[Explained] Purification of Colloidal Solutions

Colloidal solutions prepared in the laboratory, usually contains impurities of electrolytes. The presence of electrolytes in smaller concentrations stabilizes a sol but their presence in large concentrations tends to destabilize the colloidal solution. Therefore, it is necessary to purify the colloidal solution by removing the impurities of electrolytes present in them.

You should learn about the properties of colloidal solutions to understand the purification process better. 

Methods of Purification of Colloids/Colloidal Solutions

Following methods are commonly used for the purification of the colloidal systems (sols).

Dialysis

 

 

Dialysis of impure sols

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 a 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 paper. This bag is filled with impure sol to be purified and is suspended in a tank through which pure water is circulated as shown in the figure. The impurities of electrolytes present in the sol diffuse out of the bag leaving behind pure sol in the bag.

 

Electrodialysis

 

Electrodialysis of impure sol

Dialysis is a slow process. However it can be fastened by applying an electric field as shown in the figure. Under the influence of electric field, the impurity ions move faster to the oppositely charged electrodes and the process gets expedited. This process is referred to as electrodialysis.

 

Ultrafiltration

 

Ultrafiltration of impure sol

The pores of an ordinary filter paper are large enough to allow the passage of both impure particles as well as colloidal particles. therefore, and ordinary filter paper can not 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 passage of colloidal particles through it. Filter papers thus obtained are called ultra filters. Filtration through ultra filters is known as ultra filtration.

In ultra filtration, the ultra filter is supported over a wire mesh and the impure sol is poured over it. The impure particles (the electrolytes) pass through the ultra filter while the large colloidal particles are retained. The process is very slow. However, it can be expedited by applying pressure on sol side or by using a suction pump on the filtrate side. 

By using a series of graded ultra filters, impurities of different sizes can very easily be removed and it is even possible to separate colloidal particles of different sizes from one another.

 

 Ultracentrifugation

 

Colloidal solutions can also be purified by ultra centrifugation technique. The technique was discovered by Theodor Svedberg in 1925 and is mainly used for the purification of protein sols.In ultra centrifugation, the colloidal solution to be purified is rotated in a specially designed high speed instrument called ultra centrifuge. In the machine, the force exerted on colloidal particles is as great as a million times gravity. On account of it, the colloidal particles get settled down. This method is very useful in determination of the size of colloidal particles.