Tuesday, August 20, 2013

Coagulation or Flocculation of Colloidal Solutions

The colloidal sols are stable by the presence of electric charges on colloidal particles. Because of electric repulsion the particles do not come close to one another and hence do not aggregate. The removal of charge by any means will lead to the aggregation of particles and hence precipitates immediately.
The process by means of which the particles of the dispersed phase in a sol are precipitated is known as Coagulation or Flocculation.
The coagulation can be effected by different methods as follows:-
         1.      Heating or Cooling
                        In some cases, heating the sol results in coagulation. E.g. When egg is boiled, the albumin gets coagulated. Similarly, in some cases, cooling the sol results in the coagulation. E.g. Coagulation of milk i.e. on cooling milk, fat starts flowing on the surface.
2.      Mutual Action of Sols
                        When two sols carrying opposite charges are mixed together, their charges get neutralized and they get coagulated. Thus when negatively charged arsenic sulphide sol is added to the positively charged ferric hydroxide sol, in suitable proportions, coagulation of both sols take place simultaneously.
3.      Electrophoresis
When an electric current is passed through colloidal dispersion, particles migrate towards oppositely charged electrode and their charges get neutralized due to opposite charge on electrode. They thus get deposited on the electrode or are coagulated. Example: Rubber cloth can be prepared by electrophoresis of rubber latex.
4.      By Prolonged Dialysis
The stability of a colloidal sol is due to presence of small electrolyte. On prolonged dialysis the electrolyte is completely removed. As a result the colloidal sol becomes unstable and gets coagulated.
5.      Addition of Electrolyte
Presence of small concentration of appropriate electrolyte is necessary to stabilize the colloidal solutions. However, when large amount of electrolyte is added the colloidal particles get neutralized by the opposite charged ion of the electrolyte. Due to the neutralization of colloidal particles they aggregate and are coagulated.
For example: When aluminium sulphate or barium chloride is added to arsenic sulphide sol, it takes up barium ion or aluminium ion while the positively charged particles of Ferric hydroxide takes up chloride ion or sulphate ion, as the case may be. This causes neutralization of the charge on colloidal particles and lowers the stability of the sols, leading to coagulation.

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