Investigation of the self-accelerating process of ascorbic acid by in-situ circular dichroism spectroelectrochemistry

The electrode reaction process of ascorbic (Vc) was studied by in-situ circular dichroic(CD) spectroelectrochemistry with a long optical path thin layer cell on glassy carbon(GC) electrode. The spectroelectrochemical data were analyzed by the double logarithmic method together with nonlinear regression. The results suggested that the mechanism of Ve in pH 7.0 phosphate buffer solution at GC electrode was a two-electron irreversible electrooxidation followed by adsorption of the oxidation product. That is a self-accelerated process. Some kinetic parameters at free and at adsorbed electrode surface, i.e, the formal potentials, E-0' = 0.09 V, E-a(0') = 0.26 +/- 0.02 V; the electron transfer coefficient and number of transfered electron, alpha n = 0.41, alpha(a)n = 0.07;the standard heterogeneous electron transfer rate constant, k(0) = 8.0 x 10(-5) cm.s(-1), k(a)(0) = 1.9 x 10(-4) cm.s(-1) and adsorption constant, beta = 102.6 were also estimated.

In situ UV-vis and CD thin-layer spectroelectrochemistry study of the electrochemical behavior of L-noradrenaline in alkaline solution

Electrochemical redox behavior of noradrenaline in alkaline solution on a glassy carbon electrode has been investigated by in situ UV-vis and CD spectroelectrochemistry by using a long optical path thin-layer cell. The experimental data were processed by using a double logarithmic method of analysis together with nonlinear regression which confirmed that the first step in both the oxidation of noradrenaline and reduction of noradrenochrome is a two-electron irreversible process governed by an EE mechanism. The kinetic parameters of the electrode reactions, i.e., charge transfer coefficient and the number of electrons transferred, alpha(1)n(1) = 0.11 and alpha(2)n(2) = 0.23, formal potentials modified with kinetics, E-1(0') = 0.65 (+/- 0.01) V and E-2(0') = 0.72V and standard rate cnstants, k(1)(0) = 7.0(+/-0.5)x10(-5) cm s(-1), for the first and second steps in the oxidation process of noradrenaline, and similarly, alpha(1)n(1) = 0.33, alpha(2)n(2) = 0.58, E-1(0') = 0.37(+/-0.01) V, E-0' = -0.25 (+/-0.01) V and k(1)(0) approximate to k(2)(0) = 1.06 (+/-0.05)x10(-4) cm s(-1) for the first and second steps in the reduction process of noradrenochrome were also determined.

Preparation of a composite film electrode containing 12-tungstosilicic acid and its electrocatalytic reduction for nitrite

A novel organic-inorganic composite film was formed by attaching Keegin-type heteropolyanion, SiW12O404- (devoted briefly as SiW12), on a glassy carbon electrode derivatized by 4-aminophenyl group. The composite film has an ionic bonding character between SiW12 and the surface amino group, which greatly improves the Blm stability and exhibits a more reversible electrochemical behavior. The modified electrode offers an excellent and stable electrocatalytic response for the reduction of nitrite. Possible mechanism was provided for the reaction of nitrite with SiW12O404-/aminophenyl composite film.

Electrocatalytic oxidation and flow amperometric detection of hydrazine at an electropolymerized 4-vinylpyridine/palladium film electrode

A poly(4-vinyl)pyridine (PVP)/Pd film electrode was constructed for the electrocatalytic detection of hydrazine. The preparation of the PVP/GC electrode was performed by electropolymerization of the monomer 4-vinylpyridine onto the surface of a glassy carbon electrode. Subsequently, palladium is electrodeposited onto the polymer modified electrode surface. The ion-exchange function of PVP polymer is helpful to this process in view of the tetrachlorapalladate anion. Compared with the Pd/GC electrode, the modified electrode displays a better mechanical stability in a flowing stream. The PVP/Pd film electrode exhibits higher sensitivity when detecting hydrazine with a detection limit of 0.026 ng (S/N=3).

Electrochemical reactions of cyanocobalamin in acidic media

Electrochemical reactions of cyanocobalamin, CN-Cbl[Co(III)], were studied at glassy carbon electrodes in acidic media by means of cyclic voltammetry and differential pulse polarography. It was found that in pH 0 solution, CN-Cbl[Co(III)] exists mainly in the base-off form, {CN-Cbl[Co(III)]}(base-off). It can undergo a one-electron reduction and a follow-up chemical reaction to form {H2O-Cbl[Co(II)]}(base-off). The rate-constant k of the follow-up decyanation reaction is 0.022 s(-1). {H2O-Cbl[Co(II)]}(base-off) is further reduced to obtain H2O-Cbl[Co(I)]. (C) 1997 Elsevier Science S.A.

Study on the Property of Vitamin B-12-Modified Electrode and its Electrocatalysis for the Reduction of Oxygen

It was found that vitamin B-12 could be strongly adsorpted on the anodized glassy carbon electrode to form a vitamin Thy-modified glassy carbon electrode. The modified electrode is stable in a wide pH range. The electrochemical characteristics of the modified electrode were studied in details. In addition, it was found that the reduction of oxygen could be catalyzed by the modified electrode to form H2O2. An EC mechanism was suggested for the process, and the follow up chemical reaction might he the rate determined step.

Preparation of highly dispersed gold microparticles and their electrocatalytic activity for the oxidation of formaldehyde

The electrochemical preparation of highly dispersed Au microparticles on the surfaces of glassy carbon (GC) electrodes and their electrocatalytic activities for the oxidation of formaldehyde were studied. It was found that the reduction of Au3+ to Au is controlled by diffusion and the formation mechanism of Au microparticles on the GC surfaces corresponds to an instantaneous nucleation and diffusion-controlled three dimensional growth process. The particle size is about 80-90 nm in diameter after the electrochemical ageing treatment. These highly dispersed Au microparticles have high surface areas and exhibit better electrocatalytic activity than that of bulk-form Au toward the electrochemical oxidation of formaldehyde in alkaline media.

Study of cupric hexacyanoferrate-modified platinum electrodes using probe beam deflection and electrochemical quartz crystal microbalance techniques

The ion exchange mechanism accompanying the oxidation/reduction processes of cupric hexacyanoferrate-modified platinum electrodes in different aqueous electrolyte solutions has been studied by means of in situ probe beam deflection and the electrochemical quartz crystal microbalance technique. The results demonstrate that the charge neutrality of the film during the reoxidation/reduction process is accomplished predominantly by the movement of cations, but anions and/or solvent are also participator(s). Moreover, in KHC8H4O4 (potassium biphthalate) solution, the EQCM data obtained from chronoamperometry experiment are more complicated than those in KCl and K2SO4 solutions. (C) 1997 Elsevier Science Ltd.

A study of the electrochemical characteristics of the Co(salen)-Nafion modified electrode

A Co(salen)-Nafion modified electrode was prepared by immersing a glassy carbon electrode coated with the Nafion film into the aqueous solution with saturated Co(salen), The modified electrode showed a stable electrochemical reaction of Co(salen) at about 0 V(vs, SCE), The result of XPS indicated the valence of cobalt in Co(salen) changes from +2 before to +3 after Co(salen) enters the Nafion film, It is due to forming axis coordination of cobalt with sulfonic group in Nafion film, It was found that the mode of electron transfer in Co(salen)-Nafion modified electrode was controlled by physical diffusion and electron hopping, It was also found that the modified electrode could catalyze the reduction of O-2 to H2O2. The real catalyst may be the adduct of Co-I(salen) and O-2.

Detection of menadione sodium bisulfite (vitamin K-3) by reversed-phase high performance liquid chromatography with series dual-electrode amperometric detector

A reversed-phase high-performance liquid chromatography with series dual glassy carbon electrodes for the amperometric detection of water-soluble menadione is described. The complex post-column derivatization reaction and the high background currents were avoided. The menadione sodium bisulfite was reduced at -0.3 V vs. SCE at the upstream (generator) electrode and oxidized at +0.2V vs. SCE at the downstream (collector) electrode. The mobile phase was 0.2moll(-1) HAc-NaAc aqueous buffer (pH 5.50) and 40% (v/v) methanol. The linear response was in the range of 35 ng to 15 mu g, with a detection Limit of 15 ng (S/N=3). The correlation coefficient was 0.9997 (n=6). The electrochemical detection with series dual electrodes has a higher selectivity for menadione (vitamin K-3) compound than with UV detection.

Detection of hydrazine, methylhydrazine, and isoniazid by capillary electrophoresis with a palladium modified microdisk array electrode

A palladium particle-modified carbon fiber microdisk array electrode was designed and employed in capillary electrophoresis for the simultaneous detection of hydrazine, methylhydrazine, and isoniazid. The Pd-modified microdisk electrode had high catalytic ability for hydrazines and exhibited good reproducibility and stability. The response for hydrazine was linear over 3 orders of magnitude with a correlation coefficient of 0.993. The detection limits far hydrazine, methylhydrazine, and isoniazid were 1.2, 2.1, and 6.2 pg, respectively.

In situ electrochemical scanning tunneling microscopy investigation of renewing graphite surface accompanied by electrochemical reaction

In situ electrochemical scanning tunneling microscopy (ECSTM) has been employed to follow the renewal process of a graphite electrode accompanied by flavin adenine dinucleotide (FAD) electrochemical reaction which involves adsorption of the reduced form (FADH(2)) and desorption of the oxidized form (FAD). The renewal process initiates from steps or kinks on the electrode surface, which provide high active sites for adsorption. This renewal depends on the working electrode potential, especially in the range near the FAD redox potential. Our experiment suggests that delamination of the graphite surface is caused by interaction between the substrate and adsorbed molecules. A simple model is proposed to explain this phenomenon.

Scanning tunneling microscopy characterization of electrode materials in electrochemistry

Ex situ and in situ STM characterization of the electrode materials, including HOPG, GC, Au, Pt and other electrodes, is briefly surveyed and critically evaluated. The relationship between the electrode activity and surface microtopography is discussed.

STM investigation of HOPG superperiodic features caused by electrochemical pretreatment

The surface topography of highly oriented pyrolytic graphite (HOPG) which has been subjected to anodized treatment at a low potential (less positive) has been exhaustively studied using scanning tunneling microscopy (STM). Characterization of graphite surface has revealed that a small percentage of the surface (5%-10%) shows superperiodic features. In this case, the typical topographical features are triangular networks with the observed periodicities in the plane ranging from approximately 26 to 240 nm with peak-to-peak amplitudes out of the plane extending from 7 to 35 Angstrom. We show that this triangular network can be attributed to the injection of electrolyte and solvent in the earliest stages of oxidation of the basal HOPG electrode surface. A simple model is proposed to explain this phenomenon.

DETERMINATION OF AMINOPYRINE AND ITS METABOLITE BY CAPILLARY ELECTROPHORESIS ELECTROCHEMICAL DETECTION

An electrochemical pretreatment regime for a cylindrical carbon fibre microelectrode was optimized for the determination of aminopyrine (AM) and its metabolite 4-aminoantipyrine (AAN) by capillary electrophoresis (CE)-electrochemical detection (ED). Under optimized conditions, a response of high sensitivity and stability was obtained for AM and AAN at a detection voltage as low as 0.9 V following CE-ED, by which AM and AAN were separated satisfactorily. The calibration graph was linear over three orders of magnitude and the limits of detection for AM and AAN were in the femtomole range.