Cobalti-cyanic Acid, H₃Co(CN)₆

Cobalti-cyanic Acid, Hydrogen Cobalti-cyanide, H₃Co(CN)₆.H₂O, the free acid may be prepared by decomposing a concentrated aqueous solution of the potassium salt with sulphuric acid, a slight excess of the latter being added. The mixture is warmed, and upon addition of alcohol potassium sulphate separates out, the hydrogen cobalti-cyanide remaining in solution. Evaporation and reerystallisation of the resulting product yields the pure acid free from sulphuric acid. It may also be obtained by decomposing the copper or lead salts in aqueous solution with hydrogen sulphide; and by decomposing the cineol derivative with water. Hydrogen cobalti-cyanide crystallises from concentrated aqueous solutions in minute, colourless, transparent needles. It has a strongly acid reaction, and readily decomposes carbonates. It dissolves many metals when in a fine state of division, yielding stable salts, hydrogen gas being evolved. It readily absorbs moisture from the air, its crystals deliquescing. It is also soluble in alcohol, but not in ether. At 100° C. its combined water is expelled, and at a somewhat higher temperature hydrogen cyanide is evolved, the acid becoming yellowish in colour. At 190° C. it assumes a green colour, becoming blue at 250° C., and when strongly heated becomes incandescent, leaving a black, carburised residue.