Alums

Alums:

The name alum is given to the double salt of potassium aluminium sulphate [K₂SO₄. Al₂(SO₄)₃.24.H₂O]. Now a days it is used for all the double salts with M'₂SO₄.M"₂(SO₄)₃.24H₂O, where M' is univalent metal ion or [NH₄]⁺ and M" is trivalent metal ion.

Examples:

Potash alum [K₂SO₄.Al₂(SO₄)₃.24.H₂O]; Sodium alum [Na₂SO₄.Al₂(SO₄)₃.24.H₂O] , Ammonium alum [(NH₄)₂SO₄.Al₂(SO₄)₃.24.H₂O], Chrome alum [K₂SO₄.Cr₂(SO₄)₃.24.H₂O].

Alums in general are more soluble in hot water than in cold water and in solutions they exhibit the properties of constituent ions.

Preparation:

The alunite the alum stone is the naturally occurring form and it is K₂SO₄. Al₂(SO₄)₃.4Al(OH)₃. When alum stone is treated with excess of sulphuric acid, the aluminium hydroxide is converted to aluminium sulphate. A calculated quality of potassium sulphate is added and the solution is crystallised to generate potash alum. It is purified by recrystallisation.

K₂SO₄.Al₂(SO₄)₃ .4Al(OH)₃ + 6H₂SO₄  → K₂SO₄ + 3Al₂(SO₄)₃ + 12 H₂O

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

Properties

Potash alum is a white crystalline solid it is soluble in water and insoluble in alcohol. The aqueous solution is acidic due to the hydrolysis of aluminium sulphate it melts at 365 K on heating. At 475 K loses water of hydration and swells up. The swollen mass is known as burnt alum. Heating to red hot it decomposes into potassium sulphate, alumina and sulphur-trioxide.

Potash alum forms aluminium hydroxide when treated with ammonium hydroxide.

K₂SO₄.Al₂(SO₄)₃.24 H₂O +6NH₄OH   →  K₂SO₄ + 3(NH₄)₂SO₄ + 24 H₂O + 3Al(OH)₃

Uses of Alum:

·           It is used for purification of water

·           It is also used for water proofing and textiles

·           It is used in dyeing, paper and leather tanning industries

·           It is employed as a styptic agent to arrest bleeding.