Kinetics of the chemical oxidation of polysulfide anions in aqueous solution.

The kinetic properties of the chemical oxidation of aqueous polysulfide solutions have been studied in phosphate-buffered systems at pH 7-12, at temperatures between 20 and 40 degrees C, and ionic strength between 0.05 and 0.50 M. Polysulfide solutions were mixed with a buffer solution of known dissolved oxygen concentration, after which the decrease in the oxygen concentration of the solution was measured in time. The rate of oxygen consumption can be described by the empirical relation d[O2]/dt= -k[Sx(2-)][O2](0.59) . The reaction rate constant k is moderately dependent on pH and goes through a maximum at pH 10. The rate of oxygen consumption for polysulfide solutions is approximately four times higher than for sulfide solutions. At pH values below 9, reaction products were formed according to Sx(2-)+3/2O2-->S2O3(2-)+(x-2)S(0) . At pH values higher than 9, more thiosulfate and additional sulfide were formed, which is attributed to the low chemical stability of the sulfur of oxidation state zero, formed upon polysulfide oxidation. Our results strongly suggest that hydrolysis of this 'nascent' elemental sulfur to HS- and S2O3(2-) is faster than hydrolysis of crystalline inorganic sulfur or colloidal particles of biologically produced sulfur, and has a significant contribution already at 30 degrees C and pH 10.