Characterization of oxidation progress by chemiluminescence: A study of polyethylene with pro-oxidant additives

Non-isothermal chemiluminescence measurements in nitrogen and isothermal measurements in oxygen were used for the evaluation of degradation in pre-oxidized polyethylene either pure or containing Mn-based pro-oxidant additives. The results were compared with infrared spectroscopy data. Chemiluminescence measurements of pure polyethylene and polyethylene with additive made it possible to calculate the set of rate constants, based on the Bolland-Gee oxidation scheme. The oxidation rate constants of polyethylene with additive were significantly higher, while the activation energy of the process appeared lower (65 kJ mol⁻¹), than those of pure polyethylene. The method provides an access to study oxidation processes during the induction period of oxidation when infrared spectroscopy cannot provide sufficient information.     

Computational modeling of the mechanisms of the free radical-chain reaction of alkanes with oxygen. The oxidation of isobutane,n-butane,and isopentane

A general computational method for obtaining complete solutions of time-dependent kinetic equations has been developed and applied to free radical-initiated reactions of alkanes with oxygen. The method has been applied to the low-temperature, peroxide-initiated oxidations of isobutane and isopentane. Using available independently measured and estimated values for the rate constants and activation parameters for each of the 20 proposed reaction steps for the oxidation of isobutane, the rates and products have been calculated for both the liquid phase and gas phase in the range of 100°–155°C. The calculated rates and products of oxidation agree with published experimental values. The oxidation of isopentane was examined by a 32-reaction model. The rate constants were estimated using values for the appropriate rate steps in the oxidation of n-butane and isobutane. The calculation of the oxidation rate and products agree with our experiments.     

Computational techniques applied to the study of the oxidation kinetics of iron and molybdenum cyanocomplexes by peroxynitrous acid

The oxidation kinetics of iron and molybdenum cyanocomplexes by peroxynitrous acid were studied by computational techniques. The rate constants of ONOOH decomposition and substrate oxidation were calculated by fitting the experimental data to the solution of the ordinary differential equations mechanism. There is a linear relationship between the rate constants which is solved by varying the initial concentration of one of the reactive species. The rate constants of the steps involved in ONOOH decomposition were also determined following the same procedure and considering a linear relationship between the rate constants. The results are consistent with the values previously found using the conventional method of integration, and they validate the prior steady state assumption. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of inclusion complex on nitrous acid reaction with flavonoids

The kinetic of the nitrous acid reactions with quercetin and catechin has been studied using spectrophotometric method in aqueous solution. The results show that these antioxidants participate in oxidation reactions with nitrous acid which is derived from protonation of nitrite ion in mild acidic conditions. Corresponding o-quinones as relatively stable products were detected by spectrophotometric techniques. pH dependence of the reactions has been examined and the rate constants of reactions were obtained by non-linear fitting of kinetic profiles. The effect of β-cyclodextrin on the oxidation pathway was another object of this study. It is shown that β-cyclodextrin has an inhibitory effect on the oxidation reaction. The rate constants of oxidation reactions for complexed forms and their stability constants were obtained based on changes in the reaction rates as a function of β-cyclodextrin concentration.     

Voltammetric study of the oxidation of quercetin and catechin in the presence of cyanide ion

The reaction of electrochemically generated o-benzoquinones from oxidation of quercetin and catechin as Michael acceptors with cyanide ion as nucleophile has been studied using cyclic voltammetry. The reaction mechanism is believed to be EC; including oxidation of catechol moiety of these antioxidants followed by Michael addition of cyanide ion. The observed homogeneous rate constants (k _obs) for reactions were estimated by comparing the experimental voltammetric responses with the digitally simulated results based on the proposed mechanism. The effects of pH and nucleophile concentration on voltammetric behavior and the rate constants of chemical reactions were also described.     

Conversions of carbon monoxide oxidation over Pt−Rh alloy catalysts calculated by a new gas chromatographic technique

Summary Catalytic fractional conversions of carbon monoxide to carbon dioxide over Pt−Rh alloy catalysts in the presence of excess oxygen, under steady-state or non steady-state conditions, as well as corresponding rate constants for the CO oxidation reaction were measured by using the reversed-flow gas chromatographic technique. From the variation of the conversions with temperature, maximum values of conversions were found, which depend on the catalysts nature (Pt content), while from the variation of the rate constants with temperature, activation energies for the CO oxidation reaction were determined, which also depend on the catalyst Pt content. The results suggest a synergism between Pt and Rh in the Pt−Rh bimetallic catalysts in accordance with previous works, showing that reversed-flow gas chromatography can be used with simplicity and accuracy for the kinetic study of the CO oxidation reaction, which is of technological importance for the control of air pollution.     

Characterization of the antioxidative activity of novel nontoxic neuropeptides by using capillary electrophoresis

In the present study, we have monitored the oxidation process of novel nontoxic neuropeptides and determined its rate constants, which describe the antioxidative potential of the peptides. A capillary electrophoretic method was implemented which ensures the simultaneity of analysis of reactants and products in a short time of analysis. The rate constants of oxidation of the four novel peptides, 4-methoxy-L-tyrosinyl-gamma-L-glutamyl-L-cysteinyl-glycine (UPF1), D-serinyl-gamma-L-glutamyl-L-cysteinyl-glycine (UPF6), 4-methoxy-L-tyrosinyl-alpha-L-glutamyl-L-cysteinyl-glycine and D-serinyl-alpha-L-glutamyl-L-cysteinyl-glycine, designed by us, were compared with those of oxidation of glutathione (reduced glutathione) by using capillary electrophoresis. The second-order rate constants were similar for all peptides if the oxidation was carried out with hydrogen peroxide (k(II) = 0.208 - 0.236 x 10(3)/M.min). The rate constants were also determined for the mixtures of peptides. When the oxidation is caused by hydroxyl radical (OH*), the gamma-glutamate containing peptides (UPF1 and UPF6) exhibited two to four times higher antioxidative activity (k(II) = 4.428 and 2.152 x 10(3)/M.min, respectively). The results suggest that the antioxidative potential of the peptides studied is not determined by the formation of disulphide bridge alone.     

Spectrophotometric Determination of the Stability Constant of the Inclusion Complexes of Some Catechol Derivatives with β‐Cyclodextrin Based on Their Reaction with Iodate

The effects of β‐cyclodextrin (β‐CD) inclusion on the kinetics of the oxidation of several cathechol derivatives, including 4‐tert‐butylcatechol, 3‐methylcatechol and 3‐methoxycatechol, with iodate was studied spectrophotometrically. The rate of the oxidation reactions decreased by increasing β‐CD concentration as a result of inclusion. The stability constants for the inclusion complexes of the investigated compounds were determined based on the changes in the rate constants as a function of β‐CD concentration at pH 3.0. The rate constants for the free and complexed forms and also the stability constants for the inclusion complexes were calculated. The role of the hydrophobic effect was evaluated by studying the influence of the presence of different amounts of ethanol on the β‐CD: guest interaction. In a given H₂O‐EtOH mixture the stability of β‐CD complexes shows the order of 4‐tert‐butylcatechol ≈ 3‐methylcatechol ≈ 3‐methoxycatechol. Increasing ethanol content caused a decrease in the stability constant of the inclusion complexes and an increase in observed rate constants.     

Steric Effects on Reaction Rates. Rate and equilibrium constants for oxidation of cyclanols

Equilibrium constants for oxidations of cyclanols by cyclohexanone in benzene have been determined in the presence of aluminium isopropoxide. The free energies of the equilibrium (ΔG_ox) are correlated with equilibrium constants for dissociation of cyanohydrins, rate constants for cyclanol oxidation with chromic acid, ketone reduction with sodium borohydride and trifluoroethanolysis of tosylates.     

Investigation of n-hexane oxidation on the surface of activated charcoal

n-Hexane oxidation process on the surface of activated charcoal with UV and without UV initiation has been investigated. The charcoal was chemically activated and treated at high temperatures with oxidizing agents. Kinetic parameters of n-hexane oxidation such as reaction rate constants, reaction orders, intermediate concentrations and adsorption constants were determined. An oxidation process on the surface of activated charcoal is assumed.     

The Electrochemical Measurement of Rate Constants for the Catalyzed Oxidation and Reduction of Water in microheterogeneous systems

The appearent first-order rate constants for the oxidation of water by iron tris (bipyridyl) (Fe (bpy)³⁺₃), and reduction of water by methylviologen (MV⁺) catalyzed by a stabilized RuO₂ or Pt-sol, respectively, were measured. Rate constants for water oxidation at pH 7 of 0.4 s^?1 and water reduction at pH 4.7 and pH 1 of up to 460 s^?1 for different sols were found.     

Evaluation of antioxidants using oxidation reaction rate constants

An evaluation method for the capacity of antioxidants to protect drugs against oxidation is presented. As a new viewpoint, to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the drug-protection capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics instead of standard oxidation (or reduction) potentials. Sodium sulfite, sodium bisulfite and sodium pyrosulfite were used as models for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient air was continually infused into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations of the antioxidants were determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants. It was found that the degradation of sodium sulfite, sodium bisulfite or sodium pyrosulfite obeyed pseudo zero-order kinetics in the buffer solutions. Because of the ionization equilibrium, these three antioxidants have the same ion form in solutions at a definite pH value and therefore their apparent rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25°C are (1.34 ± 0.03) × 10⁻³ at pH 6.8, (1.20 ± 0.02) × 10⁻³ at pH 4.0 and (6.58 ± 0.02) × 10⁻³ mol·L⁻¹·h⁻¹ at pH 9.2, respectively. Translated from Acta Chimica Sinica, 2006, 64(6): 496–500 (in Chinese)     

Oxidation of Iron and Nickel with 3,5-Di-<Emphasis Type="Italic">tert</Emphasis>-butyl-1,2-benzoquinone in Coordinating Solvents

The final products of oxidation of iron and nickel with 3,5-di-tert-butyl-1,2-benzoquinone in DMSO were identified, and the formal-kinetics relationships of the process were elucidated. The apparent equilibrium constants, enthalpies, and entropies of adsorption of the reactants on the metal surfaces and the rate constants and activation energies of the reactions were determined.     

Interrelation between the electrochemical activity of alkyl-and thioalkylphenols and their antioxidant action

The oxidation potentials of alkyl-and thioalkylphenols of various structures and the rate constants for their reactions with styrene peroxide radicals were determined. The overall inhibiting activity of the substances in a model reaction of the thermal autooxidation of lard was studied. The oxidation potential of phenols was found to correlate with the O-H bond energy, the rate constants for phenol reactions with styrene peroxide radicals and phenoxyl radical reactions with Tetralin molecules, and the effectiveness of the overall inhibiting action of these compounds.     

Evolution of HCN by thermal oxidative degradation from nylon 66 at high temperatures including burning

Work on the evolution of HCN from nylon 66 was extended to temperatures from ca. 300 to 695°C. Below ca. 300°C the evolution of HCN is governed by chemical decomposition, and above 300°C the evolution is controlled by diffusion. Above 530°C oxidation of HCN becomes noticeable and ignition occurs at 590°C. The rate constants for all of the temperature ranges and for the oxidation of HCN are given in terms of Arrhenius equations. The activation energy for the oxidation of HCN before ignition (590°C) is reached is 47 kcal/mole, and beyond this point, the oxidation is controlled by diffusion. The rate constants increase linearly with oxygen concentration as long as HCN oxidation is negligible; however, they pass through maxima if HCN oxidation is appreciable (some HCN is evolved even in the absence of oxygen). A new flash degradation apparatus has been constructed for these high-temperature ranges and a degradation mechanism has been proposed which is in satisfactory agreement with the experimental results.     

Stationary Kinetics of Light Olefin Oxidation by Hydrogen Peroxide in Aqueous Alkali Solutions in the Presence of Fe(III) Oxide

The steady-state kinetics of ethylene and propylene oxidation by hydrogen peroxide in the presence of Fe(III) oxide in aqueous solutions with the permanent adding of H₂O₂ to the reaction medium was studied. The use of an original method for the study of the steady-state reaction kinetics with gas chromatographic detection of substrate consumption from the gas phase made it possible to estimate the apparent rate constants of ethylene oxidation, the ratio of the rate constants of propylene and ethylene oxidation, the reaction orders with respect to the substrate and oxidant concentration, the dependence of the apparent rate constant of ethylene oxidation on the catalyst weight and on the pH of solution, and the apparent activation energy of the process under condition of substrate distribution between the gas and liquid phases. It was found that the kinetic isotope effect in ethylene oxidation is almost absent when completely deuterated ethylene is used.     

Free radical intervention,deamination and decarboxylation in the ruthenium(III)-catalysed oxidation of L-arginine by alkaline permanganate – a kinetic study

The kinetics of the Ru^III catalysed oxidation of L-arginine by alkaline permanganate was studied spectrophotometrically using a rapid kinetic accessory. The reaction follows a two stage process. In both the stages the reaction is first order with respect to [oxidant] and [catalyst] with an apparent less than unit order in [substrate] and [alkali]. The data suggest that oxidation proceeds via formation of a complex between the active Ru^III species and L-arginine, which then reacts with one mole of permanganate in a slow step to yield a L-arginine free radical, followed by a fast step to form the products. The reaction constants involved in the mechanism were evaluated. There is a good agreement between observed and calculated rate constants under different experimental conditions for both stages of reaction. The activation parameters for the slow step were calculated and are discussed.     

Methanol electrooxidation in alkaline solutions on platinum-based electrodes: Classical and dynamical approach

The electrochemical oxidation of methanol has been carefully studied due to its application in fuel cells. In this work electrooxidation of methanol was investigated on bare platinum electrode, the platinum electrode covered with Nafion and platinum supported on zeolite 13X. Along with classical electrochemical methods, attractor reconstruction was used to make rough distinction among possible reaction mechanisms on different forms of Pt. The obtained transient voltammogram records were used to calculate apparent rate constants for methanol oxidation limiting steps in transient period. All samples contributed to methanol oxidation by basically same reaction mechanism, but with significantly different apparent rate constants.     

Oxidation of 2,6-Di-tert-butylphenol by dioxygen catalyzed by tetrasodium phthalocyaninatocobalt(II) tetrasulfonate in aqueous micellar media

The oxidation of 2,6-di-tert-butylphenol by dioxygen has been investigated in aqueous micellar aggregates of cetyltrimethylammonium bromide (CTAB) using tetrasodium phthalocyaninatocobalt(II) tetrasulfonate (CoPcTsNa₄) as catalyst. The CTAB/CoPcTsNa4 system showed enhanced catalytic activity in the oxidation of 2,6-di-tert-butylphenol compared to that observed in the oxidation reaction in the absence of CTAB. 2,6-Di-tert-butyl-1,4-benzoquinone and 3,5,3′,5′-tetra-tert-butyl-4,4-diphenoquinone were identified as reaction products. The initial rate constants of auto-oxidation reaction was found to increase with increasing the pH range from 7.0 to 13.0. The rate constants k_obs of auto-oxidation reaction showed linear dependence on catalyst concentration. The rate of auto-oxidation reaction was found to fit a Michealis-Menten kinetic model for the saturation of catalyst sites with increasing 2,6-di-tert-butylphenol concentration and dioxygen pressure. Tetrasodium phthalocyaninatocobalt(II) tetrasulfonate in aqueous micellar solution of CTAB was found to be mainly monomeric.     

Oxidation of magnesium with diphenylbismuth and diphenylantimony chlorides in polar solvents

The intermediate and final products of the reactions of magnesium with diphenylantimony and diphenylbismuth chlorides were identified, and the formal kinetic relationships of the process were elucidated. The apparent equilibrium constants, enthalpies, and entropies of adsorption of the reagents on the metal surface, and also the rate constants and activation energies of the reactions in dimethylformamide-p-xylene mixture were determined. Probable schemes of magnesium oxidation with the organobismuth and organoantimony chlorides were considered.