Purification of Colloidal Solution

When a colloidal solution is prepared it contains certain impurities. These impurities are mainly electrolytic in nature and they tend to destabilize the colloidal solutions.

Colloidal solutions are purified by the following methods:

1. Dialysis
2. Electrodialysis

Dialysis

The process of dialysis is based on the fact that colloidal particles cannot pass through parchment or cellophane membrane while the ions of the electrolyte can. The colloidal solution is taken in a bag of cellophane which is suspended in a tub full of fresh water. The impurities diffuse out leaving pure colloidal solution in the bag. This process of separating the particles of colloids from impurities by means of diffusion through a suitable membrane is called dialysis.

Electrodialysis

The dialysis process is slow and to speed up its rate, it is carried out in the presence of an electrical field. When the electric field is applied through the electrodes, the ions of the electrolyte present as impurity diffuse towards oppositely charged electrodes at a fast rate. The dialysis carried out in the presence of electric field is known as electrodialysis.

The most important use of dialysis is the purification of blood in the artificial kidney machine. The dialysis membrane allows the small particles (ions, etc.) to pass through, whereas large size particles like haemoglobin do not pass through the membrane.