Hydrogen Isotope Fractionation in Aqueous Alkaline Earth Chloride Solutions

Abstract

The D/H ratio of hydrogen gas in equilibrium with aqueous alkaline earth (Mg, Ca, Sr or Ba) chloride solutions measured at 25◦C using a hydrophobic platinum catalyst, was found to be higher than the D/H ratio equilibrated with the applied pure water. The hydrogen isotope effect between such solutions and pure water changes with the molality of the solutions. The order of the D/H ratios in alkaline earth chlorides is found to be BaCl2 > SrCl2 ≥ CaCl2 ≥ MgCl2. The hydrogen isotope effect in the aqueous chloride solutions of Mg, Ca, Sr or Ba ions is significantly larger than that in the aqueous chloride solutions of Li, Na, K or Cs ions. For MgCl2 and CaCl2 solutions, the hydrogen isotope effect is opposite to the oxygen isotope effect. The results are compared with the free energy change of transfer from H2O to D2O, and are discussed for the vapour pressure ratio of H2O and D2O of CaCl2 solutions.