Monday, August 19, 2013

Purification of Colloidal Solutions

The colloidal solutions prepared by various methods contains appreciable amount of electrolyte and other soluble Impurities. These Impurities may destablise the colloidal sol. Hence, they are to be removed. A very important method of removal of soluble impurities from sol is Dialysis.

Dialysis
Animal membranes (bladder) or those made of parchment paper and cellophane sheet, have very fine pores. These pores permit ions (or small particles) to pass through, but not the large colloidal particles. Thus particles of true solution can pass through parchment membrane but colloidal particles cannot pass through these membranes. A bag made of such membrane is filled with the colloidal solution and is then suspended in fresh water (distilled water). After some time whole of crystalloid in solution passing out leaving the colloidal behind. The distilled water is renewed frequently to avoid accumulation of the crystalloid as otherwise they may start diffusing back into the bag. “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 above process can be quickened if an electric field is applied around the membrane, the process is then known as Electrodialysis.

Ultrafiltration
In this method, colloidal solutions are purified by carrying out filtration through special type of graded filters called Ultra – filters. These filter papers allow electrolytes to pass through them but do not allow colloidal particles. These filter papers are made from ordinary filter papers impregnating them with colloidal particles. In order to quicken the filtration increased pressure or suction is required.
Ultra-Centrifugation
In this method, the colloidal solution is place in a high-speed centrifugal machine. On centrifuging, the colloidal particles settle down. The impurities remain in the centrifugate and are removed. The settled colloidal particles are mixed with water to form the colloidal solution again.

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