A reddish brown sol is obtained by adding small quantity of FeCl₃ solution to freshly prepared and well washed Fe(OH)₃ ppt. Fixed layer and mobile layer are represented as

Fe(OH)₃ sol adsorbs common ion of FeCl₃ and Fe(OH)₃ from FeCl₃, i.e. Fe³⁺ and positively charged sol is the fixed part of the electrical double layer. Thus, Cl^- (to balance charge of Fe³⁺) ions are on the mobile part of the electrical double layer.

$\begin{array}{c}\left[\text{Fe}{\left(\text{OH}\right)}_3\right]{\text{Fe}}^{3+}⋮3{\text{Cl}}^{-}\\ \text{ Fixed \hspace{0.33em}\hspace{0.33em}\hspace{0.33em} Mobile }\end{array}$