A colloidal solution is prepared by adding a small amount of silver nitrate solution to an excess of potassium iodide solution. What is the charge on the colloidal particles and which ion is the potential determining ion?

Solution

1. The reaction is ${\text{AgNO}}_3+\text{KI}\to \text{AgI}+{\text{KNO}}_3$.
2. Since $\text{KI}$ is in excess, the $\text{AgI}$ precipitate adsorbs the common ion ${\text{I}}^{-}$ from the dispersion medium.
3. The adsorbed ${\text{I}}^{-}$ ions act as the potential determining ions.
4. The resulting colloidal particles carry a negative charge.
5. Hence the answer is (B).