Potassium nitroprusside, K₂[Fe(CN)₅NO]

Potassium nitroprusside, K₂[Fe(CN)₅NO], may be obtained by the interaction of a ferrous salt with the nitrite and cyanide of potash: -

5KCN + 2FeSO₄ + KNO₂ + H₂O = K₂[Fe(CN)₅NO] + Fe(HO)₂ + 2K₂SO₄,

or by the action of potassium nitrite on ferricyanides at 100° C.

A convenient method consists in decomposing the barium salt with the calculated amount of potassium sulphate, and, after filtering off the insoluble barium sulphate, evaporating at low temperature under reduced pressure. The residue is purified by recrystallisation from 95 per cent, alcohol, when it separates as pale pink, anhydrous crystals.