Class 12 Chemistry Surface Chemistry Coagulation


 It is process of settling of colloidal particles. Also called precipitation of sol

Coagulation of Lyophobic Sols- 


Coagulation of lyophobic sols can be done by the following methods:

  • By electrophoresis - The colloidal particles move towards oppositely changed electrodes get discharged and precipitate.
  • By mixing two oppositely charged sols - Oppositely charged sols when mixed together in almost equal proportion, neutralise their charges and get partially or completely precipitated.
  • By Boiling- When a sol is boiled the adsorbed layer is disturbed due to increased number of collisions with the molecules of the dispersion medium. This reduces the charge on the particles and they ultimately settle down in the form of a precipitate.
  • By Persistent dialysis - On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely. Colloids become unstable and coagulate.
  • By addition of electrolyte - When excess of electrolyte is added, colloidal particles precipitate as colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralisation leading to their coagulation.

Example- A negatively charged ion when added to a positively charged sol causescoagulation. The negatively charged ion is called coagulating ion/flocculative ion as it neutralises the colloid to cause coagulation.


Coagulation of Lyophillic Sols-

Lyophilic sols are stable because of charge and solvation of colloidal particles. So we remove these two factors to coagulate them. This is done by

  • Addition of an electrolyte
  • Addition of a suitable solvent


Protection of colloids -

  • Lyophilic sols are more stable than lyophobic sols
  • Lyophilic colloids have a unique ability to protect lyophobic colloids from electrolytes
  • When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) form a layer around the particles of lyophobic sol
  • Lyophilic colloids are also called protective colloids


Hardy-Schulze Rule-

  • The greater the valency of the flocculating ion added, the greater is its precipitation.
  • For negative sols, when positive ions are added

           Al3+> Ba2+>Na+ is the order in terms of flocculating power

  • For positive sols, when negative ions are added

           [Fe(CN)6] 4-> PO43->SO42->Cl- is the order in terms of flocculating power

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