Carbonyl ferrocyanic acid, H₃[Fe(CN)₅.CO]

Hydrogen carbonyl ferrocyanide, or carbonyl ferrocyanic acid, H₃[Fe(CN)₅.CO].H₂O, is obtained by the action of hydrogen sulphide on the copper salt in solution. Upon evaporation in the dark over potassium hydroxide, the acid separates in colourless tabular crystals. It is acid to litmus, and is sufficiently powerful to decompose alkali carbonates.

The heat of combustion of carbonyl ferrocyanic acid is 809,800 calories, the products being nitrogen, carbon dioxide, water, and ferric oxide. The heat of formation of the anhydrous acid is -48,600 calories; the acid is thus less strongly endothermic than ferrocyanic acid (-122,000 calories).