(io) Mix with one-half its weight sodium carbonate and one-quarter its weight of dry, clean, sand, and ignite.
Platinum is now largely used in photography, in the different platino type processes and in platinum toning, and is a sufficiently valuable metal to repay the worker to save what he can of it.
The following reniarks concerning the recovery of platinum residues, are given in Lyonel Clark's book on platinum toning : " The old toning baths, and the washing water the prints are passed through after toning, are especially rich in the metal, and these liquids should be poured into any bottle, and an ounce or so of the ordinary ferrous oxalate developing solution added. The bottle should be stood aside, and the platinum will be found to be rapidly thrown down in the metallic state. The major portion of the liquid is decanted off, and the residue filtered and dried, and can be sent to the refiners at once, or it can be converted into platinum chloride for subsequent conversion into chloroplatinite of potassium. In case any iron should be present, it is best to treat the filtrate with a fairly strong solution of sulphuric acid; this will dissolve out any iron in the form of the sulphate without injuring the platinum; the residue should be washed and dried and dissolved in aqua regia (nitric acid one part, hydrochloric acid, four parts), with gentle warming. As little aqua regia as possible should be used, only just sufficient to dissolve
the platinum, as it has subsequently all to be evaporated off. The dissolution of the platinum should be conducted out of doors, or in a well-ventilated room, as the nitrous oxide fumes given off are most unpleasant, and rapidly rust iron or steel objects. The residue left after evaporation is chloroplatinic acid, and can be weighed, and at once used for the production of the chloro platinite of potassium.
Gold obtain the gold from old toning baths, add sulphate of iron either in crystals or in solution. This will precipitate the gold in a black powder, which is well washed and dried, and then dissolved in nitro-muriatic acid, when a solution of gold chloride will be the re sult. This can be evaporated to dryness, dissolved in distilled water, and again evaporated to remove all traces of acid. Another way is to reduce the black powder in a crucible, but an enormous heat is required.