Detection of Platinum

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.