Potassium Cobalto-cyanide, K₄Co(CN)₆

Potassium Cobalto-cyanide, K₄Co(CN)₆, as mentioned above, is produced when cobaltous cyanide is dissolved in excess of potassium cyanide solution. It may be precipitated from solution by the addition of alcohol. Its aqueous solution contains the ion Co(CN)₆'''', so that the formula for the salt is K₄Co(CN)₆ rather than 4KCN.Co(CN)₂.

When an air-free aqueous solution of potassium cobalto-cyanide is boiled, hydrogen is evolved, the volume of which equals that of the oxygen absorbed if the solution is rapidly oxidised in air, but to twice that volume during slow oxidation in air. An explanation for this apparent anomaly is given below.