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Electrodialysis: Dialysis is a slow process. However, it can be expedited by applying an electric field. Under the influence of electric field, the impurity ions move faster to the oppositely charged electrodes and the process gets quickened. This process is referred to as electrodialysis. (a) Dialysis (b) Electrodialysis 2. Ultrafiltration: The pores of an ordinary filter paper are large enough to allow the passage of both impurity particles as well as colloidal particles. Therefore an ordinary filter paper cannot be used for removing the impurities of electrolytes from an impure sol. However, if the pore size of ordinary filter paper is reduced, it can be used for separating the impurities from impure sols. This is achieved by treating an ordinary filter paper with collodion or gelatin followed by its hardening by dipping it in formaldehyde solution. This treatment reduces the pore size and enables it to check the passage of colloidal particles through it. Filter papers thus obtained are called ultrafilters. Filtration through ultrafilters is called ultrafiltration.
Ultrafiltration In ultrafiltration, the ultrafilter is supported over a wire mesh and the impure sol is poured over it. The impurity particles (electrolytes) pass through the ultrafilter while the larger colloidal particles are retained. The process is very slow. However, it can be expedited by applying pressure on sol side or by using a suction pump on the filtrate side. By using a series of graded ultrafilters, impurities of different size can easily be removed and it is even possible to separate colloidal particles of different size from one another. 3. Ultra-centrifugation: Ultracentrifugation involves the separation of colloidal particles from the impurities by centrifugal force. The impure sol is taken in a tube and the tube is placed in an ultra-centrifuge. The tube is rotated at high speeds. On account of this, the colloidal particles settle down at the bottom of the tube and the impurities remain in the solution. This solution is termed as centrifugate. The settled colloidal particles are removed from the tube and are mixed with an appropriate dispersing medium. Thus, the pure sol is obtained.
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