Coagulation is the process by which a colloid precipitates out of a solution. The precipitation is brought about by induced aggregation. For example., an iron (III) hydroxide sol can be made to aggregate by addition of an ionic solution. A positively charged particle of iron (III) hydroxide gathers a layer of anions around it. The thickness of this layer is determined by the charge on the anions. The greater the magnitude of the negative charge, the more compact the layer of charge. For example, phosphate ions gather more closely to the positively charge iron (III) particle than do chloride ions.
A. Layers of ions surrounding a charged particle of iron (III) hydroxide. Fe(OH)3 surrounded by Cl- ions
B. Fe(OH)3 surrounded by PO43- ions
If the ion layer is gathered close to the colloidal particles, the overall charge is effectively neutralized and two colloidal particles can approach close enough to aggregate and precipitate out.
The coagulation of colloids by an electrolyte takes place only when the electrolyte has a certain minimum concentration. Different electrolytes have different coagulation values. Smaller the coagulation value of the electrolyte, larger is its coagulating power. According to Hardy and Schulze, coagulation of colloids by electrolytes is governed by two factors, namely :
i) Ions carrying charge opposite to that of the colloidal particles are effective in bringing about coagulation.
ii) Coagulation power of an electrolyte is directly proportional to the valency of its ions.
These two observations made by Hardy and Schulze are called the Hardy Schulze rule. On the basis of this rule, it is possible to predict that for colloidal sols consisting of negatively charged particles such as (AS2S3)Al3+ ions are more effective than Ba2+ or Na+ ions. For positively charged colloidal particles, PO43- is much more effective than Cl- to bring about coagulation
1) Curdling of milk
Milk is a colloidal suspension in which the particles are prevented from aggregating because they have electric charges of the same sign. When milk sugar (lactose) ferments, ions responsible for curdling are formed.
2) When river water meets the concentrated ionic solution of an ocean, coagulation of the colloidal suspension of the soil in river water occurs.
