Alums

Alums are a family of double salts named after the archetype, potash alum, potassium aluminium sulphate [K₂SO₄.Al₂(SO₄)₃.24H₂O]. The name is now applied generally to any double salt of the general formula M'₂SO₄.M''₂(SO₄)₃.24H₂O, where M' is a univalent metal ion (or the ammonium ion, [NH₄]⁺) and M'' is a trivalent metal ion. Besides potash alum, common examples are sodium alum [Na₂SO₄.Al₂(SO₄)₃.24H₂O], ammonium alum [(NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O] and chrome alum [K₂SO₄.Cr₂(SO₄)₃.24H₂O]. Alums are generally more soluble in hot water than cold, and in solution each behaves as its separate constituent ions.

Preparation of potash alum. The naturally occurring mineral alunite (alum stone), K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃, is treated with excess sulphuric acid, converting the aluminium hydroxide present into aluminium sulphate; a calculated quantity of potassium sulphate is then added, and the solution is crystallised (and recrystallised, for purity) to give potash alum: K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃ + 6H₂SO₄ → K₂SO₄ + 3Al₂(SO₄)₃ + 12H₂O, and then K₂SO₄ + Al₂(SO₄)₃ + 24H₂O → K₂SO₄.Al₂(SO₄)₃.24H₂O. …