Nitroprussic acid, H₂[Fe(CN)₅NO]

Hydrogen nitroso ferricyanide or nitroprussic acid, H₂[Fe(CN)₅NO], is obtained by decomposition of the silver salt with hydrochloric acid or by the action of dilute sulphuric acid upon the barium salt. It is also fortned when nitric oxide is bubbled through an acidified solution of potassium ferrocyanide. The reaction proceeds in two stages, namely, (a) oxidation to the ferricyanide, and (b) substitution of the cyanogen radicle by NO: -

H₃[Fe(CN)₆] + NO = H₂[Fe(CN)₅NO] + HCN.

Concentration in vacuo yields dark-red needles, which readily deliquesce. Its most important salt is Sodium nitroprusside.