Experimental estimation of the bisulfite isomer quotient as a function of temperature: Implications for sulfur isotope fractionations in aqueous sulfite solutions

Bisulfite (HSO3−) and sulfite (SO32−) compounds play key roles in numerous geochemical and biochemical processes extending from the atmosphere to the subseafloor biosphere. Despite decades of spectroscopic investigations, the molecular composition of HSO3− in solution remains uncertain and, thus, the role of bisulfite in (bio)chemical and isotope fractionation processes is unclear. We report new experimental estimates for the bisulfite isomer quotient (Qi = [(HO)SO2−]/[(HS)O3−]; [] = concentration) as a function of temperature from the interpretation of Raman spectra collected from aqueous NaHSO3 solutions contained in fused silica capsules. In pure NaHSO3 solutions (1Na+:1HSO3−, stoichiometric) over [NaHSO3] = 0.2-0.4 m (moles/kg H2O), the following relationship is obtained: ln(Qi)=878.59(±32.98)T-1.9642(±0.1081),where T = 278-358 K (5-85 °C) (based on 50 determinations; additional significant figures are provided to avoid rounding errors). This relationship suggests that the minor isomer, (HS)O3−, may comprise 23-38% (±3%) of the mole fraction of HSO3− over 5-85 °C, respectively, which is higher in relative abundance than has generally been understood previously. We additionally provide estimates for Qi in NaHSO3 solutions containing a total ionic strength of µ = 1.0 m (0.2 m NaHSO3 + 0.8 m NaCl) and different pH (3.3-4.5), but do not appear to resolve any significant differences in Qi as a function of these additional variables. These new values of Qi are employed to re-assess the bulk sulfur isotope fractionations among bisulfite and other S(IV) compounds as a function of temperature, which appear to be highly dependent on the amount of (HS)O3− present. Our new constraints on the bisulfite isomer quotient may allow for a detailed assessment of the molecular composition and isotope mass balance of S(IV) solutions containing HSO3−, and may be useful in the further investigation of the mechanisms and isotope fractionations associated with a number of processes that involve bisulfite compounds. These may include the intracellular enzymatic transformations of sulfite and bisulfite compounds that occur as part of dissimilatory sulfate reduction, which is a major and geologically important form of anaerobic respiration.