Question

Which of the following are methods to coagulate a colloidal sol?

• A. Addition of Electrolytes
• B. Persistent dialysis
• C. Mutual coagulation
• D. All of the above

Answer 1

The correct option is D All of the above

Why do the particles of a colloidal dispersion not aggregate or settle down quickly? This is because of the existence of similar and nearly equal electrical charge on the particles. If we can manage to lower or neutralize the charge somehow, we can get the particles to coalesce and form aggregates corresponding to the suspension range. The process of settling down of colloidal particles is called coagulation or flocculation of colloidal solutions. It is generally brought about by following treatments.
(i) By the mutual action of sols (mutual precipitation) : When the two oppositely charged sols like As₂S₃ and Fe(OH)₃ sols are mixed in nearly equal amounts, the charge on one sol is neutralized by the opposite charge on the other. This causes the dispersed phase of both the sols to settle down.
(ii) By persistent dialysis: When a colloidal dispersion is made, traces of electrolytes are always present and there is a need for purification. We can remove the excess electrolyte by dialysis. But if we persist with prolonged dialysis or repetitive dialysis, the electrolyte particles are removed to the extent of precipitating the colloidal system.
(iii) By the addition of electrolytes: The presence of an electrolyte is essential for stabilising a colloidal dispersion but if we add large amounts of an electrolyte to a stable colloid, this would again lead to the disruption of a delicate equilibrium of charges thus causing precipitation.