Question

Coagulation of colloidal solutions.

Answer 1

The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If the charge is removed, the particles aggregate or flocculate and settle down under the foce of gravity.

• Coagulation or flocculation: It is the process of aggregating the colloidal particles together so as to form bigger particles which may settle down as a precipitate. It is usually done by addition of suitable electrolytes. 

• Difference between Flocculation and Coagulation: When a small amount of the electrolyte is added, i.e., when the concentration of the electrolyte added is low, the process is called flocculation. It can be reversed on shaking. However, at higher concentration, coagulation takes place and the process cannot be reversed simply by shaking. 

• Coagulation value or Precipitation value or Flocculation value: It is the minimum amount of the electrolyte in millimoles that must be added to one litre of the colloidal sol to bring about complete coagulation or precipitation. Thus. smaller is the flocculation value of an electrolyte, greater is its coagulating or precipitating power.

\(\frac{Coagulating\,power\,of\,electrolyte\,1}{Coagulating\,power\,of\,electrolyte\,2}=\frac{Coagulating\,value\,of\,2}{Coagulating\,value\,of\,electrolyte\,1}\)

• Hardy-Schulze rule: It states that “Greater is the valency of the oppositely charged ion of the electrolyte being added, faster is the coagulation. Further, the coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the effective ion.” Thus, for a negatively charged sol, like As₂ S₃ , the order is Al³⁺ > Ba²⁺ > Na⁺ and for a positively charged sol, like Fe(OH)₃ , the order is

[Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻]

Coagulation can also be caused by electrophoresis, mutual precipitation (mixing colloidal sols of opposite charge), prolonged dialysis or by heating or cooling the sol.