To prepare a pure sample of potash alum. [K2SO4.Al2(SO4)3.24H2O] Principle:
Potash alum is prepared by dissolving an equi molar mixture of aluminum sulphate and potassium sulphate in minimum
amount of water containing a little amount of sulphuric acid and then subjecting the solution to crystallisation; octahedral crystals of potash alum separate out.
Beaker (100 ml)
Conical flask (100mL)
Aluminium sulphate: 10 g
Ethanol : 20 mL
Potassium sulphate : 2.5 g
Dilute sulphuric acid : 1 mL
a.Take 20 ml of distilled water in a 100 ml beaker and heat it to about 40°c. Add about 1 ml of dilute sulphuric acid ,
warm this and dissolve 10 g of aluminum sulphate adding
small amounts at a time.
b.Weigh 2.5 g of powdered potassium sulphate (given in the packet) and add it to the above solution stir well.
c. Heat the solution with constant stirring till potassium sulphate dissolves completely on a water bath. Test the
solution at regular intervals of time far crystallisation point.
d.Allow the solution to cool to room temperature.
e.On cooling, white crystals of potash alum separate out.
f. Decant the mother liquor carefully and shake gently with a mixture of (1 cc alcohol + 1 cc water) to the
g. Filter the crystals, dry these between the folds of a filter paper and note the yield.
Weight of crystals obtained = ..................... g
Colour of the crystals = ......................
The crystals of potash alum are octahedral in shape.
a. Cool the solution slowly to get good crystals. Avoid rapid cooling. b. Do not disturb the solution while cooling.