Activity-based analysis of potentiometric pH titrations

Abstract

The discrepancy between concentrations and activities is a predicament well known to the analytical chemist. Because of the difficulty of determining activity coefficients, the standard technique for quantitative equilibrium studies is to work under a particular 'constant ionic strength' by adding an excess of an inert salt. Under such conditions, activity coefficients are approximately constant and can be taken into the equilibrium constants which are defined for the chosen ionic strength (I). Here we propose a fundamentally different approach. Throughout the numerical analysis of the titration data, activity coefficients for all individual species are approximated by well-known equations based on the work of Debye-Hückel. The computational analysis of the measurements strictly obeys the law of mass conservation and obeys the law of mass action only approximately. The main novelty is that now the addition of inert salts is no longer required and measurements are done at minimal I. Consequently, the the thermodynamic equilibrium constants are now determined much more robustly based on experiments taken at low I. The approach has been tested and validated with the two very well investigated 3protic phosphoric and citric acids. In summary: the technique of artificially keeping ionic strength constant has been made replaced by improved computational analysis.

Stabilization of the Computation of Stability Constants and Species Distributions from Titration Curves

“…Since these are dependent on the established chemical equilibrium, it is assumed that once a stable equilibrium system has been reached at any point on the titration curve, the determination of the activity can be dispensed with in favor of observing the concentrations in the case of equilibrium. Similar to the contribution of Karimvand et al [24], this article proposes an approach where thermodynamic equilibrium constants are determined directly from the analysis of potentiometric pH titrations. However, this approach avoids the use of activity coefficients by determining the individual points of the potentiometric titration at any time in thermodynamic equilibrium.…”