The pK1* for the dissociation of H2S in various ionic media

The stoichiometric pK1∗ for the dissociation of H2S has been determined from emf measurements in NaCl from 0.1 m to saturation at 5, 25 and 45°C and in KCl solutions at 5 and 25°C. The values of pK1∗ were also measured in NaCl with small amounts of added MgCl2 and CaCl2 to I = 6.0 m at 25 °C. The results were used to determine Pitzer coefficients for the interactions of HS− ions with Na+, K+, Mg2+ and Ca2+. Values of the pK1∗, estimated using these coefficients, were found to be in excellent agreement with the measured values. Estimates of pK1∗ in seawater and Dead Sea water calculated from the Pitzer coefficients were in excellent agreement with measured values from 5 to 25°C. Thus, it is possible to characterize the thermodynamics of the first dissociation of H2S in most natural waters from 0 to 50°C and I = 0 to 6 m.