Ammonium Cobalt Alum, (NH₄)₂SO₄.Co₂(SO₄)₃

Ammonium Cobalt Alum, (NH₄)₂SO₄.Co₂(SO₄)₃.24H₂O, was first prepared by Marshall, who, from a peculiar change in colour observed in the electrolysis of a solution of copper-cobalt potassium sulphate, was induced to electrolyse a solution of ammonium and cobalt sulphates, acidified with sulphuric acid in a divided cell.

The solution passes through a series of changes of colour, ultimately becoming greenish blue in consequence of the oxidation of the cobalt to the trivalent state. This suggested the formation of an alum, and by continuing the electrolysis for several days, and then concentrating the solution, blue octahedral crystals were obtained, a few crystals showing faces of dodecahedra and cubes. When thoroughly dry the crystals are not unstable, but their solution is quickly reduced, although the addition of dilute sulphuric acid increases its stability.