Purification of colloidal solutions:-

The colloidal solution when prepared generally contain excessive amount of electrolytes and some other soluble impurities. While the presence of traces of electrolyte is essential for the stability of the colloidal solution, large quantities co-agulute it. It is therefore necessary to reduce the conc. of these impurities to a minimum level. It is called purifications of sol. These impurities are removed by the following methods:

1. Dialysis:-

The process of separation the particles of colloids from those of crystalloids by means of diffusion through a suitable membrane, while the ions of the electrolytes can pass through them. The colloidal solution is taken in a bag made of parchment or cellophane membrane. this bag is suspended in  water where continuous flow of water is maintained. The impurities slowly diffuse out of the bag leaving the colloids inside. The ordinary process of dialysis is very slow; hence the two electrodes are applied on the both sides of the cellophane bag to accelerate the process of dialysis. It is called Electro dialysis.

2. Ultra filtration:- in this method, the colloidal solutions are purified by carrying out filtration through special filter papers of very high porosity called ultra-filters. Ultra-filter papers are prepared by soaking the ordinary filter paper in a collodion, hardening by formaldehyde and then drying. Collodion is a 4% solution of nitrocellulose in a mixture of alcohol and ether. These filter papers allow the electrolytes to pass through them but do not allow colloidal particles to pass.