The MC activity coefficient function for acid–base equilibria. Part 3. Improvement on the MC function by mathematical treatment

Abstract

The empirical relationship log f_Bjf_H⁺/f_BjH⁺=n_jilog f_Bif_H⁺/f_BiH⁺ for protonation equilibria in aqueous sulphuric acid, between any two weak bases i and j has been tested. The weak bases examined are primary nitroanilines, amides, tertiary amines, indoles, benzophenones, sulphoxides, and hydroxy- and alkoxy-benzenes. A mathematical treatment which accounts for all the indicators, independently of their structure or acidity range, has been used. The results obtained show in all cases the validity of the relationship examined. Activity coefficient functions have been calculated by using both structurally similar indicators and all the compounds. The results show the identity between the particular functions and the general one. A single function [M_Cf(X)‘generatrix function’] able to give the dependence on acid concentration of {log ([BH⁺]/[B])– log [H⁺]} of any indicator is obtained. The analysis of relative n_ji values for different or structurally similar indicators show the limitation of classing indicators by series.