THE ACTIVITY-COMPOSITION RELATIONSHIP OF OXYGEN AND HYDROGEN ISOTOPESIN AQUEOUS SALT-SOLUTIONS .1. VAPOR-LIQUID WATER EQUILIBRATION OF SINGLE SALT-SOLUTIONS FROM 50-DEGREES-C TO 100-DEGREES-C

The differences between oxygen and hydrogen isotope activity and compo sition ratios of water in single salt solutions (NaCl, KCl, MgCl2, CaC l2, Na2SO4, and MgSO4) were determined by means of a vapor-liquid wate r equilibration method over the temperature range of 50 to 100-degrees -C. A parallel equilibration technique of pure water and salt solution s with the same isotopic composition at the same experimental conditio ns enabled the precise determination of the isotope salt effects. Hydr ogen isotope activity ratios of all of the salt solutions studied were appreciably higher than composition ratios. That is, D/H ratio of wat er vapor in isotope equilibrium with a solution increases as salt is a dded to the solution. Magnitudes of the hydrogen isotope effects are i n the order CaCl2 greater-than-or-equal-to MgCl2 > MgSO4 > KCl almost- equal-to NaCl > Na2SO4 at the same molality. Except for KCl solutions at 50-degrees-C, oxygen isotope activity ratios in the solutions were lower than, or very close to, the composition ratios. The isotope effe cts 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) exhi bited oxygen isotope effects much larger than those of monovalent cati ons (Na and K). Magnitudes of the oxygen isotope effects in NaCl solut ions, and of the hydrogen isotope effects in Na2SO4 and MgSO4 solution s, may increase from 50 to 100-degrees-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-degrees-C. The res ults in this study and the literature data near room temperature were satisfactorily fitted to simple equations as a function of concentrati on of the salt solutions and temperature.