Precision modelling of conductivity data of monovalent aqueous electrolytes

Abstract

When two ions of opposite charge in an aqueous solution form an ion pair, the hydration shells of the individual ions become partly destroyed because of the weakened interaction between the ions and the water molecules. The reaction A^–+ K⁺⇌ AK +h_FH₂O is considered, where A^– is the hydrated free anion and K⁺ the cation, and AK the less hydrated contact ion pair. The equilibrium concentrations are calculated, taking into account the excess Gibbs energies that arise from ion–ion and ion–dipole interactions. The concentration of the bulk water also affects the equilibrium.

Combining this chemical model with the MSAT (mean spherical approximation transport) equations for the electrolytic conductivity, very precise fits with an rms better than 0.05% of conductivity data are obtained, up to high concentrations (e.g. 12 mol dm^–3 for HCI at 25 °C).

Using the equilibrium concentrations obtained from conductivity data, density and vapour pressure data can be reproduced quantitatively. Using this approach, molar volumes of free hydrated ions, contact ion pairs and bulk water are independent of the concentration.