To prepare crystal of double salt of potash alum (K2SO4.Al2(SO4)3.24H2O)

 Aim-To prepare crystal of double salt of potash alum (K₂SO₄.Al₂(SO₄)₃.24H₂O)

Apparatus required-

250ml beaker, glass rod, china dish, funnel, funnel stand, filter paper, wash bottle, tripod stand, burner, wire gauze etc.

Chemical required- potassium sulphate, aluminium sulphate, distilled water, ethanol and disulphuric acid etc.

Theory-

The additional compounds which are stable in solid state only but are broken down into individual constituents when dissolved in water are called double salts. Potash alum is double salt which is prepared by dissolving equ imolar quantity of potassium sulphate and aluminium sulphate  in the minimum quantity of water containing dilute sulphuric acid. The solution is filtered and heated in a china dish till point of crystallisation is reached and then cooled to obtain colourless octahedral crystal of potash alum. It is commonly known as “fitkari”.

K₂SO₄     +    Al₂(SO₄)₃.18H₂O   +    6H₂O   ---------->        K₂SO₄.Al₂(SO₄)₃.24H₂O

Procedure-

·   We take clean and dry a 250 ml beaker and   transfer 2.5g potassium sulphate crystal to it, then add 1ml dil. sulphuric acid and 20 ml of distilled water. And stir it to dissolve the crystal.

·       take 10g aluminium sulphate crystal in another cleaned 250ml beaker and add  1ml sulphuric acid (to prevent hydrolysis of (aluminium sulphate) and 20ml water.

stir to dissolve the crystal.

·       Now, these two solutions are mixed in china dish after the filtration separately and placed it on a wire gauze/sand bath, placed over a burner so that it can be heated.

·       Gently heated with burner with constantly stirring the solution with a glass rode so that it may not boil.

·       When the content of the china dish is reduced to 50% of original volume then we dip the glass rod into the concentrated solution.

·       Cool it, after sometime the crystals separate out.

·       When sufficient crystals formed decant of the mother liquor without disturbing the crystals.

·       Add small amount of ethanol and distilled water (1:1 ratio) in china dish to wash the crystal and decant of the liquid.

Result- The yield of potash alum(fitkari) is 15.722

               And expected yield is 13.631g

               Colour of the crystal is colourless/white

               Shape of the crystal is octahedral

Precaution

·       Precautions-potash alum should be colourless and not turbid.

·       Sulphuric acid should be added before adding distilled water

·       Avoid over heat the content or from boiling the content.

·       Don’t disturb the concentrated solution while cooling