Preparation of colloid

There are two principal ways of preparation of colloids:
1. Dispersion of large particles to the colloidal dimensions. There are three methods of colloidal preparation using this method : mechanical, peptization, and Briedig arc methods.
a. Mechanical method
In the mechanical method, large particles are broken down into colloidal size particles by grinding. The colloidal size particles obtained are then dispersed in the dispersing medium by stirring. Paint pigments, for example, are produced by dispersing large particles by grinding in special mills.
b. Peptization method
A few solid substances such as gelatin, glue, and starch, when brought into contact with water, disperse spontaneously and form colloidal system. Gelatin , glue, and starch particles are already in colloidal size, the water simply disperses them. Gelatin, glue, and starch particles in water are said to undergo peptization.
c. Bredig arc method
Bredig arc method is used to prepare metallic sol such as colloid of gold in water. Gold metal that will be dispersed in water is used as electrode and is immersed in water as shown in figure above.

Gold atoms are released from gold electrodes when potential difference is given between the electrodes. The gold atom disperse and condense to produce colloidal particles in water.
2. Condensation of molecules dissolved in a true solution into larger colloidal particles.For examples, clouds form when water condense and produce very small droplets of colloidal size. Many con densation methods employ chemical reactions such as redox, double decomposition adn hydrolisis reactions.
a. By oxidation : A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising agent like HNO3, Br2 etc.) through a solution of hydrogen sulphide in water.

2H2S + O2 (or any other agent) → 2H2O + 2S

b. By reduction : A number of metals such as silver, gold and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc
2AuCl3 + 3SnCl2 → 3SnCl + 2Au
Gold sol
2AuCl3 + 3HCHO + 3H2O → 2Au + 3HCOOH + 6HCl
Gold sol

The gold sol, thus prepared, has a purple colour and is called purple of cassius.

c. By hydrolysis : Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chlorides.

FeCl3 + 3H2O → Fe(OH)3 + 3HCl
Colloidal sol
Similarly silicic acid sol is obtained by the hydrolysis of sodium silicate.

d. By double decomposition : A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenious oxide in water.
As2O3 + 3H2S → As2S3 + 3H2O