Gold number is defined as the minimum amount of lyophilic colloid in milligrams which prevents the flocculation of 10ml gold sol by the addition of 1 ml of 10% NaCl solution.The concept of gold number was given by Zsigmondy. Coagulation of gold sol is indicated by colour change from red to blue when particle size just increases. More is the gold number, lesser will be the protective power of the lyophilic colloid since it means that the amount required is more. The amount is taken in terms of weight in milligrams. For ex- gold number of geletine = 0.005 – 0.01 and haemoglobin = 0.03 – 0.07. Gold number is an experimentally determined quantity.