Preparation of a Sol: Fe(OH)3 Colloid

Fe(OH)3 colloid can be prepared in two ways: by peptization method and by hydrolysis method. In the peptization method, a freshly prepared precipitate is converted into colloidal solution by the addition of a suitable electrolyte. The electrolyte added acts as peptizing agent or dispersion agent by adsorbing onto the surface of tiny particles in precipitate and keeping them suspended in solution due to electrostatic repulsion. For example, if freshly prepared ferric hydroxide precipitate is shaken with a small quantity of FeCl3 (peptizing agent) solution, a dark reddish brown colloidal solution of Fe(OH)3 is formed. On the other hand, if a concentrated solution of FeCl3 is heated, hydrolysis occurs producing colloidal Fe(OH)3 particles and aqueous HCl. Some of HCl may escape as gas. The dark brown solution turns darker as a precipitate of Fe(OH)3 forms.

FeCl₃ (aq) + 3H₂O (l) → Fe(OH)₃ (s) + 3HCl (aq)

If a concentrated FeCl3 solution is added to a large quantity of hot water, colloidal iron oxide particles form giving rise to a cherry-red color to the solution.