Chlorine isotope fractionation between chloride (Cl−) and dichlorine (Cl2)

Abstract

The use of chlorine stable isotopes (35Cl and 37Cl) can help to constrain natural processes that involve chlorine species with different oxidation states. Theoretical studies based on thermodynamic and quantum mechanical approaches predict large isotope fractionation during redox reactions but to date, these reactions have not been studied experimentally.Here, we explore the chlorine isotope fractionation during the oxidation of hydrated Cl− (redox state of -I) to Cl2 (redox state of 0) at 25°C and at 0°C. Our apparatus consists of a sealed glass reactor where liquid HCl is mixed with liquid H2O2, a strong oxidant. Following complex reaction pathways, this mixture ultimately leads to the oxidation of Cl− and to the formation of Cl2 gas. As long as it is degassing, the Cl2 gas is flushed out of solution using N2 as a vector-gas from the glass-reactor to a potassium hydroxide (KOH) solution (pH 14) where it disproportionates into soluble species: Cl− and ClO−. After each experiment, the chlorine isotopic composition was measured in the recovered KOH-trap solution, as well as in the residual HCl solution. Consistent with theoretical predictions, the produced Cl2 gas is always enriched in the heavier 37Cl as compared to the initial Cl-reservoir.The following isotope fractionation factors are obtained:At 0°C the isotopic fractionation 1000lnα(Cl2−Cl−) is 4.51 (+1.65/−0.49)‰At 25°C the isotopic fractionation 1000lnα(Cl2−Cl−) is 3.94 (+0.69/−0.18)‰.From the obtained data it is suggested that the production of Cl2 gas in our experiments is best described by a closed-system distillation. Our results are in agreement with published theoretical ab-initio calculations.