Aggregation of methyl orange in aqueous acidic solutions below room temperature

Abstract

The absorptivity decrease observed for aqueous acidic methyl orange as a function of time, caused by aggregation of the dye, occurs in two steps clearly discernible at lower temperatures. An immediate substantial decrease, the rate of which strongly depends on both temperature and initial dye concentration, is followed by a slower one. In between both steps an actual equilibrium is observed, at which the degree of aggregation is evaluated (using the methods of D. Pugh and M. Hida). Varying the temperature does not markedly influence the nature of the aggregated species but varying the surface–volume ratio of the solution does.

For the first reaction step a third-order relation with respect to the dye concentration is found at all temperatures.