Explain the purification of colloidal solutions.

(N/A) Colloidal solutions,when prepared,often contain excessive amounts of electrolytes and other soluble impurities. While traces of electrolytes are essential for the stability of the colloidal solution,larger quantities can cause coagulation. Therefore,it is necessary to reduce the concentration of these soluble impurities to a requisite minimum.
The process used for reducing the amount of impurities to a requisite minimum is known as the purification of colloidal solutions. The purification is carried out by the following methods:
$(i)$ Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. Particles (ions or smaller molecules) in a true solution can pass through an animal membrane (bladder),parchment paper,or cellophane sheet,whereas colloidal particles cannot.
$A$ bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The molecules and ions diffuse through the membrane into the outer water,leaving behind a pure colloidal solution. The apparatus used for this purpose is called a dialyser.
$(ii)$ Electro-dialysis: Ordinarily,the process of dialysis is quite slow. It can be made faster by applying an electric field if the dissolved substance in the impure colloidal solution is an electrolyte. This process is named electro-dialysis.