The coagulating power of an electrolyte for Arsenous sulphide sol decreases in the order 

Greater is the valency of the oppositely charged ion of the electrolyte is added, the faster is the coagulation. As₂S₃ is an anionic (negative) sol. Therefore, the coagulating power of an electrolyte for arsenious sulphide depends on the coagulating power of cation which is directly proportional to the valency of the cation.

${\mathrm{Al}}^{3+}>{\mathrm{Ba}}^{2+}>{\mathrm{Na}}^{+}$