Hardy schulze’s rule states that “greater the valency of the active ion or flocculating ion, greater will be its coagulating power”.
That is, the coagulation capacity of different electrolytes depends upon the valency of the flocculating ion (the ion carrying charge opposite to the charge on the colloidal particles).
Hardy Schulze law says that,
(i) 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 or flocculating ions.
(ii) Greater is the valency of the coagulating or the flocculating ion, greater is its power to bring about coagulation.
For example, the coagulation of negatively charged arsenious sulfide sol, trivalent cations (Al3+) are far more effective than divalent (Ba2+) cations which in turn are more effective than monovalent Na+ cations.