Applied aspects of acid-base interactions and modelling equilibrium concentrations in two-component acid mixtures

Abstract

Fundamental and applied research into aqueous and non-aqueous solutions of strong and weak electrolytes remains to be highly relevant, which fact is confirmed by a large number of Russian and foreign publications. In almost all such publications, acid-base interactions are considered exclusively with regard to changes in hydrogen ion concentrations. However, the ionic strength of solutions is determined by all ions present in the system, the concentration of which varies during interactions. This is particularly true for potentiometric titration of strong and weak electrolytes not only in aqueous, but also in more complex non-aqueous solutions, which differ significantly in their basic properties (dielectric constant, ionic product, dipole moment, viscosity, etc.). In the study of equilibria, it is more feasible to develop model representations that would greatly simplify and facilitate the computation and evaluation of certain properties of the system under consideration. In this work, acid-base interactions are presented in the form of equations based on mass action laws and those describing equilibrium processes, solvent ionic product, electroneutrality and material balance in electrolyte systems. The proposed equations consider the effect of the concentrations of all charged particles in the system (not only of hydrogen ions – pH) on the ionic strength of the solution, activity coefficients and, as a consequence, the thermodynamic dissociation constant. In addition, these equations allow the dependence between the equilibrium concentrations of all charged particles and the solution acidity determined by the potentiometric method to be expressed in convenient and objective logarithmic coordinates, thus facilitating estimation of the concentration of all particles at any moment of titration.