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.
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.
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
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
· 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.
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