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.
- Potash alum is a white crystalline solid.
- It is soluble in water but insoluble in alcohol.
- The aqueous solution is acidic due to the hydrolysis of Al2 (SO4)3.
- 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.
- It is used in purification of water, water proofing of textiles and in dyeing and paper industry.
- It is also employed to arrest bleeding.