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Coagulation of Colloidal Solutions

The precipitation of a colloidal solution through induced aggregation by the addition of a suitable electrolyte is called coagulation or flocculation.
Certain minimum concentration of an electrolyte is needed to cause coagulation of a particular sol. The minimum amount of an electrolyte (in millimoles) that must be added to one liter of a colloidal solution so as to cause its complete coagulation is called the coagulation or flocculation value of the electrolyte.
According to Hardy–SchuIze rule, “The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol.”
The coagulating power of an electrolyte is directly proportional to the valency of the ions causing coagulation. Thus, for the coagulation of negatively charged sols such as As2S3, AI3+ ions are more effective than Ba2+ ions, which are more effective than Na+ ions. In the same way, for the coagulation of positively charged sols such as Fe(OH)3, Description: 37996.png ions are more effective than Description: 38004.png ions which are more effective than CF ions. Other methods used to cause coagulation of colloidal solution are as follows.
  1. By mutual precipitation: When two oppositely charged sol such as Cr(OH)3 and Sb2S3 are mixed in equimolar proportion, they neutralize each other and get coagulated.
  2. By electrophoresis: During electrophoresis of a sol, the colloidal particles move towards oppositely charged electrode. The particles touch the electrode, lose charge, and get coagulated.
  3. By repeated dialysis: The stability of a sol is due to the presence of a small amount of electrolyte. If the electrolyte is completely removed by repeated dialysis, the sol will get coagulated.
  4. By heating: Even simple heating may coagulate the sol.

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