The MC activity coefficient function for acid–base equilibria. Part 2. A critical analysis of acidity functions and the incompatibility amongst proposed empirical correlations

Abstract

Protonatiori equilibria of two weak bases X and Y in sulphuric acid are discussed and experimental linear relationships between their log [BH⁺]/[B](log I) and between their log [BH⁺][B][H⁺](log Q) values are analysed. The validity of the relationships has been tested either by using acidity functions or by comparisons with experimental data. It is shown that the utilization of acidity functions leads to different empirical correlations which are incompatible. Approximations involved in the evaluation of acidity functions and acidity ranges where experimental data have been tested were recognized to give rise to apparent compatibility, till now accepted, amongst the relationships. Evidence of the validity of log Q_Yversus log Q_X relationships, through experimental data, is given in the range 0–98% sulphuric acid, whilst log I_Yversus log I_X relationships are only apparently verified. The reasons why relationships involving log Q and log I can appear to be verified simultaneously are discussed.