REDOX CHEMISTRY OF H2S OXIDATION IN THE BRITISH-GAS STRETFORD PROCESS .1. THERMODYNAMICS OF SULFUR-WATER SYSTEMS AT 298K

The thermodynamics of aqueous sulphur-water systems are summarized in the form of potential-pH diagrams, calculated from recently reported critically assessed standard Gibbs energies of formation of the species considered. However, there is convincing evidence from the literature that a value of pK(a)(HS-) = 17-19 is appropriate, whereas a value of 13 is widely accepted; hence, the higher value of 19, corresponding to DELTAG(f)0(S2-) = 120.5 kJ mol-1, was used in these calculations, rather than DELTAG(f)0(S2-) = 86.31 kJ mol-1 quoted in the main data source. Under ambient conditions, only - 2 (sulphide), 0 (elemental sulphur) and + 6 (sulphate) oxidation states are thermodynamically stable in water, which is predicted to be oxidized by peroxodisulphate (H2S2O8/SO82-) and peroxomonosulphate (HSO5-/SO52-). However, when sulphate is excluded from the calculations to allow for the large energy of activation/slow kinetics of its formation from sulphide, then other sulphoxy species appear on the diagram for what is then a metastable system. Similarly, if all sulphoxy species (i.e. any species with oxidation states > 0) are excluded, then polysulphide ions (S(n)2-, 2 less-than-or-equal-to n less-than-or-equal-to 5) have areas of predominance at high pH, each with a narrow potential window of predominance. Hence, this information is complemented with S(n)2-/HS- activity-potential diagrams at pH 9 and 14. Some species have no area of stability even on the metastable diagrams. Hence, potential-pH diagrams are also presented for the sulphite-dithionite system (excluding elemental sulphur), and that involving peroxomonosulphate (HSO5-/SO52-) in place of peroxodisulphate (H2S2O8/SO82-).