Lyophobic sols are unstable and are easily precipitated by the addition of electrolytes. However it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatin etc. To lyophobic colloids render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as ‘protection’ and the lyophilic colloids are termed as protective colloids. It is believed that the protected action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid.
To compare the protective action of different lyophobic colloids, Zsigmondy in 1901 introduced a term known as Gold number.
Gold number of a protective colloid is the minimum weight of it in milligrams which must be added to 10ml of a substance red gold sol so that no coagulation of the gold sol takes place when 1ml of 10% sodium chloride solution is rapidly added to it.
Evidently smaller the gold number of a protective colloid, the greater is its protective action.
Instead of gold number, the protective action of a lyophilic colloid is sometimes expressed in terms of Congo rubin number discovered by Ostwald. It is defined as the minimum amount of the protective colloid in milligrams that prevents the color change of 100ml of 0.01% Congo rubin dye to which 0.16 g equivalent of KCl is added.
