source : hubpages.com

Hardy schulze rule

“Greater is the valency of coagulating ion added, the greater is its power to cause precipitation ”. In the coagulation of a negative colloidal solution the coagulating power is as :


In the coagulation of a positive colloidal solution the following power is as :


Protection of colloids:

Lyophobic colloidal solutions  are coagulated on addition of electrolytes while lyophillic colloidal solutions are not coagulated by electrolyte . If small amount of lyophillic colloidal solution is present along with a lyophobic colloidal solution, the addition of a small amount of electrolyte does not coagulate either of them. It means  presence of lyophillic colloidal solution prevents a lyophobic colloidal solution from coagulation. This phenomenon is known as protection & the lyophillic colloid used for this purpose is known as protective colloid.

When a lyophillic colloid is added to a lyophobic colloidal solution, the particles of lyophobic colloidal solution adsorbs the particles of lyophillic colloid around them & behaves like lyophillic colloidal solution. This prevent them from coagulation.

Gold Number:

” The Gold number is defined as the number of milligrams of the protective colloid in dry state which when added to 10 ml of a colloidal solution of gold, prevents the gold solution from coagulation on addition of 1 ml of 10%  NaCl solution.”

Gold number of different lyophillic colloids are different. Lesser is the gold number greater is the protective power.

Question : The coagulation of 100 ml of a colloidal solution of gold is completely prevented by adding 0.25 gm of starch to it before adding 10 ml of 10% NaCl solution. Find out the gold number of starch ?


100 ml gold solution requires = 0.25 gm starch ( i.e 250 mg starch)

10 ml gold solution requires = 250 × 10 / 100 = 25 mg

10ml of 10 % NaCl solution is added to 100 ml gold solution. Thus 1 ml of 10% NaCl solution been added to 10 ml solution of gold.

Question: The coagulation of 10 ml of a colloidal solution of gold  can be completely stopped by the  addition of  0.025 gms of starch before the addition of  1 ml of 10% NaCl solution. What  is  the gold number ?


Coagulation of 10 ml of colloidal solution of gold can be stopped by the addition of

1 ml of 10% NaCl = number of milligrams of starch

= 0.025 gm =  0.025 × 1000 mg = 25 mg.