The process of removing dissolved impurities from a colloidal system, by means of diffusion through suitable membrane under the influence of an electric field is called

a dialyser bag has its pore size such that only true solution particles (or)electrolytic impuritiescan pass through but not the colloidal particles.                        When the dialyser bag contains impure colloid is kept in water the ionic impurities diffuse out of the bagleaving behind pure colloid.This process can be accelarated by applying electric field. This is called electrodialysis