Question

Give reason for the following:
For ferric hydroxide sol the coagulating power of phosphate ion is more than chloride ion.

Answer 1

Ferric hydroxide is a positively charged sol, therefore it will coagulate upon addition of an anion. According to Hardy-Schulze rule, greater the valence of the flocculating ion (charge on ion) added, the greater is its power to cause precipitation/coagulation. Therefore, phosphate ion $P{O_4}^{3-}$ being trivalent will coagulate the sol more readily than mono valent chloride ion $C{l}^{-}$.