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1.
Pulse radiolysis technique has been employed to study the reactions of oxidizing (√OH, N3√) and reducing radicals (e−aq, CO2√−, acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts (λmax=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals (λmax=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ√+/PMZ and ABTS√−/ABTS2− as the reference couple, different methods are employed to determine the one-electron reduction potential of o-vanillin and the average value is estimated to be 1.076±0.004 V vs. NHE at pH 6. The phenoxyl radicals of o-vanillin were able to oxidize ABTS2− quantitatively. The eaq− is observed to react with o-vanillin with rate constant value of 2×1010 dm3 mol−1 s−1. CO2√− and acetone ketyl radical are also observed to react with o-vanillin by electron transfer mechanism and showed the formation of transient absorption bands with λmax at 350 and 390 nm at pH 4.5 and 9.7, respectively. The pKa of the one-electron reduced species was determined to be 8.1. The results indicate that the aldehydic group is the most preferred site for electron addition. 相似文献
2.
Research on the radiolysis of vitamins is of considerable interest since these compounds are important nutritional constituents in foods and in dietetic supplements. In spite of these considerations there are few data and very often difficult to compare for the radiolytic behavior of vitamins. In this work we focused our attention on to the study of the radiolysis of cyanocobalamin (vitamin B12) in solid state and in aqueous solutions. The procedure was followed by HPLC and UV-spectroscopy. The results obtained in aqueous solutions showed a dependence of the decomposition as a linear function of the dose. The G of decomposition for a 1×10−5 M solution was 3.3. In the solid state the vitamin was very stable towards the irradiation in the conditions used in this study with a G=2.1×10−3. A study made with Serratia marcescens as a microbiological contaminant showed that at the sterilization dose there is a destruction of the vitamin in aqueous solution. In the solid state the degree of decomposition was 7%. 相似文献
3.
The results of the radiolysis of aqueous solutions of thiamine (vitamin B1) are presented. The yields for decomposition of thiamine and the product of radiolytic products were determined. The G values decrease as the dose increases. Some radiolytic products were identified. Decomposition of thiamine was slightly dependent on the presence of oxygen and on the pH of the solution. At pH 4.4 with a concentration of 2.5 × 10−4 mol L−1 of thiamine in an oxygen free aqueous solution, the G0 value for decomposition is 5.0. 相似文献
4.
A new, highly sensitive and simple kinetic method for the determination of thyroxine was proposed. The method was based on the catalytic effect of thyroxine on the oxidation of As(III) by Mn(III) metaphosphate. The kinetics of the reaction was studied in the presence of orthophosphoric acid. The reaction rate was followed spectrophotometrically at 516 nm. It was established that orthophosphoric acid increased the reaction rate and that the extent of the non-catalytic reaction was extremely small. A kinetic equation was postulated and the apparent rate constant was calculated. The dependence of the reaction rate on temperature was investigated and the energy of activation and other kinetic parameters were determined.
Thyroxine was determined under the optimal experimental conditions in the range 7.0 × 10−9 to 3.0 × 10−8 mol L−1 with a relative standard deviation up to 6.7% and a detection limit of 2.7 × 10−9 mol L−1. In the presence of 0.08 mol L−1 chloride, the detection limit decreased to 6.6 × 10−10 mol L−1. The proposed method was applied for the determination of thyroxine in tablets. The accuracy of the method was evaluated by comparison with the HPLC method. 相似文献
5.
J.M. Meinders H.J. Busscher 《Colloids and surfaces. A, Physicochemical and engineering aspects》1993,80(2-3):279-285
The residence-time-dependent desorption during the deposition of polystyrene particles 736 nm in diameter on glass was studied in situ using a parallel-plate flow chamber and automated image analysis. Comparison of successively grabbed images yielded the initial desorption rate coefficient, and final desorption rate coefficient and a relaxation time for the transition from the initial to the final desorption state, i.e. ageing of the bonds. Desorption experiments were performed from suspensions with different potassium nitrate concentrations (1, 10 and 50 mM) and at varying shear rates (15–200 s−1. The initial desorption rate coefficient β0 ranging from 1 × 10−3 to 20 × 10−3s−1, and the final desorption rate coefficient β∞, ranging from 0.01 × 10−3 to 0.65 × 10−3s−1 were both larger than the desorption rate coefficients calculated neglecting a possible residence time dependence. These desorption rate coefficients, β, ranged from 0.005 × 10−3 to 0.40 × 10−3s−1. The relaxation times, at which the adhesion of the polystyrene particles entered a more irreversible state of adhesion compared with their initial state of adhesion, varied from 100 to 1000 s. The desorption rate coefficients as well as the relaxation time showed major variations with the shear rate and the ionic strength of the suspension. At high ionic strength, the initial and final desorption rate coefficients increase and the relaxation time decreases with increasing shear rate, whereas at low ionic strength the desorption rates decrease and the relaxation time increases with increasing ionic strength. This study provides direct evidence that the interaction forces between adhering particles and a collector surface change over time. 相似文献
6.
A laser-induced fluorescence (LIF) immunoassay technique based on capillary electrophoretic (CE) separation is demonstrated. The analysis of digoxin in serum at clinically useful concentration levels of 10−9 to 10−10 M is achieved using this technique. The chemistry presented here using digoxigenin-labeled B-phycoerythrin was selected as a convenient model for the exploration of CE-LIF-based immunoassays. The LIF system described here exhibits detection limits in the low 10−11 M range for several common fluorophores. The data presented in this report are one of the first examples of nanomolar quantitative analysis in a human serum matrix by CE. 相似文献
7.
The phophorescence of biacetyl induced by an energy transfer to biacetyl from triplet benzene produced in the pulse radiolysis of benzene-biacetyl mixtures has been studied. The time required to reach the maximum intensity of phosphorescence, tmax, after the electron pulse, varies as a function of biacetyl pressure at constant benzene pressure (40 torr), which gives the lifetime of triplet benzene τ = (6.7 ± 3.2) × 10−6 s and the rate constant of the energy transfer kC6H6*(T1) + biacetyl = (1.6 ± 0.7) × 10−10 cm3 molecule−1 s−1. 相似文献
8.
A. I. Alonso A. M. Urtiaga S. Zamacona A. Irabien I. Ortiz 《Journal of membrane science》1997,130(1-2):193-203
In this work it is reported that the kinetic modelling of the separation of cadmium from phosphoric acid by non-dispersive solvent extraction. Using Cyanex 302 as selective extractant, the extraction step was carried out in a hollow fibre module containing polypropylene fibres, whereas the concentration step required a ceramic module with tubular channels due to the high acidity of the backextraction agent. Application of the methodology previously reported by the authors led to the development of a kinetic model with three design parameters, i.e., equilibrium constant of the extraction reaction (K'e = 6 × 103 mol−2/l−2), membrane mass transport coefficient of the extraction module Kme=8.33×10−8 m/s) and of the backextraction module (Kms=3.33×10−8 m/s), that described satisfactorily the behaviour of the separation-concentration system. Thus, in this work a new application of the non-dispersive solvent extraction technology is presented, characterising at the same time the behaviour and parameters of a new type of contactor, i.e., a tubular ceramic module. 相似文献
9.
Chen X Siems WF Asbury GR Yount RG 《Journal of the American Society for Mass Spectrometry》1999,10(12):75-1340
The photochemical reaction of azide derivatives induced by ultraviolet (UV) laser in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) is reported. A novel synthesized class of azide aromatic derivatives, spin-labeled photoaffinity non-nucleoside adenosine triphosphate (ATP) analogs which are useful probes in study of muscle contraction mechanism, is used in this investigation. In the negative ion MALDI spectra of these ATP analogs, “fingerprint” peaks corresponding to [M − 10 − 1]−, [M − 12 − 1]−, [M − 16 − 1]−, [M − 26 − 1]−, [M − 28 − 1]−, [M − 41 − 1]−, and [M − 42 − 1]− were observed with relative intensities depending on the MALDI matrix. Only the [M − 16 − 1]− is present in the similar mass spectra of the analog in which the azido group is replaced by a hydrogen. A model is suggested for the photochemical reactions of azide derivatives under UV laser irradiation. The photoreaction fingerprint information is diagnostically useful in characterization of azido compounds, especially for spin-labeled photoaffinity non-nucleoside ATP analogs. 相似文献
10.
The radiation polymerization of vinylpyrrolidone (VP) in aqueous solution was studied. For pulse radiolysis experiments, absorption and light scattering detection were used to measure the nature and concentration of reacting species. Some aspects of these detection methods in relation to the kinetic measurements in the radiation-induced polymerization reactions were discussed. The absorption spectra of intermediates were taken. The decay of the transient species follows second-order kinetics with the rate constant depending on the VP concentration. The growth of the absorbance and intensity of scattered light in the concentrated VP solution (9,4 × 10−1 mol dm−3) occur both in the same timescale. 相似文献
11.
The rate coefficients for the reactions of C2H and C2D with O2 have been measured in the temperature range 295 K T 700 K. Both reactions show a slightly negative temperature dependence in this temperature range, with kC2H+O2 = (3.15 ± 0.04) × 10−11 (T/295 K)−(0.16 ± 0.02) cm3 molecule−1 s−1. The kinetic isotope effect is kC2H/kC2D = 1.04 ± 0.03 and is constant with temperature to within experimental error. The temperature dependence and the C2H + O2 kinetic isotope effect are consistent with a capture-limited metathesis reaction, and suggest that formation of the initial HCCOO adduct is rate-limiting. 相似文献
12.
Matthias Roßberg Werner Strube Jrg Wollbrandt Eberhard Linke 《Journal of photochemistry and photobiology. A, Chemistry》1994,80(1-3):61-65
In this paper, we investigate the rotationally resolved spectra of hot CF radicals generated after IR multiphoton dissociation (IRMPD) of CFCl3 or CF2Cl2 and subsequent UV photodissociation. It is shown that these conditions are advantageous for the spectroscopy of transitions involving high rotational quantum numbers and hot bands. Thus molecular constants of CF for the first vibrationally excited state of the electronic ground state (Av=77.1 cm−1, Bv=1.389 cm−1, Dv=6.570×10−6 cm−1) are determined for the first time or are calculated more accurately. The spectroscopic method used was resonance-enhanced multiphoton ionization (REMPI) spectroscopy. 相似文献
13.
A thin film of mixed-valent CuPtCl6 is deposited on a glassy carbon electrode by continuous cyclic scanning in a solution containing 3×10−3 M CuCl2+3×10−3 M K2PtCl6+1 M KCl in the potential range from 700 to −800 mV. The cyclic voltammetry is used to study the electrochemical behaviors of nitrite on CuPtCl6/GC modified electrode and the electrode displays a good catalytic activity toward the oxidation of nitrite. The effects of the film thickness, pH, the electrode stability and precision have been evaluated. Experiments in flow-injection analysis are performed to characterize the electrode as an amperometric sensor for the detection of nitrite. The modified electrode shows a wide dynamic range, quite a low detection limit and short response time. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1×10−7–2×10−3 M with a detection limit of 5×10−8 M. 相似文献
14.
Vasudev N. Shetti D. Srinivas P. Ratnasamy 《Journal of molecular catalysis. A, Chemical》2004,210(1-2):171-178
Alkali metal ions, when present during the synthesis of TS-1, lead to inactive oxidation catalysts. However, when added, in small amounts, to the reaction medium during the epoxidation of allyl alcohol or allyl chloride by H2O2 over TS-1, they increase the selectivity for the epoxide. To probe this phenomenon in detail, the influence of pH and alkali and alkaline earth ions on the structure and catalytic activities of the oxo-Ti species generated in H2O2. TS-1 and TiMCM-41 systems have been investigated using EPR and diffuse reflectance UV-Vis spectroscopies. In acidic and neutral pH, two types of superoxo-Ti species (Ti(O2√−)), A and B, are observed over TS-1. In the presence of alkali and alkaline metal ions or at high pH, Ti3+ ions and a new type of Ti(O2√−) species, A′ are observed. Only the B-type species, however, is observed on TiMCM-41. The A-type are more reactive than the B-type. Epoxide selectivities approaching 100% can be achieved by a proper control of the reaction medium. 相似文献
15.
The kinetics of the association reaction of CF3 with NO was studied as a function of temperature near the low-pressure limit, using pulsed laser photolysis and time-resolved mass spectrometry. CF3 radicals were generated by photolysis of CF3I at 248 nm and the kinetics was determined by monitoring the time-resolved formation of CF3NO. The bimolecular rate constants were measured from 0.5 to 12 Torr, using nitrogen as the buffer gas. The results are in very good agreement with recent data published by Vakhtin and Petrov, obtained at room temperature in a higher pressure range and, therefore, the two studies are quite complementary. A RRKM model was developed for fitting all the data, including those of Vakhtin and Petrov and for extrapolating the experimental results to the low- and high-pressure limits. The rate expressions obtained are the following: k1(0) = (3.2 ± 0.8) × 10−29 (T/298)−(3.4±0.6) cm6 molecule−2 s−1 for nitrogen used as the bath gas and k1(∞) = (2.0 ± 0.4) × 10−11 (T/298)(0±1) cm3 molecule−1 s−1. RRKM calculations also help to understand the differences in reactivity between CF3 and other radicals, for the same association reaction with NO. 相似文献
16.
Rita Terzian Nick Serpone Marye Anne Fox 《Journal of photochemistry and photobiology. A, Chemistry》1995,90(2-3):125-135
Photo-oxidations of environmental organics in illuminated TiO2 dispersions have implicated surface-bound •OH radicals and/or valence band holes. To explore the implications of the former oxidizing entity, six isomeric xylenols (dimethylphenols) were examined by pulsed (nanoseconds to milliseconds) radiolysis methods. The spectral and kinetic characteristics of formation and decay of the transients formed by the reaction of N3•, •OH and H• radicals with these xylenols were assessed in buffered (pH 4, 10−3 M phosphate) aqueous media, where the xylenols exist in their protonated form (pK ≈ 10.19–10.65). The products from the reaction of N3• with 2,6- and 3,4-xylenol were exclusively the corresponding dimethylphenoxyl radicals, formed via electron transfer followed by deprotonation. In contrast, except with 3,4-xylenol, the principal radical intermediates formed initially upon reaction with •OH were the corresponding •OH adducts, the dihydroxydimethylcyclohexadienyl radicals. 3,4-Xylenol was examined in the pH range 4–10. At pH 8 the initial •OH adduct (dihydroxy-3,4-dimethylcyclohexadienyl radical) was subsequently transformed (about 20%–40%) via water elimination into the dimethylphenoxyl radical. In contrast, at pH 9 and 10 the •OH adduct and the dimethylphenoxyl radical were formed concurrently (about 60% •OH adduct and about 40% dimethylphenoxyl species), the latter through an inner-sphere electron transfer pathway. The switch in behaviour from pH 8 to pH 9 suggests that the pKa of the dihydroxy-3,4-dimethylcyclohexadienyl radical is about 8–9, about 2 pK units below the pKa of the parent substrate (10.4). A mechanism for the conversion of the •OH adduct to the dimethylphenoxyl radical is proposed. Reaction of 2,6-xylenol with H• radicals gave exclusively the H• adduct (hydroxycyclohexadienyl radical), whose spectral characteristics are similar to those of the related •OH adduct. 相似文献
17.
A novel polymeric membrane sensor sensitive to (4-sulphophenylazo-)1-naphthylamine (SPAN) based on the use of tris(bathophenanthroline) Ni(II)–SPAN ion pair as an ion exchanger in plasticised PVC membrane is described. The sensor exhibits a linear calibration plot with near-Nernstian anionic slope of −55.0±0.3 mV log[SPAN]−1 over the concentration range 10−6–10−2 mol l−1 at pH 7. The sensor shows working range over the pH 6–8, response time of 20 s for 10−5 mol l−1 and operational lifetime of 8 weeks. The sensor is used for quantification of micro quantities of nitrite ion by a prior conversion into the more lipophilic SPAN ion, which is measured with adequate sensitivity, and high selectivity using SPAN sensor. Validation of the method according to the quality assurance standards shows good performance characteristics. The sensor is satisfactory utilised for potentiometric determination of nitrite ion in wastewater samples and meat products. The results are favourably compared with data obtained using the standard spectrophotometric procedure involving the same reaction. 相似文献
18.
The reactions of hydroxyl radical, hydrogen atom and hydrated electron intermediates of water radiolysis with N-isopropylacrylamide (NIPAAm) were studied by pulse radiolysis in dilute aqueous solutions. OH, H and eaq− react with NIPAAm with rate coefficient of (6.9±1.2)×109, (6.6±1)×109, and (1.0±0.2)×1010 mol−1 dm3 s−1. In OH and H radical addition to the double bond mainly -carboxyalkyl type radicals form, (OHCH2CH√C(N-i-C3H7)O and CH3CH√C(N-i-C3H7)O). In reaction of eaq− oxygen atom centered radical anion is produced (CH2CHC√(N-i-C3H7)O−), the anion undergoes reversible protonation with pKa=8.7. There is also an irreversible protonation on the β-carbon atom that produces the same radical as forms in H atom reaction (CH3CH√C(N-i-C3H7)O). The -carboxyalkyl type radicals at low NIPAAm concentration (0.1–1 mmol dm−3) mainly disappear in self-termination reactions, 2kt,m=8.4×108 mol−1 dm3 s−1. At higher concentrations the decay curves reflect the competition of the self-termination and radical addition to monomer (propagation). The termination rate coefficient of oligomer radicals containing a few monomer units is 2kt≈2×108 mol−1 dm3 s1. 相似文献
19.
Christine Kempter Wilhelm P tter Norbert Binding Holger Kl ning Ute Witting Uwe Karst 《Analytica chimica acta》2000,410(1-2):47-64
A new approach for the synthesis and application of tailor-made derivatizing agents for liquid chromatography is presented. The s-triazine ring serves as the backbone for these substances. Synthesis is performed in three steps based on substitution reactions of cyanuric chloride. A chromophor or fluorophor, a reactive group and a third moiety intended to modify the polarity of the derivatizing agent are coupled to cyanuric chloride. The three groups may be selected freely from molecules which carry functional groups which might be linked to cyanuric chloride. As an example, the synthesis and application of a respective reagent for the determination of aldehydes and ketones using HPLC with fluorescence detection is carried out. Baseline separation of the derivatives and low limits of detection (1.5×10−9 mol/l for formaldehyde, 2.0×10−9 mol/l for acetaldehyde and 3.0×10−9 mol/l for p-tolualdehyde) are obtained with the new versatile reagent. 相似文献
20.
Measurements of Rayleigh scattering and relativistic calculations of incoherent scattering functions
Recent measurements of Rayleigh scattering employing neutron capture γ-rays are presented. Experimental conditions are achieved such that the Rayleigh contribution is dominant and much larger than the other competing coherent processes. A detailed comparison with the modified relativistic form factor (MRFF) approximation is made and it is concluded that the latter overestimates the cross-section by 3–4%. New calculations of S, the incoherent scattering function, are presented in the relativistic treatment of Ribberfors and Berggren, using multiconfiguration Dirac–Fock relativistic wavefunctions. Tables of S, for Z=1–110, are shown on a momentum transfer mesh identical to previous non-relativistic calculations. S has been calculated at a representative angle θ=60° and energies compatible with the presentation mesh. For other scattering angles, the values presented in the tables are accurate to within 1–2% for momentum transfers larger than 0.1 Å−1. In the region below 0.1 Å−1 the accuracy worsens with decreasing momentum transfer, reaching 6% at 0.01 Å−1 and 10% at 0.005 Å−1. The same multiconfiguration wave functions were used to evaluate new MRFFs. The new elastic scattering cross sections differ by 3–6% compared with calculations based on single configuration wave functions. 相似文献