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.