Kinetics of the oxidation of magnetite using simultaneous TG/DSC

Kinetics of the oxidation of magnetite (Fe₃O₄) to hematite (a-Fe₂O₃) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO₃ [1], the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results. Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared by wet chemical methods. These materials are much more reactive than the material studied previously [2], which had been coarsened and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe₂O₃, was formed as an intermediate in the oxidation to the final stable form, a-Fe₂O₃. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects. This revised version was published online in July 2006 with corrections to the Cover Date.     

Non-isothermal kinetic of oxidation of tungsten carbide

Tungsten carbide, WC, has shown dissimilar thermal behavior when it is heated on changeable heating rate and flow of oxidant atmosphere. The oxidation of WC to WO₃ tends to be in a single and slow kinetic step on slow heating rate and/or low flux of air. Kinetic parameters, on non-isothermal condition, could be evaluated to the oxidation of WC to heating rate below 15°C min⁻¹ or low flow of air (10 mL min⁻¹). The reaction is governed by nucleation and growth at 5 to 10°C min⁻¹ then the tendency is to be autocatalytic, JMA and SB, respectively.     

Flow-injection analysis chemiluminescence detection combined with microdialysis sampling for studying protein binding of drug

The bovine serum album binding of streptomycin sulfate was studied in vitro using the technique of microdialysis combined with flow-injection analysis-chemiluminescence detection. The principle of the determination of streptomycin sulfate is that it increases the radiation emitted during the chemiluminescence oxidation of luminol by potassium hexacyanoferrate (III) in sodium hydroxide medium. The drug and protein were mixed in different molar ratios in 0.067 M phosphate buffer, pH 7.4, and incubated at 37 degrees C in a water bath. The microdialysis probe was utilized to sample the mixed solution at a perfusion rate of 5 mul min(-1). The concentration of unbound streptomycin sulfate in the microdialysate was determined by FIA-CL. In vitro recovery of streptomycin sulfate under experimental conditions was 22%. The data obtained by the present microdialysis-FI-CL system was analyzed using the Scatchard analysis and Klotz plot. The results show that the Scatchard plot and Klotz plot are linear, showing that studied drug has only one type of binding sites. The estimated binding parameters agreed well with literature values.     

Determination of oxidation parameters by DSC for polypropylene stabilized with hydroxytyrosol (3,4-dihydroxy-phenylethanol)

Oxidation thermal parameters on samples of polypropylene (PP) stabilized with hydroxytyrosol were determined. For comparison purposes, α-tocopherol and a synthetic phenolic commercial antioxidant (Irganox 1076), were also analyzed. Oxidation induction time (OIt) and oxidation induction temperature (OIT) were determined by differential scanning calorimetry (DSC). The addition of hydroxytyrosol 0.1 mass% to PP was enough to obtain efficient stabilization during processing. Certain decrease in stabilizing properties of natural antioxidants was observed for compression moulded materials at high temperatures. However, these samples were still efficiently stabilized in comparison to the pure material. Hydroxytyrosol showed good performance as polypropylene antioxidant and it might be considered as promising alternative to the use of phenolic synthetic compounds.     

Oxidation of Saturated Fatty Acids Esters DSC investigations

Oxidation of saturated fatty acids ethyl esters: laurate, myristate, palmitate and stearate was investigated by means of DSC techniques under isothermal and non-isothermal conditions. The activation energies of isothermal oxidation were similar to each other (112–123 kJ mol⁻¹) and no influence of carbon length on the rate of oxidative decomposition was observed. Results obtained from non-isothermal experiments were similar only for the first stage of oxidation. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Liquid-phase oxidation of thiols with chlorine dioxide

The products and kinetics of the liquid-phase oxidation of 11 aliphatic and aromatic thiols with chlorine dioxide were studied at –10—+70 °C in organic media. The rate constants and activation parameters of the reaction were determined. The influence of the thiol structure on its reactivity was studied. A strong solvent effect on the reaction rate constant was found, and the reaction mechanism was proposed.     

Thermal oxidation of nonstabilized and stabilized polymers and chemiluminescence

The chemiluminescence under isothermal and nonisothermal conditions accompanying the oxidation of some polymers (polyolefins, polyamides, and polysaccharides) has been examined from the viewpoint of its relationship with the rate of oxidation. The chemical structure of a given type of polymer plays a decisive role in the resulting shape of the chemiluminescence curves. In the presence of an antioxidant, the inhibition periods of oxidation are shifted to longer times and/or higher temperatures. This shift is directly proportional to the type and concentration of stabilizer. Attention has also been paid to the impact of the average molar mass on the oxidizability of the macromolecular material. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 648–660, 2004     

Kinetics of oxidation of fullerene C60 with dimethyldioxirane

The kinetics of the reaction of dimethyldioxirane with fullerene C₆₀ was studied, and the activation parameters logk = (8.3±0.8) – (14.2±0.9)/, ( = 2.3RT kcal mol^–1) (20—45°C) were determined. The formation of paramagnetic particles was detected.     

Surface oxidation of high-surface-area silicon carbide: FT-IR studies

The surface of ultrafine silicon carbide powders, prepared by a laser-driven gas-phase reaction was studied as a self-supporting disk by FT infrared spectrometry. After evacuation silicon and carbon atoms located at the surface give rise tovSiH andvCH bands. When heating in oxygen, subtraction spectra showed features which could be strictly correlated with a progressive growth of a silica layer: SiH and CH bands were replaced by new bands characteristic of amorphous silica and the typical band of surface silanol groups on silica (3745 cm^–1) simultaneously increased.     

The kinetics of the oxidation of cyclopentanone with decaneperoxysulfonic acid

The kinetics of the oxidation of cyclopentanone with decaneperoxysulfonic acid at 291–323 K in CCl₄ has been studied. The reaction is not autocatalytic, and its rate increases linearly with increases in the concentrations of each of the reagents. The addition of CF₃COOH does not affect the reaction rate. The observed results are explained within a scheme which is a special case of the well-known Baeyer-Villiger reaction mechanism established for peroxycarboxylic acids. The effective rate constant of the process has been determined: logk (L mol⁻¹ s⁻¹)=(7.6±1.7) — (42.1±9.6)/θ, where θ=2.30RT kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1827–1829, October, 1993.     

Flow-injection chemiluminescence detection for studying protein binding for drug with ultrafiltration sampling

The novel flow chemiluminescence (CL) system for determination of pipemidic acid was proposed, which was based on the sensitizing effect of pipemidic acid on the CL oxidation of sulfite by KMnO₄ in acid media. Combined with the technique of ultrafiltration, the flow-injection CL system was applied to study in vitro the bovine serum album (BSA) binding of pipemidic acid. The estimated association constant (K) and the number of the binding site (n) on one molecule of BSA were 8.81×10⁴ l/mol and 0.94, respectively. The method provided a fast and simple technique for the study of drug-protein interaction.     

Kinetic evidence for the identical nature of active centers for partial and deep oxidation of ethylene on silver

Kinetics of interaction of ethylene with pre-adsorbed oxygen on a silver film was studied at 473 K and different initial values of surface coverage with oxygen. Dependencies of the initial rates of partial and deep ethylene oxidation on the oxygen surface coverage manifest themselves as peaked curves with coinciding maxima. The results are considered as evidence that the processes of partial and deep ethylene oxidation on silver, when they occur at optimal conditions, proceed viaidentical active centers.     

半胱氨酸和谷胱甘肽氧化反应动力学的研究进展

本文对半胱氨酸和谷胱甘肽的氧化反应动力学的研究进展进行了综述。阐述了含硫氨基酸的生化性质、应用及其重要作用。分别对半胱氨酸、谷胱甘肽的氧化产物分析和动力学机理的研究成果进行了介绍。提出了巯基氨基酸及肽氧化反应动力学研究难点和可行的研究手段及方法。     

Prediction of the ageing of rubber using the chemiluminescence approach and isoconversional kinetics

A common scepticism towards the application of many product formulations results from the fact that their long-term stability is difficult to predict. In the present study we report on a new approach of kinetic analysis of the oxidation reactions of natural rubbers with and without stabiliser in an oxygen atmosphere at moderate temperatures using CL measurements carried out on a newly-developed instrumentation. The kinetic parameters of the oxidation process, calculated from the chemiluminescence’s signals by means of the differential isoconversional method of Friedman, were subsequently applied for the simulation of the rubber aging under different temperature profiles. The presented results are the first stage of research by using the chemiluminescence method to measure the oxidative aging of rubber and predicting the life time of rubber items.     

Review of polymer oxidation and its relationship with materials performance and lifetime prediction

All polymers are intrinsically susceptible to oxidation, which is the underlying process for thermally driven materials degradation and of concern in various applications. There are many approaches for predicting oxidative polymer degradation. Aging studies usually are meant to accelerate oxidation chemistry for predictive purposes. Kinetic models attempt to describe reaction mechanisms and derive rate constants, whereas rapid qualification tests should provide confidence for extended performance during application, and similarly TGA tests are meant to provide rapid guidance for thermal degradation features. What are the underlying commonalities or diverging trends and complications when we approach thermo-oxidative aging of polymers in such different ways? This review presents a brief status report on the important aspects of polymer oxidation and focuses on the complexity of thermally accelerated polymer aging phenomena. Thermal aging and lifetime prediction, the importance of DLO, property correlations, kinetic models, TGA approaches, and a framework for predictive aging models are briefly discussed. An overall perspective is provided showing the challenges associated with our understanding of polymer oxidation as it relates to lifetime prediction requirements.     

Homogeneous high-temperature oxidation of methane

The kinetics of homogeneous deep oxidation of methane in lean mixtures (up to 2 vol % CH₄ in air) in ceramic tubes and fixed beds of ceramic spheres was studied. Experiments with the homogeneous reaction have shown that the methane oxidation occurs via a consecutive scheme through CO formation. The reaction rate of CH₄ oxidation was found to depend upon the equivalent pass diameter with a significant reaction inhibition in packing of small tubes and spheres, reflecting the influence of mass transfer on the radical-chain termination at the ceramic surfaces. It was also found that CO oxidation practically does not depend upon the mass exchange conditions, but it is visibly inhibited by methane. Recommended kinetic equations and their parameters are presented.     

Kinetics of oxidation of glycylglycine by bromamine-T in acid medium

The kinetics of oxidation of a typical dipeptide glycylglycine (GG) by bromamine-T have been studied in HClO₄ medium at 40°C. The rate shows first-order dependence on [BAT]₀ and is fractional order in [GG]₀ which becomes independent of [substrate]₀ at higher [GG]₀. At [H⁺ ] > 0.02mol dm⁻³, the rate is inverse fractional in [H⁺ ] but is zero order at lower [H⁺ ] (≤0.02 mol dm⁻³). Variation in ionic strength or dielectric constant of the medium had no significant effect on the rate. The solvent-isotope effect was measured and = 1.45. Proton inventory studies have been made. The reaction has been studied at different temperatures (308-323 K) and activation parameters have been computed.