Thermodynamic constants of formation of disulfide anion in aqueous solutions

Abstract

This study is aimed at precise measurement of thermodynamic constants of disulfide (S22−) formation. It was demonstrated that these constants, K, for formation of longer polysulfides for reaction between hydrogen sulfide and orthorhombic cyclooctasulfur (α-S8) measured by independent techniques (potentiometry, spectrophotometry, chromatography) are in good agreement. For the most abundant polysulfides, S42−, S52− and S62−, the reported ranges of pK values are ≤0.72 pK units. On the other hand, pK2 values of the formation of S22− in this reaction measured by different techniques span from 11.46 to 14.43. The main difficulty in measurement of K2 is low abundance of disulfide in the solutions saturated with respect to α-S8. In this work, we applied chromatographic analysis for the samples, which are highly undersaturated with respect to α-S8. The results show that robust determination of K2 may be achieved at the molar ratio of S0 to S(II) ≤ 0.064. At these conditions the value of pK2 at 25 °C was found to be 12.77 ± 0.08. Based on this value, the standard Gibbs free energy of S22− formation was calculated. The standard enthalpy of formation and molar entropy were derived from the data obtained in experiments at the temperature range of 10 °C to 70 °C. Calculations based on these thermodynamic values show that while disulfide accounts for only a minor fraction of polysulfide zero-valent sulfur at saturation with α-S8, it can become the most abundant polysulfide at low S0 to S(II) molar ratios. In marine sediments and water columns low molar polysulfide S0 to S(II) ratio may exist in hydrogen sulfide-rich zero valent sulfur-poor systems as well as, due to kinetic constraints, in the zero valent sulfur-rich systems. Results of our work suggest that at the molar ratio of polysulfide S0 to S(II) > 0.064, the calculation of disulfide concentration should be based on concentrations of hydrogen sulfide and other polysulfides and the set of thermodynamic constants provided in this work. At the lower polysulfide S0 to S(II) ratios, the disulfide concentration should be either measured directly or calculated in case that it is below the detection limit of quantification method.