Fe(OH)₃ sol is formed by hydrolysis of FeCl₃. Which has maximum coagulating power to Coagulate Fe(OH)₃ sol?  

Fe(OH)₃ sol is positively charged due to adsorption of Fe³⁺ion.

${\mathrm{FeCl}}_3+3{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Fe}(\mathrm{OH}{)}_3+3\mathrm{HCl}$

$\mathrm{Fe}(\mathrm{OH}{)}_3+{\mathrm{FeCl}}_3⟶\left[\mathrm{Fe}(\mathrm{OH}{)}_3\right]{\mathrm{Fe}}^{3+}:3{\mathrm{Cl}}^{-}$

By Hardy-Schulze rule, positively charged colloidal sol is coagulated by negatively charged ions. Larger the charge larger the coagulating power.

${\mathrm{I}}^{-}<{\mathrm{SO}}_4^{2-}<{\mathrm{PO}}_4^{3-}<{\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{4-}$