There are two ways in which colloidal size particles can be obtained
(a) by building up the particles from single molecules. The methods under this category are known as condensation or aggregation methods.
(b) by disintegrating down the bulk material to the colloidal particles. Methods under this category known as the dispersion methods.
In preparing a colloidal solution, care must be exercised to see that the vessels used should be thoroughly clean and the materials used should be pure.
(1) CONDENSATION METHODS
A supersaturated solution of the disperse phase is prepared in the dispersion medium by carefully controlling the temperature and keeping the concentration of disperse phase to be low. The various methods used here are:
This method is generally used for the preparation of metal sols and involves the reduction of soluble salts of metals by reducing agents, such as formaldehyde. hydrazine or tannic acid. For example: Silver sol can be obtained
(i) MECHANICAL DISPERSION METHOD
In this method the substance is finely ground and then shaken with a dispersion medium to get a coarse suspension. This suspension is then changed to particles of colloidal size by means of colloidal mill. This mill consists of two metal discs nearly touching each other and rotating in opposite direction at very high speed (7,000 revolutions per minute). The suspension passes through the rotating discs and is broken down to particles of colloidal size. Examples; colloidal graphite (a lubricant) and printing inks are made by this method.
The process of dispersing a precipitated material into a colloidal solution by adding a small quantity of an electrolyte is called peptisation. The electrolyte added is called a peptisation agent. and mostly contains a common ion. Example; freshly prepared precipitates of ferric hydroxide when treated with a small amount of FeCI3 solution (peptizing agent) results in the formation of a dark reddish brown sol.
(iii) ELECTRO-DISPERSION OR BREDIG’S METHOD
This method involves both dispersion and condensation processes. In this method, an electric arc is produced inside water (solvent) between the two electrodes of the metal to be dispersed. The heat of the arc evaporates the metal. These vapours of the metal are condensed by the ice cold water to form a colloidal solution. A small amount of KOH is added in the dispersion medium to stabilize the sol. This method is used for preparThg hydrosols of metals. Such as Ag, Au. Cu and Pt.