Purification of Colloidal Solution -Purification of Sol
A sol prepared by any one of the above methods very often contains other material besides the colloidal particles. It is necessary to remove these impurities as they reduce the stability of the sols. The purification of sols can be done by
means of the following three methods
(ii) Electro dialysis
(iii) Ultra filtration
This method makes use of the fact that there is a significant difference in size between colloidal particles and the dissolved molecules or ions. The process of removing ions or molecules from a sol by diffusion through a permeable membrane is called dialysis. The apparatus used for dialysis is called dialyser. The membranes used are parchment paper or cellophane or cellulose nitrate or cellulose acetate or animal bladder. An ordinary dialyser consists of a cylindrical vessel closed at the lower end with the help of parchment membrane. The impure solis placed in the cylinder which is then suspended in a cellophane or cellulose nitrate or cellulose acetate or animal bladder in a vessel containing distilled water. As a result, the ions present in the impure sole pass through the membrane into water but the colloidal particles remain in the sot as they can not pass through the membrane. The water in the form of solution is removed from time to time.
Ordinary dialysis is a slow process and can be accelerated by applying electric field. This process is called electro-dialysis. The impure colloidal solution is placed between two dialysing membranes. The electrodes arc placed in the outer vessel containing pure water. When an electric field of high potential is applied across the membrane, ions present in sol move towards the electrodes and thus removed. For effective separation, a continuous supply of water should be maintained.
This is another important method for purifying sols. The separation of the sol particles from the ions of the electrolyte present in the liquid medium, by filtration through an ultrafiltration, is called ultrafiltration. The colloidal particles present in the sol can pass through an ordinary filter paper. But in ultrafiltration such membranes are used through which colloidal particles can not pass. The liquid medium and the dissolved ions of the electrolyte can easily pass through such membranes. Such membranes are made by impregnating ordinary filter paper with cellophane or a regenerated cellulose. The membranes used for ultrafdtration are of delicate constitution and can be easily broken; hence they are generally supported on wire screens. The rate of ultrafiltration can be increased by applying pressure through suction.