Thermodynamic constants of formation of disulfide anion in aqueous solutions

This study is aimed at precise measurement of thermodynamic constants of disulfide (S-2(2-)) formation. It was demonstrated that these constants, K, for formation of longer polysulfides for reaction between hydrogen sulfide and orthorhombic cyclooctasulfur (alpha-S-8) measured by independent techniques (potentiometry, spectrophotometry, chromatography) are in good agreement. For the most abundant polysulfides, S-4(2-), S-5(2-) and S-6(2-), the reported ranges of pK values are <= 0.72 pK units. On the other hand, pK(2) values of the formation of S-2(2-) in this reaction measured by different techniques span from 11.46 to 14.43. The main difficulty in measurement of K-2 is low abundance of disulfide in the solutions saturated with respect to alpha-S-8. In this work, we applied chromatographic analysis for the samples, which are highly undersaturated with respect to alpha-S-8. The results show that robust determination of K-2 may be achieved at the molar ratio of S-0 to S(II) <= 0.064. At these conditions the value of pK(2) at 25 degrees C was found to be 12.77 +/- 0.08. Based on this value, the standard Gibbs free energy of S-2(2-) 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 degrees C to 70 degrees 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 alpha-S-8, it can become the most abundant polysulfide at low S-0 to S(II) molar ratios. In marine sediments and water columns low molar polysulfide S-0 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 S-0 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 S-0 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. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study aims to accurately measure the thermodynamic constants of disulfide formation. The constants for the formation of longer polysulfides were found to be in good agreement when measured using different techniques. The study also demonstrated that the concentration of disulfide can be reliably determined under certain conditions, and the thermodynamic values obtained can be used to calculate the standard Gibbs free energy, standard enthalpy, and molar entropy of disulfide formation.