The method depends on the fact that cuprous sulphide reacts with silver salts, with the formation of equivalent amounts of metallic silver and silver sulphide, whilst under similar conditions cupric sulphide gives rise only to silver sulphide. The metallic silver in the mixture of silver and silver sulphide is determined by solution in ferric nitrate solution and subsequent estimation as chloride. The substance (0.4 to 0.5 grm.) is ground to pass a sieve of 200 meshes per lineal inch, and
... d with 50 C.C. of 5 per cent. silver nitrate solution. The reaction is rapid at first, even in the cold, but is only quantitative if allowed to proceed for some three hours on the steam-bath with frequent agitation, the products of the reaction tending to protect the material from the reagent. The precipitate is washed by decantation until the filtrate no longer gives a precipitate with hydrochloric acid, and it is then extracted two or three times at about 7 0 ' C. with 40 to 50 C.C. of a 6 per cent. solution of ferric nitrate (anhydrous). The extract is filtered each time through a hot-water funnel, and the residue finally washed.with hot water. To the filtrate nitric acid is added until it becomes colourless, and the silver is then precipitated and weighed as chloride. 2Ag = Cu&. The residual silver sulphide is roasted, dissolved in nitric acid, and the silver precipitated as chloride. If the original sample contained cuprous sulphide, but not cupric sulphide, the weight of silver chloride in this second experiment will equal that in the first, within the limits of experimental error of the method, about 1.5 per cent. If the second precipitate is appreciably heavier than the first, then the cupric sulphide in the sample is calculated from the difference between the weights of silver chloride obtained in the two experiments. 2Ag-GUS.