Colloidal solution obtained from various methods is not pure. Therefore solution needs to be purified before it can be put to further use. Dialysis and Ultra-filtration techniques are used for purification of colloids.
Dialysis
We know that colloidal particles cannot be passed through parchment paper. This property is used in dialysis. The colloidal solution kept in parchment bag is dipped in beaker containing water, so that impurities may pass into aqueous medium leaving the colloidal particles inside the bag. The aqueous solution is renewed on timely basis to avoid rediffusion of impurities back into parchment bag. Pure colloidal solution is left inside the parchment paper. To make this process faster process of electrolysis in which two electrodes are dipped in the solvent and electric field is applied. The entire process is called electro-dialysis. The same process is used for purification of blood in case of kidney failure.
Ultra-Filtration
We know that ordinary filter paper cannot be used to filter out colloidal particles as pore size of filter paper are almost equal to size of colloidal particles. The separation of solute from colloidal system can be carried out by using ultra filter which have small pore size as compared to ordinary filter. Filtration using ultra papers is known as ultra filtration.
The pores of normal filter paper can be made smaller by soaking the filter paper in a solution of gelatin or colloidon and subsequently hardening them by soaking in formaldehyde. This decreases the pore size of filter paper which would restrict the colloidal particle to pass through it. By using a series of graded ultra filter paper, it is possible to separate colloidal particles of different sizes.
It is important to note that above methods to purify colloidal solution do not produce 100% solution. Absolute purity is not required as well as traces of small amount of electrolytes (impurities) are necessary for stability of colloidal solution.