Ammonium ferrocyanide, (NH₄)₄Fe(CN)₆

Ammonium ferrocyanide, (NH₄)₄Fe(CN)₆.3H₂O, may be obtained either by the action of ammonium hydroxide upon Prussian blue, or by neutralising hydrogen ferrocyanide with ammonia. On concentrating the solution over potassium hydroxide in a vacuum, the salt crystallises out in thin, greenish-yellow plates, or the salt may be precipitated from solution with alcohol. When heated in the absence of air, ammonium ferrocyanide yields ferrous cyanide. The aqueous solution of the salt is very unstable, being decomposed on merely warming, yielding, amongst other products, ammonium cyanide. When boiled in contact with air, it yields a dull green deposit of Fe^•••₂Fe^••(NH₄)₈[Fe''(CN)₆]₄.3H₂O.

The double salt with ammonium chloride, (NH₄)₄Fe(CN)₆.NH₄Cl.3H₂O, is obtained as pale yellow or brown crystals by cooling warm, mixed solutions of ammonium ferrocyanide and ammonium chloride in the presence of sodium cyanide or of acetic acid respectively. The anhydrous salt has also been obtained.