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 color 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.