Potassium carbonyl ferrocyanide, 2K₃[Fe(CN)₅CO]

The discovery and early preparation of Potassium carbonyl ferrocyanide, 2K₃[Fe(CN)₅CO].7H₂O, have already been described. It may be prepared in the laboratory by passing moist carbon monoxide over potassium ferrocyanide at 130° C. Thus: -

K₄Fe(CN)₆ + CO + 2H₂O = K₃Fe(CN)₅.CO + NH₃ + H.COOK.

It crystallises in thin scales and rectangular prisms. 100 parts of water at 18° C. dissolve 148 parts of the salt.

The salt is more stable than potassium ferrocyanide towards oxidising media, being unaffected by potassium persulphate, and but slightly oxidised by permanganate or ozone in the absence of sulphuric acid. The salt becomes anhydrous at 120° C. The heats of solution are as follow: -

[K₃Fe(CN)₅CO] + Aq. = K₃Fe(CN)₅CO.Aq. - 5,200 calories,

[2K₃(Fe(CN)₅CO).7H₂O] + Aq. = 2K₃Fe(CN)₅CO.Aq. -21,800 calories,

whence the heat of hydration of the anhydrous salt is 5700 calories at 18° C.

Chlorine appears to oxidise the salt to an unstable carbonyl ferricyanide. Prolonged exposure to chlorine results in the formation of potassium ferricyanide.