Determination And Kinetic Analysis Of Sulfur-Oxygen Species In Reaction Process

Thiosulfate(S₂O₃^2-) was chosen as the sulfur(-II) species which has been studied in this thesis. Basing on their hydrolysis and oxidation reactions, thiosulfate itself and its oxidation products such as tetrathionate(S₄O₆^2-) and pentathionate(S₂O₃^2-), were studied by high performance liquid chromatography(HPLC) and capillary electrophoresis(CE) for sulfur-oxygen species distribution and kinetics. Mechanisms of the reactions were proposed based on the results, and the simulation work fits the experimental data well including all the characters of kinetics.Thiosulfate, sulfate (SO₄^2-), dithionate (S₂O₆^2-), trithionate (S₃O₆^2-), tetrathionate (S₄O₆^2-), pentathionate(S₅O₆^2-), hexathionate(S₆O₆^2-) and heptathionate(S7O₆^2-) were all baseline separated by HPLC and CE methods with outstanding qualitative and quantitative determination, thus they were available for kinetic analysis.Colloid sulfur(S₈), sulfite/bisulfite (H₂SO₃/HSO₃-/SO₃^2-), hydrogen sulfide (H₂S/HS^-), thiosulfate/bithiosulfate (HS₂O₃-/S₂O₃^2-), polysulfide(HSn-), HSnO₃- and polythionates(SnO₆^2-) were confirmed in the hydrolysis of thiosulfate in strong acidic solutions within the pH range 1.2 - 2.5. It was found that degradation was first order with respect to thiosulfate and the reaction rate constant was 8.8 x 10-4 min-1.In the H₂O^2-S₂O₃^2- reaction, pH is the key parametre which controls not only the reaction rate, but also the distribution of polythionates. Intrestingly, because of the hydrolysis of tetrathionate in alkaline solutions, pH 6.0-7.0 is the watershed of the maximum formation of tetrathionate in the oxidation course. It was found that the reaction kinetic was first order with respect to each of the reactants and, the mean reaction rate constant was 0.025 M^-1s^-1.In the hydrolysis of tetrathionate in alkaline solutions, thiosulfate, dithionate, trithionate, and sulfate were confirmed as terminal products with the formation of pentathionate as the intermediate. The stoichiometry equation was 4S₄O₆^2-+8OH^-→6S₂O₃^2-+S₃O₆^2-+SO₄^2-+4H₂O. The rate of hydrolysis of tetrathionate was found to be first order with respect to tetrathionate and hydroxide, and rate constant was 0.115 M^-1s^-1. Contrary to previous studies, the autocatalytic effect of thiosulfate was not found in the hydrolysis of tetrathionate.The concentration of hydrogen peroxide has no measurable effect on the degradation of tetrathionate in the H₂O^2-S₄O₆^2- reaction within the initial conditions used in the experiments. The reaction system showed obvious pH dependence and first order reaction with respect to each of the reactants with the reaction rate constant 0.106 M^-1s^-1 which was close to the sole hydrolysis of tetrathionate at alkaline solutions. Furthermore, higher pH and lower concentration of oxidant favor faster formation of thiosulfate and also its maximum concentration. Based on these observations, it is believed that hydrolysis of tetrathionate with hydroxide is the first step in the H₂O_<sup>2-S₄O₆^2- reaction system.Thiosulfate, tetrathionate and hexathionate were detected as intermediates in the hydrolysis of pentathionate in alkaline solutions, and the later two species underwent further hydrolysis to produce thiosulfate and sulfate, etc. Based on the quantitative analysis of the system and the conservation of sulfur atoms, more precise stoichiometry was proposed as 7S₅O₆^2-+20OH^-→16S₂O₃^2-+SO₄^2-+2S+10H₂O. The reaction kinetic was first order with respect to pentathionate and hydroxide with the rate constant 5.67 M^-1s^-1.On the basis of the experimental findings in H₂O_<sup>2-S2O₃^2- reaction and H₂O_<sup>2-S₄O₆^2- reaction systems, the hydrolysis of tetrathionate with hydroxide was ascertained to be the first step instead of the bimolecular reaction between tetrathionate and hydrogen peroxide to produce two molecular of HOS₂O₃- for the degradation of tetrathionate. A new mechanism for the degradation of HOS₂O₃- was also proposed by introducing sulfoxylate HOS₃O₃^- and soluble sulfur species, S(OH)2 and HS(OH).Detailed mechanisms and kinetic parameters were proposed for every reaction system, and some of the parts were just the same with each other since the similarity of the species and kinetics involving in the reaction courses. Madonna Software was used to simulate the experimental data by using the mechanisms and parameters, which gave coincidence results.