Describe Potash Alum with Properties

Potash Alum is manufactured from alunite or alum stone.

Potash Alum: K2 SO4. Al2 (SO4)3. 24 H2O

From Alunite: Alunite or alum stone is K2 SO4. Al2 (SO4)3. 4Al(OH)3. It is finely powdered and boiled with dilute sulphuric acid, the aluminium hydroxide part changes into aluminium sulphate. When a little more potassium sulphate in calculated amount is added, the alum is crystallized.


  1. Potash alum is a white crystalline solid.
  2. It is soluble in water but insoluble in alcohol.
  3. The aqueous solution is acidic due to the hydrolysis of Al2 (SO4)3.
  4. When heated, it melts at 365K and on further heating loses the whole of its water of crystallization and swells up. The swollen mass so produced is called burnt alum.


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