It is a process in which the electric charge of dispersed phase particles neutralized by colloidal particles, which results aggregation and form the particles of larger size which finally settle down as a precipitate under the action of gravity.
Generally, coagulation process occurs by the addition of electrolytes. When an electrolyte is added to a colloidal solution, the particles of the colloidal solution get neutralized by the oppositely charged particles of electrolyte. Then, the neutral particles start accumulating and form larger size particles which settle down as a precipitate.
Hence, the amount of electrolyte required to neutralize the particles of colloidal solution is depends upon the valency of ion present in colloidal solution.This observation of Hardy and Schulze, known as Hardy – Schulze law.
According to Hardy-Schulze rule –
- Valency of the active ion (or coagulating ion or flocculating ion) increases
- Coagulating power increases
- Power to cause precipitation increases
Al3+ > Ba2+ > Na+
The different electrolytes have different coagulation capacity which depends upon the valency of the active ion, known as flocculating ion (carrying charge opposite to the charge of the colloidal particles).
Here you can read more about Flocculation .
Key point of Hardy-Schulze Rule:
Coagulating power of an electrolyte α Valency of the active ions
Note – The flocculating ion are very effective in causing coagulation of the sol.