Potash Alum

Topics: Aluminium, Alum, Sulfates Pages: 2 (294 words) Published: May 10, 2013

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. 
Apparatus required:
Beaker (100 ml) 
Wire gauze 
Conical flask (100mL) 
Glass rod 
Tripod stand 
Chemicals required:
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 
crystals mixture. 

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.
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