Question

Which one of the following electrolytes is most effective for the coagulation of Fe(OH)₃ sol and why?

NaCl, Na₂SO₄, Na₃PO₄

Answer 1

Na₃PO₄ is the most effective electrolyte for the coagulation of Fe(OH)₃ sol because the coagulating power of an electrolyte increases with the valency of the oppositely charged ion. Since Fe(OH)₃ sol is positively charged, the relevant ions are anions: \[\ce{PO^{3-}4}\], \[\ce{SO^{2-}4}\], and Cl⁻. Among these, \[\ce{PO^{3-}4}\] has the highest valency, thus it has the greatest coagulating power, followed by \[\ce{SO^{2-}4}\] and then Cl⁻. This is supported by the order of coagulating power: \[\ce{PO^{3-}4}\] > \[\ce{SO^{2-}4}\] > Cl⁻.