An Anomalous Isotope Effect of ²³⁵U in U(IV)-U(VI) Chemical Exchange

Abstract

The abundance ratios of 234 U, 235 U, 236 U and 238 U were determined by mass spectrometry on uranium samples obtained from a chromatographic uranium enrichment process based on the U(IV) - U(VI) chemical exchange reaction. According to standard theory, the elementary isotope effect in processes based on equilibrium chemical exchange is proportional to the mass difference of the isotopes. In the present work this has been confirmed for 234 U, 236 U and 238 U. However, according to the observed data the fractionation between 234 U and 235 U is much smaller than the one between 235 U and 236 U in spite of the same mass difference in either isotopic pair. The anomaly of the fractionation is concluded to be due to the characteristics of 235 U, which is the only odd mass number isotope in the present system.