Ferric hydroxide sol (hydrolysis)
A freshly prepared nearly saturated solution of ferric chloride is first made. About 10 to 12 mL of the ferric chloride solution is then added dropwise to about 800 mL of boiling water. A few minutes interval is kept between, successive drops. A brilliant dark red sol is produced by the hydrolysis of ferric chloride forming ferric hydroxide and hydrochloric acid.
Ferric hydroxide forms a hydrophobic sol. Hence it is prepared indirectly by the hydrolysis of ferric chloride with boiling water. The reaction takes place as follows:
FeCl3 + 3H2O → Fe(OH)3 + 3HCl
The ferric ions (Fe3+) produced from FeCl3 solution are absorbed on the surface of the particles of Fe(OH)3. As a result, the colloidal particles of Fe(OH)3 acquire a positive charge. Hence because of similar charge on the colloidal particles, they keep on repelling each other and cannot come together or aggregate together to form bigger particles. This prevents their coagulation and is responsible for the stability of the sol. However, the HCl formed simultaneously with Fe(OH)3, destabilizes the sol. Hence HCl, being an electrolyte is removed from the colloidal sol by dialysis.
The sol needs immediate dialysis against hot water to remove the hydrochloric acid; otherwise, the sol would be coagulated. Many sols of heavy metal hydroxides can be prepared in this way.