High temperature dissociation constants of HS- and the standard thermodynamic values for S2-

The magnitude of the dissociation constant for HS- = S2- + H+ at saturation vapor pressure, 25 degreesC, and infinite dilution is log K-2a(-) = 14.0 /- 0.2. The value of DeltaG(2a)(-) = (79.9 +/- 2.3) kJ.mol(-l), calculated from this log K-2a(-), was combined with the calorimetric enthalpy DeltaN(2 a)(-) = (54.8 +/- 1.7) kJ.mol(-l), to obtain the entropy of dissociation, D eltaS(2a)(-) = (-84.2 +/- 30.0) J.K-1.mol(-1). Consistent standard thermody namic values for the aqueous species S2- Obtained from these data are as fo llows: Delta (f)G(o) = (91.9 +/- 2.3) kJ.mol(-l), Delta H-f(o) = (38.7 +/- 1.8) kJ.mol(-l), and S-o = (-16.0 +/- 10.0) J.K-l.mol(-l). The standard hea t capacity of S2-, C-p(o) = (-284 +/- 60) J.K-l.mol(-l), was calculated fro m a correlation between C-p(o) and S-o. The heat capacity of dissociation, DeltaC(p,2a)(-) = (-192 +/- 60) J.K-l.mol(-l), was calculated from C-p(o) f or HS- and this C-p(o) for S2-. The change in R 1n K-2a with temperature wa s computed up to 300 degreesC and compared with experimental data from the literature. The standard potential for S(rhomb) + 2e --> S2- at 25 degreesC and infinite dilution is E- = (-0.476 +/- 0.004) V. Published experimental data with the more negative log K-m,K-2a = -17 were rejected because, whil e valid in, for example, (12 to 17) mol.L-l NaOH solutions, they could not be calculated with confidence to the standard condition of infinite dilutio n.