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Schulz Hardy rule: Greater the valency of the active ion or flocculating ion, greater will be its coagulating power.
(i) The ions carrying the charge opposite to that of sol particles are effective in causing coagulation of the sol.
(ii) Coagulating power of an electrolyte is directly proportional to the valency of the active ions.
I. Gold number: Gold number of a protective colloid is the minimum weight of it in milligrams which must be added to 10ml of a substance red gold sol so that no coagulation of the gold sol takes place when 1ml of 10% sodium chloride solution is rapidly added to it.
II. Congo rubin number: It is defined as the minimum amount of the protective colloid in milligrams that prevents the color change of 100ml of 0.01% Congo rubin dye to which 0.16 g equivalent of KCl is added.