Hardy Schulze Rule: According to Hardy Schulze’s rule, greater the valency of the active ion (coagulating or flocculating ion), greater will be its coagulating power.
Coagulating power of an electrolyte is directly proportional to the valency of the active ions. e.g. to coagulate negative sol of As2S3, the coagulating power of different cations has been found to decrease in the order as:
Al3+ >Mg2+> Na+
Similarly, to coagulate a positive sol such as Fe(OH)3, the coagulating power of different anions has been found to decrease in the order:
[Fe(CN)6]4- > PO43- > SO42-> Cl–