Simultaneous Determination of All Species Concentrations in Multiequilibria for Aqueous Solutions of Dihydrogen Phosphate Considering Debye–Hückel Theory

Abstract

Solutions of citric acid and Na2HPO4 were studied with the dynamical approach to multiequilibria systems. This widely employed buffer has a well-defined pH profile and allows for the study of the distribution of phosphate species over a wide pH range. The dynamical approach is a flexible and accurate method for the calculation of all species concentrations in multiequilibria considering ionic strength (I) via Debye–Hückel theory. The agreement between the computed pH profiles and experiment is excellent. The equilibrium concentrations of the non-hydrogen species are reported for over 30 buffer mixtures across the entire pH range. These new concentration data enable researchers to lookup the equilibrium distribution of species at any pH. The data highlight the dramatic effects of ionic strength, and for example, the position of maximal H2PO4– concentration is shifted by almost an entire pH unit! From a more general perspective, the study allows for a discussion of the dependence of concentration quotients Qxy on ionic strength, pQxy = f(I), and for the numerical demonstration that the thermodynamic equilibrium constants Kxy,act(I) = Kxy. The analysis emphasizes the need for measurements of the concentrations of several species in complex multiequilibria systems over a broad pH range to advance multiequilibria simulations.