Colloids – Preparation and Charge on Colloidal Particles

Generally, the charge on a colloid is due to adsorption of the common ion present in excess in the dispersion medium. For a negatively charged $[AgI]I^-$ sol, $I^-$ must be in excess over $Ag^+$. Millimoles of $AgNO_3$ and KI in each mixture: (1 M, 1.5 M) → 50 and 75; (1 M, 2 M) → 50 and 100; (2 M, 1.5 M) → 100 and 75; (0.1 M, 0.1 M) → 5 and 5. The excess of KI is maximum in 50 mL of 1 M $AgNO_3$ + 50 mL of 2 M KI (100 millimol KI), so $I^-$ is adsorbed on the AgI formed, giving the $[AgI]I^-$ sol.