The changes in opacity during the coagulation of (a) thorium hydroxide, (b) stannic hydroxide and (c) zirconium hydroxide sols have been investigated. The results have been used to compare the coagulating power of electrolytes and also to study the applicability of Smoluchowski’s equation to the coagulation of these sols. It has been found that Smoluchowski’s equation is applicable upto a certain stage of coagulator concentrations only (corresponding to rapid coagulation). The effect of temperature and the influence of alcohols have also been studied. Both these factors accelerate the rate of coagulation in the case of all the sols investigated.
Basing on the theory of Smoluchowski for rapid coagulation an attempt has been made to follow theoretically the changes in the intensity of light scattered transversely (I) by a system undergoing coagulation. The opacitytime curves obtained during the coagulation of sols have been compared with I-T curves obtained theoretically. The curves experimentally obtained for rapid coagulation are similar to those theoretically deduced. However the I-t curves for slow coagulation are “S” shaped and it has been inferred that Smoluchowski’s equation is not applicable in the region of slow coagulation. This conclusion is the same as that obtained from the direct verification of Smoluchowski’s equation from the opacity data.