Apparent molal heat capacities and volumes of aqueous hydrogen sulfide and sodium hydrogen sulfide near 25 °C: the temperature dependence of H2S ionization

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Abstract
A flow microcalorimeter and vibrating tube densimeter were used to obtain apparent molal heat capacities and volumes of aqueous NaHS and Na2S from 0.1 to 1.0 mol kg−1 and of aqueous H2S from 0.03 to 0.08 mol kg−1 at 10, 25, and 40 °C. Standard state heat capacities and volumes for H2S and HS− were obtained by extrapolation to infinite dilution. Combining these results with 25 °C enthalpy data yields an expression for the temperature dependence of the equilibrium constant for H2S neutralization at pressures near those of steam saturation, H2S + OH− = HS− + H2O, log K1b = 19.84 + 930.8/T−2.800 In T, with an estimated uncertainty of ±0.47 at 300 °C. The heat capacity data for bulk aqueous Na2S suggest that the relative concentration of S2− at these molalities is small.