A freshly prepared $\mathrm{Fe}(\mathrm{OH}){}_3$ sol carries

Hydrated metal oxides such as $\mathrm{Fe}(\mathrm{OH}){}_3$ and $\mathrm{Al}(\mathrm{OH}){}_3$ form positively charged sols. The charge arises by preferential adsorption of cations such as $\mathrm{Fe}{}^{3+}$ from the medium.