EFFECT OF THE CONCENTRATION OF POLYBASIC AMINOANILIDES AND AMINOBENZIMIDAZOLES ON THE ACIDITY FUNCTION H0 OF AQUEOUS-SOLUTIONS OF HCL

The effect of the concentrations of the reactants and reaction products on the protonating ability of aqueous HCl is investigated in the example of the cyclization of polybasic aminoanilides to aminobenzimidazoles at 25-degrees-C and 50-degrees-C. It is established that the acidity function H0s of HCl solutions in organic-aqueous solvents of constant composition is equal to H0, the acidity function for an aqueous solution with the same concentration of HCl on the mole-ratio scale. This can be utilized for the quantitative prediction of H0s. The decrease in the basicity of indicators in mixed solvents is a consequence of the decrease in the activity coefficient of the unionized form of the base standardized to water gamma(B). It is observed that not only H0s but also the basicity constant of the reactant may vary during the reaction. The latter depends on the concentration of the reactant, as well as on the effect of the product concentration on the value of gamma(B) for the reactant.