Purification of Silver Residues

The dried silver residue is treated with aqua regia (Care Hood ). After action has ceased the solution is filtered through glass wool and the residue is washed to 

The silver chloride is transferred to a casserole, covered completely with concentrated hydrochloric acid, and stick zinc of high purity is added to reduce the silver to the elemental form. The metal is washed thoroughly with water until the last washings give no test for the chloride ion. 

The metal is dissolved in dilute nitric acid (1 volume concentrated acid to 1 volume water). A large volume of distilled water is added, and the solution is permitted to stand for at least 12 hours. Tin, antimony, and bismuth, if present, are precipitated here. 

The silver is dissolved in dilute (7.5 N) nitric acid. An excess of 20 per cent above the calculated weight of formic acid (85 per cent) needed for reduction to metallic silver is neutralized with a slight excess of ammonium hydroxide, and the resultant solution added drop by drop to the hot silver nitrate solution. The granular precipitate of silver is washed with hot water and dried by suction or between filter papers. 

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