Inorganic Compound of Class 12

Originally the name alum was given to the double salt, potassium aluminium sulphate, K2SO4.Al2(SO4)3.24H2O. But now this name is used for all the double salts having the composition: M2ISO4M2III(SO4)2.24H2O where MI stands for univalent basic radical like Na+, K+, Rb+, Cs+,  NH+4 , Ag+, Tl+ and MIII is a trivalent metal Al3+, Cr2+, Fe3+, Co2+ etc. The crystals of alums are made up of [MI(H2O)6]+, [MIII(H2O)6]3+ and four SO2-4 ions.

Some common examples of alums are:

(i) Potash alum (or common alum)−K2SO4.Al2(SO4)3.24H2O.

(ii)Sodium alum−Na2SO4.Al2(SO4)2.24H2O

(iii) Ammonium alum (NH4)2SO4.Al2(SO4)3.24H2O

(iv) Chrome alum K2SO4.Cr2(SO4)3.24H2O etc. The crystals of alums are generally obtained when hot solutions of equimolecular quantities of their constituent sulphates are mixed and the resulting solution is cooled. All the alums are isomorphous, form mixed crystals in all proportions and also layer−crystals. They are usually much more soluble in hot water than in cold water. In solution they give the ions of the component salts.

Alums are often used for the removal of colloidal impurities from water (as coagulants).


K2SO4.Al2(SO4)3.24H2O or KAl(SO4)2.12H2O


Commercially it is prepared as follows:

From alunite or alumstone mineral, K2SO4.Al2(SO4)3.4Al(OH)3.

The mineral alunite is calcined and treated with H2SO4 when Al(OH)3 component is converted into Al2(SO4)3. Now calculated quantity of K2SO4 is added to the solution, which is then crystallized for alum. Finally it is purified by recrystallisation.

K2SO4.Al2(SO4)3.4Al(OH)3 + 6H2SO4 K2SO4 + 3Al2(SO4)3 + 12H2O

Alunite mineral

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

potash alum


Potash alum is white crystalline solid, soluble in water but insoluble in alcohol. The aqueous solution is acidic due to the hydrolysis of aluminium sulphate. When heated, it melts at 92°C and on further heating loses the whole of its water of crystallization and swells up. The swollen mass so produced is called burnt alum (phul phatkiri).


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

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