wht is preferential adsorption of colloidal sol?explain

The colloidal particles have a tendency to preferentially adsorb a particular type of ions from the solution. A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice. This preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles. 

 For example, when colloidal sol of AgI is prepared by adding a small quantity of silver nitrate (AgNO3) solution to a large quantity of potassium iodide (KI) solution, the colloidal particles of silver iodide adsorb I from the solution to become negatively charged, (at this stage KI is in excess, and I- is common to lattice of AgI).

AgI  +      I                (AgI) : I

(Colloidal         (in excess          (Colloidal particle

particle)     in the medium)           becomes negatively charged)

But, when colloidal sol of AgI is prepared by adding a small quantity of potassium iodide (KI) solution to a large quantity of silver nitrate solution (AgNO3); the colloidal silver iodide particles adsorb Ag+ from the solution to become positively charged, (at this stage AgNO3 is in excess and Ag+ is common to lattice of AgI).

AgI  +      Ag+                (AgI) : Ag+

(Colloidal         (in excess          (Colloidal particle

particle)     in the medium)           becomes positively charged)

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