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An AgI colloidal sol is prepared by peptising AgI with AgNO3 solution. Which of the following statements about this sol are correct?

1. In electrophoresis, the dispersed phase will move towards the cathode.
2. AlCl3 is more effective than Na3PO4 for coagulating this sol.
3. Na3PO4 is more effective than AlCl3 for coagulating this sol.
4. On persistent dialysis, the colloidal sol undergoes coagulation.

Correct answer: In electrophoresis, the dispersed phase will move towards the cathode.

Solution

Peptisation with AgNO3 causes Ag+ adsorption on AgI, resulting in positively charged colloidal particles. In electrophoresis they migrate toward the cathode (A correct). By the Hardy-Schulze rule, the coagulating power depends on the charge of the counter-ion; for a positive sol, trivalent anion PO4³- (from Na3PO4) is far more effective than Al3+ cation (from AlCl3, which is the same sign as the sol). Hence C and D are also correct, but A is the most definitively answerable single statement.

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