Gold number

Gold Number is a term used in colloidal chemistry. It is defined as the minimum amount of protective colloid in milligrams which prevents a color change from red to violet of 10ml gold sol by the addition of 1 ml of 10%NaCl solution.

Coagulation of gold sol is indicated by colour change from red to blue/purple when particle size just increases. More is the gold number, less is the protective power of the lyophilic colloid since it means that the amount required is more. It was first used by Richard Adolf Zsigmondy. The amount is taken in terms of weight in milligrams.

The gold number of some colloids are given below.