Potash alum is a double salt of potassium sulphate and aluminium sulphate. Hence, it is prepared by taking the amount of K2SO4 and Al2(SO4)3.18H2O in the ratio of their molecular masses, preparing their concentrated solutions separately followed by mixing the concentrated solutions and heating the resulting solution to the crystallization point when white crystals of potash alum (octahedral in shape) are obtained.
The reaction is represented as:
K2SO4 + Al2(SO4)3.18H2O + 6H2O → K2SO4.Al2(SO4)3.24H2O
How To Perform Experiment
(i) Take about 3 g of potassium sulphate crystals in a 250 ml beaker. Add about 25 ml of distilled water. Stir with a glass rod to dissolve the salt. Heat if necessary.
(ii) Take about 12 g of aluminium sulphate crystals in another beaker. Add about 25 ml of distilled water and a few drops of dil. sulphuric acid (to prevent the hydrolysis of aluminium sulphate). Stir with a glass rod to dissolve the salt. Heat if necesary. Filter if the solution is not clear.
(iii) Mix the two solutions by transferring them into a china dish. Heat the china dish gently on a water bath to concentrate the solution up to the crystallization point.
(iv) Allow the solution in the china dish to cool undistrubed by placing it over the mouth of a beaker filled with cold water for some time. White octahedral crystals of potash alum are obtained.
(v) Separate the crystals by decanting off the mother liquor.
(vi) Wash the crystals with a small amount of ice cold water.
(vii) Finally dry the crystals either by placing them between folds of filter paper.