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Silver Nitrate

water, residue, copper, acid, solution, pure and dissolved

SILVER NITRATE (Formula, AgNO, ; molecular weight, z7o ; synonyms, nitrate of silver, argentic nitrate, lunar by dissolving pure silver in nitric acid. Six ounces of silver are dissolved in 2i fluid ounces of strong nitric acid and zo fluid ounces of water, by the aid of gentle heat. The solution is then evaporated to dryness and healed to 198 deg. C. (fusing point) to expel excess of acid. Silver nitrate can also be prepared from standard silver, contain ing copper, in the following manner : Dissolve the metal in a moderately strong solution of nitric acid and evaporate in a porcelain dish. A blue residue, containing the nitrate of silver and copper, will be the result. The dish is then heated until this residue has fused and become uni formly black, the blue copper nitrate being decomposed, and leaving black copper oxide at a temperature which is insufficient to decompose the silver nitrate. Care should be taken that the residue is not heated more than sufficient to decompose all the copper nitrate. To find out when this is effected, take a little on the end of a glass and dissolve in water filtered and tested with ammonia. If any copper be present a blue color will be produced. When satisfactory, the residue is treated with hot water, the solution filtered from the copper oxide and evaporated to crystallization.

For surgical purposes the fused nitrate is moulded into thin sticks (lunar caustic), but for chemical and photographic purposes it is dissolved in water and crystallized. Silver nitrate acts as a powerful caustic, owing to the facility with which it parts with oxygen, the silver being reduced to the metallic state when in contact with organic matter. This effect is greatly increased by exposure to light. Pure silver nitrate undergoes no change when exposed to the i light, but if organic matter be present a black deposit, containing finely-divided silver, is pro duced. It is for this reason that silver baths used for sensitizing collodion plates and albumen ized paper readily become discolored. They can, however, be easily purified. (See Silver Bath.) Recrystallized silver nitrate should be soluble in its own weight of water and in four times its weight of boiling alcohol, which, however, precipitates it on cooling. Zinc, iron, lead, tin, and

several other metals precipitate metallic silver from solutions of silver nitrate The following method of testing the purity of a commercial sample of silver nitrate is given by Clement J. Leaper : " Dissolve it in water ; completely precipitate the silver with pure hydrochloric acid ; filter and evaporate the filtrate to dryness, when no residue should remain. If there should be any remain, ignite it, when white fumes will indicate the presence of ammonium nitrate in the sample. If a residue remain after continued ignition, treat it with boiling water and test the soluble portion for potassium and sodium in the usual way (vide tests for these salts), when positive results will indicate the presence of potassium or of sodium nitrate. The insoluble portion left after treating the residue with water is dissolved in a little nitric acid diluted with water and tested for lead with sulphuretted hydrogen, and for magnesium with ammonium hydroxide and sodium phosphate." Silver nitrate is largely used in photography for a variety of purposes, but chiefly to form other silver salts. To prepare a pure solution of silver nitrate it is dissolved in water in a clean glass, bottled, and placed in the sun for a few hours. The solution will be rapidly darkened, and a brown precipitate will be formed. This will eventually fall to the bottom, and the clean solu tion may be decanted or filtered off (See Silver Bath.) To test the amount of silver nitrate in a solution, a special instrument, termed an argento meter, has been constructed. This is clearly marked with the number of grains per ounce. The following table, however, by Dawson,* will perhaps be found useful at times, when only an ordin ary hydrometer is available. (See also under Silver Tester.) Correction for Temperature.—For every to° below Go° deduct one grain from the number quoted in the table, and for every to° above 6o° add one grain to the number tabulated.