Hydrogen Chromicyanide, H₃[Cr(CN)₆]

Hydrogen Chromicyanide, Chromicyanic Acid, H₃[Cr(CN)₆], is prepared by adding tartaric acid to a solution of potassium chromicyanide, or by passing hydrogen sulphide into water in which silver or lead chromicyanide is suspended. In the latter case the insoluble sulphide is filtered off and the filtrate evaporated in vacuo. Reddish- yellow vitreous crystals are obtained which have an acid reaction. The aqueous solution on standing loses hydrogen cyanide, a change which also takes place on prolonged boiling of a solution of the acid in boiling hydrochloric acid, when the red liquid gradually becomes green, compounds of composition HCN.Cr(CN)₃ and 2HCN.Cr(CN)₃ probably being formed. Chromicyanic acid is tribasic, and forms salts analogous in composition to the ferricyanides.