Cobalt Perchlorate, Co(CoO₄)₂

Cobalt Perchlorate, Co(CoO₄)₂.5H₂O, may be obtained by dissolving cobalt carbonate or hydroxide in a solution of perchloric acid, expelling the excess of acid at 110° C., and concentrating to crystallisation. The salt separates out in long red needles which are very soluble in water, alcohol, and acetone, but are insoluble in chloroform. When dried over sulphuric acid the crystals contain five molecules of water, and melt at 143° C. They are more stable than the corresponding pentahydrate of nickel perchlorate, and do not decompose at 103° C., neither does their aqueous solution undergo hydrolytic decomposition. The solubility of cobalt perchlorate in water is as follows:

Temperature ° C.	-30.7	-21.3	0	7.5	18	26	45
Grams Co(CoO4)2 in 100 grams solution	19.02	20.72	22.90	23.35	23.75	25.95	26.33
Density of solution	-	-	1.5639	1.5658	1.5670	1.5811	1.5878

At -21.3° C. the nonahydrate, Co(CoO₄)₂.9H₂O, is formed. The hexahydrate, Co(CoO₄)₂.6H₂O, is described by Salvadori as crystallising in hexagonal prisms, which are pale pink in colour and show feeble polychroism. Heated to 100° C. the salt loses water, yielding a violet, hygroscopic powder. When rapidly heated it deflagrates without explosion but it explodes on percussion.

When ammonia and ammonium perchlorate are added to an aqueous solution of calcium perchlorate, the hexammoniate, Co(CoO₄)₂.6NH₃, is formed as a red precipitate. Boiling water effects its decomposition according to the equation:

2Co(CoO₄)₂.6NH₃ + 2H₂O = Co(OH)₂ + Co(CoO₄)₂ + 2NH₄CoO₄ +NH₃.

The anhydrous salt, Co(CoO₄)₂, cannot be isolated since the hydrated salt decomposes before all the water has been expelled.