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.
Another important point in the preparation of this sol is that as the presence of ions destabilizes the sol, the flask in which the hydrolysis is to carried out must be free from all types of ions.
To prepare a sol of ferric hydroxide
The procedure consists of three steps.
(i) Cleaning of the conical flask
The apparatus is set up.
Distilled water is taken in a round bottomed flask fitted with a funnel in its mouth. A glass tubing passes through the funnel and is held within the funnel with loose rubber stopper. Water in the flask is heated to produce steam. The conical flask is held in an inverted position over the top of the glass tubing. As the steam hits the inside of the flask, it condenses and dissolves away any ions sticking to the inside of the flask.
(ii) Preparation of ferric hydroxide sol
Prepare about 20 ml of 2% FeCl3 solution in a boiling tube (by dissolving about 0.4 g in about 20 ml of distilled water). Filter if necessary. Take about 10 ml of distilled water in the cleaned conical flask.
Heat it to boiling. To the boiling water, add FeCl3 solution drop-wise with constant stirring till a deep wine red colour is obtained which is the colloidal sol of Fe(OH)3.
(iii) Purification of the ferric hydroxide sol
Ferric hydroxide sol obtained contains HCl as impurity. This is removed by the process of dialysis.