Metallic platinum is insoluble in the individual mineral acids, but readily dissolves in aqua regia, yielding platinic chloride, PtCl4. A mixture of chloric acid with excess of hydrochloric acid has been recommended as a useful solvent for platinum, the mechanism of the reaction depending, as in aqua regia, upon the liberation of free chlorine. Thus:
5HCl + HClO3 = 3Cl2 + 3H2O.
When present in certain alloys, platinum is dissolved by nitric acid.
When fused with caustic alkalies or potassium nitrate, platinum is oxidised.
Platinum sponge dissolves in boiling potassium cyanide solution. |
Hydrogen sulphide slowly causes the deposition of a dark brown precipitate of platinic sulphide, PtS2, which is soluble in aqua regia, but insoluble in nitric acid and in hydrochloric acid separately.
Ammonium sulphide precipitates the disulphide from solution; it is soluble in excess of the yellow ammonium sulphide, yielding the thio salt, (NH4)2PtS3.
Ammonium chloride produces a yellow precipitate of ammonium chlor-platinate, (NH4)2PtCl6, which is slightly soluble in water, but insoluble in alcohol.
Platinum salts are readily reduced to the metal by ferrous sulphate and stannous chloride solutions.
Solutions of platinic chloride yield a blood-red colour with stannous chloride unless in very dilute solution, when the colour is a golden brown. On shaking with ether the colour is extracted. This reaction is useful since it is not given by iridium or palladium, nor yet by iron or gold. Filter-paper, however, gives coloration after treatment with aqua regia, so that in testing for platinum any filter-paper or organic matter of a humic nature should be destroyed by fusion with pyro- sulphate prior to extraction with aqua regia.
Oxalic acid, on the other hand, does not reduce platinic chloride although it reduces gold solutions, depositing metallic gold. This often affords a convenient method of separating the two metals.
Metallic platinum is precipitated from solutions of its salts by metallic zinc. |