Cobaltous Cyanide, Co(CN)₂

Cobaltous Cyanide, Co(CN)₂.3H₂O, may be precipitated from a solution of the chloride by addition of potassium cyanide. The precipitate, which has a reddish colour, becomes violet at 100° C. It still retains its combined water even at 200° C., but at 250° C. becomes incandescent in air and leaves a black carburised residue. The original precipitate is soluble in excess of potassium cyanide in consequence of the formation of soluble potassium cobalto-cyanide, K₄Co(CN)₆. Addition of dilute hydrochloric acid causes the reprecipitation of the cobaltous cyanide unless, in the meantime, the solution has been warmed, thereby oxidising the cobalto-cyanide to cobalti-cyanide, which is more stable (vide infra).