Surface Chemistry
8.0 Coagulation of colloidal solutions
8.0 Coagulation of colloidal solutions
Stability of the colloidal state is due to the existence of electrical charge on the particles. However, if the charge is lowered to a certain critical value or neutralized, the particles approach close enough to coalesce to form bigger particles of the suspension range. This phenomenon of change of colloidal state to suspension state is known as coagulation or flocculation of colloidal solutions. It is generally brought about by following treatments.
1. By the mutual action of sols (mutual precipitation)
When the two oppositely charged sols are mixed in approximately equal proportions, the charge on one sol is neutralized by the opposite charge on the other sol with the result the dispersed phase of both the sols are precipitated out.
2. By persistent dialysis
If the colloidal system is subjected to prolonged dialysis, the electrolyte particles are removed through the dialyser and the colloidal system becomes unstable since the stability of the colloidal system is due to the presence of traces of electrolytes.
3. By the addition of electrolytes
Although traces of an electrolyte are essential for stabilising the sols, presence of large amounts causes their coagulation or flocculation. This is due to the fact that the colloidsal particles take up oppositely charged ions from electrolytes with the result the charge on the sol particles is neutralised and thus these are precipitated. The ion carrying the opposite charge is called the flocculating ion.
The amount of electrolyte required to coagulate a fixed amount of a sol depends upon the valency of the flocculating ion. In general, greater the valency of the flocculating ion the higher is its power to cause precipitation. This is known as Hardy-Schulze rule.
Flocculating cations and anions cause coagulation of the negatively and positively charged colloids respectively. The flocculation power of a trivalent ion is nearly $500-1000$ times more than that of a monovalent ion while that of a bivalent ion is about $100-500$ times more than that of a univalent ion.
The minimum concentration of an electrolyte required to cause flocculation is known as flocculation value which is generally expressed in terms of milli-moles per litre.
