The quantity of the electrolyte which is required to coagulate a definite amount of a colloidal solution depends upon the valency of the ion having a charge opposite to that of the colloidal particles. This observation is known as Hardy Schulze law.
It can be defined as:
Greater is the valency of the oppositely charged ion of the electrolyte being added, the faster is the coagulation.
Hence, for the coagulation of negatively charged arsenious sulphide sol., trivalent cations are far more effective than divalent cations which in turn are more effective than monovalent cations. Similarly for coagulation of positively charged ferric hydroxide sol, tetravalent anions are more effective than trivalent anions which are more effective than divalent anions which in turn are more effective than monovalent anions.
The minimum amount of an electrolyte that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called the coagulation or flocculation value of the electrolyte. Thus smaller is the flocculation value of an electrolyte; greater is its coagulating or precipitating power.