ALUM

Definition

A type of double salt. Alums are double sulfates obtained by crystallizing mixtures in the correct proportions. They have the general formula:
M₂SO₄.M';₂(SO₄)₃.24H₂O
Where M is a univalent metal or ion, and M′ is a trivalent metal. Thus, aluminum potassium sulfate (called potash alum, or simply alum) is
K₂SO₄.Al₂(SO₄)₃.24H₂O
Aluminum ammonium sulfate (called ammonium alum) is
(NH₄)₂SO₄.Al₂(SO₄)₃.24H₂O
The name ‘alum’ originally came from the presence of Al3+ as the trivalent ion, but is also applied to other salts containing trivalent ions, thus, Chromium(III) potassium sulfate (chrome alum) is
K₂SO₄.Cr₂(SO₄)₃.24H₂O.

Agricultural Uses

Alum is a double sulphate of a monovalent and trivalent salt.
Alum acts as a flocculating agent. It is used in sewage treatment and in purification of drinking water. Alum is also used in the preparation of mordant and as a fireproofing agent.
Aluminum sulphate is sometimes erroneously called alum in some industries (such as in the paper industry).