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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.
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