Ferric potassium alum, K₂SO₄.Fe₂(SO₄)₃

Ferric potassium alum, K₂SO₄.Fe₂(SO₄)₃.24H₂O, may be obtained by mixing together concentrated solutions of potassium and ferric sulphates in the requisite proportions and allowing to stand for some days at 0° C., when the salt crystallises out in violet octahedra.

It cannot be obtained as readily by electrolysis as the ammonium salt, owing to the relatively slight solubility of potassium sulphate.

Addition of potassium hydroxide to a solution of the alum, and subsequent evaporation yields a crop of crystals yellowish brown in colour and possessing peculiar optical properties like tourmaline. Their composition is given as 5K₂SO₄.2Fe₂(SO₄)₂.(HO)₂.16H₂O. The crystals decompose to iron alum and a basic salt.

The densities of various concentrations of the alum in water at 17.5° C. are given as follow: -

Iron alum (per cent.)	5	10	15	20	25	30
Density	1.0268	1.0466	1.0672	1.0894	1.1136	1.1422