Ammonium Cobalt Sulphate, (NH₄)₂SO₄.CoSO₄

Ammonium Cobalt Sulphate, (NH₄)₂SO₄.CoSO₄.6H₂O, is obtained by evaporation of mixed solutions of ammonium and cobalt sulphates in equivalent proportions. The crystals are red, exhibiting pleo- chroism, and of density 1.873. Their solubility in water is as follows:

Temperature ° C	0	10	18	23	35	45	60	75
Grams of (NH4)2SO4.CoSO4. in 100 grams of water	8.9	11.6	15.2	17 1	19.6	25 0	34.5	43.3

An anhydrous double salt, of composition 2(NH₄)₂SO₄.3CoSO₄, is obtained by heating anhydrous cobalt sulphate13 with excess of ammonium sulphate. In contact with water it readily hydrates with evolution of heat.

Lithium sulphate does not appear to unite with cobalt sulphate on heating in the anhydrous condition, but sodium sulphate yields the double salt, CoSO₄.3Na₂SO₄, which decomposes at 425° C. With potassium sulphate two compounds are obtained, namely, CoSO₄.K₂SO₄, which decomposes above 560° C., and 2CoSO₄.K₂SO₄, which crystallises out at 736° C.