Abstract
Abstract Based on the results of hydrogen isotope fractionation in aqueous alkali halide solutions at 25 °C, reported in previous papers, the hydrogen isotope effect between water molecules hydrated to a cation or an anion and bulk water molecules is estimated with the assumption that the equilibrium constant of the hydrogen isotope exchange reaction between the hydration water of sodium ion and bulk water is unity. The orders of the hydrogen isotope effects of alkali metal cations and halide anions are found to be Cs+ > K+ > Na+ > Li+, and I− > Br− > Cl− for a constant hydration number, respectively, and the hydrogen isotope effect is mainly influenced in anions rather than in cations. The oxygen isotope effect for alkali and halide ions is evaluated from the literature. The hydrogen and oxygen isotopic effects for the cations and anions show the same trend, but the magnitude of the hydrogen isotope effect for anions is significantly larger than that of the oxygen isotope effect. The free energy of transfer of alkali metal and halide ions from H2O to HDO is calculated from the hydrogen isotope effect for the individual ions. The calculated values are compared with the free energy of transfer from H2O to D2O obtained by electromotive force measurements, and are compared with the free energy of transfer from H216O to H218O calculated from the oxygen isotope effect.
Published Version
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