Scanning electrochemical microscopy study of electron transfer across the water/1,2-dichloroethane interface induced by common ions

In this work, we report the reverse electron transfer reaction between TCNQ in 1, 2-dichloroethane (DCE) and ferrocyanide in water. This process is a thermodynamic unfavorable reaction and the reverse electron transfer reaction can only be obtained by scanning electrochemical microscopy(SECM) in the presence of suitable potential-determining ions, which govern the interfacial potential difference. In our case, the potential determining ions are tetrabutylammonium ion(TBA(+)) and tetraphenylarsonium ion (TPAs+). The effects of the concentrations of TBA(+) and TPAs+ in two phases and other parameters have been studied in detail. The apparent heterogeneous rate constants(k(i)) were obtained under different values of K-p(K-p=c(i)(w)/c(i)(o)) for both cases by fitting the SECM approach curves with theoretical ones and the results showed that they were controlled by the interfacial potential differences. The relationship between apparent heterogeneous rate constants and the interfacial potential differences obeys Butler-Volmer theory.

Kinetic isotope effects of proton transfer reaction for amines by ultrasonic relaxation methods: Unexpected evidence from the results in ethylamine solutions

Ultrasonic absorption coefficients for ethylamine in heavy water (D2O) and in light water (H2O) have been measured in the frequency range from 0.8 to 220 MHz at 25 degrees C. A single relaxational process has been observed in these two kinds of solutions. From the concentration dependence of the ultrasonic relaxation parameters, and following the reaction mechanism proposed by Eigen et al. for ethylamine in H2O, the causes of the relaxations have been attributed to a perturbation of an equilibrium associated with a deuteron or proton transfer reaction. The rate and equilibrium constants have been estimated from deuterioxide or hydroxide ion concentration dependence of the relaxation frequency, and the kinetic isotope effects have been determined. In addition, the standard volume changes of the reactions have been calculated from the concentration dependence of the maximum absorption per wavelength, and the adiabatic compressibility has also been determined from the density and sound velocity for ethylamine in D2O and in H2O, respectively. These results are compared with those for propylamine and butylamine and are discussed in relation to the different kinetic properties between D2O and H2O, the reaction radii derived by Debye theory, and the structural properties of the reaction intermediate.

EQCM characterization of self-assembled kinetics of 4-pyridyl hydroquinone at Pt surface and its ion transfer in electrochemical redox

The adsorbed kinetics, proton transportation in electrochemical redox process of 4-pyridyl hydroquinone (4PHQ) self-assembled monolayer (SAM) modified Pt electrode were studied by electrochemical quartz crystal microbalance (EQCM) in situ. It proved that the electrode was modified by a monolayer and underwent a rapid electron transfer. It was a slow adsorbed kinetic process. The ion transfer in the electrochemical redox at the SAM-modified electrode surface mainly involved into the hydrate hydrogen ion.

In-situ circular dichroism spectroelectrochemical study on the redox process of norepinephrine

The redox process of norepinephrine in pH = 7.0 phosphate buffer solution at glassy carbon electrode was studied by circular dichroism spectroelectrochemistry with a long optical path thin layer cell. The spectroelectrochemical data were analyzed with the double logarithm method. According to the double logarithsmic plot results, the mechanism of electrochemical oxidation of norepinephrine is an irreversible process with a subsequent chemical reaction (EC) to form a norepinephrinechrome. Both of norepinephrinequinone and norepinephrinechrome are followed E mechanisms. Some kinetic parameters about the electrochemical process, i.e. the electron transfer coefficient and number of electron transfered, alpha n = 0.38, the formal potential, E-1(0)' = 0.20 V, the standard heterogenous electron transfer rate constant, k(1)(0) = 1.2 x 10(-4) cm s(-1) for the oxidation of norepinephrine, alpha n = 0.37, E-2(0)' = 0.25 V and k(2)(0) = 4.4 x 10(-5) cm . s(-1) for the reduction of norepinephrinequnone and alpha n = 0.33, E-3(0)' = -0.25V and k(3)(0) = 1.1 x 10(-4) cm . s(-1) for the reduction of norpinephrinechrome, were also estimated.

Isotope effect in proton-transfer and association-dissociation reactions for butylamine by ultrasonic relaxation methods

Ultrasonic absorption coefficients were measured for butylamine in heavy water (D2O) in the frequency range from 0.8 to 220 MHz and at concentrations from 0.0278 to 2.5170 mol dm(-3) at 25 degrees C; two kinds of relaxation processes were observed. One was found in relatively dilute solutions (up to 0.5 mol dm(-3)), which was attributed to the hydrolysis of butylamine. In order to compare the results, absorption measurements were also carried out in light water (H2O). The rate and thermodynamic parameters were determined from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. The isotope effects on the diffusion-controlled reaction were estimated and the stability of the intermediate of the hydrolysis was considered while comparing it with the results for propylamine in H2O and D2O. Another relaxation process was observed at concentrations greater than 1 mol dm(-3) in D2O. In order to examine the solution characteristics, proton NMR measurements for butylamine were also carried out in D2O. The chemical shifts for the gamma- and delta-proton in butylamine molecule indicate the existence of an aggregate. From profiles of the concentration dependence of the relaxation frequency and the maximum absorption per wavelength of sound absorption, the source of the relaxation was attributed to an association-dissociation reaction, perhaps, associated with a hydrophobic interaction. The aggregation number, the forward and reverse rate constants and the standard volume change of the reaction were determined. It was concluded from a comparison with the results in H2O that the hydrophobic interaction of butylamine in D2O is stronger than that in H2O. Also, the isotope effect on this reaction was interpreted in terms of the solvent structure.

A double logarithmic method for studying the adsorption effects in an irreversible electrochemical process

The adsorption of an electroinactive product greatly influences an irreversible electrochemical reaction in three ways, including self-block, self-inhibition, and self-acceleration, and changes not only the heterogeneous electron-transfer rate constant but also the modified formal potential and electron-transfer coefficient of the electrochemical reaction. In order to study these adsorption effects, a double logarithmic method was suggested to be used in processing the potential-controlled thin layer spectroelectrochemical data. The result shows three types of double logarithmic plots for three kinds of adsorption effects. These double logarithmic plots can be a diagnostic criterion of the adsorption effects and enable us to determine some thermodynamic and kinetic parameters. The combination of nonlinear regression with double logarithmic method is a convenient way to examine the suggested mechanism and to extract more information from the limited experimental data. Some examples are given to test the theoretical results. (C) 1999 The Electrochemical Society. S0013-4651(98)05-012-5. All rights reserved.

A double logarithmic method in thin-layer spectroelectrochemistry for distinction among mechanisms of an irreversible electrochemical process

A simple double logarithmic method in potential-controlled thin-layer spectroelectrochemistry for an irreversible electrochemical process has been studied by numerical analysis and examined by experimental examples. This simple algorithm has a novel function offering some important information about the mechanism of a complex electrochemical process directly from a limited amount of potential-spectrum data, and can be used to distinguish different reaction mechanisms such as E, EC, EE, as well as to determine the electron-transfer coefficient, a, and the kinetically modified E-0'. Combination of the double logarithmic method with nonlinear regression provides a powerful tool to examine the proposed mechanism and also to estimate other thermodynamic and kinetic parameters. (C) 1999 The Electrochemical Society. S0013-4651(98)06-090-X. All rights reserved.

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.

Studies on extraction and mass transfer of samarium and neodymium in hollow fiber membrane (HFM) with HEH/EHP

The membranes of polyvinylidene fluoride, which were synthesized by our laboratory, were used to study the transfer and extraction performances of Nd(III) and Sm(III) with the extraction system of HEH/EHP-kerosene. The results show that the membrane material was suitable to the study on membrane extraction, and could offer a good transfer performance in the membrane construction parameters selected, The extraction reaction in the membrane module was the same as that in liquid-liquid process, HEH/EHP ammoniated for increasing the mass transfer coefficient was almost the same with increasing the concentration of HEH/EHP, and H+ was still transferred first at higher pH range of feed solution when HEH/EHP was ammoniated, The controlling model of the membrane extraction process was the diffusion model accompanied by interfacial reaction, The controlling function of interfacial reaction would increase gradually with the increasing of the membrane pore size. The mass transfer coefficient increased when extraction and stripping were carried out simultaneously.

Structure of [C8H7](+) and intramolecular proton transfer reactions in 3-phenyl-1-butyn-3-ol by MS/MS technique

The electron impact mass spectrum (EIMS) of 3-phenyl-1-butyn-3-ol was reported in this paper. Collision-induced dissociation (CID) was used to study the gas phase ion structure of [C8H7](+) formed by the fragmentation of ionized 3-phenyl-1-butyn-3-ol, and that it has the same structure as m/z 103 ions generated by cinnamic acid and alpha-methylstyrene. Deuterium labelling, metastable ion (MI) and CID experimental results indicate the formation of m/z 103 ion resulting from molecular ion of 3-phenyl-1-butyn-3-ol, which is a stepwise procedure via twice proton transfers, rather than concerted process during the successive elimination of methyl radical and neutral carbon monoxide accompanying hydrogen transfer. Moreover, in order to rationalized these fragmentation processes, the bimolecular proton bound complex between benzyne and acetylene intermediate has been proposed.

The transfer of chloride ion across an anion exchange membrane

The transfer of chloride ions into a low resistance anion exchange membrane (AEM) was investigated by cyclic voltammetry (CV) and electrochemical impedance spectra. In all cases, concentration polarization of Cl- ions is exterior to the membrane. It controls the flux and produces the limiting currents: either steady state or transient (peak type) current. In CV experiments, when the size of the holes in the membrane was much smaller than the distance between membrane holes, the Cl- anion transfer showed steady state voltammetric behavior. Each hole in the membrane can be regarded as a microelectrode and the membrane was equivalent to a microelectrode array in this condition. When the hole in the membrane was large or the distance between membrane holes was small, the CV curve of the Cl- anion transfer across the membrane showed a peak shape, which was attributed to linear diffusion. In AC impedance measurement, the impedance spectrum of the membrane system was composed of two semicircles at low DC bias, corresponding to the bulk characteristics of the membrane and the kinetic process of ion transfer, respectively. The bulk membrane resistance increases with increasing DC bias and only one semicircle was observed at higher DC bias. The parameters related to kinetic and membrane properties were discussed.

STUDY OF THE ELECTRODE PROCESS OF HEMOGLOBIN AT A POLYMERIZED AZURE A FILM ELECTRODE

The electrochemically polymerized azure A film electrode is reported. The resulting film on a platinum electrode surface was analyzed with electron spectroscopy for chemical analysis (ESCA). The heterogeneous electron transfer processes of hemoglobin at the polymerized azure A film electrode have been investigated using in situ UV-visible spectroelectrochemistry. The formal potential (E-degrees') and electron transfer number (n) of hemoglobin were calculated as E = 0.088 V versus NHE (standard deviation +/- 0.5, N = 4) and n = 1.8 (standard deviation +/- 0.5, N = 4). Exhaustive reduction and oxidation electrolysis are achieved in 80 and 380 seconds, respectively, during a potential step between -0.3 and +0.3 V. A formal heterogeneous electron-transfer rate constant (k(sh)) of 3.54(+/- 0.12) X 10(-6) cm/s and a transfer coefficient (alpha) of 0.28(+/- 0.01) were obtained by cyclic voltabsorptometry, which indicated that the poly-azure A film electrode is able to catalyze the direct reduction and oxidation of hemoglobin.

APPLICATIONS OF RAPID SCAN SPECTROELECTROCHEMISTRY - DETERMINATION OF HETEROGENEOUS ELECTRON-TRANSFER RATE-CONSTANT OF BILIVERDIN OXIDATION

The rapid scan spectrometer was used to determine the heterogeneous electron transfer rate parameters for the oxidation of Biliverdin in DMF by single potential step thin layer spectroelectrochemical techniques and yielded an average formal heterogeneous electron transfer rate constant K(s, h)0' = 2.45 (+/-0.12) x 10(-4) cm s-1, electrochemical transfer coefficient alpha = 0.694+/-0.008. The oxidation process of Biliverdin was also studied and the formal potential E0 = 0.637 V (vs. Ag/AgCl) was obtained.

EFFECT OF PH ON THE ELECTRON-TRANSFER OF CYTOCHROME-C ON A GOLD ELECTRODE MODIFIED WITH BIS(4-PYRIDYL)DISULPHIDE

The adsorption of bis(4-pyridyl)disulphide (PySSPy) and 4.4'-bipyridyl (PyPy) on a gold electrode was studied using cyclic voltammetry. The adsorption isotherms and equilibrium constants (1 X 10(6) mol-1 l for PyPy and 6 x 10(6) Mol-1 l for PySSPy) were determined. The effect of pH on the electrochemical behaviour of cytochrome c was studied on the PySSPy-modified gold electrode. The results show that cytochrome c can only transfer electrons on a deprotonated electrode surface. When the pH is decreased, the standard heterogeneous rate constant of cytochrome c on the modified gold electrode decreases and the electrochemical behaviour changes from a quasi-reversible to an irreversible process.

STUDY ON ELECTRODE PROCESS OF MYOGLOBIN AT A POLYMERIZED AZURE A FILM ELECTRODE

The electrochemically polymerized azure A film electrode was firstly reported in this paper. A quasi-reversible electrode processes of myoglobin with the formal heterogeneous electron transfer rate constant (k(sh)) of 1.73 x 10(-4) cm.s-1 at the polymerized azure A modified electrode have been achieved using in-situ UV-visible spectroelectrochemistry. The adsorption of myoglobin on the polymerized azure A film electrode surface was confirmed by XPS. With simultaneously studying of cyclic voltammetry and in-situ cyclic voltabsorptometry, the attribution of the voltammetry responses of myoglobin at the film electrode has been studied. The mechanism for the heterogeneous electron transfer of myoglobin at the polymerized azure A film modified electrode has been proposed as well.