Determining the Oxygen Isotope Equilibrium Fractionation Between Sulfite and Water

The kinetics of oxygen isotope exchange between water and sulfite have been well studied. However, the scientific community still lacks reliable information for the oxygen isotope equilibrium fractionation between these two species. One reason for the lack of this data is the technical difficulty of extracting sulfite from solution for oxygen isotope analysis. The experimental challenge arises from the fact that sulfite coexists in solution with other S(IV) species, i.e. sulfur dioxide, bisulfite, and pyrosulfite. Extraction techniques such as precipitation of sulfite as barium sulfite or silver sulfite by the addition of barium chloride or silver nitrate induce conversion of non-sulfite species, i.e. sulfur dioxide and pyrosulfite, to sulfite. Each S(IV) species is expected to have a different oxygen isotope equilibrium with water. The wholesale conversion of all S(IV) species to sulfite may induce incorporation of additional oxygen from water and kinetic oxygen isotope effects. Such a conversion would overprint the oxygen isotope signature of sulfite in equilibrium with water and compromise experimental results. We report the first results from our experimental approach to overcome this limitation by using a solid sulfite phase (Ag2SO3, BaSO3, Na2SO3) that is in equilibrium with sulfite in solution. Our goal is to determine both the oxygen isotope equilibrium fractionation between water and sulfite, as well as the isotope equilibrium fractionation between solids and dissolved sulfite. Elucidation of the oxygen isotope equilibrium fractionation between sulfite and water would enable a better understanding of reductive and oxidative sulfur cycling, where sulfite plays a pivotal role as intermediate in both microbial and abiotic processes.