Identifying Silver Minerals

1. Reduction to the Metal on Charcoal. Silver can frequently be reduced to a metallic globule from its compounds by heating the powdered mineral on charcoal with sodium carbonate. The resulting globule is bright both when hot and cold. It is malleable. No accompanying coating is formed on the charcoal. This resulting globule is bright both when hot and cold. It is malleable. No accompanying coating is formed on the charcoal. This test for silver is frequently complicated by the presence of lead, arsenic or antimony in the mineral. Usually the mineral should be carefully roasted on charcoal in the oxidizing flame before attempting the reduction in order to remove the last two; otherwise a brittle globule will result. In many cases the only satisfactory test for silver is the fire assay.
2. Precipitation as Silver Chloride. When a silver mineral is dissolved in nitric acid and to the solution a few drops of hydrochloric acid is added, a white curdy precipitate of silver chloride, AgCI, formed. The test is quite delicate, and if there is only a trace of silver in the solution its presence will be indicated by amilky-blue coloration. The precipitate is white at first but darkens on exposure to light. It is soluble in ammonium hydroxide. Frequently when a silver mineral is treated with nitric acid a precipitate will result at once. This may be metantimonic acid, lead sulfate, etc., and should be filtered off before making the silver test.