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Coagulation or precipitation:-it is a process of aggregating together the colloidal particle so as to change them into large sized particles which ultimately settles as a precipitate.

  • The coagulation is generally brought about by the addition of electrolytes.
  • The ion which is responsible for neutralization of charge on the colloidal particles is called the coagulating ion or Flocculating ion.

Coagulation or Flocculation Value:-The minimum of an electrolyte ( in millimoles) that must be added to one litre of a colloidal solution leads to bring about the complete coagulation is called coagulation or Flocculation Value.

Hardy schulze law:-

  1. The effective ions of the electrolyte in bringing about coagulation are those which carry charge opposite to that of the colloidal particles. These ions are called coagulating ions.
  2. Greater is the valency of the coagulating or the Flocculating ion, greater is its power to bring about the coagulation.


Electrolyte NaCl KCl HCl MgCl2 BaCl2  
Cation Na+ K+ H+ Mg2+ Ba+2
Coagulation value 52 4g.5 30.8 0.72 0.6g

Methods of Coagulation:-

  • By electrophoresis:- in electrophoresis, the particles of dispersed phase move towards oppositely charged electrode and get neutralized.
  • By prolonged dialysis:-the stability of a colloidal sol is due to presence of a small amount of the electrolyte on prolonged dialysis electrolyte is completely removed and colloidal sol is get coagulated.

Another important methods of coagulation are, by heating or cooling, By mutual precipitation etc.

Protective action of lyophilic colloids:-

  • Lyophilic sols like those of metals (Au, Ag etc) are unstable and are easily precipitated by addition of electrolytes.
  • The lyophobic sols can be protected from coagulation by adding a small amount of lyophilic sols this process is called as “protection” and lyophilic colloids are termed as “protective colloids”.

Gold Number:-the minimum mass of protective colloid is the added to 10 ml of a standard red gold sol (Containing 0.0053 to 0.0058) so that no coagulation of the gold soltakes place is called the Gold Number.


Q.1      What do you mean by coagulating value ?

Ans.     The minimum amount of an electrolyte (in milimoles) that must be added to one litre of colloidal solutionto as to bring about the complete coagulation is called as the coagulation or precipitation Value.

Q.2       For the coagulation of 100ml of arsenious sulphide sol, 5Ml of lM Nacl is required what is the Floculation value of NaCl?

Ans.     5 ml of lM NaCl contains

thus 100 ml of As2S3 sol require NaCl for complete coagulation = 5 milimoles
100 ml of the required NaCl for complete coagulation = 50 milimoles
Flocculation Value of NaCl =50

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