Purification Of Colloidal Solutions

Surface Chemistry of Class 12

Purification Of Colloidal Solutions

Colloid solutions particularly lyophobic sols, usually contain some impurities of electrolytes and other soluble substance (from the dispersion medium). The presence of a small quantity of the electrolytes is sometimes essential for the stability of the colloidal sol but their presence in excess results in the coagulation (or precipitation) of the colloidal solution on standing. Thus presence of excess of impurities make the sol unstable. Consequently, it is quite necessary to purify or reduce their concentrations in colloidal sols. Methods generally used for this purpose are as follows:

(I) Dialysis

This method is based on he fact that the pores of membranes (like parchment paper or animal membrane) are very small and allow the free passage of only the dissolved molecules or ions of the true solution, but colloidal particles are too big to pass through these pores. Thus if a colloidal sol containing impurities is kept in a parchment bag, the crystalloids (or electrolytes) pass through the pores while colloidal particles are retained in the parchment bag. The process of separating crystalloids from colloids by means of diffusion of the former through an animal or vegetable membrane, is called dialysis and the apparatus used for effecting such a separation is called dialyser.

(II) Ultra filtration

The pores of ordinary filter paper large enough for the colloidal particles to pass through, so they can not be employed for filtering sols. However, a filter paper impregnated with collodion or gelatin followed by hardening of the material deposited in pores by formaldehyde, serves the purpose. Due to this treatment the pore−size is reduced to such an extent that the colloidal particles are retained by it: while electrolytes (or true solutions) can pass through. Such a filter paper is known as ultrafilter paper. By using the impregnating solution of different concentrations a series of graded ultrafilted can be obtained. With such ultrafilters, solute impurities of different sizes can effectively be removed. The sol is poured over the ultrafilter which permits solution of electrolytes (which are of much smaller sizes than the pore−size of ultrafilter) to pass through but retains the colloidal particles in the form of a slime. slime is then brought in contact with water when the former disperses spontaneously to form a pure colloidal solution. The process of separating crystalloids from colloidal sols by using utrafilters is called ultrafiltration.

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