Cyanoplatinic Acid, HPt(CN)₄

On adding hydrogen peroxide (20-volume solution) to a fairly concentrated solution of cyanoplatinous acid and evaporating over a water-bath, a white substance is first produced which, upon further evaporation, becomes pink, brown, and finally dull olive-green. Cyanoplatinic acid, HPt(CN)₄.2H₂O, is left. On treating this last-named product with water it passes through the same colour changes, but in the reverse order, before dissolving, showing that the colours correspond to the formation of different hydrates:

2H₂Pt(CN)₄ + H₂O₂ = 2HPt(CN)₄ + 2H₂O.

The aqueous solution of this acid liberates iodine from potassium iodide, the reaction being quantitative if sufficient time (several hours) is allowed. Thus:

2HPt(CN)₄ + 2KI = 2KHPt(CN)₄ + I₂.

Sulphur dioxide reduces the acid solution to cyanoplatinous acid. Heated to 120° C. the solid green acid is converted into a light yellow residue consisting mainly of platinum tricyanide, Pt(CN)₃.