Colloidal solutions prepared by the above mentioned methods usually contain the impurities of electrolytes. The presence of electrolytes in smaller concentrations stabilizes a sol but their presence in large concentration tends to destabilize the colloidal solution. Therefore, it is necessary to purify colloidal solutions by removing the impurities of electrolytes present in them. Following methods are generally used for the purification of colloidal solutions (sols).
1. Dialysis: We have already seen that an animal membrane allows the passage of crystalloids but retains the larger colloidal particles. This property of animal membranes is utilized for the purification of sols. The process involved is called dialysis. It may be defined as follows.
The process of separating the impurity particles of true solution dimensions (crystalloids) from an impure sol by means of diffusion through a suitable membrane such as parchment paper or cellophane membrane is called dialysis.
The apparatus used in this method is called dialyser. It consists of a bag made of parchment or cellophane. The bag is filled with the impure sol to be purified and is suspended in a tank through which pure water is circulated. The impurities of electrolytes present in the sol diffuse out of the bag leaving behind pure sol in the bag.
Electrodialysis: Dialysis is a slow process. However, it can be expedited by applying an electric field. Under the influence of electric field, the impurity ions move faster to the oppositely charged electrodes and the process gets quickened. This process is referred to as electrodialysis.
(a) Dialysis (b) Electrodialysis
2. Ultrafiltration: The pores of an ordinary filter paper are large enough to allow the passage of both impurity particles as well as colloidal particles. Therefore an ordinary filter paper cannot be used for removing the impurities of electrolytes from an impure sol. However, if the pore size of ordinary filter paper is reduced, it can be used for separating the impurities from impure sols. This is achieved by treating an ordinary filter paper with collodion or gelatin followed by its hardening by dipping it in formaldehyde solution. This treatment reduces the pore size and enables it to check the passage of colloidal particles through it. Filter papers thus obtained are called ultrafilters. Filtration through ultrafilters is called ultrafiltration.