Alums

Originally the name alum was given to the double salt, potassium aluminium sulphate, K₂SO₄.Al₂(SO₄)₃.24H₂O. But now this name is used for all the double salts having the composition: M₂ISO₄M₂III(SO₄)₂.24H₂O where MI stands for univalent basic radical like Na+, K+, Rb+, Cs+,  NH⁺₄ , Ag+, Tl+ and MIII is a trivalent metal Al³⁺, Cr²⁺, Fe³⁺, Co²⁺ etc. The crystals of alums are made up of [MI(H₂O)₆]+, [MIII(H₂O)₆]³⁺ and four SO^2-₄ ions.

Some common examples of alums are:

(i) Potash alum (or common alum)−K₂SO₄.Al₂(SO₄)₃.24H₂O.

(ii)Sodium alum−Na₂SO₄.Al₂(SO₄)₂.24H₂O

(iii) Ammonium alum (NH4)₂SO₄.Al₂(SO₄)₃.24H₂O

(iv) Chrome alum K₂SO₄.Cr₂(SO₄)₃.24H₂O etc. The crystals of alums are generally obtained when hot solutions of equimolecular quantities of their constituent sulphates are mixed and the resulting solution is cooled. All the alums are isomorphous, form mixed crystals in all proportions and also layer−crystals. They are usually much more soluble in hot water than in cold water. In solution they give the ions of the component salts.

Alums are often used for the removal of colloidal impurities from water (as coagulants).

POTASH ALUM OR COMMON ALUM,

K₂SO₄.Al₂(SO₄)³.24H₂O or KAl(SO₄)².12H₂O

Preparation

Commercially it is prepared as follows:

From alunite or alumstone mineral, K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃.

The mineral alunite is calcined and treated with H₂SO₄ when Al(OH)₃ component is converted into Al₂(SO₄)₃. Now calculated quantity of K₂SO₄ is added to the solution, which is then crystallized for alum. Finally it is purified by recrystallisation.

K₂SO₄.Al₂(SO₄)₃.4Al(OH)₃ + 6H₂SO₄ K₂SO₄ + 3Al₂(SO₄)₃ + 12H₂O

Alunite mineral

K₂SO₄ + Al₂(SO₄)₃ + 24H₂O K₂SO₄.Al2(SO₄)3.24H₂O

potash alum

Properties

Potash alum is white crystalline solid, soluble in water but insoluble in alcohol. The aqueous solution is acidic due to the hydrolysis of aluminium sulphate. When heated, it melts at 92°C and on further heating loses the whole of its water of crystallization and swells up. The swollen mass so produced is called burnt alum (phul phatkiri).

Uses

It is used in purification of water, water proofing of textiles and in dyeing and paper industry. It is also employed as a styptic to arrest bleeding.