Excess ${\mathrm{FeCl}}_3$ is hydrolysed with hot water. The sol formed is most readily coagulated by

${\mathrm{FeCl}}_3+3{\mathrm{H}}_2\mathrm{O}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_3\downarrow +3\mathrm{HCl}$

$\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$ precipitated adsorbs the common ion, ${\mathrm{Fe}}^{+3}$ from the excess electrolyte, ${\mathrm{FeCl}}_3$ surrounding it. It forms a positive sol. 

$\mathrm{Fe}{\left(\mathrm{OH}\right)}_3/{\mathrm{Fe}}^{+3}$

It is most effectively coagulated by the anion with maximum valency. 

${\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{-4}>{\mathrm{SO}}_4^{-2}>{\mathrm{X}}^{-}$