The coagulating power of electrolytes having ions Na+, Al3+ and Ba2+ for arsenic sulphide sol increases in the order:

As2S3 is an anionic sol (negative sol). Hence, coagulation will depend upon the coagulating power of the cation. Now, according to the Hardy-Schulze rule, the coagulating power of cation is directly proportional to the valency of the cation. Hence, the order is Na+<Ba2+<Al3+.