Gold Number

 Gold number of a protective colloid is the minimum weight of it in milligrams which must be added to 10 mL of a substance (red gold sol) so that no coagulation of the gold sol takes place when 1 mL of 10% sodium chloride solution is rapidly added to it.

or

Weight of the dried protective agent in milligrams, which, when added to 10 mL of a standard gold sol (0.0053 to 0.0058%), is just sufficient to prevent a colour change from red to blue on the addition of 1 mL of 10% sodium chloride solution, is equal to the gold number of protective colloid. 

Protective power ∝ 1/Gold number

Note: Gold number is a practical value, it cannot be obtained theoretically.