High Temperature Dissociation Constants of HS^- and the Standard Thermodynamic Values for S²^-

Abstract

The magnitude of the dissociation constant for HS- = S2- + H+ at saturation vapor pressure, 25 °C, and infinite dilution is log K2a⊖ = −14.0 ± 0.2. The value of ΔG2a⊖ = (79.9 ± 2.3) kJ·mol-1, calculated from this log K2a⊖, was combined with the calorimetric enthalpy ΔH2a⊖ = (54.8 ± 1.7) kJ·mol-1, to obtain the entropy of dissociation, ΔS2a⊖ = (−84.2 ± 30.0) J·K-1·mol-1. Consistent standard thermodynamic values for the aqueous species S2- obtained from these data are as follows: ΔfG° = (91.9 ± 2.3) kJ·mol-1, ΔfH° = (38.7 ± 1.8) kJ·mol-1, and S° = (−16.0 ± 10.0) J·K-1·mol-1. The standard heat capacity of S2-, Cp° = (−284 ± 60) J·K-1·mol-1, was calculated from a correlation between Cp° and S°. The heat capacity of dissociation, ΔCp,2a⊖ = (−192 ± 60) J·K-1·mol-1, was calculated from Cp° for HS- and this Cp° for S2-. The change in R ln K2a with temperature was computed up to 300 °C and compared with experimental data from the literature. The standard potential for S(rhomb) + 2e → S2- at 25 °C and infinite dilution is E⊖ = (−0.476 ± 0.004) V. Published experimental data with the more negative log Km,2a = −17 were rejected because, while valid in, for example, (12 to 17) mol·L-1 NaOH solutions, they could not be calculated with confidence to the standard condition of infinite dilution.