Disproportionation of Aqueous Sulfur and Sulfide: Kinetics of Polysulfide Decomposition

Thermodynamically, aqueous polysulfide solutions are unstable, and sulfur, dissolved as polysulfide species, Sx2-, x = 2−5, will decompose to thiosulfate, S2O32- in accord with the net consumption of dissolved sulfur: Sdissolved + OH- → 1/4S2O32− + 1/2HS- + 1/4H2O. The distribution of species, and measured and modeled rate of polysulfide decomposition, are investigated for a wide range of polysulfide solution compositions from 25 to 85 °C. The observed decomposition of sulfur concentration, CS, is consistent with an activation energy for sulfur decomposition of 104(±5) kJ mol-1 in accord with the following rate of sulfur loss: −dCS/dt = −kf[S52-][OH-]/[HS-]; kf = 10-(10.7-5430/T(K)), where CS is the concentration of dissolved zerovalent sulfur distributed among all polysulfide species. At temperatures up to 85 °C, polysulfide solutions are extremely stable under conditions of high polysulfide concentration (>5 m K2S4) and low KOH concentrations. Under these conditions, zerovalent sulfur dissolved in these solutions should be stable on the order of years.