Assay. 1. The method of assaying silver by cupellation has been explained under Assay and Cupellation; and that method is alone applicable when the alloy contains a very small quantity of silver, as a few ounces only per ton. When the reverse is the case, as with the silver of commerce, the following is a much more accurate method:—
2. Humid assay of silver.—a. Dissolve 10 gr. of the silver for assay in 100 gr. of nitric acid, sp. gr. 1·28, by the aid of heat, the solution being made in a tall stoppered glass tube, furnished with a foot; then place it in a very delicate balance, bring it into an exact state of equilibrium, and add the test solution (see below), gradually and cautiously, until the whole of the silver be thrown down, the number of grains now required to restore the equilibrium of the balance or scales gives the exact quantity of pure silver in 1000 parts of the sample.—Obs. To ensure accuracy, after each addition the stopper should be placed in the tube, and the latter violently agitated for a short time, when the liquor will rapidly clear and enable us to see when the operation is concluded. We must then, as a check, add a small quantity of a solution of nitrate of silver to the liquor in the tube, after having first carefully taken the weight. If too much of the test liquor has been added, this will produce a fresh precipitate, and the assay cannot then be depended on.—Instead of weighing the quantity of test liquor used, a tube graduated into 100 parts, and holding 1000 gr., may be employed, every division of which, required to throw down the silver, will represent the 1⁄10th of a grain. See Alkalimetry and Acidimetry.
b. The precipitate of chloride of silver may be collected in a paper filter, and be dried, washed, fused, and weighed. The previous weight of the paper, deducted from the gross weight of the filter and its contents, gives the quantity of chloride of silver present, which
multiplied by 0·75278, gives the weight of the pure silver in the sample.
Test liquor. Dissolve 54·27 (541⁄4) gr. of pure sea salt in 9945·73 gr. (or 22 oz. and 3203⁄4 gr. avoirdupois) of distilled water; filter, and keep the liquor in a stoppered bottle for use. Pure sea salt is obtained by boiling together, for a few minutes, in a glass vessel, a solution of common salt with a little pure bicarbonate of soda; then adding to the filtered liquor sufficient hydrochloric acid to render it neutral to litmus and turmeric paper, and, lastly, evaporating and crystallising.
Obs. The presence of mercury, lead, or sulphuret of silver, interferes with the accuracy of the above assay. When mercury is present, the precipitate blackens less readily by exposure to light; and when it contains 4⁄1000 or 5⁄1000 of chloride of mercury, it remains of a dead white; with 3⁄1000 it is not sensibly discoloured by the diffused light of a room, with 2⁄1000 only slightly darkened, with 1⁄1000 more so, but with pure chloride of silver, the effect is very rapid and intense. When this metal is present, which is, however, seldom the case, the assay sample must be placed in a small crucible, and exposed to a full red heat, before solution in the acid. Another method, proposed by M. Levol, and modified by M. Gay-Lussac, is to add to the nitric solution of the silver sufficient acetate of ammonia or crystallised acetate of soda to saturate all the nitric acid existing in the liquor, either in the free state or combined with the silver. When the alloy contains lead, shown by the precipitated chloride being partly soluble in water, it may either be laminated and subjected to the action of acetic acid before solution in the nitric acid; or, the test solution of chloride of soda should be replaced by one of chloride of lead; (139·355 gr. of the latter are equiv. to 58·732 gr. of the former). The presence of sulphuret of silver is detected whilst dissolving the sample in nitric acid, by the black flakes which may be observed floating about in the liquor in an insoluble state. These flakes may be dissolved by fuming nitric acid, or by adding pure concentrated sulphuric acid to the solution, which should be then heated for about a 1⁄4 hour in a steam-bath. When thus treated, the precipitate produced by the test liquor represents the whole of the silver contained in the alloy.
Dr Gräger[164] gives the following process of the preparation of pure metallic silver:—He dissolved the alloy of silver in nitric acid, taking care to use as small a quantity of the acid as possible. The solution is then transferred to a large-sized porcelain basin, and gradually neutralised with previously lixiviated chalk free from chlorine. The neutralised liquid is next boiled, and chalk again added
to it, while boiling, until the fluid has become colourless. (In order to test more accurately, a drop of the liquid is poured on a piece of white filtering-paper, and next to that drop is placed one of a solution of ferrocyanide of potassium; as long as the well-known red colouration, copper reaction, hereby ensues, chalk is added.) The fluid is next filtered to separate the carbonate of copper, and the filtrate (a solution of nitrate of silver, and nitrate of lime) is again boiled, and either further treated with carbonate of lime, or better still, with carbonate of soda. The bright yellow-coloured precipitate thereby ensuing, a mixture of carbonate of silver, and carbonate of lime, is washed, dried, and ignited, leaving a greyish-white mass of metallic silver, mixed with carbonate of lime. This mixture is treated with dilute hydrochloric acid, washed with distilled water, and then fused along with borax, yielding pure silver. The bright green coloured carbonate of copper can be used as a pigment for painting purposes.
[164] ‘Chemical News,’ xxv, No. 641, 119.
Krüger says:—In frequently experimenting with silver salts, a mixture of solution of precipitates is obtained, in which silver exists in all kinds of combination. When such mixtures are shaken up with an ethereal solution of phosphorus, the solution and the precipitate soon separate, the former being more or less yellow, the latter intensely black. If light-coloured particles are perceived in the precipitate, the quantity of phosphorus solution employed is not sufficient, and more must be added till the precipitate is uniformly black. The precipitate is next filtered, washed, and dried, or placed while still damp, in a porcelain dish and boiled with potash solution. Pure metallic silver is thus obtained. If a solid fused mass is required, pure potassium hydrate is fused in a crucible, and to this the dried precipitate is added in small portions.