Colloidal sols can be purified by

(a) In dialysis, sol under purification is placed in a bag made of cellophane or parchment. This is suspended in water. Small molecules and ins pass through the membrane, but the sol is retained. When an electric field is applied during dialysis, it is called electrodialysis.

(b) Colloidal sols can also be purified by using special type of graded filters called ultra-filters and method is called ultra-filtration. These ultra-filter papers allow only the electrolytes to pass through under the influence of suction or pressure pump.

(c) In ultra-centrifugation method, colloidal sol is placed in ultra-centrifuge. Under high speed, colloidal sols settle down at the bottom, while impurities remain in solution.