Anion-exchange chromatographic study of the chlorine isotope effect accompanying hydration

The single-stage separation factor for chlorine isotopes (35Cl and 37Cl) was determined to be 1.00034 by anion exchange chromatography on a 4.5-m column operated in reverse breakthrough manner at 25 °C. This value is in good agreement with those obtained in our previous works. It was confirmed that the lighter isotope (35Cl) was preferentially fractionated into the resin phase, whereas the heavier isotope (37Cl) partitioned into the aqueous phase. This observation, however, contradicts the experimental results for Cl isotope fractionation during NaCl precipitation and the recent theoretical results on Cl isotope fractionation: the 37Cl isotope selectively enriched into the solid phase and not into the aqueous phase. This discrepancy is discussed based on the theory of isotope distribution between two phases. It is suggested that the chromatographic results reflect an isotope effect accompanying hydration rather than an isotope effect due to a phase change, whereas the reverse is the case for the results in the NaCl precipitation study.