Fe(OH)3 is a positive sol, hence the best coagulant for precipitation of Fe(OH)3 is Na3[Fe(CN)6] The precipitating effect of an ion on dispersed phase of opposite charge increases with the valence of ions. According to Hardy-Schulze rule, greater is the valency of ions, greater is its power to bring about coagulation. Flocculation value =‌
1
‌ Coagulation power ‌
‌‌... (i) Na3[Fe(CN)6]⟶3Na++[Fe(CN)6]3− Na2SO4⟶2Na++SO42− Cr(OH)3⟶Cr3++3OH− Al(OH)3⟶Al3++3OH− The ion with highest negative valency will coagulate positive sol of Fe(OH)3 is [Fe(CN)6]3−. As it has highest coagulating power from relation (i), it would have lowest flocculating value and hence, is the best agent for coagulation.