On addition of one mL of 10% NaCl solution to 10mL gold sol in presence of 0.25 gm of starch, the coagulation is just prevented, starch has gold number :

minimum milligrams of starch required to protect 10ml of gold sol from coagulation by the addition of 1ml of 10% NaClis called gold number of starch10ml gold sol required 0.25 grams of starch Gold number of starch =0.25 X 103 = 250 mg