On addition of 0.5 mL of solution of 10% NaCl to 20 mL of red gold sol in presence of 0.025 g of starch, the coagulation is just prevented. The gold number of starch is
0.1
Minimum mass (mg) of lyophilic sol to be added to 10 ml of red gold sol which can prevent from coagulation on addition upon 1 ml of 10% NaCl solution is called gold number.
For 20 ml of red gold sol -------- 0.5 ml of 10% NaCl required
For 10 ml of red gold sol -------- 0.5×1020 = 0.25 ml is required
For 0.25 ml of 10% NaCl-------- 0.025 g of starch is required
For 1 ml of 10% NaCl----------- 0.025×10.25 = 0.1 mg starch is required (gold number)