Ferric hydroxide sol is more readily conagulated by `Na_(3)PO_(4)` in comparison to KCl. Why ?

Explanation

Ferric hydroxide sol is positively charge and to cause its congulation, ions carrying negative charge are needed. Since `PO_(4)^(3-)` ions have higher negative charge than `Cl^(-)` ions therefore, `Na_(3)PO_(4)` coagulates the ferric hydroxide sol most efficiently. This is according to Hardy-Schulze law. For detalis, consult Secction 23.