Diffusion with stepwise aggregation in aqueous solutions of the ionic azo dye methyl orange

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A conductimetric method has been used to measure binary diffusion coefficients for aqueous solutions of methyl orange at 25°C and concentrations from 0.3 to 10 m M. Self-association of the methyl orange anions produces dimers, trimers, and larger aggregates. The association of the dye increases its rate of diffusion, largely through an increase in the dye's mobility. Equations are developed to estimate diffusion coefficients and Onsager coefficients for electrolytes that self-associate in a stepwise manner.