3702. To Recover Silver from Solution

3702.    To Recover Silver from Solution. When a silver solution gets out of order, and cannot bo rendered fit for use again, the silver may bo recovered by adding to the solution any acid that will neutralize the alkali; if nitric or sulphuric acid be used, the silver precipitates as cyanide, but if hydrochloric acid be used, the" silver will be precipitated as a chloride; in either case the solution should be diluted, or a portion of the precipitate will bo redissolved. The precipitate is allowed to deposit, the clear liquor decanted, and the vessel filled with water to wash the precipitate, which is afterwards collected upon a filter and dried, and then mixed with twice its weight of carbonate of potash, and fused in a Hessian crucible for 15 minutes, or until the fused fluid ceases to of fervesce. On removing the crucible, and pouring the whole into an iron ladle, when cool the silver will bo found in the metallic state at the bottom of the ladle. In these operations, when pouring the acid into the cyanide solution, great care must be taken not to inhale the fumes given off, which are very abundant and poisonous. The operation should bo done in the open air, and even then it is bad. Instead of throwing down the silver by an acid, it is better to evaporate the solution to dryness, and to fuse the product as described; in which case the cyanide is an excellent reducing flux, requiring no addition of carbonate of potash, and saves the necessity of evolving poisonous fumes.

3703. Test for Free Cyanide of Potassium in Solutions

3703.    Test for Free Cyanide of Potassium in Solutions. If wo dissolve a small quantity of sulphate of copper and add to it an excess of ammonia, there is produced a deep blue color. Cyanide of potassium will destroy the blue color, in a fixed chemical proportion. To obtain this proportion, take ten grains of pure cyanide of potassium and dissolve in water; then take a certain quantity, say 100 grains, of sulphate of copper, and convert it into ammoniuret, the whole measuring a given quantity, and pour from an alkalimeter this blue liquor into the cyanide of potassium till it ceases to destroy the color, then mark the number of graduations required, and that amount of copper solution will represent 10 grains cyanide of potassium - a quantitative test will thus bo got for the full cyanide of potassium in the solution, and should bo used as follows: Say that the color of GO graduations of the blue solution was destroyed by the 10 grains of cyanide of potassium ; then, to test the quantity of free cyanide of potassium in the plating solution, take 60 graduations of the blue liquor in any convenient vessel, and add to it from an alkalimeter the plating solution, till the color of the blue liquor is destroyed, then note the quantity which contains 10 grains free cyanide, from which the quantity in the whole solution may be calculated.