OLD TONING BATHS.
These will contain both gold and silver. They should be kept in a jar or tub until a sufficient quantity for treatment has been obtained, the collected solution being then concentrated by evaporation in a dish. After concentration, granu lated zinc should be added, and the whole allowed to stand overnight. In the case of the acetate bath, both the gold and silver are deposited in the metallic state, but with the suiphocyanide the gold alone appears to be deposited in the metallic state, and the silver as a double sulpho cyanide of silver and zinc. The zinc should be treated with nitric acid and heated ; the residual zinc is then dissolved out, the silver is converted into the insoluble cyanide, and the gold is unaffected. After decanting, the precipitate should he treated with ammonia, which dissolves the cyanide of silver, leaving behind the gold. The latter should be washed several tunics with water and then boiled with aqua rivia ; on evaporating to dryness the gold is obtained as chloride ready for use. The ammoniacal solution of silver cyanide should be treated with nitric acid in slight excess, whereby the cyanide is precipi tated. This is then collected on a filter paper and added to Precipitate A.
PLEcovEnv OF PLATINUM, URANIUM, ETC.
Platinotype clippings should be burnt to an ash and treated with aqua regia ; the solution is diluted, filtered off, and added to any old platinum toning or developing baths. The baths may be concentrated and treated with zinc, as in the recovery of gold from ordinary toning baths, the platinum being converted into platinic chloride in an exactly similar manner. Old uranium toning baths should be heated with aqua reqia to destroy the ferri cyanide, and ammonia then added in ex cess. The precipitate should be collected on a filter paper, washed several times with hot water, and finally boiled with the least excess of acetic acid. The solution is evaporated to dryness to expel excess of acid ; the resulting acetate may then be used for the preparation of a new bath. in preparing pure solutions and in wash ing precipitates distilled water should always be used. Tap water contains chlorides, which give a precipitate with silver nitrate, or sometimes there is even sufficient organic matter to cause a reduc tion with salts of gold and silver.