A new approach based on rapid, chemical derivatization in a single phase was used to determine the disproportionation constants and the underlying thermodynamics of inorganic polysulfides in aqueous solutions. This method resolves the dispute over the existence of hexasulfide in aqueous solutions and establishes the presence of even higher polysulfide chains in water. The Gibbs free energies of formation (G(Sn)(o)2-) for the polysulfide species are 77.4, 71.6, 67.4, 66.1, 67.2, 70.5, and 73.6 kJ/mol for n = 2-8, respectively. Our approach is based on single phase, fast methylation of polysulfides with methyl trifluoromethanesulfonate (methyl triflate) and subsequent determination of the dimethylpolysulfides by HPLC. Two independent methods were used in order to confirm quantitative equivalence between the observed distribution of dimethylpolysulfides and the polysulfide distribution in the water: (i) Kinetic studies of each competing reaction step showed that the kinetics of the derivatization are faster than each of the competing reactions that may lead to disproportionation and deviation of the observed distribution of dimethylpolysulfides from that of the aqueous polysulfides. (ii) Determination of isotope mixing during the derivatization of a mixture of two solutions, one containing polysulfide of natural isotopic distribution and the second containing 34S-rich polysulfide revealed that polysulfide mixing during derivatization is rather low. The systematic error due to redistribution of pentasulfide during derivatization is 3% based on isotope dilution tests and less than 5% of total zero-valent sulfur based on kinetic considerations.

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We present the first observations on the occurrence of inorganic polysulfides in an oxygen rich aquatic system. Inorganic polysulfides were found both in the hypolimnion and the epilimnion of a freshwater lakesLake Kinneret. The presence of these compounds in oxic systems resolves the enigma concerning the mechanism of formation of dimethyl disulfide, dimethyltrisulfide, and dimethyltetrasulfide in oxygen rich aquatic systems and marine water. The abundance of low molecular weight organic and inorganic polysulfides relative to the a priori postulated dominance of the pentasulfide family is explained by the low level of polysulfides in oxygen-rich aquatic systems. Thermodynamic calculations show that for trace levels of reduced sulfur compounds, dimethyl disulfide becomes the dominant polysulfide form.

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OCS formation by the reaction of inorganic polysulfides with carbon monoxide, which are both abundant in natural aquatic systems, was studied as a model abiotic route for OCS formation in the dark. The net OCS accumulation rate was a function of a bimolecular formation reaction and simultaneous OCS hydrolysis kinetics. The reaction of polysulfides with CO in the dark was found to be first order with respect to CO concentration and first order with respect to the molar sum of the polysulfide species generated by the disproportionation of the dissolved polysulfide precursors. The pH dependence of the OCS production rate was controlled by the pH-dependent disproportionation of polysulfide precursors. Lower temperatures, intermediate redox potentials, and moderately basic pH conditions increase the steady-state concentration of OCS. The speciation of polysulfides in aqueous solutions is still disputed. Some authors claim that hexasulfide is one of the dominant species while others believe that pentasulfide is the largest sulfide species in aqueous systems. Despite the disagreement between different models for speciation of polysulfides, the proposed rate law agreed very well with the thermodynamic data based on four and on five polysulfide species, with only minor differences in the preexponential kinetic coefficients.

Page 1867. The expression for OCS hydrolysis rate and the activation energy in the second paragraph were incorrectly written. The expression should be k h,tot ) (6.52) × 10 12 e (-11988/T) + (1.39) × 10 18 e -11591/T [OH -] corresponding to activation energies of 99.67 and 96.37 kJ/mol. There was no propagation of this error in the paper. Only the data for 5 and 30°C, which were directly derived in our tests, were used for calculation of OCS formation rate (at the corresponding temperatures). We thank X.-X. Li and X.-H. Wei of the Peking University, Beijing, for bringing the error to our attention.

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Inorganic polysulfides are important intermediates in the formation of dimethylpolysulfides and possibly other volatile sulfur compounds of environmental significance. Currently, direct determination of these ions in the concentration range of natural systems is practically impossible, particularly under oxic conditions. Polysulfide quantification by derivatization with methyl iodide or d6-methyl iodide is emerging as a valuable alternative method for studies of polysulfide formation in natural systems. This manuscript presents detailed studies aimed at the evaluation of this method. We determined the conversion of the inorganic polysulfides to dimethylpolysulfides by methylation with methyl iodide. Close to 100 per cent of the molar concentration of polysulfide salts were converted to organic polysulfides for very low concentrations of dissolved polysulfide solutions, but only a small recovery was obtained for high concentrations of polysulfide precursors or when the solubility limit was exceeded. The recovery of polysulfides based on the calculated dissolved polysulfide concentration exceeds 1,000 per cent for very low dissolved concentrations of polysulfides. This unexpected dependence is attributed to continuous inorganic polysulfide formation from hydrogen sulfide and sulfur precipitate concurrent with, and in fact driven by, the methylation process.