Which of the following cations will have minimum flocculation value for arsenic sulphide sol?

The minimum amount of an electrolyte that, must be added to one litre of a colloidal solution so as to bring about complete coagulation is called the coagulation or flocculation value of the electrolyte. Greater is the valency of the coagulating or the flocculating ion, smaller is the quantity of the electrolyte which is required to coaguate a definite amount of coagulating ion. Thus, for coagulation of negatively charged arsenious sulphide solution, trivalent cations \(\left(\mathrm{Al}^{3+}\right)\) are far more effective than divalent \(\left(\mathrm{Ba}^{2+}\right)\) cations which in turn are more effective than monovalent \(\left(\mathrm{Na}^{+}\right)\)cations. Thus, the order of flocculating value for arsenic sulphide solution : \(\mathrm{Na}^{+}>\mathrm{Ca}^{2+}=\mathrm{Mg}^{2+}>\mathrm{Al}^{3+}\)