Most effective coagulant for As₂S₃ sol is

Solution:

Arsenic sulphide (As₂S₃) sol is a negatively charged sol. According to the Hardy-Schulze rule, for the coagulation of a negatively charged sol, a cation is required, and the coagulating power of the cation increases with its valency.

Let's analyze the valency of the cations from the given options:

1. K₄[Fe(CN)₆] dissociates to give K⁺ ions (valency +1).

2. MgCl₂ dissociates to give Mg²⁺ ions (valency +2).

3. AlPO₄ dissociates to give Al³⁺ ions (valency +3).

4. KCl dissociates to give K⁺ ions (valency +1).

Since Al³⁺ has the highest positive charge (+3), it will be the most effective coagulant for the negatively charged As₂S₃ sol.