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By Sunil Bhardwaj
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A sol contains some electrolyte during its preparation. A small amount of electrolyte is essential for stability of the sol, but excess of it can cause coagulation. Therefore, it is essential to remove excess of electrolyte.
The purification of the sol is done by Dialysis process.
Dialysis: The process of removing the dissolved electrolytes from the sol by means of a membrane is called dialysis. A dialyser consists of a bag made of parchment or cellophane and suspended in running water in a trough. The mixture of the colloid and the crystalloid is taken in the bag and apparatus fitted as shown in Fig. 
After sometime, whole of the crystalloid in solution passes out while the colloid remains in the bag. It is further observed that the above process of separation can be accelerated if an electric field is applied around the parchment bag. The process is then called electrodialysis.
Because of their bigger size, colloidal particles are unable to pass through the parchntent bag while ions of the crystalloid are carried away by the running water in the dialyser. Further on applying the electric field. the movement of the ions is accelerated. 
An important application of dialysis is in the purification of blood in the artificial kidney machine. The dialysis membrane permits small particles to pass through but colloidal haemoglobin does not pass through.
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