PRECISION MODELING OF CONDUCTIVITY DATA OF MONOVALENT AQUEOUS-ELECTROLYTES

When two ions of opposite charge in an aqueous solution form an ion pa ir, the hydration shells of the individual ions become partly destroye d because of the weakened interaction between the ions and the water m olecules. The reaction A(-) + K+ half arrow right over half arrow left AK + h(F)H(2)O is considered, where A(-) is the hydrated free anion a nd K+ the cation, and AK the less hydrated contact ion pair. The equil ibrium concentrations are calculated, taking into account the excess G ibbs energies that arise from ion-ion and ion-dipole interactions. The concentration of the bulk water also affects the equilibrium. Combini ng this chemical model with the MSAT (mean spherical approximation tra nsport) equations for the electrolytic conductivity, very precise fits with an rms better than 0.05% of conductivity data are obtained, up t o high concentrations (e.g. 12 mol dm(-3) for HCl at 25 degrees C). Us ing the equilibrium concentrations obtained from conductivity data, de nsity 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.