Hardy-Schultz Rule

     The coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum amount of an electrolyte (millimoles) 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 electrolyte, greater is its coagulation or precipitating power.
Different electrolytes have different coagulation values. The coagulation behaviour of various electrolytes was studied by Hardy and Schultz. They gave a generalisation known as Hardy – Schultz Law, which states,        
“Greater the valency of oppositely charged ions of the electrolyte being added, the faster is coagulation”.    
So, for coagulation of sols carrying negative charge Al³⁺ ion is more effective than Ba²⁺ions or Na⁺ ions. Similarly for coagulation of positively charged sols ion is more effective than  or  ions.
Thus in case of positively charged sol the coagulating power of anions is in the order of

  and in case of negatively charged sols, the coagulating power of cations is in the order of  Al³⁺ > Ba²⁺ > Na⁺