- Nonaqueous Heterogeneous Oxidation of Sulfur Dioxide
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The nonaqueous oxidation of SO2 on various solid catalysts was investigated in the presence of oxygen and ozone.The amount of product formed was investigated as a function of catalyst mass and the duration of experiment.For alumina catalysts a surface saturation value of approximately 0.5 mg of SO42-/m2 of alumina was found for a variety of different aluminas independent of surface area, surface pH, and water content.This result corresponds to a monolayer of SO42- if a surface area of 0.3 nm2/SO42- is assumed.For an activated carbon catalyst a surface saturation value of 0.25 mg of SO42-/m2 of carbon was found under wet nonaqueous conditions.For the activated carbon catalysts, unlike the alumina catalysts, the presence of at least a few layers of readily desorbable water is a significant variable for the oxidation of sulfur dioxide.
- Halstead, Judith Ann,Armstrong, Roger,Pohlman, Bruce,Sibley, Scott,Maier, Robert
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- Kinetics and mechanism of the oxidation of sulfite by chlorine dioxide in a slightly acidic medium
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The sulfite-chlorine dioxide reaction was studied by stopped-flow method at I = 0.5 M and at 25.0 ± 0.1 °C in a slightly acidic medium. The stoichiometry was found to be 2 SO32- + 2-ClO2 + H2O → 2SO42- + Cl- + ClO 3- + 2H+ in ClO2 excess and 6SO 32- + 2·ClO2 → S2O 62- + 4SO42- + 2Cl- in total sulfite excess ([S(IV)] = [H2SO3] + [HSO 3-] + [SO32-]). A nine-step model with four fitted kinetic parameters is suggested in which the proposed adduct SO3C1O22- plays a significant role. The pH-dependence of the kinetic traces indicates that SO32- reacts much faster with ClO2 than HSO3- does.
- Horvath, Attila K.,Nagypal, Istvan
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- Kinetics and Mechanism of the Chlorite-Thiourea Reaction in Acidic Medium
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The kinetics of the reaction between chlorite and thiourea in acidic medium have been studied.The stoichiometry of the reaction for pH > 3 is 2ClO2- + SC(NH2)2 + H2O --> 2Cl- + OC(NH2)2 + SO42- + 2H+; for pH + and CO2.The reaction is characterized by an induction period after which chlorine dioxide is produced.For ->/ - + SC(NH2)2 + H+ --> HOCl + HOSC(NH)NH2.Computer simulation using this mechanism gives good agreement with experiment.
- Epstein, Irving R.,Kustin, Kenneth,Simoyi, Reuben H.
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- Aqueous photochemical reactions of chloride, bromide, and iodide ions in a diode-array spectrophotometer. Autoinhibition in the photolysis of iodide ions
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The aqueous photoreactions of three halide ions (chloride, bromide and iodide) were studied using a diode array spectrophotometer to drive and detect the process at the same time. The concentration and pH dependences of the halogen formation rates were studied in detail. The experimental data were interpreted by improving earlier models where the cage complex of a halogen atom and an electron has a central role. The triiodide ion was shown to exert a strong inhibiting effect on the reaction sequence leading to its own formation. An assumed chemical reaction between the triiodide ion and the cage complex interpreted the strong autoinhibition effect. It is shown that there is a real danger of unwanted interference from the photoreactions of halide ions when halide salts are used as supporting electrolytes in spectrophotometric experiments using a relatively high intensity UV light source.
- Kalmar, Jozsef,Doka, Eva,Lente, Gabor,Fabian, Istvan
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- Density fingering in acidity fronts: Effect of viscosity
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We have studied experimentally how the change in solution viscosity affects the convective instability in the acid-catalyzed chlorite oxidation of tetrathionate. The increase in viscosity leads to the decrease of instability; the wavelength of the emerging patterns increases while the fingers evolve slower. We have observed only a minor change in the longtime behavior: the region where a single cell develops slightly increases and the frequency of the occurrence of tip splitting decreases on increasing viscosity.
- Rica, Tamás,Horváth, Dezso,Tóth, ágota
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- Multicomponent convection in the chlorite-tetrathionate reaction
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The effect of chemical composition on the density fingering arising in the acid-catalyzed reaction of chlorite and tetrathionate ions is investigated experimentally. The increase in the concentration of reactants increases the region of convective instabi
- Bánsági Jr., Tamás,Horváth, Dezso,Tóth, ágota
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- Free Radical Reactions Involving Cl(.), Cl2(-.), and SO4(-.) in the 248 nm Photolysis of Aqueous Solutions Containing S2O8(2-) and Cl(-)
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Rate constants and other properties are reported for photochemical reactions initiated by laser photolysis (248 nm) of S2O8(2-)(aq) in acidic solutions containing Cl(-)(aq) at 297 +/- 2 K. The absorption spectra of SO4(-.)(aq) and Cl2(-.)(aq) are measured and compared to literature values. Equilibrium constants and rate constants involving Cl(.) (aq), Cl2(-.)(aq), H2O, SO4(-.)(aq), and S2O8(2-)(aq) are determined by analyzing the formation and decay of Cl2(-.)(aq) and SO4(-.)(aq) in a self-consistent chemical mechanism. For the first time, the anticipated pH dependence is observed unambiguously in the pseudo-first-order decay rate constant of Cl2(-.). The rate constants for the reactions Cl(.)(aq) + Cl2(-.)(aq) -> Cl2(aq) + Cl(-)(aq) and Cl2(-.)(aq) + Cl2(-.)(aq) -> Cl2(aq) + 2Cl(-)(aq) are determined; rate constants for the latter are measured as a function of temperature and ionic strength. In addition, the ionic strength dependence of the reaction SO4(-.)(aq) + Cl(-)(aq) -> SO4(2-)(aq) + Cl(.)(aq) is reported. Rate constants for the reactions of Cl(.)(aq) and Cl2(-.)(aq) with S2O8(2-)(aq) are reported for the first time. The present and previous (when available) experimental results are generally in good agreement within the context of the comprehensive chemical mechanism for this system.
- Yu, Xiao-Ying,Bao, Zhen-Chuan,Barker, John R.
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- Pattern formation during polymerization of acrylamide in the presence of sulfide ions
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When polyacrylamide gel is prepared from a mixture of acrylamide, N,Na?2-methylene-bisacrylamide and triethanolamine with a peroxodisulfate initiator in the presence of sulfide, spatial pattern formation occurs in the gel if water is spread on top of the gel after it solidifies. These patterns, typically arrays of spots or stripes, persist for many hours. Analysis of the effects of temperature, composition, stirring, and other factors suggests that the pattern formation occurs during polymerization due to convection driven by the highly exothermic polymerization reaction. The role of sulfide ion, which is essential for the pattern formation, appears to be to terminate the polymer chain elongation while maintaining the overall exothermic chain reaction. ? 1999 American Chemical Society.
- Orbaì?n, Mikloì?s,Kurin-Csoì?rgei, Krisztina,Zhabotinsky, Anatol M.,Epstein, Irving R.
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- Oscillatory peroxodisulfate reduction on Pt and Au electrodes under high ionic strength conditions, caused by the catalytic effect of adsorbed OH
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Four electrochemical oscillations of different types (named oscillations ?±, ?2, ?3, and ??), which are different from previously reported Frumkin-type oscillations in low ionic strength electrolytes, appear for peroxodisulfate (S2O82-) reduction on Pt and Au electrodes under high ionic strength conditions. Impedance measurements for oscillation ?3 that appears in a region of the most positive potentials have shown that it is classified as hidden negative differential resistance (hidden-NDR or HNDR) oscillators. Detailed electrochemical measurements have suggested that the NDR arises from a decrease (with a negative potential shift) in the surface coverage of adsorbed OH, which acts as a catalyst for the dissociative adsorption of S2O82- (the first step of its reduction). The NDR is hidden by the desorption (with the negative potential shift) of adsorbed sulfate (SO42-) produced by the dissociative adsorption and reduction of S2O82-, which works as a site-blocking agent. Mathematical simulation based on this model has reproduced the appearance of oscillation ?3. The result supports our previously proposed concept of autocatalytic effect of adsorbed OH and catalytic effect of adsorbed halogen on the dissociative adsorption of H2O2, showing the generality of the electrocatalytic effect of adsorbed OH and halogen.
- Nakanishi, Shuji,Sakai, Sho-Ichiro,Hatou, Michiru,Mukouyama, Yoshiharu,Nakato, Yoshihiro
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- Detailed kinetics and mechanism of the oxidation of thiocyanate ion (SCN-) by peroxomonosulfate ion (HSO5-). Formation and subsequent oxidation of hypothiocyanite ion (OSCN-)
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The haloperoxidase-catalyzed in vivo oxidation of thiocyanate ion (SCN -) by H2O2 is important for generation of the antimicrobial hypothiocyanite ion (OSCN-), which is also susceptible to oxidation by strong in vivo oxidizing agents (i.e., H2O 2, OCl-, OBr-). We report a detailed mechanistic investigation on the multistep oxidation of excess SCN- with peroxomonosulfate ion (HSO5- in the form of Oxone) in the range from pH 6.5 to 13.5. OSCN- was detected to be the intermediate of this reaction under the above conditions, and a kinetic model is proposed. Furthermore, by kinetic separation of the consecutive reaction steps, the rate constant of the direct oxidation of OSCN- by HSO 5- was determined: k2 = (1.6 ± 0.1) × 102 M-1 s-1 at pH 13.5 and k 2H = (3.3 ± 0.1) × 103 M -1 s-1 at pH 6.89. A critical evaluation of the estimated activation parameters of the elementary steps revealed that the oxidations of SCN- as well as the consecutive OSCN- by HSO 5- are more likely to proceed via 2e--transfer steps rather than 1e- transfer.
- Kalmár, József,Lente, Gábor,Fábián, István
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- POLYMER-SUPPORTED METAL COMPLEXES AS CATALYSTS FOR THE OXIDATION OF THIOSALTS BY MOLECULAR OXYGEN: V. THE POLYTHIOSEMICARBAZIDE-Cu(II) COMPLEX.
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Polythiosemicarbazide obtained by the condensation of N,N prime -diaminopiperazine with methylene bis(4-phenyl isothiocyanate) has been complexed with copper(II) and used to catalyse the oxidation of S//2O//3**2** minus and S//4O//6**2** minus by molecular oxygen in a batch reactor, employing the catalyst as a 2% (w/v) slurry. The oxidation rate of S//2O//3**2** minus was several times higher than that of S//4O//6**2** minus , while the rates of both the thioanions were significantly influenced by the thioanion concentration, the pH and the temperature. The product distribution pattern in batch oxidation may be modelled by a three-stage reaction mechanism. In stage I, a part of the S//2O//3**2** minus in the feed is rapidly oxidized to S//4O//6**2** minus by an electron-transfer redox reaction with the polymeric Cu(II) complex, while in stage II S//2O//3**2** minus and S//4O//6**2** minus are oxidized to SO//4**2** minus by O//2 on the catalyst by series-parallel reaction paths. In stage III, which begins with the disappearance of S// 2O//3**2** minus , S//4O//6**2** minus is oxidized by O//2 to SO//4**2** minus on the catalyst.
- Chanda,Grinshpun,O'Driscoll,Rempel
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- A three-variable model for the explanation of the supercatalytic effect of hydrogen ion in the chlorite-tetrathionate reaction
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It has been shown that not only the slow direct but also the indirect (HOCl-catalyzed) reaction between chlorite and tetrathionate ions is second order with respect to hydrogen ion. Since the direct reaction was found to be orders of magnitude slower than the parallel HOCl-catalyzed pathway, a three-variable model is derived from the previously published five-step model taking into account the experimentally determined H+ concentration dependence of its rate coefficients by neglecting the direct reaction. The new three-variable model indicates that the supercatalytic effect of the hydrogen ion in the HOCl-catalyzed pathway arises from the pH dependence of the individual reactions of the five-step model. The new three-variable model also accounts for the continuous change of the stoichiometric ratio of the reactants and provides a simple kinetic law for involving it in the partial differential equation systems widely used in the study of spatiotemporal behavior of the chlorite-tetrathionate reaction.
- Horvath, Attila K.
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- Promoted dissociative adsorption of hydrogen peroxide and persulfate ions and electrochemical oscillations. Caused by a catalytic effect of adsorbed bromine
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Absorbed bromine on Pt and Au electrodes acts as a catalyst for dissociative adsorption of H2O2 and S2O82-, similar to adsorbed OH (and adsorbed iodine). This fact is revealed by analyses of newly found negative differential resistances and electrochemical oscillations, appearing in a potential region where the surface coverage of adsorbed bromine (θBr) decreases steeply. The present result further supports the generality of a catalytic effect of adsorbed electronegative species such as OH, Br, and I on the dissociative adsorption of peroxides such as H2O2 and S2O82-.
- Nakanishi, Shuji,Sakai, Sho-Ichiro,Hatou, Michiru,Fukami, Kazuhiro,Nakato, Yoshihiro
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- Diffusion-Driven Front Instability in a Three-Dimensional Medium
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Lateral instability in reaction-diffusion fronts of the chlorite oxidation of tetrathionate is studied experimentally in three dimensions. A simple two-variable model based on the empirical rate law of the reaction is utilized to reproduce the experimental observations. The onset of instability does not change by extending the system from two to three dimensions; the natural wavelength associated with the cellular structure, however, slightly decreases.
- Toth, Agota,Veisz, Bernadett,Horvath, Dezso
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- Fluctuations and Stirring Rate Effects in the Chlorite-Thiosulfate Reaction
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The stoichiometry, kinetics, and mechanism of the reaction between chlorite and thiosulfate have been studied at 25 deg C and pH 6-9.In excess thiosulfate, the stoichiometry is 4S2O3(2-) + ClO2(1-) + 2H2O = 2S4O6(2-) + OH(1-) + Cl(1-).In excess chlorite, sulfate is produced as well, the stoichiometry being mixture of the above reaction and S2O3(2-) + 2ClO2(1-) +H2O = 2SO4(2-) +2Cl(1-) + 2H(1+).The rate of OH(1-) is d(δ)/dt = kexpt, but the rate constant kexpt varies with the / ratio, being about twice as large in excess S2O3(2-) as in excess ClO2(1-).A mechanisms is proposed involving the complex intermediates S2O3ClO(1-) and S2O3Cl(1-), a key reaction between the simpler intermediates SO3(2-) and ClO(1-), and the supercatalytic chlorite-tetrathionate reaction.At pH ca. 11 in unbuffered solution, the reaction behaves as a clock reaction, with an initial rise in pH followed by an abrupt drop.The reaction time, however, varies irreproducibly, even in identically prepared samples.Careful analysis of the reaction time distribution and its variation with temperature, volume, reactant concentrations, and stirring rate leads to the conclusion that the switch from net OH(1-) production to net H(1+) generation is induced by random fluctuations within the solution.The implications of this interpretation are discussed.
- Nagypal, Istvan,Epstein, Irving R.
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- Determination of reaction energy values for biological pyrite oxidation by calorimetry
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The reaction energy values for pyrite oxidation by chemolithotrophic leaching bacteria (pure cultures of Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and mixed cultures of L. ferrooxidans and T. thiooxidans) were calorimetrically measured in batch experiments. The obtained data were compared with the nonconservative values for the corresponding abiotic processes that were calculated from standard enthalpies and chemical analyses of the leaching products. Due to the different abilities of the investigated species to oxidize the sulphur moiety of pyrite, the measured reaction energy values ranged from - 1100 to - 1600 kJ/mol with an accuracy of 4-16%. In samples with T. ferrooxidans and with the mixed cultures, no significant difference between the calorimetric and theoretical reaction energy values occurred. In contrast, pure cultures of L. ferrooxidans exhibited measured values which were up to 200 kJ/mol lower than the theoretical ones. It is highly unlikely that this difference may be explained by a higher energy conservation efficiency of L. ferrooxidans compared to the one of T. ferrooxidans or T. thiooxidans. Besides use in efficiency studies, the collected data can be used to determine the activity of leaching bacteria in natural biotopes by calorimetric measurements.
- Rohwerder, Thore,Schippers, Axel,Sand, Wolfgang
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- Kinetics and mechanism of the reaction between thiosulfate and chlorine dioxide
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The reaction between thiosulfate and chlorine dioxide in slightly alkaline medium has been studied by stopped- flow techniques. The reaction cannot be studied under pseudo-first-order condition, thus a new approach based on the improved calibration and use of stopped-flow spectrophotometers was applied. The reaction starts with irreversible formation of ?S2O3ClO22- radical. The main path of the reaction produces tetrathionate and chlorite through the formation of light absorbing tetrathionate radical (?S4O63-). Any of the reactant present in excess slightly modifies the 1:1 stoichiometry, and sulfate as well as chloride ions are also formed. A detailed mechanism based on a rigorous simultaneous fitting of the experimental data is proposed.
- Horvath, Attila K.,Nagypal, Istvan
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- The photocatalytic autoxidation of sulfur oxoanions by water-soluble porphyrin complexes
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The photocatalytic autoxidation of S4O62- and S2O32- by water- soluble porphyrin complexes Fe(2-TMPyP), Mn(4-TMPyP), and Co(2-TMPyP) were compared under illumination with a 419 nm visible light in oxygen-saturated aqueous solutions at room temperature. The process involves S2O32- as an intermediate product and SO42- is the final oxidation product. The trend in photocatalytic activity of the porphyrin complexes for S4O62- oxidation is Co(2-TMPyP) > Mn(4-TMPyP) > Fe(2-TMPyP). For S2O32- oxidation, the trend is Co(2-TMPyP) > Fe(2-TMPyP) > Mn(4-TMPyP).
- Chen, Shen-Ming
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- Alkali metal and ammonium difluoromono(isonicotinoylhydrazinato)cuprate(II) trihydrate
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Alkali metal and ammonium difluoromono(isonicotinoylhydrazinato)cuprate(II) trihydrates, M[CuF2(pyinhH)].3H2O (where M = Li, Na, K, Rb and NH4, pyinhH2 = isonicotinoylhydrazine), have been synthesized and charac
- Lal, Ram A.,Kumar, Arvind
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- Photoelectrochemical Oxidation of Sulfur Dioxide in Strong Acid Solution: Iodide-Mediated Oxidation at Illuminated Metal Dichalcogenide Electrodes
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In strong acid solution, n-type MoS2, MoSe2, or WS2 can be used as a photoanode to effect the oxidation of SO2 to SO4(2-).The oxidation can be driven with visible light, hν > 1.1 eV, with an output photovoltage of ca. 0.6 V relative to Er(SO4(2-)/SO2) in 6 M H2SO4.The oxidation of SO2, however, requires the presence of I(-) as a mediator serving to (1) alter the MY2/liquid energetics to shift the band edges to a more negative potential to yield a photovoltage relative to Er(SO4(2-)/SO2) and (2) improve the kinetics for SO2 oxidation presumably through the intermediate formation of I3(-).It is noteworthy that MY2 is stable in the presence of strong acid, even concentrated H2SO4; the photooxidation of SO2 can be sustained without photoanodic corrosion of the MY2 at current densities of 20 mA/cm2 and at potentials ca. 0.6 V more negative than Er(SO4(2-)/SO2).The cathode reaction is H2 evolution, and the overall reaction is 2H2O + SO2 -> H2SO4 + H2 that comprises one part of a hybrid cycle for splitting H2O to H2 + 1/2 O2.The cycle is completed, in principle, by thermolyzing H2SO4 to H2O + SO2 + 1/2 O2.Thus, light and heat can be used to effect water splitting.Fundamentally, the noteworthy finding is that I(-) allows a good rate (current) for SO2 oxidation and also favorably affects the energetics to improve the photovoltage.This new concept may be exploited to illustrate processes that can be uniquely done at semiconductor photoelectrodes.The efficiency for conversion of 632.8-nm light (ca. 50mW/cm2) in 6 M H2SO4/ca.1 M SO2/5 mM I(-) is ca. 8percent using an n-type WS2 photoanode.
- Calabrese, Gary S.,Wrighton, Mark S.
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- Chemical Oscillators in Group VIA: The Cu(II)-Catalyzed Reaction between Thiosulfate and Peroxodisulfate Ions
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Sustained oscillations in redox potential, pH, and the concentration of dissolved O2 are reported in the Cu(II)-catalyzed reaction between K2S2O8 and Na2S2O3 in a stirred tank reactor.A range of steady states is found, most of which can be unambiguously designed as either high or low pH, but there is no evidence of bistability.A free-radical mechanism in which Cu(I), Cu(III), and radicals SO4.- and S2O3.- play key roles appears to be capable of accounting both for the observed kinetics (first order in S2O8(2-) and Cu(II), zeroth order in S2O3(2-)) for the batch reaction and for the oscillations in the flow system.
- Orban, Miklos,Epstein, Irving R.
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- New Chlorite Oscillators: Chlorite-Bromide and Chlorite-Thiocyanate in a CSTR
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The reactions between ClO2(1-) and Br(1-) and between ClO2(1-) and SCN(1-) show sustained oscillations in a stirred tank reactor.The former system possessesa broad region of bistability and is qite sensitive to the rate of stirring.The chlorite-thiocyanate reaction, in contrast, does not exhibit bistability.Oscillations without bistability have also been found in the eactions of CO2(1-) with two thiourea derivatives.
- Alamgir, Mohamed,Epstein, Irving R.
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- Low-temperature oxidation of sulfur dioxide by molecular oxygen in aqueous solutions of non-transition metals trifluoroacetates
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Oxidation of SO2 was shown to occur in aqueous solutions of Li, Na, and K trifluoroacetates at room temperature. Oxidative activity of the non-transition metal trifluoroacetates increases on decreasing pH of solution and at equal pH values, it follows the sequence: Li Na K.
- Vishnetskaya,Murtazina, Yu. V.,Skrepleva, I. Yu.,Mel'Nikov, M. Ya.
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- Diffusion-driven front instabilities in the chlorite-tetrathionate reaction
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An extensive study on the instabilities of planar fronts leading to the formation of cellular structures has been carried out in the acid-catalyzed chlorite-tetrathionate reaction.A simple two-variable model based on the empirical rate law of the reaction is developed to describe the observed pattern formation.The calculated onset of instability and the size of the patterns in the cellular fronts are in good agreement with experimental observations.
- Horvath, Dezso,Toth, Agota
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- The catalytic and photocatalytic autoxidation of Sx/2- to SO4/2- by water-soluble cobalt porphyrin
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The catalytic and photocatalytic transformation of S42-, S32-,S22- and S2- to SO42- by water-soluble cobalt tetrakis (N-methyl-4-pyridyl) porphyrin (Co(4-TMPyP)) was performed by a two-step process. The catalytic autoxidation of Sx2- (x = 2, 3 and 4) to S2O32- by Co(4-TMPyP) in the presence of oxygen was followed by the photocatalytic oxidation of S2O32- by Co(4-TMPyP) under illumination in presence of oxygen at a strong basic solution. The direct transformation of Sx2- to SO42- was also performed. The catalytic activity of Co(4-TMPyP) was compared to Fe(2-TMPyP) and Mn(4-TMPyP). All three catalysts have activity towards autoxidation of S42- to S2O32-. The trend in photocatalytic oxidation activity of the porphyrin complexes is Co(4-TMPyP) > Fe(2-TMPyP) > Mn(4-TMPyP). The electrocatalytic reactions of Sx2- by Co(4-TMPyP) were also observed.
- Chen, Shen-Ming,Chiu, Shiu-Wen
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- Preparation, Characterization, and Electrocatalytic Properties of mvRuO/RuCN and RP Hybrid Film-Modified Electrodes
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Ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate, or mvRuO/RuCN) and iron(III) ruthenocyanide (ruthenium purple, or RP) hybrid films have been prepared using consecutive cyclic voltammetry, and the deposition process and the films' elect
- Chen, Shen-Ming,Hsueh, Sheh-Hung
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- Electron transfer reactions of peroxydisulfate and fluoroxysulfate reactions with the cyanide complexes M(CN)n4- (M=Fe(II), Ru(II), Os(II), Mo(IV), and W(IV))
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The stoichiometry and the kinetics of oxidation of the cyanide complexes M(CN)n4- (M = Fe(II), Ru(II), Os(II), Mo(IV), and W(IV)) by the peroxydisulfate ion, S2O82-, and by the much more strongly oxidizing fluoroxysulfate ion, SO4F-, were studied in aqueous solutions containing Li+. Reactions of S 2O82- with -M(CN)4- are known to be strongly catalyzed by Li+ and other alkali metal ions, and this applies also to the corresponding reactions of SO4F -. The primary reactions of S2O82- and SO4F- have both been found to be one-electron processes in which the equally strong O-O and O-F bonds are broken. The primary reaction of S2O82- consists of a single step yielding M(CN)n3-, SO4-, and SO 42-, whereas the primary reaction of SO4F - comprises two parallel one-electron steps, one leading to M(CN)n3-, SO4-, and F- and the other yielding M(CN)n-12- CN-, SO 4- and F-. The relationship between the rate constants and the standard free energies of reaction for the Li +-catalyzed reactions of SO4F- and S 2O82 with M(CN)n4-, and for the uncatalyzed reactions of S2O82- with bipyridyl and phenanthroline complexes MLn2+ (M = Fe(II), Ru(II), and Os(II)) studied previously, suggests that the intrinsic barrier for all three sets of reactions is similar, i.e., unaffected by the Li+ catalysis, and that the electron transfer and the breakage of the O-O and O-F bonds are concerted processes.
- Klaening, Ulrik,Byberg, Jorgen R.,Daasbjerg, Kim,Sehested, Knud,Appelman, Evan H.
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- Complex Oligooscillatory Behavior in the Reaction of Chlorite with Thiocyanate
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The reaction of chorite and thiocyanate has been studied in the pH range 1-4.The stoichiometry of the reaction is 2ClO2- + SCN- + H2O -> SO42- + 2Cl- + HOCN + H+.In excess ClO2-, ClO2(aq.) is formed as a product at the end of the reaction.ClO2 is formed from the reaction of excess ClO2- with HOCl: 2ClO2- + HOCl + H+ -> 2ClO2 + Cl- + H2O.At pH less then 4 and in excess SCN-, ClO2 is formed as an intermediate but is totally consumed at the end of the reaction.The reaction is very complex with oligooscillatory behavior in which ClO2 concentration goes through two maxima during the course of the reaction.The reaction is catalyzed by acid in pH range 2-4 but is retarded by acid in pH less than 2.The rate-determining step involves the reaction Cl(III) + SCN- + H+ -> HOSCN + HOCl.The acid retardation is due to the fact that ClO2- is more reactive than HClO2 and SCN- is a better nucleophile than isothiocyanic acid, HNCS.The reaction is autocatalytic in HOCl.The autocatalysis can be explained by using the asymmetric intermediate, Cl2O2, which produces two HOCl molecules after a two-electron reduction.The reaction of ClO2(aq) with SCN- was also studied, and it gives an autocatalytic rate of decay of ClO2.The mechanism involves initially forming ClO2- in an one-electron reduction followed by HOCl autocatalysis.Direct reaction between ClO2- and SCN- could be followed by using the FeSCN2+-SCN- reaction, which also showed acid retardation and HOCl autocatalysis.A 21-reaction mechanism was used to simulate the ClO2-SCN- reaction while a 24-reaction scheme was used to simulate the ClO2--SCN- reaction.There is resonable agreement between experiments and simulations in both cases.
- Chinake, Cordelia R.,Mambo, Elisabeth,Simoyi, Reuben H.
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- The Bromate-Sulfide System. A Particularly Simple Chemical Oscillator.
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In a batch system, elementary bromine rapidly oxidizes the sulfide ion, HS-, mostly to elementary sulfur and only a little further even though the redox potentials suggest the sulfide could be oxidized all the way to sulfate, SO42-.A stoichiometric excess of acidic bromate, BrO3-, does oxidize batch sulfide all the way to sulfate.The potential of a platinum electrode in such a system exhibits two successive autocatalytic rises with an intervening plateau; visible evidence of elementary bromine does not appear until the start of the second rise.In a flow reactor, this ostensibly simple system exhibits oscillations with constant periods but somewhat erratic amplitudes.We are initiating a study to elucidate the mechanism of the reaction, which seems to differ in behavior from bromate-driven oscillators involving elements other than sulfur.
- Simoyi, Reuben H.,Noyes, Richard M.
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- S-oxygenation of thiocarbamides II: Oxidation of trimethylthiourea by chlorite and chlorine dioxide
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The kinetics of the oxidation of a substituted thiourea, trimethylthiourea (TMTU), by chlorite have been studied in slightly acidic media The reaction is much faster than the comparable oxidation of the unsubstituted thiourea by chlorite. The stoichiometry of the reaction was experimentally deduced to be 2C1O2- + Me2N(NHMe)C=S + H2O → 2Cl- + Me2N(NHMe)C=O + SO42 + 2H+. In excess chlorite conditions, chlorine dioxide is formed after a short induction period. The oxidation of TMTU occurs in two phases. It starts initially with S-oxygenation of the sulfur center to yield the sulfinic acid, which then reacts in the second phase predominantly through an initial hydrolysis to produce trimethylurea and the sulfoxylate anion. The sulfoxylate anion is a highly reducing species which is rapidly oxidized to sulfate. The sulfinic and sulfonic acids of TMTU exists in the form of zwitterionic species that are stable in acidic environments and rapidly decompose in basic environments. The rate of oxidation of the sulfonic acid is determined by its rate of hydrolysis, which is inhibited by acid. The direct reaction of chlorine dioxide and TMTU is autocatalytic and also inhibited by acid. It commences with the initial formation of an adduct of the radical chlorine dioxide species with the electron-rich sulfur center of the thiocarbamide followed by reaction of the adduct with another chlorine dioxide molecule and subsequent hydrolysis to yield chlorite and a sulfenic acid. The bimolecular rate constant for the reaction of chlorine dioxide and TMTU was experimentally determined as 16 ± 3.0 M-1 s-1 at pH 1.00.
- Chigwada, Tabitha R.,Simoyi, Reuben H.
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Read Online
- Oxidation of Sulfur Dioxide in Sodium and Calcium Fluorides
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Abstract: It was established that the oxidation reactions of sulfur dioxide occur in aqueous solutions of sodium fluoride and in suspensions of calcium fluoride at room temperature and atmospheric pressure. It was found that the solutions of NaF and CaFs
- Ivanova, M. S.,Savitskaya, Yu. V.,Tomskii, K. O.,Vishnetskaya, M. V.
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p. 513 - 517
(2021/08/23)
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- Transition-State Interactions in a Promiscuous Enzyme: Sulfate and Phosphate Monoester Hydrolysis by Pseudomonas aeruginosa Arylsulfatase
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Pseudomonas aeruginosa arylsulfatase (PAS) hydrolyzes sulfate and, promiscuously, phosphate monoesters. Enzyme-catalyzed sulfate transfer is crucial to a wide variety of biological processes, but detailed studies of the mechanistic contributions to its catalysis are lacking. We present linear free energy relationships (LFERs) and kinetic isotope effects (KIEs) of PAS and analyses of active site mutants that suggest a key role for leaving group (LG) stabilization. In LFERs PASWT has a much less negative Br?nsted coefficient (βleaving groupobs-Enz = 0.33) than the uncatalyzed reaction (βleaving groupobs = 1.81). This situation is diminished when cationic active site groups are exchanged for alanine. The considerable degree of bond breaking during the transition state (TS) is evidenced by an 18Obridge KIE of 1.0088. LFER and KIE data for several active site mutants point to leaving group stabilization by active site K375, in cooperation with H211. 15N KIEs and the increased sensitivity to leaving group ability of the sulfatase activity in neat D2O (βleaving groupH-D = +0.06) suggest that the mechanism for S-Obridge bond fission shifts, with decreasing leaving group ability, from charge compensation via Lewis acid interactions toward direct proton donation. 18Ononbridge KIEs indicate that the TS for PAS-catalyzed sulfate monoester hydrolysis has a significantly more associative character compared to the uncatalyzed reaction, while PAS-catalyzed phosphate monoester hydrolysis does not show this shift. This difference in enzyme-catalyzed TSs appears to be the major factor favoring specificity toward sulfate over phosphate esters by this promiscuous hydrolase, since other features are either too similar (uncatalyzed TS) or inherently favor phosphate (charge).
- Van Loo, Bert,Berry, Ryan,Boonyuen, Usa,Mohamed, Mark F.,Golicnik, Marko,Hengge, AlVan C.,Hollfelder, Florian
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p. 1363 - 1378
(2019/03/11)
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- Synthesis, characterization of a ternary Cu(II) Schiff base complex with degradation activity of organophosphorus pesticides
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Organophosphorus pesticides are a class of the most important groups of insecticides and are widely applied to pest and plant diseases control in agriculture. The extensive application of organophosphorus pesticides can subsequently release organophosphorus pesticides into environment which may pose a seriously adverse impact to non-target organisms and humans. However, we could not ban organophosphorus pesticides at present as they are very efficient. Therefore, the new technique of degradation of organophosphorus pesticides must be studied. In this paper, a ternary copper Schiff base complex 2Cu(C14H8NO3Cl)(C12H8N2)·3CH3OH was synthesized and characterised by physico-chemical and spectroscopic methods. The degraded effects of 4 pesticides by the complex were investigated. The results showed that the degraded rates of every organophosphorus pesticide were increased. However, the effects of degradation were varied because of the different molecule structures of organophosphorus pesticides. Gas chromatography-mass spectrometry (GC–MS) and ion chromatography were used to analyze the degradation products by the complex, and then the catalytic mechanism of degradation was proposed. The study results suggest that the Schiff base copper complex, as a potent catalyzer, may find its applications in catalytic degradation area.
- Li, Xin,Zhang, Dandan,Liu, Zihui,Xu, Ying,Wang, Dongfeng
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p. 280 - 289
(2017/11/23)
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- Design and construction of a bifunctional magnetically recyclable 3D CoMn2O4/CF hybrid as an adsorptive photocatalyst for the effective removal of contaminants
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Herein, a magnetic microsphere CoMn2O4 (MS-CoMn2O4) with a 3D architecture was constructed directly on cellulose fiber (CF) substrates from wastepaper by a solvothermal synthesis method with further calcination treatment. The designed hybrid shows excellent dual functions including rapid catalytic oxidation of tetracycline (TC)/methylene blue (MB) and a high adsorption capacity. What's more, the hybrid is easily recycled using an external magnetic field. In comparison with that of pure MS-CoMn2O4, the enhanced adsorption ability and photocatalytic activity of MS-CoMn2O4/CFs can mainly be attributed to the introduced cellulose fiber supporter in the hybrid system. MS-CoMn2O4 incorporated CFs can improve the efficient separation of photogenerated electron-hole pairs and the transport pathway of electrons. More importantly, introduction of CFs can help to enrich and further improve the degradation efficiency of organic contaminants. The possible mechanism for the enhancement of the photocatalytic activity has been elucidated in detail. The reusability analysis revealed that the MS-CoMn2O4/CF hybrid exhibited superb cycling stability after 5 cycles. This study provides novel insights into the design and construction of high capacity sorbents as strongly adsorptive photocatalysts to perform catalytic degradation of organic contaminants.
- Zheng, Jianhua,Hu, Yandong,Zhang, Lei
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p. 25044 - 25051
(2017/09/29)
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- Degradation of the azo dye Acid Red 1 by anodic oxidation and indirect electrochemical processes based on Fenton's reaction chemistry. Relationship between decolorization, mineralization and products
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Solutions of 236 mg dm-3 Acid Red 1 (AR1), an azo dye widely used in textile dying industries, at pH 3.0 have been comparatively treated by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) at constant current density (j). Assays were performed with a stirred tank reactor equipped with a Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode for H2O2 generation from O2 reduction. The main oxidizing agents were hydroxyl radicals produced at the anode from water oxidation in all methods and in the bulk from Fenton's reaction between generated H2O2 and 0.5 mmol dm-3 Fe2+ in EF and PEF. For each anode, higher oxidation power was found in the sequence AO-H2O2 A total of 11 aromatic intermediates, 15 hydroxylated compounds, 13 desulfonated derivatives and 7 short-linear carboxylic acids were identified. NH4+, NO3- and SO42- ions were released during azo dye degradation. From the products detected, a comprehensive reaction sequence for AR1 mineralization is proposed. The relationship between decolorization, mineralization and products formed is finally discussed.
- Florenza, Xavier,Solano, Aline Maria Sales,Centellas, Francesc,Martínez-Huitle, Carlos Alberto,Brillas, Enric,Garcia-Segura, Sergi
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p. 276 - 288
(2015/02/19)
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- Enhanced photocatalytic degradation of methylene blue under vacuum ultraviolet irradiation
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A vacuum ultraviolet (VUV) lamp was used as the irradiation of photocatalysis to enhance the degradation of methylene blue (MB), a model dye compound. In contrast to photocatalysis under 254 nm UV irradiation (UV/TiO 2), photocatalysis under VUV irradiation (VUV/TiO2) exhibited much higher efficiency in MB degradation and mineralization and needed less TiO2 dosage. The rate constant of MB degradation in VUV/TiO2 (0.0793 min-1) is about 4 times of that of UV/TiO2 (0.0205 min-1). The MB mineralization rate is also greatly increased from 12.5% in UV/TiO2 to 47.7% in VUV/TiO 2 after 60 min of irradiation. Multiple advanced oxidation processes including photocatalytic oxidation and VUV photo-oxidation coexist in VUV/TiO2 to substantially produce highly reactive species (such as OH and energetic photons), which is responsible for efficient degradation and mineralization of MB.
- Huang, Haibao,Leung, Dennis Y.C.,Kwong, Philip C.W.,Xiong, Jing,Zhang, Lu
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p. 189 - 194
(2013/02/25)
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- Oxidation of thiocyanate with H2O2 catalyzed by [RuIII(edta)(H2O)]-
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The [RuIII(edta)(H2O)]- (edta4- = ethylenediaminetetraacetate) complex is shown to catalyze the oxidation of thiocyanate (SCN-) with H2O2 mimicking the action of peroxidases. The kinetics of the catalytic oxidation process was studied by using stopped-flow and rapid scan spectrophotometry as a function of [RuIII(edta)], [H2O2], [SCN-], pH (3.2-9.1) and temperature (15-30 °C). Spectral analyses and kinetic data are suggestive of a catalytic pathway in which hydrogen peroxide reacts directly with thiocyanate coordinated to the RuIII(edta) complex. Catalytic intermediates such as [RuIII(edta)(OOH)]2- and [Ru V(edta)(O)]- were found to be non-reactive in the oxidation process under the specified conditions. Formation of SO 42- and OCN- was identified as oxidation products in ESI-MS experiments. A detailed mechanism in agreement with the spectral and kinetic data is presented. The Royal Society of Chemistry 2013.
- Chatterjee, Debabrata,Paul, Barnali,Mukherjee, Rupa
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supporting information
p. 10056 - 10060
(2013/08/23)
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- Bisulfite-driven autocatalysis in the bromate-thiosulfate reaction in a slightly acidic medium
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The thiosulfate-bromate reaction has been studied by high-performance liquid chromatography, monitoring the concentrations of thiosulfate and tetrathionate simultaneously. It is found that concentration-time curves of both species display a sigmoidal shape in a slightly acidic, well-buffered medium. Unlike the previously reported complex reaction systems involving bromate, this nonlinear dynamical behavior originates from neither proton nor bromine(III) autocatalysis under our experimental conditions. We demonstrated that sulfur(IV) species significantly accelerates the reaction; therefore, it acts as an autocatalyst. To the best of our knowledge, no reaction system has yet been reported among the pH-driven oxysulfur-oxyhalogen systems, where sulfur(IV) has such a remarkable role. On the basis of the simultaneous evaluation of [S 2O32-] and [S4O6 2-] time series, an eight-step kinetic model is proposed to account for the experimental observations. The model employed here may serve as a solid starting point to extend it for other oxysulfur-oxyhalogen systems where such a seemingly general phenomenon may become observable.
- Wang, Zhen,Gao, Qingyu,Pan, Changwei,Zhao, Yuemin,Horvath, Attila K.
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p. 12062 - 12064
(2013/01/15)
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- General pathway of sulfur-chain breakage of polythionates by iodine confirmed by the kinetics and mechanism of the pentathionate-iodine reaction
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The pentathionate-iodine reaction has been studied spectrophotometrically at T = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95-5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S5O6I- along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S5O6I -, including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S 5O62- + 10I2 + 14H2O → 5SO42- + 20I- + 28H+. A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included.
- Xu, Li,Csek?, Gy?rgy,Kégl, Tamás,Horváth, Attila K.
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p. 7837 - 7843
(2012/08/28)
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- Kinetics of oxidation of nitrosodisulfonate anion radical with a metallo-superoxide
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The metal bound superoxide in μ-superoxo-bis[pentaamminecobalt(iii)] 5+ (1) oxidizes the nitrosodisulfonate anion radical (NDS 2-) by two electrons. Oxidized NDS2- quickly decomposes to SO42- and NO. 1 is itself reduced to the corresponding hydroperoxo complex which also decomposes fast to Co(ii), NH4 + ions and oxygen. 1.5 moles of volatile products formed per mole of 1 mixed with excess NDS2-. In the absence of superoxide in a bridged complex, e.g. the μ-amido-bis[pentaamminecobalt(iii)]5+ complex fails to oxidize the nitroxyl radicals, NDS2-, TEMPO and 4-oxo TEMPO. With excess NDS2- over 1, the reaction is first-order with respect to [1], [NDS2-] and inverse first order in [H+]. The activation entropy, ΔS≠, is largely negative, increased ionic strength decreased the rate and a Bronsted plot is fairly linear with a negative slope. Oxidant μ-superoxo-bis[(ethylenediamine) (diethylenetriamine)cobalt(iii)]5+ has ligands sterically more crowded though more basic than ammonia in 1. It oxidizes NDS2- much more slowly. No solvent kinetic isotope effect (kH2O/D2O ≈ 1) could be seen; a spin-adduct formation by the conjugate base of 1 followed by electron transfer is postulated. The Royal Society of Chemistry 2012.
- Mandal, Kaustab,Banerjee, Rupendranath
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p. 2714 - 2719
(2012/04/04)
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- Mechanism of decomposition of the human defense factor hypothiocyanite near physiological pH
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Relatively little is known about the reaction chemistry of the human defense factor hypothiocyanite (OSCN-) and its conjugate acid hypothiocyanous acid (HOSCN), in part because of their instability in aqueous solutions. Herein we report that HOSCN/OSCN- can engage in a cascade of pH- and concentration-dependent comproportionation, disproportionation, and hydrolysis reactions that control its stability in water. On the basis of reaction kinetic, spectroscopic, and chromatographic methods, a detailed mechanism is proposed for the decomposition of HOSCN/OSCN- in the range of pH 4-7 to eventually give simple inorganic anions including CN -, OCN-, SCN-, SO32-, and SO42-. Thiocyanogen ((SCN)2) is proposed to be a key intermediate in the hydrolysis; and the facile reaction of (SCN) 2 with OSCN- to give NCS(=O)SCN, a previously unknown reactive sulfur species, has been independently investigated. The mechanism of the aqueous decomposition of (SCN)2 around pH 4 is also reported. The resulting mechanistic models for the decomposition of HOSCN and (SCN) 2 address previous empirical observations, including the facts that the presence of SCN- and/or (SCN)2 decreases the stability of HOSCN/OSCN-, that radioisotopic labeling provided evidence that under physiological conditions decomposing OSCN- is not in equilibrium with (SCN)2 and SCN-, and that the hydrolysis of (SCN)2 near neutral pH does not produce OSCN-. Accordingly, we demonstrate that, during the human peroxidase-catalyzed oxidation of SCN-, (SCN)2 cannot be the precursor of the OSCN- that is produced.
- Kalmar, Jozsef,Woldegiorgis, Kelemu L.,Biri, Bernadett,Ashby, Michael T.
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p. 19911 - 19921
(2012/01/31)
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- On the complexity of kinetics and the mechanism of the thiosulfate- periodate reaction
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The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.
- Rauscher, Evelin,Cseko, Gyoergy,Horvath, Attila K.
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p. 5793 - 5802
(2011/07/29)
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- One- versus two-electron oxidation with peroxomonosulfate ion: reactions with iron(II), vanadium(II), halide ions, and photoreaction with cerium(III)
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The kinetics of the redox reactions of the peroxomonosulfate ion (HSO 5-) with iron(ll), vanadium(IV), cerium(lll),chloride, bromide, and iodide ions were studied. Cerium(lll) is only oxidized upon illumination by UV light and cerium
- Lente, Gabor,Kalmar, Jozsef,Baranyai, Zsuzsa,Kun, Aliz,Kek, IIdikoe,Bajusz, David,Takacs, Marcell,Veres, Lilla,Fabian, Istvaen
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p. 1763 - 1773
(2009/06/17)
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- Simultaneous evaluation of different types of kinetic traces of a complex system: Kinetics and mechanism of the tetrathionate - bromine reaction
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The bromine-tetrathionate reaction has been studied in the presence of phosphoric acid/dihydrogen phosphate buffer at T = 25 ± 0.1°C and at I = 0.5 M ionic strength with both stopped-flow technique and a conventional diode array spectrophotometer. The sto
- Varga, Denes,Horvath, Attila K.
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p. 9988 - 9996
(2010/02/16)
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- Kinetics and mechanism of the oxidation of tetrathionate by iodine in a slightly acidic medium
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The iodine-tetrathionate reaction has been reinvestigated spectrophotometrically at T = 25.0 ± 0.1°C and at an ionic strength of 0.5 M adjusted by sodium acetate as a buffer component in both the absence and presence of the iodide ion in the pH range of 4.25-5.55. The reaction was found to be independent of pH within the range studied, and it was clearly demonstrated that the reaction proceeds via an intermediate S4O 6I- formed in a pre-equilibrium. Iodide dependence of the kinetic curves strongly suggests that the iodide ion has to be involved in this equilibrium. Further reactions of the intermediate, including its hydrolysis and reaction with iodide, leads to the strict stoichiometry characterized by S 4O62- + 7I2 + 10H2O → 4SO42- + 14I- + 20H+. A seven-step kinetic model with three fitted kinetic parameters is suggested and discussed. A rate equation is also derived from which a sound explanation of the iodide dependence of the apparent rate coefficient is presented. Furthermore, it has also been pointed out that formation of the triiodide ion alone is not sufficient to take the retardation effect of the iodide ion into account quantitatively.
- Kerek, Attila,Horvath, Attila K.
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p. 4235 - 4241
(2008/10/09)
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- Kinetics and mechanism of the decomposition of tetrathionate ion in alkaline medium
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The kinetics of the alkaline decomposition of tetrathionate has been studied spectrophotometrically in the pH range of 9.2-12.2 using both phosphate/hydrogen phosphate and carbonate/hydrogen carbonate as buffer systems and by adjusting the ionic strength to I = 0.5 M with sodium perchlorate at T = 25.0 ± 0.1°C. Matrix rank analysis of the spectra recorded between 265 and 330 nm shows the presence of three independent absorbing species. Besides tetrathionate, thiosulfate and trithionate are identified as absorbing products of the decomposition, but sulfite and a trace amount of sulfate are also formed during the alkaline degradation process. With pentathionate, sulfoxylic acid (S(OH)2), S2O3OH-, and S3O3OH- as key intermediates, a 10-step kinetic model is proposed with six fitted kinetic parameters to take all the important characteristics of the experimental curves into account. On the basis of the stoichiometric measurements and the model proposed, it is also enlightened that the product distribution of the reaction continuously varies with pH; thus the kinetic traces cannot be evaluated by assuming a single stoichiometry in contrast to recent studies. Buffer dependence of the decomposition is also discussed.
- Varga, Denes,Horvath, Attila K.
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p. 7654 - 7661
(2008/10/09)
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- Three autocatalysts and self-inhibition in a single reaction: A detailed mechanism of the chlorite-tetrathionate reaction
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The chlorite-tetrathionate reaction has been studied spectrophotometrically in the pH range of 4.65-5.35 at T = 25.0 ± 0.2°C with an ionic strength of 0.5 M, adjusted with sodium acetate as a buffer component. The reaction is unique in that it demonstrates autocatalysis with respect to the hydrogen and chloride ion products and the key intermediate, HOCl. The thermodynamically most-favorable stoichiometry, 2S4O6 2- + 7ClO2- + 6H2O → 8SO 42- + 7Cl- + 12H+, is not found. Under our experimental conditions, chlorine dioxide, the chlorate ion, or both are detected in appreciable amounts among the products. Initial rate studies reveal that the formation of chlorine dioxide varies in an unusual way, with the chlorite ion acting as a self-inhibitor. The reaction is supercatalytic (i.e., second order with respect to autocatalyst H+). The autocatalytic behavior with respect to Cl- comes from chloride catalysis of the chlorite-hypochlorous acid and hypochlorous acid-tetrathionate subsystems. A detailed kinetic study and a model that explains this unusual kinetic behavior are presented.
- Horvath, Attila K.,Nagypal, Istvan,Epstein, Irving R.
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p. 9877 - 9883
(2008/10/09)
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- Novel non-aqueous Fe(III)/Fe(II) redox couple for the catalytic oxidation of hydrogen sulfide to sulfur by air
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A series of Fe(iii) salts and organic solvents have been screened to develop novel non-aqueous catalysts for the conversion of H2S to sulfur. FeCl3/95% N-methylpyrrolidinone/5% H2O was found to be a most efficient non-aqueous system. The process chemistry, the proposed mechanism of catalytic oxidation of H2S, and the electrochemistry are discussed. The Royal Society of Chemistry 2006.
- Hua, Guoxiong,Zhang, Qingzhi,McManus, Derek,Slawin, Alexandra M. Z.,Woollins, J. Derek
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p. 1147 - 1156
(2007/10/03)
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- S-oxygenation of thiocarbamides I: Oxidation of phenylthiourea by chlorite in acidic media
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The oxidation of 1-phenyl-2-thiourea (PTU) by chlorite was studied in aqueous acidic media. The reaction is extremely complex with reaction dynamics strongly influenced by the pH of reaction medium. In excess chlorite concentrations the reaction stoichiometry involves the complete desulfurization of PTU to yield a urea residue and sulfate: 2ClO2 + PhN(H)CSNH 2 + H2O → SO42- + PhN(H)CONH2 + 2Cl- + 2H+. In excess PTU, mixtures of sulfinic and sulfonic acids are formed. The reaction was followed spectrophotometrically by observing the formation of chlorine dioxide which is formed from the reaction of the reactive intermediate HOCl and chlorite: 2ClO2- + HOCl + H+ → 2ClO2(aq) + Cl- + H2O. The complexity of the ClO2 - - PTU reaction arises from the fact that the reaction of ClO 2 with PTU is slow enough to allow the accumulation of ClO 2 in the presence of PTU. Hence the formation of ClO2 was observed to be oligooscillatory with transient formation of ClO2 even in conditions of excess oxidant. The reaction showed complex acid dependence with acid catalysis in pH conditions higher than pKa of HClO 2 and acid retardation in pH conditions of less than 2.0. The rate of oxidation of PTU was given by -d[PTU]/dt = k1[ClO2 -][PTU] + k2[HClO2][PTU] with the rate law: -d[PTU]/d/ = [Cl(III)]τ[PTU]0/Ka1 + [H + ] [K 1Ka1 + k2[H + ]]; where [Cl(III)]T is the sum of chlorite and chlorous acid and Ka1 is the acid dissociation constant for chlorous acid. The following bimolecular rate constants were evaluated; k1= 31.5±2.3 M-1 s -1 and k2 = 114±7 M-1 s-1. The direct reaction of ClO2 with PTU was autocatalytic in low acid concentrations with a stoichiometric ratio of 8:5; 8ClO2 + 5PhN(H)CSNH2 + 9H2O - 5SO42- + 5PhN(H)CONH2 + 8Cl- + 18H+. The proposed mechanism implicates HOCl as a major intermediate whose autocatalytic production determined the observed global dynamics of the reaction. A comprehensive 29-reaction scheme is evoked to describe the complex reaction dynamics.
- Chigwada, Tabitha R.,Chikwana, Edward,Simoyi, Reuben H.
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p. 1081 - 1093
(2007/10/03)
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- Catalytic activity of CuS nanoparticles in hydrosulfide ions air oxidation
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The most efficient technique for H2S removal from the wastewaters is the catalytic aeration of the wastewaters, i.e., H2S oxidation in air-saturated solutions in the presence of catalysts. Photophysical characteristics of colloidal CuS nanoparticles synthesized in various conditions and stabilized in aqueous solutions with sodium polyphosphate were studied. Hydrosulfide ions air oxidation in aqueous solutions at room temperatures and 1 atm proceeded with small rates in the absence of catalysis and increased, substantially upon the injection of CuS nanoparticles into a reacting mixture. The rate of HS- catalytic oxidation grew at an increase in molar CuS concentration, initial concentration of Na2S, volume fraction of oxygen in gas mixture bubbled into the reactor, and pH of a solution (≤ 11.9). A scheme for the mechanism of HS- catalytic oxidation was proposed. According to the scheme, HS- oxidation is a chain radical reaction initiated on the surface of CuS nanoparticles and propagated further in the bulk of a solution.
- Raevskaya,Stroyuk,Kuchmii,Kryukov
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p. 259 - 265
(2008/10/09)
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- Kinetics and mechanisms of S(IV) reductions of bromite and chlorite ions
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The reaction of bromite with aqueous S(IV) is first order in both reactants and is general-acid catalyzed. The reaction half-lives vary from 5 ms (p[H+] 5.9) to 210 s (p[H+] 13.1) for 0.7 mM excess S(IV) at 25°C. The proposed mechanism includes a rapid reaction (k1 = 3.0 × 107 M-1 s-1) between BrO2- and SO32- to form a steady-state intermediate, (O2BrSO3)3-. General acids assist the removal of an oxide ion from (O2BrSO3)3- to form OBrSO3-, which hydrolyzes rapidly to give OBr- and SO42-. Subsequent fast reactions between HOBr/OBr- and SO32- give Br- and SO42- as final products. In contrast, the chlorite reactions with S(IV) are 5-6 orders of magnitude slower. These reactions are specific-acid, not general-acid, catalyzed. In the proposed mechanism, ClO2- and SO3H-/SO2 react to form (OClOSO3H)2- and (OClOSO2)- intermediates which decompose to form OCl- and SO42-. Subsequent fast reactions between HOCl/OCl- and S(IV) give Cl- and SO42- as final products. SO2 is 6 orders of magnitude more reactive than SO3H-, where k5(SO2/ClO2-) = 6.26 × 106 M-1 s-1 and k6(SO3H-/ClO2-) = k6(SO3H-/ClO2-) = 5.5 M-1 s-1. Direct reaction between ClO2- and SO32- is not observed. The presence or absence of general-acid catalysis leads to the proposal of different connectivities for the initial reactive intermediates, where a Br-S bond forms with BrO2- and SO32-, while an O-S bond forms with ClO2- and SO3H-.
- Huff Hartz, Kara E.,Nicoson, Jeffrey S.,Wang, Lu,Margerum, Dale W.
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- Destruction of malodorous compounds using heterogeneous photocatalysis.
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Malodorous sulfur-containing compounds (thiophene, methyl disulfide, trimethylene sulfide, and propylene sulfide; model compounds for industrial wastewater) were photocatalytically oxidized over TiO2 in an annular plug-flow reactor. A MS online system monitored the formation of products and byproducts in real time. Mineralization of the sulfur-containing compounds was confirmed by mass balance of CO2 and SO42-. Dilute concentrations of trimethylene sulfide and propylene sulfide were completely mineralized. For thiophene and methyl disulfide, however, partial oxidation was observed, generating SO2 and SO as byproducts. Sensory analysis showed that for trimethylene sulfide and propylene sulfide, odor intensity after TiO2/UV treatment was below the olfactive threshold limit of the human panel. Both GC-FID and sensory analysis indicated that the photocatalytic process efficiently destroyed the malodorous compounds.
- Jardim
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p. 2788 - 2792
(2007/10/03)
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- Destructive oxidation of mirex
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The perchlorinated aliphatic substance Mirex (perchloropentacyclo[5.3.0.02,6.03,9.0 4,8]decane) once had widespread use as an insecticide and still has limited application as a termiticide. Mirex resists direct oxidation by ruthenium tetraoxide. It undergoes a six-electron reduction in acetonitrile at -1·2 V from the Ag/AgCl reference potential. One or more chlorines are readily substituted by methoxide at 90°C. Both the reduced and the methoxylated derivatives are oxidized at room temperature by alkaline hypochlorite or persulfate in the presence of a homogeneous ruthenium catalyst. Only a trace amount of cyclohexane-extractable residue remains, but not all of the chlorine is released as ionic chloride. This implies that the oxidation products include some unidentified water-soluble organochlorine substances.
- Beattie, James K.,Fleming, Douglas S.
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p. 973 - 976
(2007/10/03)
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- Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance raman spectroscopy
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The existence of an electron-transfer pathway in the reaction of ·OH radical with aromatic molecules in water has been established, for the first time, using time-resolved resonance Raman spectroscopy as a diagnostic tool and p-dimethoxybenzene as a model system. In the currently accepted mechanism, the cation radical is produced by ·OH addition to the ring, followed by loss of OH-. The present work demonstrates that this process competes with direct electron transfer. A generalized reaction mechanism has been proposed in terms of potential energy diagrams to explain two-step formation of the cation radical. In this reaction mechanism, the electron- transfer component and the rate of OH- elimination from the ·OH adduct both depend on the ionization potential (IP) of the molecule. The cation radical yield by electron transfer increases from 6% in p-dimethoxybenzene to 30% in p-anisidine and 85% in p-phenylenediamine. For neutral molecules with IP > 8 eV, the ·OH addition is the first step in the chemistry, and for IP 7 eV, it is the electron transfer. In the intermediate IP range, both processes occur simultaneously.
- Tripathi
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p. 4161 - 4166
(2007/10/03)
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- The Kinetics of Formation of Chromium(III) and Iron(III) Polynuclear Hydroxo Complexes in Nitrate Solutions
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The kinetics of Cr3+, Fe3+, and Cr3+-Fe3+ hydrolysis in nitrate solutions at various temperatures is studied by spectrophotometry and potentiometry. The kinetic parameters of formation of polynuclear hydroxo complexes in the systems are determined by using the transformation coefficients.
- Kopylovich,Baev,Chernik
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p. 1552 - 1556
(2008/10/08)
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- Kinetics of retardation in persulphate-Fe3+-bisulphite initiated polymerization of acrylamide
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The kinetics of persulphate-Fe3+-bisulphite initiated polymerization has been studied at 30±0.2°C in oxygen atmosphere. It has been found that the polymerization kinetics is significantly altered by Fe3+ concentration. At lower concentration (2.0 × 10-4 mol dm-3) of Fe3+ the rate of polymerization follows the rate equation Rp ∝ , [monomer] [S2O82-] while at higher concentration (8.0 × 10-4 mol dm-3) the rate equation becomes Rp ∝ [M][HSO3-]. Besides affecting the polymerization kinetics Fe3+ also decreases the molecular weight of the resulting polymers. The effects of added salts and temperature on the polymerization process have also been investigated and energy of activation is calcualted to be 42.6 kJ mol-1. Retarding effect of various aliphatic alcohols, nitro compounds and nitrophenols has been studied in terms of retardation constants.
- Bajpai,Sharma
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p. 1091 - 1096
(2007/10/03)
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- Kinetics and mechanism of the oxidation of the sulphite ion by the Mn(III)-cydta complex ion
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The reinvestigated oxidation of S(IV), HSO3-/SO32- ions, by - confirmed that S(IV) is oxidized in two parallel paths; the order with respect to is one of the paths and two in the other.The nature of the dependence of the rate on +> is also confirmed.However, the rapid scan of the reaction mixture and measuremtnt of the initial absorbance of the reaction mixture at different wavelengths at the beginning of the reaction suggest an outer-sphere mechanism.The rate parameters are of the same order as obtained in known reactions of an outer-sphere mechanism and this mechanism is further supported by the Marcus cross relation.Key words: kinetics, outer-sphere mechanism, -, SO32-.
- Chandrawat, Uttra,Prakash, Aditya,Mehrotra, Raj N.
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p. 1531 - 1537
(2007/10/03)
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- Structure/Function Relationships in Ligand-Based SO2/O2 Conversion to Sulfate As Promoted by Nickel and Palladium Thiolates
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The dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Pd(II), (bme-daco)Pd(II) or Pd-1, whose structure was determined by X-ray crystallography (monoclinic P21/m space group with a = 6.1680(10) ?, b = 15.715(5) ?, c = 6.5930(10) ?, β = 107.0
- Darensbourg, Marcetta Y.,Tuntulani, Thawatchai,Reibenspies, Joseph H.
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p. 6287 - 6294
(2008/10/08)
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- Kinetic Evidence for the Redox Cycling of Manganese(II,III) in the Presence of (1-)ON(NO)SO3(1-) in Aqueous Media
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Redox cycling of Mn(II/III) in the presence of (1-)ON(NO)SO3(1-) in the absence and presence of oxygen was studied by following the increase and decrease of Mn(III) spectrophotometrically as a function of , , and pH.The autoxidation reaction exhibits typical autocatalytic behaviour in which the induction period depends on the concentrations of M(III) and (1-)ON(NO)SO3(1-).Under acidic conditions the reaction does not take place in the absence of Mn(III) or O2.The concentrations of O2, Mn(II) and (1-)ON(NO)SO3(1-) determine whether only oxidation of Mn(II) to Mn(III), or reduction of Mn(III) to Mn(II), or both in one or two redox cycles is observed.Preliminary simulations of some of the observed kinetic traces are presented.
- Prinsloo, Frans F.,Pienaar, Jakobus J.,Eldik, Rudi van
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p. 293 - 300
(2007/10/02)
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