good work impossible. The pipette is next held
over the vessel into which the 100 c.c. are to be put, and the liquid
allowed to run out. When the bulb is nearly empty, the flow should be
checked by replacing the finger, and the liquid allowed to escape slowly
until the lower mark is reached. The pipette is then withdrawn; it is in
the withdrawing that the disadvantage of this particular form[5] makes
itself felt. It must be withdrawn very steadily, as the slightest shock
causes the remaining column of liquid to vibrate, whereby air is drawn
in and the liquid is forced out.
This disadvantage is got rid of by making the mouth of the jet the lower
limit, or, in other words, allowing the instrument to empty itself.
There are two forms of such pipettes; in the one generally recommended
in Gay-Lussac's silver assay (the last shown in fig. 29) the nose is
replaced by a jet. This is most conveniently filled by stopping the jet
with the finger, and allowing the liquid to flow in a fine stream into
the neck until the pipette is filled, and then working as just
described. The other form is the one in general use; in fact, a long
nose to a pipette is so convenient that it may almost be said to be
necessary. But the accuracy is slightly diminished; a long narrow tube
makes a poor measuring instrument because of the amount of liquid it
finally retains. A defect possessed by both forms is the retention of a
drop of varying size in the nozzle. Whatever method is adopted for
removing this drop must be always adhered to. The most convenient form
is the one last described, and the most useful sizes are 100 c.c., 50
c.c., 20 c.c., 10 c.c., and 5 c.c. Ten c.c. pipettes graduated into
tenths of a cubic centimetre are very useful: those are best in which
the graduation stops short of the bottom.
All measurements should be made at the ordinary temperature; and, before
being used, the pipette should be rinsed out with a cubic centimetre or
so of the solution to be measured. After using, it should be washed out
with water.
~Burettes~ differ mainly from pipettes in having the flow of liquid
controlled from below instead of from above. The best form is that known
as Mohr's, one kind of which is provided with a glass stopcock, while
the other has a piece of india-rubber tube compressed by a clip. The
latter cannot be used for solutions of permanganate of potash or of
iodine, or of any substance which acts on india-rubber; but in other
respects there is little to choose between the two kinds. A burette
delivering 100 c.c., and graduated into fifths (_i.e._, each division =
0.2 c.c.), is a very convenient size. For some kinds of work, 50 c.c.
divided into tenths (_i.e._, each division = 0.1 c.c.) may be selected.
Burettes may be fixed in any convenient stand; they must be vertical and
should be so placed that the assayer can read any part of the graduated
scale without straining. When not in use, they should be kept full of
water. When using a burette, the water must be run out; the burette is
next rinsed with some of the solution to be used, and drained; and then
it is filled with the solution. Next squeeze the india-rubber tube so as
to disentangle air-bubbles and, by smartly opening the clip, allow the
tube and jet to be filled; see that no bubbles of air are left. Then
run out cautiously until the level of the liquid in the burette stands
at zero. In reading the level with very dark-coloured liquids it is
convenient to read from the level A (fig. 27), and, provided it is done
in each reading, there is no objection to this. The accuracy of the
reading of a burette is sensibly increased by the use of an Erdmann
float. This is an elongated bulb, weighted with mercury, and fitting
(somewhat loosely) the tube of the burette. It floats in the solution,
and is marked with a horizontal line; this line is taken as the level of
the liquid. If the burette is filled from the top, the float rises with
aggravating slowness, and this is its chief disadvantage. The float must
come to rest before any reading is made.
[Illustration: FIG. 30.]
A convenient plan for filling a burette from below is shown in fig. 30.
The diagram explains itself. The bottle containing the standard solution
is connected with the burette by a syphon arrangement through the glass
tube and T-piece. The flow of liquid into the burette is controlled by
the clip. When this clip is opened, the burette fills; and when it is
closed, the burette is ready for use in the ordinary way.
~Measuring Gases.~--Lange's nitrometer (fig. 69) is a very convenient
instrument for many gasometric methods. It requires the use of a fair
quantity of mercury. In fig. 31, there is a representation of a piece of
apparatus easily fitted up from the ordinary material of a laboratory.
It is one which will serve some useful purposes. It consists of a
wide-mouthed bottle fitted (by preference) with a rubber cork. The cork
is perforated, and in the perforation is placed a glass tube which
communicates with the burette. The burette is connected by a rubber tube
and a Y-piece, either with another burette or with a piece of ordinary
combustion-tube of about the same size. The wide-mouthed bottle contains
either a short test-tube or an ordinary phial with its neck cut off. In
working the apparatus the weighed substance is put in the bottle and the
re-agent which is to act on it, in the test-tube; the cork is then
inserted. The liquid in the two burettes is next brought to the same
level, either by pouring it in at A or running it out at B. The level of
the liquid in the apparatus for correcting variation in volume is then
read and noted. Next, after seeing that the level of the liquid in the
burette has not changed, turn the bottle over on its side so that the
re-agent in the test-tube shall be upset into the bottle. Then, as the
volume of the gas increases, lower the liquid in the burette by running
it out at B, and at the same time keep the level in A half an inch or so
lower than that in the burette. When the action has finished bring the
liquid in the two vessels to the same level and read off the burette.
This part of the work must always be done in the same manner.
[Illustration: FIG. 31.]
_The volume corrector for gas analysis_ is a graduated glass tube of 120
c.c. capacity inverted over a narrow glass cylinder of mercury. It
contains 0.2 or 0.3 c.c. of water and a volume of air, which, if dry and
under standard conditions, would measure 100 c.c. The actual volume
varies from day to day, and is read off at any time by bringing the
mercury inside and outside to the same level. This is done by raising or
lowering the tube, as may be required. Any volume of gas obtained in an
assay can be corrected to standard temperature and pressure by
multiplying by 100 and dividing by the number of c.c. in the corrector
at the time the assay is made.
FOOTNOTES:
[5] It is best to use this form with a glass stopcock, or with an
india-rubber tube and clip, after the manner of a Mohr's burette.
CHAPTER VI.
RE-AGENTS.--ACIDS, ETC.
~Acetic Acid~, H[=A=c] or C_{2}H_{4}O_{2}. (sp. gr. 1.044, containing 33
per cent. real acid).--An organic acid, forming a class of salts,
acetates, which are for the most part soluble in water, and which, on
ignition, leave the oxide or carbonate of the metal. It is almost always
used in those cases where mineral acids are objectionable. To convert,
for example, a solution of a substance in hydrochloric acid into a
solution of the same in acetic acid, alkali should be added in excess
and then acetic acid. Many compounds are insoluble in acetic acid, which
are soluble in mineral acids, such as ferric phosphate, ferric arsenate,
zinc sulphide, calcium oxalate, &c., so that the use of acetic acid is
valuable in some separations. The commercial acid is strong enough for
most purposes, and is used without dilution.
~"Aqua Regia"~ is a mixture of 1 part by measure of nitric acid and 3
parts of hydrochloric acid. The acids react forming what is practically
a solution of chlorine.[6] The mixture is best made when wanted, and is
chiefly used for the solution of gold and platinum and for "opening up"
sulphides. When solutions in aqua regia are evaporated, chlorides are
left.
~Bromine~, Br. (sp. gr. 3.0). Practically pure bromine.--It is a heavy
reddish-brown liquid and very volatile. It boils at 60° C., and,
consequently, must be kept in a cool place. It gives off brown
irritating vapours, which render its use very objectionable. Generally
it answers the same purpose as aqua regia, and is employed where the
addition of nitric acid to a solution has to be specially avoided. It is
also used for dissolving metals only from ores which contain metallic
oxides not desired in the solution.
~"Bromine Water"~ is simply bromine shaken up with water till no more is
dissolved.
~Carbonic Acid~, CO_{2}.--A heavy gas, somewhat soluble in water; it is
mainly used for providing an atmosphere in which substances may be
dissolved, titrated, &c., without fear of oxidation. It is also used in
titrating arsenic assays with "iodine" when a feeble acid is required
to prevent the absorption of iodine by the alkaline carbonate. It is
prepared when wanted in solution, by adding a gram or so of bicarbonate
of soda and then as much acid as will decompose the bicarbonate
mentioned. When a quantity of the gas is wanted, it is prepared, in an
apparatus like that used for sulphuretted hydrogen, by acting on
fragments of marble or limestone with dilute hydrochloric acid.
~Citric Acid~ (H_{3}[=C=i] or C_{6}H_{8}O_{7}.H_{2}O) is an organic acid
which occurs in colourless crystals, soluble in less than their weight
of water. The solution must be freshly prepared, as it gets mouldy when
kept. It forms a comparatively unimportant class of salts (citrates). It
is used in the determination of phosphoric acid, chiefly for the purpose
of preventing the precipitation of phosphates of iron and alumina by
ammonia, and in a few similar cases. The commercial crystals are used;
they should be free from sulphuric acid and leave no ash on ignition.
~Hydrochloric Acid~, HCl in water, (sp. gr. 1.16. It contains 32 per
cent. of hydrogen chloride).--It is sometimes called "muriatic acid,"
and when impure, "spirit of salt." The acid solution should be
colourless and free from arsenic, iron, and sulphuric acid. It forms an
important family of salts, the chlorides. It is the best acid for
dissolving metallic oxides and carbonates, and is always used by the
assayer when oxidising agents are to be avoided. The acid is used
without dilution when no directions are expressly given to dilute it. It
has no action on the following metals: gold, platinum, arsenic, and
mercury; it very slightly attacks antimony, bismuth, lead, silver, and
copper. Tin is more soluble in it, but with difficulty; whilst iron,
zinc, nickel, cobalt, cadmium, and aluminium easily dissolve with
evolution of hydrogen and the formation of the lower chloride if the
metal forms more than one class of salts. All the metallic oxides,
except a few of the native and rarer oxides, are dissolved by it with
the formation of chlorides of the metal and water.
~Dilute Hydrochloric Acid~ is made by diluting the strong acid with an
equal volume of water. This is used for dissolving precipitates obtained
in the general course of analysis and the more easily soluble metals.
~Hydrofluoric Acid, HF.~--A solution in water may be purchased in
gutta-percha or lead bottles. It is of variable strength and doubtful
purity. It must always be examined quantitatively for the residue left
on evaporation. It is used occasionally for the examination of
silicates. It attacks silica, forming fluoride of silicon, which is a
gas. When the introduction of another base will not interfere with the
assay, the substance may be mixed in the platinum dish with fluoride of
ammonium, or of potassium, or of calcium, and hydrochloric acid,
instead of treating it with the commercial acid. It is only required in
special work. The fumes and acid are dangerous, and, of course, glass or
porcelain vessels cannot be used with it.
~Iodine, I.~--This can be obtained in commerce quite pure, and is often
used for standardising. It is very slightly soluble in water, but
readily dissolves in potassium iodide solution. It closely resembles
chlorine and bromine in its properties, and can be used for dissolving
metals without, at the same time, attacking any oxide which may be
present. It is chiefly used as an oxidizing agent in volumetric work,
being sharp in its reactions and easily detected in minute quantities.
It cannot be used in alkaline solutions, since it reacts with the
hydrates, and even with the carbonates, to form iodides and iodates.
Iodine is soluble in alcohol.
~Nitric Acid, HNO_{3}.~ (Sp. gr. 1.42; boiling point 121° C.; contains
70 per cent. by weight of hydrogen nitrate).--It is convenient to
remember that one c.c. of this contains 1 gram of real acid. It combines
the properties of an acid and of an oxidising agent. One c.c. contains
0.76 gram of oxygen, most of which is very loosely held, and easily
given up to metals and other oxidisable substances. Consequently it will
dissolve many metals, &c., upon which hydrochloric acid has no action.
All sulphides (that of mercury excepted) are attacked by it, and for the
most part rendered soluble. It has no action on gold or platinum, and
very little on aluminium. The strong acid at the ordinary temperature
does not act on iron or tin; and in most cases it acts better when
diluted. Some nitrates being insoluble in nitric acid, form a protecting
coat to the metal which hinders further action. Where the strong acid
does act the action is very violent, so that generally it is better to
use the dilute acid. When iron has been immersed in strong nitric acid
it not only remains unacted on, but assumes a _passive_ state; so that
if, after being wiped, it is then placed in the dilute acid, it will not
dissolve. Tin and antimony are converted into insoluble oxides, while
the other metals (with the exception of those already mentioned)
dissolve as nitrates. During the solution of the metal red fumes are
given off, which mainly consist of nitrogen peroxide. The solution is
often coloured brown or green because of dissolved oxides of nitrogen,
which must be got rid of by boiling. Generally some ammonium nitrate is
formed, especially in the cases of zinc, iron, and tin, when these are
acted on by cold dilute acid. Sulphur, phosphorus, and arsenic are
converted into sulphuric, phosphoric, and arsenic acids respectively,
when boiled with the strong acid.
~Dilute Nitric Acid.~--Dilute 1 volume of the strong acid with 2 of
water.
~Oxalic Acid~, H_{2}[=O] or (H_{2}C_{2}O_{4}.2H_{2}O.)--This is an organic
acid in colourless crystals. It forms a family of salts--the oxalates.
It is used in standardising; being a crystallised and permanent acid, it
can be readily weighed. It is also used in separations, many of the
oxalates being insoluble. For general use make a 10 per cent. solution.
Use the commercially pure acid. On ignition the acid should leave no
residue.
[Illustration: FIG. 32.]
~Sulphuretted Hydrogen.~ Hydrosulphuric acid, SH_{2}.--A gas largely
used in assaying, since by its action it allows of the metals being
conveniently classed into groups. It is soluble in water, this liquid
dissolving at the ordinary temperature about three times its volume of
the gas. The solution is only useful for testing. In separations, a
current of the gas must always be used. It is best prepared in an
apparatus like that shown in fig. 32, by acting on ferrous sulphide with
dilute hydrochloric acid. When iron has to be subsequently determined in
the assay solution, the gas should be washed by bubbling it through
water in the smaller bottle; but for most purposes washing can be
dispensed with. The gas is very objectionable, and operations with it
must be carried out in a cupboard with a good draught. When the
precipitation has been completed, the apparatus should always be washed
out. The effect of this acid on solutions of the metals is to form
sulphides. All the metallic sulphides are insoluble in water; but some
are soluble in alkaline, and some in acid, solutions. If sulphuretted
hydrogen is passed through an acid solution containing the metals till
no further precipitation takes place, a precipitate will be formed
containing sulphides insoluble in the acid. On filtering, adding ammonia
(to render the filtrate alkaline), and again passing the gas, a further
precipitate will be obtained, consisting of sulphides insoluble in an
alkaline solution, but not precipitable in an acid one; the filtrate may
also contain sulphides not precipitable in an acid solution, which are
soluble in an alkaline one; these will be thrown down on neutralising.
Again, the metals precipitated in the acid solution form sulphides which
may be divided into groups, the one consisting of those which are
soluble, and the other of those which are not soluble, in alkalies. This
classification is shown in the following summary:--
1. _Precipitable in an acid solution._
(a) Soluble in Alkalies.--Sulphides of As, Sb, Sn, Au, Pt, Ir, Mo, Te,
and Se.
(b) Insoluble in Alkalies.--Sulphides of Ag, Pb, Hg, Bi, Cu, Cd, Pd, Rh,
Os, and Ru.
2. _Not precipitated in an acid solution, but thrown down in an alkaline
one._
Sulphides of Mn, Zn, Fe, Ni, Co, In, Tl, and Ga.
These can again be divided into those which are dissolved by dilute
acids and those which are not.
3. _Not precipitated in an acid or alkaline solution, but thrown
down on neutralising the latter._
Sulphides of V and W.
Sulphuretted hydrogen is a strong reducing agent. Ferric salts are
thereby quickly reduced to ferrous; in hot solutions nitric acid is
decomposed. These changes are marked by a precipitation of sulphur, and
the student must be careful to pass the gas sufficiently long, and not
be too hasty in concluding that no sulphide will form because it does
not at once make its appearance. The best indication that it has been
passed long enough is the smell of the gas in the solution after
shaking.
~Sulphurous Acid~, H_{2}SO_{3}.--The reagent used may be regarded as a
saturated solution of sulphur dioxide in water. It may be purchased, and
keeps for a long time. It may be made by heating copper with sulphuric
acid and passing the gas formed into water. The heat should be withdrawn
when the gas is coming off freely. It is used as a reducing agent, and
should not be diluted.
~Sulphuric Acid~, H_{2}SO_{4}. (Sp. gr. 1.84, containing 96 per cent. of
real acid, H_{2}SO_{4}.)--This acid forms insoluble sulphates with salts
of lead, strontium, and barium. It has a high boiling point, 290° C.,
and, when evaporated with salts of the more volatile acids, converts
them into sulphates. When nitrates or chlorides are objectionable in a
solution, evaporation with sulphuric acid removes them. In working with
this acid caution is necessary, since, on mixing with water, great heat
is evolved; and, if either the acid or water has been previously heated,
a serious accident may result. In diluting the acid it should be poured
into cold water. Glass vessels containing boiling sulphuric acid should
be handled as little as possible, and should not be cooled under the
tap. The action of diluted sulphuric acid on metals closely resembles
that of dilute hydrochloric acid. Magnesium, aluminium, iron, zinc,
nickel, cobalt, manganese, and cadmium dissolve, with evolution of
hydrogen, in the cold acid, or when warmed. The action of hot and strong
sulphuric acid is altogether different; it acts as an oxidising agent,
and is itself reduced to sulphur dioxide or even to sulphur. The
following metals are attacked in this way:--copper, bismuth, mercury,
silver, antimony, tin, and lead. Gold, platinum, and arsenic are not
affected. This property is made use of in parting silver from gold and
platinum. Metallic sulphides are similarly attacked; but this method of
opening up minerals has the disadvantage of giving rise to the formation
of anhydrous sulphates of iron, &c., which are not readily dissolved
when afterwards diluted. The use of sulphuric acid in assaying is (for
these reasons) to be avoided. Its chief use is as a drying agent, since
it has a strong affinity for water. Air under a bell jar may be kept dry
by means of a basin of sulphuric acid, and gases bubbled through it are
freed from water-vapour.
~Dilute Sulphuric Acid.~--This is made by diluting 1 volume of the
strong acid with 4 of water.
~Tartaric Acid~, H_{2}[=T] or C_{4}H_{6}O_{6}.--A crystallised organic
acid, soluble in less than its own weight of water, or in less than
three parts of alcohol. It is used for the same purposes as citric acid
is. The solution is made when required.
BASES, SALTS, &c.
~Alcohol~, C_{2}H_{6}O. (Commercial alcohol of sp. gr. 0.838; it
contains 84 per cent. by weight of alcohol.)--It should burn with a
non-luminous flame and leave no residue. It is used for washing
precipitates where water is inapplicable, and for facilitating drying.
~Ammonia~, NH_{3}. (Commercial ammonia, a solution having a sp. gr. of
0.88 to 0.89, and containing about 33 per cent. of ammonia.)--It is used
as an alkali (more commonly than soda or potash), since an excess of it
is easily removed by boiling. The salts of ammonium formed by it may be
removed by igniting, or by evaporating in a porcelain dish with an
excess of nitric acid. It differs in a marked way from soda or potash in
its solvent action on the oxides or hydrates of the metals. Salts of the
following metals are soluble in an ammoniacal solution in the presence
of ammonic chloride:--copper, cadmium, silver, nickel, cobalt,
manganese, zinc, magnesium, sodium, potassium, and the alkaline earths.
~Dilute Ammonia~ is made by diluting 1 vol. of commercial ammonia with 2
of water. The dilute ammonia is always used; but in assays for copper a
stronger solution (1 of strong ammonia to 1 of water) is required.
~Ammonic Carbonate~ (Am_{2}CO_{3}) is prepared by dissolving one part of
the commercial sesquicarbonate of ammonia in four parts of water, and
adding one part of strong ammonia.
~Ammonic Bicarbonate~ (HAmCO_{3}) is prepared by saturating a solution
of the sesquicarbonate of ammonia with carbon dioxide.
~Ammonic Chloride~, AmCl.--Use the commercial salt in a 20 per cent.
solution in water. The salt should leave no residue on ignition.
~Ammonic Molybdate.~--The solution is prepared as follows:--Dissolve 100
grams of the powdered commercial salt in 200 c.c. of dilute ammonia, and
pour the solution in a slow stream into 750 c.c. of dilute nitric acid;
make up to 1 litre, and allow the mixture to settle before using. It is
used for the purpose of separating phosphoric oxide from bases and from
other acids, and also as a test for phosphates and arsenates. In using
this solution the substance must be dissolved in nitric acid, and a
considerable excess of the reagent added (50 c.c. is sufficient to
precipitate 0.1 gram P_{2}O_{5}); when the phosphate is in excess no
precipitate will be got. The precipitate is phospho-molybdate of
ammonia.
~Ammonic Nitrate~ (AmNO_{3}) is used in the separation of phosphoric
oxide by the molybdate method, and occasionally for destroying organic
matter. It is soluble in less than its own weight of water. The solution
is made when wanted.
~Ammonic Oxalate~ (Am_{2}C_{2}O_{4}.2H_{2}O) is used chiefly for the
separation of lime. The solution is made by dissolving 15 grams of the
salt in 100 c.c. of water.
~Ammonic Sulphide~ may be purchased in the state of a strong solution.
It is yellow, and contains the disulphide, S_{2}Am_{2}. It serves the
same purpose as is obtained by passing a current of sulphuretted
hydrogen through an ammoniacal solution; but has the disadvantage of
loading the solution with sulphur, which is precipitated when the
solution is subsequently acidified. It is useful for dissolving the
lower sulphide of tin (SnS).
~Baric Carbonate~ (BaCO_{3}) is sometimes used for precipitating the
weaker bases. It should be prepared when wanted by precipitating a
solution of baric chloride with ammonic carbonate and washing. The moist
precipitate is used without drying.
~Baric Chloride~, BaCl_{2}.2H_{2}O.--A crystallised salt, soluble in
2-1/2 parts of water. It is used for the detection and separation of
sulphates. Make a 10 per cent. solution.
"~Black Flux.~"--A mixture of finely divided carbon with carbonate of
potash or with carbonates of potash and soda. It is prepared by heating
tartar or "rochelle salt" until no more combustible gas is given off.
One gram will reduce about 2 grams of lead from litharge.
~Borax~, Na_{2}B_{4}O_{7}.10H_{2}O.--It is chiefly used as a flux in dry
assaying, as already described. It is also used in testing before the
blowpipe; many metallic oxides impart a characteristic colour to a bead
of borax in which they have been fused.
~Calcium Chloride.~--The crystallised salt is CaCl_{2}.6H_{2}O; dried at
200° C. it becomes CaCl_{2}.2H_{2}O, and when fused it becomes
dehydrated. The fused salt, broken into small lumps, is used for drying
gases. It combines with water, giving off much heat; and dissolves in a
little more than its own weight of water. Strong solutions may be used
in baths in which temperatures above the boiling-point of water are
required. One part of the salt and 2 of water give a solution boiling at
112°, and a solution of 2 parts of the salt in 1 of water boils at 158°.
The salt is very little used as a reagent.
~Calcium Fluoride~ or "~Fluor Spar~," CaF_{2}.--The mineral is used as a
flux in dry assaying; it renders slags which are thick from the presence
of phosphates, &c., very fluid. Mixed with hydrochloric acid it may
sometimes be used instead of hydrofluoric acid.
~Calcium Carbonate~, CaCO_{3}.--It is precipitated in a pure state by
ammonic carbonate from a solution of calcium chloride. It is used for
standardising. In the impure state, as marble or limestone, it is used
in the preparation of carbonic acid.
~Calcium Hydrate~ or ~"Lime Water."~--This is used in testing for carbon
dioxide and in estimating the amount of that gas present in air. It may
be made by slaking quicklime and digesting the slaked lime with water.
One hundred c.c. of water at 15° C. dissolves 0.1368 grams of the
hydrate (CaH_{2}O_{2}), and hot water dissolves still less. "_Milk of
lime_" is slaked lime suspended in water.
~Cobalt Nitrate~ (Co(NO_{3})_{2}.6H_{2}O) is used in a 10 per cent.
solution for the detection of oxides of zinc, aluminium, &c.; on
ignition with which it forms characteristically coloured compounds.
~Copper~, Cu.--Pure copper, as obtained by electrolysis, can be
purchased. This only should be used.
~Copper Oxide~, CuO.--It occurs as a black, heavy, and gritty power, and
is used for the oxidation of carbon and hydrogen in organic substances.
It should be ignited and cooled out of contact with air just before
using, since it is hygroscopic. Oxide of copper which has been used may
be again utilised after calcination.
~Copper Sulphate~ (CuSO_{4}.5H_{2}O) contains 25.4 per cent. of copper.
It is used in the outer cell of a Daniell-battery. The commercial salt
is used for this purpose. The re-crystallised and pure salt is used for
preparing the anhydrous sulphate, which is used for detecting moisture
in gases. For this purpose it is dried at 200° C. till no trace of
green or blue colour remains. It must be prepared when wanted. It may be
conveniently used in the form of pumice-stone, saturated with a solution
of the salt and dried. Traces of moisture develop a green colour.
~Ferric Chloride~, Fe_{2}Cl_{6}. (When crystallised,
Fe_{2}Cl_{6}.6H_{2}O.)--The solution is prepared as described under
iron. The commercial salt contains arsenic, and, since the chief use of
ferric chloride is for the determination of this substance, it must be
purified (_see_ under ARSENIC).
~Ferric Sulphate~ (Fe_{2}(SO_{4})_{3}) is a yellowish white deliquescent
salt. It is used as an indicator in volumetric silver assaying, and for
the separation of iodine from bromine. It may be purchased as iron alum,
Am_{2}Fe_{2}(SO_{4})_{4}.24H_{2}O. But it is best prepared by adding
strong sulphuric acid to ferric hydrate in equivalent proportions. Use
it as a solution containing 2 or 3 per cent. of iron.
~Ferrous Sulphate~, FeSO_{4}.7H_{2}O.--The granulated form is best, and
can be purchased pure. It is used for standardising. It keeps better in
crystals than in solution. It is readily soluble in water, but the
solution is best made with the help of a little free acid. As a re-agent
use a 10 per cent. solution. The crystals should be clear bluish-green;
if their colour is dark green, brown, or blue, they should be rejected.
~Ferrous Sulphide~ (FeS) is used for the preparation of sulphuretted
hydrogen. It may be purchased and broken in small lumps, nut-size, for
use.
"~Fusion Mixture~" (K_{2}CO_{3}.Na_{2}CO_{3}) is a mixture of potassic
and sodic carbonates in the proportions of 13 of the former to 10 of the
latter, by weight. It is hygroscopic. A mixture of the bicarbonates is
better, being purer and less apt to get damp.
~Gallic Acid~ (C_{7}H_{6}O_{5}.H_{2}O) is an organic acid, occurring as
a pale fawn-coloured crystalline powder, soluble in 100 parts of cold
water, or in 3 parts of boiling water. It is used for the determination
of antimony. A 10 per cent. solution in warm water is made when
required.
~Hydrogen~ (H) is a gas. It is obtained by acting on zinc with dilute
hydrochloric or sulphuric acid. It is used as a reducing agent, and for
providing an atmosphere free from oxygen. It reduces metallic oxides at
a high temperature. It must be freed from water; and special precautions
should be taken to prevent an admixture with air. It is generally
required in a current which can be continued for an hour or more without
interruption. The preparation can be conveniently carried out in the
apparatus shown (fig. 33). A quart bottle is half filled with sheet
zinc, and connected with bulbs filled with sulphuric acid, and with a
calcium chloride tube. The last is connected with the apparatus through
which the gas has to be passed. Dilute hydrochloric acid mixed with a
few cubic centimetres (20 c.c. to 1 pint) of stannous chloride sol. to
fix any dissolved oxygen, is placed in the funnel, and let into the
bottle by opening the stopcock when required. Care must be taken to let
the hydrogen escape for some time before starting the reduction.
[Illustration: FIG. 33.]
~Gold~, Au.--Gold, obtained by cupelling and "parting," is for most
purposes sufficiently pure. It is best kept in the shape of foil. When
the purer metal is required, gold should be dissolved in aqua regia, the
solution evaporated to a paste, diluted, allowed to stand, and filtered.
The filtered solution is acidified with hydrochloric acid, warmed, and
precipitated with sodium sulphite. The precipitate is collected, washed,
and fused on charcoal.
~Iron~, Fe.--The soft wire (thin) is used for standardising. Rods are
used in dry assays as a desulphurising agent. Steel must not be used,
since it is not pure, and contains a variable amount of iron.
~Lead~, Pb.--Granulated lead or lead-foil is used in the dry assay for
silver and gold, and in the preparation of lead salts. It can be
obtained very pure, but always contains more or less silver, 1 or 2
milligrams in 100 grams. The amount of silver it contains must be
determined and recorded.
~Lead Acetate~ (Pb[=A=c]_{2}.3H_{2}O, or
Pb(C_{2}H_{3}O_{2})_{2}.3H_{2}O) is used as a test, specially for the
detection and estimation of sulphuretted hydrogen. Prepare a 10 per
cent. solution for use.
~Lead Nitrate~ (Pb(NO_{3})_{2}) can be purchased pure. It is used for
standardising.
~Lead Dioxide~ (PbO_{2}) occurs as a dark-brown powder. It is used as an
oxidizing agent and for absorbing sulphurous oxide. It can be prepared
by digesting red lead with warm dilute nitric acid; washing and drying
the residue.
"~Litharge~," PbO.--It can be purchased as a yellow heavy powder. It is
used in dry assaying as a flux, as a desulphurising agent, and also as a
source of lead. It always contains some silver, the amount of which must
be determined.
~Litmus.~--This is an organic colouring matter which is turned red by
acids and blue by alkalies. For ordinary purposes it is best used as
litmus paper, which may be purchased in small books. A solution is
prepared by digesting 15 or 20 grams of the commercial litmus in 100
c.c. of water on the water bath. After being allowed to settle, it is
filtered and made just faintly red with acetic acid. Then there is
added a drop or two of a solution of soda and 10 c.c. of alcohol. It
should be kept in a loosely-covered bottle.
~Magnesia~, MgO.--It may be purchased as "calcined magnesia." It is used
for making "magnesia mixture," and should be kept in a corked
wide-mouthed bottle.
"~Magnesia Mixture.~"--Dissolve 22 grams of magnesia in about a quarter
of a litre of dilute hydrochloric acid, avoiding excess. Add 5 grams of
magnesia, boil, and filter. Add 300 grams of ammonic chloride, and 250
c.c. of strong ammonia; and dilute with water to 2 litres. It should be
kept in a stoppered winchester.
~Magnesium Sulphate~, MgSO_{4}.7H_{2}O.--It can be purchased very pure,
and is occasionally used as a standard salt.
~Manganese Dioxide~, MnO_{2}.--It is used in the preparation of
chlorine. The commercial article is not pure, but is sufficiently so for
this purpose.
~Marble~, CaCO_{3}.--Fragments of the white crystalline variety only
should be used. It is used as a source of lime and of carbon dioxide.
~Mercury~, Hg.--This can be purchased pure. It should have a bright
surface, flow without a tail, and leave no residue on ignition. It is
used as a standard; for amalgamation; and as a confining liquid in gas
analysis.
~Mercuric Chloride~ (HgCl_{2}) may be purchased pure. Make a 5 per cent.
solution in water. It is used for destroying an excess of stannous
chloride; for removing sulphuretted hydrogen from solution; and as a
test for stannous salts.
~Microcosmic Salt~, HAmNaPO_{4}.8H_{2}O.--When fused NaPO_{3} is formed.
It is used in testing for metallic oxides and silica before the
blowpipe. The crystals are sometimes used as a standard for phosphoric
acid.
"~Nessler's Solution.~"--Mode of preparation: Dissolve 35 grams of
potassium iodide in 100 c.c. of water; dissolve 17 grams of mercuric
chloride in 300 c.c. of water, and pour this solution into that of the
iodide till a permanent precipitate is produced; make up to 1 litre with
a 20 per cent. solution of potash; add mercuric chloride till a
precipitate is again formed; allow to settle and decant. It is used for
detecting ammonia.
~Nitre.~--This is potassium nitrate.
~Platinum Chloride~, 2HCl.PtCl_{4}. (In the crystallised form it has
6H_{2}O).--It may be made as follows:--Take 5 grams of clean platinum
scrap and dissolve in a flask at a gentle heat in 50 c.c. of
hydrochloric acid with the occasional addition of some nitric acid;
evaporate to a paste; and then dissolve in 100 c.c. of water. It is used
for separating and determining potassium.
~Phenolphthalein~ is an organic compound used as an indicator; more
especially in determining the weaker acids, it cannot be used in the
presence of ammonia. Dissolve half a gram in 100 c.c. of dilute alcohol.
~Potassium Bicarbonate~, KHCO_{3}.--It may be purchased pure; on
ignition it leaves the carbonate, K_{2}CO_{3}, which may be used as a
standard.
~Potassium Cyanide~, KCN.--It is used in the dry assay as a reducing
agent. The commercial salt is very impure. Purchase that sold as
potassic cyanide (gold) which contains about 95 per cent. of KCN. It is
used for copper assaying and occasionally in separation. Make a 10 per
cent. solution when wanted.
~Potassium Bichromate~, K_{2}Cr_{2}O_{7}. It may be purchased nearly
pure. It is used as an oxidising agent, for determining iron; and as a
test solution. For this last purpose a 10 per cent. solution is
prepared.
~Potassium Chlorate~ (KClO_{3}) can be purchased pure. It is used with
hydrochloric acid as a substitute for aqua regia.
~Potassium Ferrocyanide~ (K_{4}Fe(CN)_{6}.3H_{2}O), or "yellow prussiate
of potash," is used as a test; as an indicator; and for the
determination of zinc. Make a 5 per cent. solution.
~Potassium Ferricyanide~ (K_{6}Fe_{2}(CN)_{12}), or "red prussiate of
potash," is used for testing; and as an indicator. Make a 5 per cent.
solution when wanted, as it decomposes on keeping.
~Potassium Hydrate~, KHO. Purchase that purified with alcohol. It is an
alkali, and is used for absorbing carbonic acid, &c.
~Potassium Iodide~, KI. It may be purchased nearly pure. It is used as a
test and for dissolving iodine. It should be used in a 10 per cent.
solution freshly made. The solution decomposes on exposure to light,
with separation of iodine.
~Potassium Nitrate~ (KNO_{3}) can be purchased pure. It is used in the
dry way as an oxidizing agent. It is very fusible. It decomposes at a
low temperature into potassium nitrite (KNO_{2}) and free oxygen; and at
a higher temperature leaves potash (K_{2}O). It oxidizes sulphur and
carbon with explosive violence. This action may be moderated by mixing
the nitre with carbonate of soda, common salt, or some other inert body.
~Potassium Nitrite~, KNO_{2}.--The commercial article is not pure, but
is sufficiently so for the purpose required. A saturated solution is
used in the separation of cobalt; the solution is made when wanted.
~Potassium Permanganate~, KMnO_{4}.--This salt can be purchased
sufficiently pure. It is much used as an oxidizing agent.
~Potassium Bisulphate~ (KHSO_{4}) is used as a dry reagent for opening
up minerals. It fuses; and at a much higher temperature is converted
into potassium sulphate with loss of sulphuric acid.
~Potassium Sulphocyanate~ (KCNS) is used for the detection and
determination of traces of ferric iron; as also in the separation of
silver and copper from some of the other metals. Make a 10 per cent.
solution. It should show no colour on the addition of hydrochloric acid.
"~Red Lead~" (Pb_{3}O_{4}) is used in the dry assay as a flux instead of
litharge, from which it differs in containing a little more oxygen. When
acted on by nitric acid a brown residue of lead dioxide is left, nitrate
of lead going into solution. Like litharge it always carries silver;
about 2 milligrams in 100 grams.
~Silver~, Ag.--Pure silver in foil is required as a standard. It may be
prepared as follows:--Dissolve scrap silver in dilute nitric acid and
decant off from any residue; dilute the solution with hot water and add
hydrochloric acid until there is no further precipitate, stir; allow the
precipitate to settle; decant and wash; dry the precipitate, mix it with
twice its bulk of carbonate of soda and fuse the mixture in a crucible
until tranquil; clean the button and roll or hammer it into foil.
~Sodium Acetate~, NaC_{2}H_{3}O_{2}.3H_{2}O.--The crystals may be
purchased sufficiently pure. Make a 20 per cent. solution in water. It
is used for replacing mineral acids by acetic acid.[7]
~Sodium Acetate and Acetic Acid.~--A solution is used in the
determination of phosphates and arsenates; 100 grams of the salt is
dissolved in 500 c.c. of acetic acid, and diluted with water to one
litre.
~Sodium Bicarbonate~ (NaHCO_{3})is used as a flux in dry methods. On
ignition it leaves the carbonate (Na_{2}CO_{3}), which is used as a
standard reagent. Make a 20 per cent. solution of the carbonate for use.
It should be free from chlorides or sulphates, or if impure the amount
of impurities must be determined.
~Sodium Hydrate~, NaHO. It may be purchased in sticks, which should be
kept in a well-corked bottle. It is sometimes called "caustic soda." It
is a strong alkali. It is used for neutralizing acid solutions and for
separations where ammonia is unsuitable. Make a 5 per cent. solution for
use.
~Sodium Hyposulphite~, Na_{2}S_{2}O_{8}.5H_{2}O.--It may be purchased
pure. It is generally known as "hypo." It is used as a standard.
~Sodium Sulphite~ (Na_{2}SO_{3}.7H_{2}O) is used as a reducing agent.
~Sodium Phosphate~, Na_{2}HPO_{4}.12H_{2}O. The crystals may be
purchased pure, but they effloresce in dry air with loss of water. It
is used as a standard and for precipitating magnesia, &c. Make a 10 per
cent. solution.
~Stannous Chloride~, SnCl_{2}.2H_{2}O.--The crystals are best purchased.
If kept dry and free from air they are fairly permanent. A solution is
made by dissolving 20 grams in 10 c.c. of hydrochloric acid and diluting
to 1 litre. The solution is not permanent. It is a strong reducing
agent, and is chiefly used in solution for this purpose.
~Tin~, Sn.--Grain tin should be purchased. It is not pure, but contains
99.5 per cent. of the metal. The chief impurity is copper. It can be
used as a standard. When acted on with hot hydrochloric acid it slowly
dissolves (more rapidly in contact with platinum) and forms stannous
chloride.
~Uranium Acetate~, UO_{2}(C_{2}H_{3}O_{2})_{2}.H_{2}O.--It is best
purchased in crystals. The solution is used for the determination of
phosphates and arsenates. A solution of 3 per cent. strength is
occasionally used as an indicator.
~Uranium Nitrate~, UO_{2}(NO_{3})_{2}.6H_{2}O.--This salt is very
soluble in water and is sometimes used instead of the acetate, which is
somewhat difficult to dissolve.
"~Water~," H_{2}O.--Spring or well water is sufficiently pure for most
purposes, 100 c.c. will leave a residue of from 10 to 30 milligrams, so
that where a salt has to be dissolved out, evaporated, and weighed it
should be replaced by distilled water. Rain water, melted snow, &c.,
always leave less residue than spring water; but in other respects they
are often dirtier. Distilled water is best prepared in the office, a
glass or tin condenser being used.
~Zinc~, Zn.--It is sold in a granulated form or in sticks. It generally
contains over 1 per cent. of lead, with a little iron and arsenic. It is
used for separating metals from their solutions, and generally as a
reducing agent. For the preparation of hydrogen, and in most other
cases, scrap sheet zinc may be used.
~Zinc Oxide~, ZnO.--The commercial oxide sometimes contains carbonate.
~Zinc Sulphate~, ZnSO_{4}.7H_{2}O.--It is occasionally used as a
standard, and can be purchased nearly pure.
FOOTNOTES:
[6] 3HCl + HNO_{3} = Cl_{2} + NOCl + 2H_{2}O.
[7] NaC_{2}H_{3}O_{2} + HCl = H_{4}C_{2}O_{2} + NaCl.
CHAPTER VII.
FORMULÆ, EQUATIONS, ETC.
Formulæ and equations are a kind of short hand for expressing briefly
and in the language of the atomic theory the facts of chemical
composition and reaction. The convenience of this method of expressing
the facts justifies a short description of it here.
On comparing the percentage composition of a series of compounds the
proportions in which the elements combine appears to be regulated by no
simple law. For example:
Realgar. Orpiment. Mispickel. Pyrites.
Arsenic 71.4 60.9 46.0 --
Sulphur 28.6 39.1 19.6 53.3
Iron -- -- 34.4 46.7
------ ------ ------ ------
100.0 100.0 100.0 100.0
But if in these examples the composition is calculated, not on 100
parts, but on 107, 246, 163, and 120 parts respectively, evidence of a
simple law becomes apparent.
Realgar. Orpiment. Mispickel. Pyrites.
Arsenic 75.0 150.0 75.0 --
Sulphur 32.0 96.0 32.0 64.0
Iron -- -- 56.0 56.0
------ ------ ------ ------
107.0 246.0 163.0 120.0
It will be seen that the proportion of arsenic is 75 or twice 75, that
of iron is 56, and that of sulphur 32 or some simple multiple of 32. The
series of examples might be extended indefinitely, and it would still be
found that the "combining proportions" held good. The number 75 is
spoken of as the "combining weight," or, more frequently, as the "atomic
weight" of arsenic. Similarly 56 is the atomic weight of iron, and 32
the atomic weight of sulphur. The importance of this law of chemical
combination is altogether independent of the atomic theory; but this
theory furnishes the simplest explanation of the facts. According to it
a chemical compound is made up of exactly similar groups of particles.
The particles of each elementary substance are all alike, but differ
from those of other elements in weight. Ultimate particles are called
_atoms_, and the groups of atoms are called _molecules_. The atomic
weight of any particular element is the weight of its atom compared with
the weight of an atom of hydrogen. The atom of sulphur, for instance, is
32 times as heavy as the atom of hydrogen, and the atomic weight of
sulphur is 32. The _molecular weight_ is the sum of the atomic weights
of the group. The molecule of pyrites contains two atoms of sulphur and
one of iron: on referring to the table of atomic weights it will be seen
that the atomic weights are--sulphur 32, and iron 56. The molecular
weight, therefore, is 32+32+56--that is, 120. The meaning of this
is, 120 parts by weight of iron pyrites contain 64 parts of sulphur and
56 parts of iron; and this is true whether the "parts by weight" be
grains or tons.
_The symbol or formula of an atom_ is generally the initial letter or
letters of the Latin or English name of the substance. The atom of
hydrogen is written H, that of oxygen O, of sulphur S, of iron (ferrum)
Fe, and so on. A list of these symbols is given in the table of atomic
weights.
_The formula of a molecule_ is obtained by placing together the symbols
of the contained atoms. Thus, Fe represents an atom of iron, S an atom
of sulphur, while FeS represents the molecule of sulphide of iron as
containing one atom of each element.
When more than one atom of an element is present this is shown by
writing a figure under and after the symbol; thus, FeS_{2} represents a
molecule with one atom of iron and two atoms of sulphur, Fe_{2}S_{3}
similarly shows one with two atoms of iron and three of sulphur. When a
group of atoms is enclosed in brackets, a figure after and under the
bracket multiplies all within it; for example, Pb(NO_{3})_{2} is another
way of writing PbN_{2}O_{6}. Sometimes it is convenient to represent the
atoms of a molecule as divided into two or more groups; this may be done
by writing the formulæ of the groups, and separating each simple formula
by a full stop. Slaked lime, for instance, has the formula CaH_{2}O_{2};
or, as already explained, we may write it Ca(HO)_{2}; or, if for
purposes of explanation we wished to look on it as lime (CaO) and water
(H_{2}O), we could write it CaO.H_{2}O. A plus sign (+) has a different
meaning; CaO + H_{2}O indicates quantities of two substances, water and
lime, which are separate from each other. The sign of equality (=) is
generally used to separate a statement of the reagents used from another
statement of the products of the reaction; it may be translated into the
word "yields" or "becomes." The two statements form an equation.
Ignoring the quantitative relation, the meaning of the equation CaO +
H_{2}O = CaO.H_{2}O is: "lime and water yield slaked lime." By referring
to a table of atomic weights we can elicit the quantitative relations
thus:--
CaO + H_{2}O = CaH_{2}O_{2}
| | |
V V V
Ca = 40 H_{2} = 2 = 1×2 Ca = 40
O = 16 O = 16 H_{2} = 2 = 1×2
-- -- O_{2} = 32 = 16×2
56 18 --
74
Or, putting it in words, 56 parts of lime combine with 18 parts of water
to form 74 parts of slaked lime. This equation enables one to answer
such a question as this:--How much lime must be used to produce 1 cwt.
of slaked lime? for, if 74 lbs. of slaked lime require 56 lbs. of lime,
112 lbs. will require (56×112)/74, or about 84-3/4 lbs.
As another example having a closer bearing on assaying take the
following question:--"In order to assay 5 grams of 'black tin' (SnO_{2})
by the cyanide process, how much potassic cyanide (KCN) will be
required?" The reaction is
SnO_{2} + 2KCN = Sn + 2KCNO
| |
V V
Sn = 118 K = 39
O_{2} = 32 C = 12
--- N = 14
150 --
65×2 = 130
What is sought for here is the relation between the quantities of
SnO_{2} and KCN. Note that a figure before a formula multiplies all that
follows up to the next stop or plus or equality sign. The question is
now resolved to this: if 150 grams of oxide of tin require 130 grams of
cyanide, how much will 5 grams require?
150 : 130 :: 5 : _x_
_x_ = 4.33 grams.
A problem of frequent occurrence is to find the percentage composition
of a substance when its formula has been given. For example: "What
percentage of iron is contained in a mineral having the formula
2Fe_{2}O_{3}.3H_{2}O?" Bringing this formula together we have
Fe_{4}H_{6}O_{9}. Find the molecular weight.
Fe_{4} = 224 = 56×4
H_{6} = 6 = 1×6
O_{9} = 144 = 16×9
---
374
Then we get: 374 parts of the mineral contain 224 of iron. How much will
100 contain?
374 : 224 :: 100 : _x_
_x_ = 59.89.
And the answer to the question is 59.89 per cent.
Again, suppose the question is of this kind:--"How much crystallised
copper sulphate (CuSO_{4}.5H_{2}O) will be required to make 2 litres of
a solution, 1 c.c. of which shall contain 0.0010 gram of copper?"
A litre is 1000 c.c., so, therefore, 2 litres of the solution must
contain 0.001 gram × 2000, or 2 grams. How much crystallised copper
sulphate will contain this amount of metal?
Cu = 63.3
S = 32.0
O_{4} = 64.0 = 16×4
5H_{2}O = 90.0 = 18×5
-----
249.3
If 63.3 grams of copper are contained in 249.3 grams of sulphate, in how
much is 2 grams contained.
63.3 : 249.3 :: 2 grams : _x_
_x_ = 7.8769 grams.
The answer is, 7.8769 grams must be taken.
As a sample of another class of problem similar in nature to the last
(but a little more complicated) take the following:--"What weight of
permanganate of potash must be taken to make 2 litres of a solution, 100
c.c. of which shall be equivalent to 1 gram of iron?" In the first place
the 2 litres must be equivalent to 20 grams of iron, for there are 20 ×
100 c.c. in two litres. In the titration of iron by permanganate
solution there are two reactions. First in dissolving the iron
Fe + H_{2}SO_{4} = FeSO_{4} + H_{2}
|
V
56
and second, in the actual titration,
10FeSO_{4} + 2KMnO_{4} + 9H_{2}SO_{4}= 2MnSO_{4}
| + 5Fe_{2}(SO_{4})_{3} + 2KHSO_{4} + 8H_{2}O
V
K = 39
Mn = 55
O_{4}= 64
---
158 × 2 = 316
As before, attention is confined to the two substances under
consideration--viz., Fe and KMnO_{4}. In the second equation, we find
316 parts of the permanganate are required for 10 molecules of FeSO_{4};
and in the first equation 56 parts of iron are equivalent to one
molecule of FeSO_{4}, therefore 560 of iron are equivalent to 316 of
permanganate; and the question is, How much of the permanganate will be
equivalent to 20 grams of iron?
560 : 316 :: 20 grams : _x_.
_x_= 11.286 grams.
The answer is 11.286 grams.
Very similar to this last problem is the question suggested under the
head "Indirect Titration" (p. 43). "If 100 c.c. of the standard
permanganate solution are equivalent to 1 gram of iron, how much
peroxide of manganese will they be equivalent to?" The equation for
dissolving the iron is already given; the second equation is
2FeSO_{4} + MnO_{2} + 2H_{2}SO_{4}
| = Fe_{2}(SO_{4})_{2} + MnSO_{4} + 2H_{2}O
|
V
Mn = 55
O_{2} = 32
--
87
It will be seen that 87 grams of peroxide of manganese are equivalent to
112 grams of iron. How much then is equivalent to 1 gram of iron?
112 : 87 :: 1 gram : _x_
_x_ = 0.7767 gram.
It is sometimes convenient to calculate the formula of a substance from
its analysis. The method of calculating is shown by the following
example. Required the formula of a mineral which gave the following
figures on analysis:--
Cupric oxide (CuO) 10.58
Ferrous oxide (FeO) 15.69
Zinc oxide (ZnO) 0.35
Sulphuric oxide (SO_{2}) 28.82
Water (H_{2}O) 44.71
------
100.15
First find the molecular weights of CuO, FeO, &c., and divide the
corresponding percentages by these figures. Thus, CuO = 63.3+16 = 79.3
and 10.58 divided by 79.3 gives 0.1334. Similarly FeO = 56+16 = 72 and
15.69 divided by 72 gives 0.2179. Treated in the same way the oxide of
zinc, sulphuric oxide and water give as results 0.0043, 0.3602 and
2.484.
Classify the results as follows:--
Bases. Acids. Water.
CuO 0.1334 SO_{3} 0.3602 H_{2}O 2.484
FeO 0.2179
ZnO 0.0043
---------- ------------- ------------
RO 0.3556 RO_{3} 0.3602 R_{2}O 2.484
The figures 0.3556, 0.3602 and 2.484 should be then divided by the
lowest of them--_i.e._, 0.3556; or where, as in this case, two of the
figures are very near each other the mean of these may be taken--_i.e._,
0.3579. Whichever is taken the figures got will be approximately 1, 1
and 7. The formula is then RO.SO_{3}.7H_{2}O in which R is nearly 2/5ths
copper, 3/5ths iron and a little zinc.
This formula requires the following percentage composition, which for
the sake of comparison is placed side by side with the actual results.
Calculated. Found.
Cupric oxide 11.29 10.58
Ferrous oxide 15.37 15.69
Zinc oxide nil 0.35
Sulphuric oxide 28.47 28.82
Water 44.84 44.71
----- ------
99.97 100.15
Trimming the results of an analysis to make them fit in more closely
with the calculations from the formula would be foolish as well as
dishonest. There can be no doubt that the actual analytical results
represent the composition of the specimen much more closely than the
formula does; although perhaps other specimens of the same mineral would
yield results which would group themselves better around the calculated
results than around those of the first specimen analysed. It must be
remembered that substances are rarely found pure either in nature or in
the arts; so that in most cases the formula only gives an approximation
to the truth. In the case of hydrated salts there is generally a
difficulty in getting the salt with exactly the right proportion of
water.
PRACTICAL EXERCISES.
The following calculations may be made:--
1. Calculate standards in the following cases--
(a) Silver taken, 1.003 gram. Standard salt used, 100.15 c.c.
(b) Iron taken, 0.7 gram. Bichromate used, 69.6 c.c.
2. Calculate percentages:--
(a) Ore taken, 1 gram. Solution used, 65.2 c.c. Standard, 0.987
gram.
(b) Ore taken, 1 gram. Barium sulphate got, 1.432 gram. Barium
sulphate contains 13.73 per cent. of sulphur, and the percentage
of sulphur in the ore is wanted.
(c) Barium sulphate is BaSO_{4}. Calculate the percentage of sulphur
it contains, for use in the preceding question.
3. A method of estimating the quantity of peroxide in a manganese ore
is based on the following reactions:--
(1) MnO_{2} + 4HCl = MnCl_{2} + Cl_{2} + 2H_{2}O.
(2) Cl + KI = KCl + I.
To how much MnO_{2} is 1 gram of Iodine (I) equivalent?
4. A mineral has the following composition:--
Carbonic acid (CO_{2}) 19.09
Copper oxide (CuO) 71.46
Water (H_{2}O) 9.02
What is its formula?
5. How much copper is contained in 1.5 gram of crystallized copper
sulphate (CuSO_{4}.5H_{2}O)? How much of these crystals must be taken to
give 0.4 gram of copper?
6. How much ferrous sulphate crystals (FeSO_{4}.7H_{2}O) must be taken
to yield 2 litres of a solution, 100 c.c. of which shall contain 0.56
gram of iron?
7. Galena is PbS, and hæmatite Fe_{2}O_{3}. What percentages of metal do
these minerals contain?
CHAPTER VIII.
SPECIFIC GRAVITY.
The relation of the weight of a substance to its volume should be kept
in mind in all cases where both weight and volume are dealt with.
Students are apt to imagine that on mixing equal volumes of, say,
sulphuric acid and water, an acid of half the strength must be obtained.
If the statement of strength is in parts by weight this will lead to
considerable error. For example, 100 c.c. of sulphuric acid containing
98 per cent. by weight of real acid, will, if diluted with 100 c.c. of
water, yield a solution containing not 49 per cent. by weight, but about
63.5 per cent. of the acid. The reason is this: the 100 c.c. of
sulphuric acid weighs 184 grams, and contains 180.32 grams of real acid,
while the 100 c.c. of water weighs only 100 grams; the mixed water and
acid weighs 284 grams, and contains 180.32 of real acid, which is
equivalent to nearly 63.5 per cent. by weight. If, however, the method
of statement be volumetric, it would be correct to say that doubling the
volume halves the strength: if 100 c.c. of brine contains 10 grams of
salt, and is diluted with water to 200 c.c., it would be of one-half the
former strength, that is, 100 c.c. of the solution would contain 5 grams
of salt.
This confusion is avoided by always stating the strengths as so many
grams or "c.c." in 100 c.c. of the liquid. But obviously it would be
advantageous to be able to determine quickly the weight of any
particular substance corresponding to 1 c.c. or some other given volume.
Moreover, in descriptions of processes the strengths of acids and
solutions are frequently defined neither by their gravimetric nor
volumetric composition, but by a statement either of specific gravity or
of the degrees registered by Twaddell's or Beaumé's hydrometer. Thus, in
the description of the process of gold parting, one writer gives: "The
acid should be of 1.2 specific gravity"; and another says: "The acid
must not be stronger than 32° Beaumé."
These considerations justify an account of the subject in such a work as
this. And on other grounds the determination of a specific gravity is
one of the operations with which an assayer should be familiar.
The meaning of "specific gravity" is present in the mind of every one
who uses the sentence "lead is heavier than water." This is meaningless
except some such phrase as "bulk for bulk" be added. Make the sentence
quantitative by saying: "bulk for bulk lead is 11.36 times heavier than
water," and one has the exact meaning of: "the specific gravity of lead
is 11.36." A table of the specific gravities of liquids and solids shows
how many times heavier the substances are than water.
It is better, however, to look upon the specific gravity (written
shortly, sp. g.) as the weight of a substance divided by its volume. In
the metric system, 1 c.c. of water at 4° C. weighs with sufficient
exactness 1 gram; consequently, the sp. g., which states how many times
heavier than water the substance is, also expresses the weight in grams
of one c.c. of it. So that if a 100 c.c. flask of nitric acid weighs,
after the weight of the flask has been deducted, 120 grams, 1 c.c. of
the acid weighs 1.2 gram, and the sp. g. is 1.2. The specific gravity,
then, may be determined by dividing the weight of a substance in grams
by its volume in c.c.; but it is more convenient in practice to
determine it by dividing _the weight of the substance by the weight of
an equal volume of water_. And since the volumes of all substances,
water included, vary with the temperature, the temperature at which the
sp. g. is determined should be recorded. Even then there is room for
ambiguity to the extent that such a statement as the following, "the
specific gravity of the substance at 50° C. is 0.9010," may mean when
compared with water at 50° C. or 4° C., or even 15.5° C. For practical
purposes it should mean the first of these, for in the actual
experiments the water and the substance are compared at the same
temperature, and it is well to give the statement of results without any
superfluous calculation. In the metric system the standard temperature
is 4° C., for it is at this point that 1 c.c. of water weighs exactly 1
gram. In England, the standard temperature is 60° F. (15.5° C.), which
is supposed to be an average temperature of the balance-room. The
convenience of the English standard, however, is merely apparent; it
demands warming sometimes and sometimes cooling. For most purposes it is
more convenient to select a temperature sufficiently high to avoid the
necessity of cooling at any time. Warming to the required temperature
gives very little trouble.
~Determination of Specific Gravity.~--There is a quick and easy method
of determining the density or sp. g. of a liquid, based upon the fact
that a floating body is buoyed up more by a heavy liquid than by a light
one. The method is more remarkable for speed than accuracy, but still
is sufficiently exact. The piece of apparatus used for the purpose is
endowed with a variety of names--sp. g. spindle, hydrometer, areometer,
salimeter, alcoholimeter, lactometer, and so on, according to the
special liquid upon which it is intended to be used. It consists of a
float with a sinker at one end and a graduated tube or rod at the other.
It is made of metal or glass. Generally two are required, one for
liquids ranging in sp. g. from 1.000 to 2.000, and another, which will
indicate a sp. g. between 0.700 and 1.000. The range depends on the size
of the instrument. For special work, in which variations within narrow
limits are to be determined, more delicate instruments with a narrower
range are made.
[Illustration: FIG. 34.]
In using a hydrometer, the liquid to be tested is placed in a cylinder
(fig. 34) tall enough to allow the instrument to float, an
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