Cobalt Sulphate, CoSO₄

Cobalt Sulphate, CoSO₄, is obtained in the anhydrous condition by raising any of its hydrates to dull redness, or by simply maintaining the heptahydrate at 250° C. until all the water has been expelled; by evaporation of a solution of any of its hydrates in concentrated sulphuric acid; or by heating a hydrate with ammonium sulphate.

By the last of these methods the salt is obtained in stable, purple- red, truncated octahedra, of density 3.65. With ammonia it yields the hexammoniate, CoSO₄.6NH₃. Heated in a tube open at both ends, anhydrous cobalt sulphate begins to decompose at 718° C., yielding cobalt monoxide.

Anhydrous cobalt sulphate is very stable in air at ordinary temperatures, and dissolves but slowly in hot water. The solution can be more readily prepared by dissolving the oxides or carbonate in dilute sulphuric acid. Upon evaporation at room temperatures the hepta-hydrate, CoSO₄.7H₂O, crystallises out. A convenient method of preparing this hydrate consists in dissolving tricobalt tetroxide in hydrochloric acid, adding an equivalent of sulphuric acid and allowing to crystallise after evaporating off the hydrogen chloride. Obtained in this manner the crystals are acidic, but may be freed from acid by precipitation from aqueous solution with alcohol and recrystallisation from pure water.

The crystals are red in colour, of density 1.924, and isomorphous with their iron and nickel analogues. They melt at 96° to 98° C., and effloresce upon exposure to air, losing one molecule of water. Practically insoluble in absolute ethyl alcohol, the crystals readily dissolve in methyl alcohol and in water, the solubility in the last-named solvent being as follows:

Temperature ° С.	0	10	20	40	60	80	100
Grams CoSO4 in 100 grams H2O	25.55	30.55	36.21	48.85	60.4	70.0	83.0

The heptahydrate is found in nature as the mineral cobalt vitriol or bieberite.

A concentrated solution of cobalt sulphate maintained at 25° C. deposits monoclinic crystals of the hexahydrate, CoSO₄.6H₂O, isomorphous with its zinc sulphate analogue. Density 2.019.

The pentahydrate, CoSO₄.5H₂O, was stated to result on exposing the heptahydrate over concentrated sulphuric acid. Density 2.134.

The tetrahydrate, CoSO₄.4H₂O, is obtained by exposing finely powdered heptahydrate to desiccation over sulphuric acid until a constant weight is arrived at. It is also produced by pouring the concentrated solution into sulphuric acid. Density 2.327.

The trihydrate, CoSO₄.3H₂O. The existence of this hydrate has not as yet been definitely ascertained.

The dihydrate, CoSO₄.2H₂O, is obtained by boiling the heptahydrate in a fine state of division in absolute alcohol. Density 2.712.

The monohydrate, CoSO₄.H₂O, is precipitated on adding excess of concentrated sulphuric acid to an aqueous solution of cobalt sulphate; by evaporation of an acid solution of the salt on the sand-bath until sulphur dioxide is evolved; and by heating the heptahydrate to 100° C. until no further loss in weight accrues; by decomposition of purpureo-cobalt chloride in a small quantity of water with concentrated sulphuric acid until the salt has passed into solution. The whole is then heated to 220° C. on a sand-bath. On cooling the acid is decanted, the salt washed with cold water, and finally with alcohol.

As prepared in this manner cobalt sulphate monohydrate is a red, crystalline powder, very slowly soluble in water. Exposed to moist air it slowly absorbs water. At red heat it loses water, becoming anhydrous. Density 3.125.

Several of these have been described as resulting on addition of ammonia, cobalt carbonate or calcium carbonate to solutions of cobalt sulphate under varying conditions. The products obtained consist of blue, bluish green, and lilac precipitates of variable composition.

Numerous double salts of cobalt sulphate have been prepared. Chief amongst these are salts of the type M₂SO₄.CoSO₄.6H₂O, where M stands for ammonium or an alkali metal - potassium, rubidium, or caesium. Each of these salts has been obtained and its crystallographic constants determined. The series is isomorphous with the corresponding ferrous and nickel double salts, and with their double selenates; the individual salts are discussed below. Other double salts are cupric cobaltous sulphate, CuSO₄.2CoSO₄.3H₂O, and cobaltous nickel sulphate, (Co, Ni)SO₄.H₂O, in which the cobalt and nickel appear interchangeable.