Disproportionation of Aqueous Sulfur and Sulfide:  Kinetics of Polysulfide Decomposition

Thermodynamically, aqueous polysulfide solutions are unstable, and sulfur, dissolved as polysulfide species, S x 2-, x = 2−5, will decompose to thiosulfate, S 2 O 3 2- in accord with the net consumption of dissolved sulfur: S dissolved + OH - → 1 / 4 S 2 O 3 2 − + 1 / 2 HS - + 1 / 4 H 2 O. 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, C S, is consistent with an activation energy for sulfur decomposition of 104(±5) kJ mol -1 in accord with the following rate of sulfur loss: −d C S /d t = − k f [S 5 2- ][OH - ]/[HS - ]; k f = 10 -(10.7-5430/ T (K)), where C S 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 K 2 S 4 ) and low KOH concentrations. Under these conditions, zerovalent sulfur dissolved in these solutions should be stable on the order of years.