Peptization is a process of passing of a precipitate into colloidal particles on adding suitable electrolyte. The electrolyte added is known as peptizing agent. A few examples of sols obtained by peptization are given below:
A reddish brown colored colloidal solution is obtained by adding small quantity of ferric chloride solution to the freshly precipitated ferric hydroxide.
A precipitate of silver iodide can be peptized by shaking with a dilute solution of silver nitrate or KI.
On adding insufficient quantity of very dilute HCl solution to the freshly precipitated aluminium hydroxide, a sol of aluminium hydroxide is obtained.
Cause of peptization:
As the electrolyte is added to a freshly precipitated substance, the particles of the precipitate preferentially adsorb one particular type of ions of the electrolyte and get dispersed due to electrostatic repulsion. This gives particles of colloidal size. Freshly prepared precipitates are preferred where the particles are ready supposed to be in the colloidal solution.
