The activity-composition relationship of oxygen and hydrogen isotopes in aqueous salt solutions: I. Vapor-liquid water equilibration of single salt solutions from 50 to 100°C

The differences between oxygen and hydrogen isotope activity and composition ratios of water in single salt solutions (NaCl, KCl, MgCl₂, CaCl₂, Na₂SO₄, and MgSO₄) were determined by means of a vapor-liquid water equilibration method over the temperature range of 50 to 100°C. A parallel equilibration technique of pure water and salt solutions with the same isotopic composition at the same experimental conditions enabled the precise determination of the isotope salt effects. Hydrogen isotope activity ratios of all of the salt solutions studied were appreciably higher than composition ratios. That is, D H ratio of water vapor in isotope equilibrium with a solution increases as salt is added to the solution. Magnitudes of the hydrogen isotope effects are in the order CaCl₂ ≥ MgCl₂ > MgSO₄ > KCl ≈ NaCl > Na₂SO₄ at the same molality. Except for KCl solutions at 50°C, oxygen isotope activity ratios in the solutions were lower than, or very close to, the composition ratios. The isotope effects observed are all linear with the molalities of the salt solutions, and either decrease with temperature or are almost constant over the temperature range. Salt solutions of divalent cations (Ca and Mg) exhibited oxygen isotope effects much larger than those of monovalent cations (Na and K). Magnitudes of the oxygen isotope effects in NaCl solutions, and of the hydrogen isotope effects in Na₂SO₄ and MgSO₄ solutions, may increase from 50 to 100°C. Our results agree with most of those from the literature near room temperature, but are at notable variance with those by Truesdell (1974) around 100°C. The results in this study and the literature data near room temperature were satisfactorily fitted to simple equations as a function of concentration of the salt solutions and temperature.