Quasi-reversible Electrode Process Research Articles

Electrochemical Kinetics of Diffusion Problems with a Moving Phase Boundary

The growth and dissolution of nuclei and a deposit film on a solid electrode are considered. The diffusion problems of the growth and dissolution of hemispherical nuclei and a thin deposit film are solved. Equations are derived to describe the I–V characteristics and chronoamperograms of reversible, irreversible, and quasi-reversible electrode processes, and the dependences of the peak current, the amount of deposit on an electrode, the potential of the peak current, and the full width at half-maximum of an I–V characteristic on the preelectrolysis parameters are determined. The formation of a new phase at the solid–solution interface and the mass-exchange processes at the weakly soluble film–solution interface are discussed. Anodic films are experimentally studied. In the case of nuclei or a thin anodic film, the amount of compound formed on the electrode surface linearly depends on the ionic concentration and the preelectrolysis time.

Adsorptive square-wave voltammetry of quasi-reversible electrode processes with a coupled catalytic chemical reaction

There are several strategies for enhancing the sensitivity of electroanalytical methods. Usually, those strategies are based on the selection of the voltammetric technique, the inclusion of an accumulation step, and the eventual addition of a catalytic chemical reaction that regenerates the electroactive species. Square-wave voltammetry (SWV) is one of the most sensitive techniques. In the case of electroanalytical applications, it is typically preceded by an electrochemical or adsorptive pre-concentration step.

Fabrication of electrochemical interface based on boronic acid-modified pyrroloquinoline quinine/reduced graphene oxide composites for voltammetric determination of glycated hemoglobin

A voltammetric sensor for determination of glycated hemoglobin (HbA1c) was developed based on the composites of phenylboronic acid-modified pyrroloquinoline quinine (PBA-PQQ) and reduced graphene oxide. After the electrodeposition of reduced graphene oxide (ERGO) on the glassy carbon (GC) electrode, PQQ multilayer was decorated on the surface of the ERGO/GC electrode via potential cycling. Further modification with PBA would lead to the formation of the working electrode, namely PBA-PQQ/ERGO/GC electrode. PQQ on the electrode exhibited a quasi-reversible electrode process with 2-electron transfer and 2-proton participation, and the electron transfer efficiency was further enhanced by the introduction of ERGO layer. The complexation of PBA with HbA1c through specific boronic acid–diol recognition could cause the change of the oxidation peak current of PQQ on the electrode, which was utilized for HbA1c detection. Under the optimized conditions, the PBA-PQQ/ERGO/GC electrode provided high selectivity and high sensitivity for HbA1c detection with a linear range of 9.4–65.8μgmL−1 and a low detection limit of 1.25μgmL−1. The fabricated sensor was also successfully applied to determine the percentages of HbA1c in whole blood of healthy individuals.

Electrochemical behavior of progesterone at an ex situ bismuth film electrode

An ex situ bismuth film electrode (BiFE) was used for the electrochemical study of the hormone progesterone. Two peaks were observed by cyclic voltammetry, at −1.68V and −1.47V, respectively, associated with the reduction and oxidation reactions of progesterone in 0.1molL−1 Britton–Robinson solution at pH 12.0, in agreement with a quasireversible electrode process. In relation to the mechanism, an adsorption-controlled reaction rate with one electron involved in the electrochemical step was observed. Square-wave adsorptive stripping voltammetry was used for the analytical procedures after accumulation of the progesterone for 60s at −1.0V. A peak current at −1.63V related to the reduction of progesterone was obtained, which increased linearly with the hormone concentration in the range of 0.40–7.90μmolL−1 (R2=0.9983). The limit of detection attained was 0.18μmolL−1. The method was applied in the determination of progesterone in four pharmaceutical samples with satisfactory results being obtained compared with a spectrophotometric method.

Electrochemistry of Nanocrystalline 3C Silicon Carbide Films

Silicon carbide (SiC) films have been used frequently for high-frequency and powder devices but have seldom been applied as the electrode material. In this paper, we have investigated the electrochemical properties of the nanocrystalline 3C-SiC film in detail. A film with grain sizes of 5 to 20 nm shows a surface roughness of about 30 nm. The resistivity of the film is in the range of 3.5-6.2 kΩ cm. In 0.1 M H(2)SO(4) solution, the film has a double-layer capacitance of 30-35 μF cm(-2) and a potential window of 3.0 V if an absolute current density of 0.1 mA cm(-2) is defined as the threshold. Its electrochemical activity was examined by using redox probes of [Ru(NH(3))(6)](2+/3+) and [Fe(CN)(6)](3-/4-) in aqueous solutions and by using redox probes of quinone and ferrocene in nonaqueous solutions. Diffusion-controlled, quasi-reversible electrode processes were achieved in four cases. The surface chemistry of the nanocrystalline 3C-SiC film was studied by electrochemical grafting with 4-nitrobenzenediazonium salts. The grafting was confirmed by time-of-flight secondary ion mass spectroscopy. All these results confirm that the nanocrystalline 3C-SiC film is promising for use as an electrode material.

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF 6) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion–BMIMPF 6/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion–BMIMPF 6 composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10 −4 mol/L to 1.1 × 10 −2 mol/L and with a detection limit of 1.6 × 10 −5 mol/L (3 σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.

Zinc Barbiturate Complexes with Bidentate N-Donor Ligands: Syntheses, Crystal Structures, Spectroscopic, Thermal and Voltammetric Studies

Abstract Two new bis(5,5-diethylbarbiturato) (barb) complexes of zinc, [Zn(barb)2(en)] (1) and [Zn(barb)2(bpy)] ・H2O (2) [en = ethylenediamine, bpy = 2,2’-bipyridine], have been prepared and characterized by elemental analysis, IR spectroscopy, thermal analysis and single crystal X-ray diffraction. Complexes 1 and 2 crystallize in monoclinic space groups. The zinc(II) ions in complex 1 are tetrahedrally coordinated by two barb and an en ligand. The barb ligands are N-coordinated while the en ligand acts as a bidentate chelating ligand. One carbonyl oxygen atom of each barb ligands in complex 2 participates in the bonding with the zinc ion with remarkably long Zn-O bonds resulting in a highly distorted octahedral geometry. Molecules of complex 1 are connected via N-H ···O hydrogen bonds, involving hydrogen atoms of both barb and en ligands, while molecules of complex 2 are linked by N-H ···O and O-H ···O hydrogen bonds and also aromatic π(bpy) ···π(bpy) stacking interactions. The voltammetric behavior of complexes 1 and 2 was investigated in aqueous solution by cyclic voltammetry using a NH3/NH4Cl buffer. The cyclic voltammogram of 1 shows a cathodic peak at −1.260 V and an anodic peak at −1.068 V due to a quasi-reversible two-electron process. Complex 2 yields two reduction peaks at −1.312 and −1.412 V. The former corresponds to a quasi-reversible electrode process of the zinc(II) ions in complex 2, while the latter is attributed to the reduction of the coordinated bpy ligands.

Electroreduction of a Series of Alkylcobalamins: Mechanism of Stepwise Reductive Cleavage of the Co−C Bond

The electrochemical (EC) reduction mechanism of methylcobalamin (Me-Cbl) in a mixed DMF/MeOH solvent in 0.2 M tetrabutylammonium fluoroborate electrolyte was studied as a function of temperature and solvent ratio vs a nonaqueous Ag/AgCl/Cl(-) reference electrode. Double-potential-step chronoamperometry allowed the rate constant of the subsequent homogeneous reaction to be measured over the temperature range from 0 to -80 degrees C in 40:60 and 50:50 DMF:MeOH ratios. Activation enthalpies are 5.8 +/- 0.5 and 7.6 +/- 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively. Digital simulation and curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 x 10(3) s(-1) give E degrees' = -1.466 V, k degrees = 0.016 cm/s, and alpha = 0.77 at 20 degrees C for a quasi-reversible electrode process. Digital simulation of the results of Lexa and Savéant (J. Am. Chem. Soc. 1978, 100, 3220-3222) shows that the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible step following the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants of these reactions. An electron coupling matrix element of H(kA) = (4.7 +/- 1.1) x 10(-4) eV ( approximately 46 J/mol) is calculated for the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy for homolytic cleavage of Me-Cbl is calculated as 31 +/- 2 kcal/mol. The voltammetry of the ethyl-, n-propyl-, n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentials were correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL2DZ) calculations.

Cyclic voltammetric and EPR spectral studies of copper(II)-chloro and -bromo complexes in organo and organo-aqueous media

The electrochemical behaviour of copper(II)-chloro and -bromo complex species has been investigated in different organo (ethyl alcohol, DMSO, DMF and AN) and organo-aqueous media containing 0.1 M tetrabutyl ammonium perchlorate (TRAP) and 0.2 M KCl/KBr, respectively. The cyclic voltammetric parameters are calculated. Both the complex species involve two redox steps. The first couple (Cu 2 + / + ) involves a single electron, diffusion-controlled quasi-reversible electrode process, while the second step involves irreversible reduction of Cu I -halide species to metallic copper. Also, the reduction potential for the first couple (E p c l ) becomes more positive and that of the second step (E p c 2 ) becomes more negative with increasing percentage of a given organic solvent, indicating that the copper(I)-halo species becomes stabilized with respect to copper(II)-halo species. The EPR parameters (g∥, g⊥ and A∥) for the frozen solution spectra of CuCl 2 .2H 2 O and CuBr 2 .H 2 O in ethyl alcohol, DMSO, DMF and AN, each containing 0.1 M TBAP at liquid nitrogen temperature (77 K) are calculated and the results are discussed.

Cyclic voltammetric investigations of copper(II)-chloro and -bromo complexes in aqueous medium

The electrochemical behavior of copper(II)-chloro and -bromo complex species has been studied in different concentrations of chloride and bromide ions in aqueous solutions by means of cyclic voltammetry. The cyclic voltammetric parameters are calculated. Both the complex species involve two redox steps. The first couple (Cu 2 + / + ) involves a single-electron, diffusion-controlled quasi-reversible electrode process (EC mechanism), while the second step involves irreversible reduction of Cu(I)-halide species to metallic copper. Also, the reduction potential for the first couple (E p c 1 ) becomes more positive and that of the second step (E p c 2 ) becomes more negative with increasing concentration of chloride/bromide ions.

Square-wave voltammetry of quasi-reversible electrode processes with coupled homogeneous chemical reactions

The electrochemical behaviour of systems complicated by electrode kinetic and quasi-reversible preceding or following homogeneous chemical reactions under square-wave voltammetry (SWV) conditions is analysed theoretically. The results are discussed in detail, considering the influence of rate and equilibrium constants, together with experimentally controlled parameters such as f and E sw. Both kinetic stages act synergistically for the case of a CE mechanism, but the EC reaction scheme exhibits more complex behaviour, especially for reversible and quasi-reversible electrochemical reactions, which present a minimum response of current for the quasi-reversible chemical reactions. These curves are characteristic for each system, providing not only the bases for their distinction but also for the extraction of kinetic and thermodynamic information.

Elimination voltammetry. Experimental verification and extension of theoretical results

The theoretical results of elimination voltammetry with linear scan have been verified experimentally for the case of reversible and irreversible processes and for the potential range expansion. All experiments were performed with a hanging mercury drop electrode. An experimental extension of elimination for cyclic voltammetry reverse scans of reversible and irreversible electrode processes and for forward and reverse scans of quasireversible electrode processes has been made. The equations for elimination of reversible, charging and kinetic currents, I r, I c and I k respectively, were tested for six cases: (A) simultaneous elimination of I k and I c with I r conservation; (B) simultaneous elimination of I c and I r with I k conservation; (C) simultaneous elimination of I r and I k with I c conservation; (D) elimination of I k with I r conservation; (E) simultaneous elimination of I c and a deeper suppression of the irreversible current foot with I r conservation; (F) elimination of I c with I r conservation. Scan rate ratios using the base number 2 were used. It was proved that the case C could be used with advantage for sensitive detection of the reversibility loss of the electrode process and for the distinction and quantitative measurement of the irreversible electrode process in the presence of a reversible one. The cases A, D and E were proved to be successful for the potential range expansion.

Total Synthesis of a Miniferredoxin

A miniferredoxin has been designed based on theDesulfovibrio gigasferredoxin II structure and has been successfully synthesized. The 31 amino acid apoprotein was synthesized via standard Fmoc solid phase peptide synthesis andin vitrocluster insertion carried out. The UV-visible spectrum of the miniferredoxin (peak at 300 nm and a shoulder at 405 nm) shows the same features as that of theD. gigasferredoxin. Cyclic voltammetry indicated a quasireversible electrode process with a midpoint potential of −370mV vs NHE, which demonstrates that the miniferredoxin is redox active. From these and EPR studies, we propose the incorporation of a Fe4S4cluster.

Quasireversible Linear Potential Sweep Voltammetry: An Analytic Solution for Rational .alpha.

An analytic solution is presented to the problem of predicting the shape of a linear-potential-sweep voltammogram for a quasireversible electrode process under the conditions usually encountered in voltammetric experiments. It is necessary to assume that the transfer coefficient is a rational number, but in practice, this is not an important restriction. The solution takes the form of a series of exponential functions, the coefficients of which are calculable by a simple recursion formula

Electrochemical detection of cobalt in hair following on-column derivatization with 1-(2-pyridylazo)-2-naphthol-6-sulphonic acid and reversed-phase liquid chromatography

A method for the specific determination of cobalt based on reversed-phase liquid chromatography with amperometric detection via on-column complex formation has been developed. A water-soluble chelating agent, 1-(2-pyridylazo)-2-naphthol-6-sulphonic acid (PAN-6S), is added to the mobile phase and aqueous cobalt solutions are injected directly into the column to form in situ the cobalt—PAN-6S chelate, which is then separated from other metal PAN-6S chelates and subjected to reductive amperometric detection at a moderate potential of −0.3 V. Because the procedure eliminates the interference of oxygen and depresses the electrochemical reduction of the mobile phase-containing ligand PAN-6S, by virtue of the quasi-reversible electrode process of the cobalt—PAN-6S complex, a low detection limit of 0.06 ng can be readily obtained. Interference effects were examined for sixteen common metal species, and at a 5- to 8000-fold excess by mass no obvious interference was observed. The feasibility of the method as an approach to the specific analysis of cobalt in a hair sample has been demonstrated.

Redox Potentials of Cobalt (III/II) Complexes with Sulfonated Heterocyclic Azo Compounds

Abstract Cyclic voltammograms of the cobalt(III) complexes with sulfonated azo compounds indicated one quasi-reversible electrode process at the Pt or Au electrode at −0.3—0 V vs. SCE in aqueous solutions, which was assigned to a metal-centered redox reaction. The formal potentials are linearly correlated with the logarithmic protonation constants of the ligands.

Theoretical and experimental investigation of steady-state voltammetry for quasi-reversible heterogeneous electron transfer on a mercury oblate spheroidal microelectrode.

A mercury microelectrode formed by electroreduction of mercury on an inlaid gold microdisk is experimentally shown to be well modeled by oblate spheroidal geometry when the ratio of the semiminor axis to the semimajor axis of the protruding drop is less than 1. The validity of the geometry is established by comparison of the experimentally determined coefficient in the steady-state diffusion current equation with the theoretical value for oblate spheroidal geometry. Spherical cap geometry is also shown to be an equally valid geometric model; however, theoretical treatment for this system is more difficult. The theory of a quasi-reversible electrode process is developed and applied to the determination of the electrode parameters of the RuIII(NH3)6/RuII(NH3)6 electrode reaction on a mercury oblate spheroidal microelectrode. Results agree well with others found in the literature for the same process on a mercury electrode.

High-performance liquid chromatography of the antihistamine pyrilamine and its N-oxide using electrochemical detection

The electrochemical behavior of the over-the-counter antihistamine drug pyrilamine and its N-oxide analogue, have been studied by several voltammetric methods. Cyclic voltammograms of pyrilamine maleate in 0.1 M ammonium acetate at pH 7.0 indicated a quasi-reversible electrode process by observing a wave at +0.85 V and +1.30 V in the initial anodic sweep followed by a wave at -1.30 V versus Ag/AgCl. Differential pulse and hydrodynamic voltammetry of pyrilamine and the N-oxide were examined to determine oxidation potentials for use in high-performance liquid chromatography with electrochemical detection (HPLC-ED). Differentiation between pyrilamine and its N-oxide was achieved in HPLC-ED analyses at a detection potential of +0.7 V and +0.9 V versus Ag/AgCl with tandem ultraviolet detection at 254 nm. Utility of the HPLC-ED method was demonstrated by the analysis of pyrilamine and the N-oxide in microbial biotransformation samples.

Linear sweep voltammetry at the tubular graphite electrode Part III. Quasi-reversible processes

The theory of linear sweep voltammetry at the tubular graphite electrode for quasi-reversible electrode processes has been developed. A general solution for quasi-reversible reactions, from which the solution for irreversible reactions follows as the limiting case, has been presented. The initial boundary value problem has been solved by changing the convective-difusion differential equations to an integral equation which is solved numerically. The current—potential curves have been calculated theoretically and verified experimentally. The dependence of peak current on velocity has also been studied.

The analysis of electrode impedances complicated by the presence of a constant phase element

The electrical double-layer at a solid electrode does not in general behave as a pure capacitance but rather as an impedance displaying a frequency-independent phase angle different from 90°. Ways are indicated how to analyse the interfacial impedance if such a complication arises in the presence of a faradaic process, both on the supposition that the double-layer behaviour is due to surface inhomogeneity and on the supposition that it is a double-layer property per se. As examples, the equations derived are successfully applied to a totally irreversible and an ac quasi-reversible electrode process at a gold electrode.