Purification of Colloids: Dialysis and Electrodialysis
After the preparation of the colloid, say, by a precipitation technique, it becomes necessary to remove the excess electrolyte from the sol to impart stability to the sol. One could think of filtering the colloidal system, wash the colloid and re-disperse. But even the finest filter paper will retain little or none of the colloid. Electrolytes or other soluble materials in sols are removed by putting the sol in a membrane of cellophane or parchment surrounded by water as shown in Figure. The salts can pass through the membrane of parchment and are gradually washed away by the running water whereas the colloid-is left inside the Vessel. This process of separation of colloid and Graham’s `crystalloid’ is called dialysis. Graham himself insisted on the importance of dialysis. The assembly in which the dialysis is carried out is called a dialyzer. The process of separating the colloidal particles from those of crystalloids by diffusion of mixture through a parchment or animal membrane is known as dialysis
The membrane through which dialysis is earned out serves as a sieve of extremely fine pore size so that only the small molecules or ions can pass but not the relatively larger colloidal particles or large molecules of high polymers. The membrane also controls such selective retention and passage due to the difference in diffusion and absorption. The dialysis can be frequently improved and made faster if the assembly is subjected to a suitable electric field. The migration of the ion of the electrolytes through the membrane becomes quite fast in the electric field and makes the process rapid. This is known as electrodialysis.