The arsenious sulphide sol has negative charge. The maximum coagulationg power for precipitating it is of :
(a) 0.1 N Zn(NO3)2
(b) 0.1 N Na3PO4
(c) 0.1 N ZnSO2
(d) 0.1 N AlCl3
(d) More is the valence of effective ion, greater is its coagulating power. The Hardy-Schulze rule.