Presence of small concentrations of appropriate electrolyte is necessary to stabilize the colloidal solutions. However, if the electrolytes are present in higher concentration, then the ions of the electrolyte neutralize the charge on the colloidal particles may unite together to form bigger particles which are then precipitated. The precipitation of a colloid through induced aggregation by the addition of some suitable aggregation by the addition of some suitable electrolyte is called coagulation or flocculation.
The coagulation of a colloidal solution by an electrolyte does not tales place until the added electrolyte has certain minimum concentration in the solution. The minimum concentration of the electrolyte in millimoles that must be added to one litre of the sol so as to bring about complete coagulation value of the electrolyte for the sol.
Different electrolytes have dissimilar coagulation values. Smaller the coagulation value of the electrolyte larger is its coagulating or precipitating power. This can be expressed as under.
The coagulation behaviour of various electrolytes was studied in details by Hardy and Schulze. They experience that:
The ions carrying charges opposite to that of sol particles are effective in causing the coagulation of the sol. such ions are called flocculating ions or active ions.
Coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the active ions. This implies that greater the valency of the flocculating ion greater is the power to cause precipitation.
Thus, for the coagulation of sols carrying negative charge (like As2S3 sol), Al3+ ions are more effective then Ba2+ or Na+ ions. Similarly, for the coagulation of sols carrying positive charge, such as Fe(OH)3 sol PO43- ions are more effective than SO42- or Cl- ions. The two observations given above are collectively called Hardy Schulze rule.
Coagulation of colloidal solutions can also be attained by the subsequent techniques:
By mutual precipitations: when two oppositely charged sols such as Fe(OH)3 and As2S3 are mixed and equimolar properties, they neutralize each other and may get coagulated. Sometimes the sols may get coagulated due to the mutual destruction of stabilizing agents.
By electrophoresis: we know that during electrophoresis the sol particles move towards the oppositely charged electrodes. If the process is carried for a long time, the particles will touch the electrode, lose their charge and get coagulated.
By repeated dialysis: the stability of colloidal is due to the presence of a small amount of electrolyte. Of the electrolyte is completely removed by repeated dialysis, the sol will get coagulated.
By heating: the sol may be coagulated even by simple heating.