Potassium aquo ferricyanide, K₂Fe(CN)₅

Potassium aquo ferricyanide, K₂Fe(CN)₅.H₂O, is obtained by the prolonged action of chlorine upon potassium ferrocyanide solution. At first potassium ferricyanide is formed, which undergoes further decomposition, the chlorine abstracting one (CN) group, water taking its place. Thus: -

2K₄Fe(CN)₆ + Cl₂ = 2K₃Fe(CN)₆ + 2KCl,

followed by

K₃Fe(CN)₆ + Cl₂ = K₃Fe(CN)₅Cl + CN.Cl.K₃Fe(CN)₅Cl + H₂O = K₂Fe(CN)₅.H₂O + KCl.

The same salt is produced by the action of a mixed solution of sodium chlorate and hydrochloric acid upon potassium ferrocyanide.

The solution is intensely violet, but the salt is readily precipitated by addition of alcohol. Reduction with alkaline hydroxylamine chloride solution yields potassium aquo penta-ferrocyanide, K₃Fe(CN)₅.H₂O, whilst with alkaline potassium sulphite the sulphito penta-ferrocyanide, K₅Fe(CN)₅.SO₃, is obtained.

In earlier literature this salt is referred to as potassium perferri-cyanide, the formula, K₂Fe(CN)₆, being ascribed to it.