Cobaltic Sulphate, Co₂(SO₄)₃

Cobaltic Sulphate, Co₂(SO₄)₃.18H₂O, is obtained by electrolysis of cobaltous sulphate in a platinum dish, which serves as anode, the cathode consisting of a platinum wire immersed in a porous cell containing dilute sulphuric acid. The salt is deposited as blue silky needles round the anode. It is soluble in water, yielding an unstable blue solution which evolves oxygen, the salt being reduced to cobaltous sulphate. Upon exposure to air the crystals gradually decompose. In contact with hydrochloric acid, chlorine is evolved.

Cobalt Alums, M₂SO₄.Co₂(SO₄)₃.24H₂O

Cobaltic sulphate, like the sulphates of rhodium and iridium, unites with the sulphates of the alkali metals to yield a series of well-defined, crystalline salts known as alums. These are isomorphous with those of iron, manganese, chromium, and aluminium, and form an interesting link between these metals and the central vertical column in Group VIII of the Periodic Table, of which column cobalt is the first member.