Preparation of Potash Alum

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BACKGROUND

Potash alum is otherwise known as potassium aluminium sulphate which is mostly used in water purification, dyeing, leather tanning and inbaking powder. It forms regular octahedron crystals with flattened corners.

Potash alum is soluble in water and it turns litmus paper red. Alums are prepared by adding the concentrated hot solutions of either potassium or ammonium sulphate in to the hot solution of aluminium sulphate in equimolar concentration. The final reaction mixture is then allowed to cool. On cooling the alum crystallizes which are collected by filtration and then dried at room temperature.¹

Aluminium sulphate contains 18 number of water molecules (Al (SO₄)₃,18H₂O). This is to be considered while calculating the quantity of aluminium sulphate to be used in the preparation.

Alum in pharmacopoeia is either potash alum or Ammonium alum.

Potash alum: K₂SO₄, Al (SO₄)₃,24H₂O

Ammonium alum: (NH₄)₂ SO_4, Al₂ (SO₄)₃,24H₂O

Aim: Aim of the experiment is to do the preparation of Potash alum.

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REQUIREMENTS

Sample:     Ammonium sulphate,

Aluminium Sulphate,

Ice

Apparatus: Two 250ml beaker,

China Dish,

Funnel,

Funnel Stand,

Glass Rod,

Wash Bottle,

Tripod Stand,

Wire-Guaze.

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PROCEDURE

In a clean 250ml beaker, 2.5g of potassium sulphate crystals were transferred and 20ml of water was added and stirred till crystals get dissolved.

Then another beaker was taken and 10g of aluminium sulphate crystals were transferred to which 20ml of water and 1ml of dilute H₂SO₄ was added to prevent hydrolysis of aluminium sulphate.

Heated for about 5mins, if milkiness persists, filter the solution. The two solutions are further mixed in a china dish and the dish was placed on a wire guaze which is placed over burner.

Stirred the solution with glass rod and concentrated till the crystallisation point is reached and the beaker was placed over cold water.

Almost immediately the crystals of potash alum was separated out,the mother liquor has to be decanted off and the crystals has to be washed with a small quantity of ice cold water.

Use

1. It can be used for water purification and also as an antiseptic.
   
2. It can be used as an astringent.
   
3. It can be used as a deodorant.
   

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CONCLUSION

The colour of the crystals were observed to be white small crystals having octahedron shape.

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REFERENCES

1. Inorganic medicinal and pharmaceutical chemistry., Block, Roche, Soine, Wilson.
   
2. Indian Pharmacopoeia., 1996, 2016.
   
3. Available at: www.learncbse.in/prepare-pure-sample-potash-alum/.