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Fourier-transformed large amplitude alternating current voltammetry (FTACV) provides a sensitive analytical tool for the discrimination of electrode reactions that are complicated by surface heterogeneity. In this paper, it is shown how the FTACV response at a dual-electrode system comprised of different electrode materials having different heterogeneous charge transfer (k01 and k02) can be resolved into its individual electrode kinetics components without prior knowledge of the electrode size ratio (θ1:θ2). This is possible when one process is reversible and the other is quasi-reversible; achievable by careful selection of the FTACV frequency. The applicability of the FTACV method over a wide range of electrode kinetic values and size ratios is considered for conditions under which numerical simulations based on a 1D diffusion model are adequate to describe the mass transport problem.

Theory for cyclic square wave voltammetry of electrode reactions with coupled chemical reactions following the electron transfer is presented. Theoretical voltammograms were calculated following systematic variation of empirical parameters to assess their impact on the shape of the voltammogram. From the trends obtained, diagnostic criteria for this mechanism were deduced. When properly applied, these criteria will enable non-experts in voltammetry to assign the electrode reaction mechanism and accurately measure reaction kinetics.

International audience; Electrochemical analysis of species known to passivate electrode surfaces remains challenging. We previously proposed a new method dealing with sampled-current voltammetry performed on an electrode array to mimic polarography at a dropping mercury electrode for the detection of copper. In this work, we study the effectiveness of this method to circumvent electrode fouling with the analysis of phenol solutions at high concentrations (10-2 mol L-1), known to polymerize on electrode surface during its oxidation. Electrode arrays well-adapted to the analysis with such system are prepared by photolithography and characterized by X-Ray photoelectron spectroscopy, X-ray diffraction and voltammetry. While analyses performed in conventional linear voltammetry or sampled-current voltammetry on a single electrode are considerably affected by electrode fouling, a linear calibration plot was achievable using our method. Modelling of the electrochemical signal showed that the current depends only on the applied potential and a parameter characteristic of the passivation phenomenon. It also underlined that sampled-current voltammetry on electrode array can circumvent the problem of passivation by a judicious choice of the sampling time.

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. In this manuscript, the theory of SWV for a quasi-reversible electrode process coupled to a catalytic chemical reaction between an adsorbed reagent and a soluble product is presented. The dependences of the dimensionless net peak current and its peak potential on the value of the standard charge transfer rate constant are described. The variation of the SWV parameters such as frequency and potential pulse amplitude are discussed. The effect of the chemical and electrochemical kinetics on the voltammetric profile is analyzed. Fil: Vettorelo, Sabrina Noel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Garay, Fernando Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina

An apparatus for the measurement of steady-state microelectrode voltammetry at elevated temperatures is described. The scope of this experimental approach as a method for the determination of diffusion coefficients at variable temperatures in the case of simple one-electron processes is demonstrated. Diffusion coefficients over a range of temperatures are derived for N,N,N′N′-tetramethylphenylenediamine in both acetonitrile and water solvents, tris-4-bromophenylamine in acetonitrile, and ferrocene in acetonitrile and dimethylformamide from which activation energies for diffusion are obtained. Diffusion coefficient values are used to derive Stokes-Einstein radii for each species in solution. The electroreductions of o-bromonitrobenzene in dimethylformamide and 9-chloroanthracene in acetonitrile are studied as a function of temperature and activation energies estimated for carbon-halide bond cleavage in the corresponding radical anions.

Both the strongly implicit procedure and the alternating direction implicit finite difference (ADI) technique are used to simulate mass transport to a microdisc electrode located in a channel flow, taking account of diffusion in three dimensions and the parabolic laminar convective flow over the electrode surface. The effect of the latter on the shape of the disc diffusion layer is qualitatively identified. The flow rate dependence of the transport-limited disc current is predicted and shown to be in quantitative agreement with experiments conducted on the oxidation of ferrocene in acetonitrile solution. © 1995 American Chemical Society.

Graphene modified electrodes have been fabricated by electrodeposition from an aqueous graphene oxide solution onto conducting Pt, Au, glassy carbon, and indium tin dioxide substrates. Detailed investigations of the electrochemistry of the [Ru(NH(3))(6)](3+/2+) and [Fe(CN)(6)](3-/4-) and hydroquinone and uric acid oxidation processes have been undertaken at glassy carbon and graphene modified glassy carbon electrodes using transient cyclic voltammetry at a stationary electrode and near steady-state voltammetry at a rotating disk electrode. Comparisons of the data with simulation suggest that the transient voltammetric characteristics at graphene modified electrodes contain a significant contribution from thin layer and surface confined processes. Consequently, interpretations based solely on mass transport by semi-infinite linear diffusion may result in incorrect conclusions on the activity of the graphene modified electrode. In contrast, steady-state voltammetry at a rotating disk electrode affords a much simpler method for the evaluation of the performance of graphene modified electrode since the relative importance of the thin layer and surface confined processes are substantially diminished and mass transport is dominated by convection. Application of the rotated electrode approach with carbon nanotube modified electrodes also should lead to simplification of data analysis in this environment.

The use of single walled carbon nanotube (SWNT) band electrodes in a channel flow cell, for low concentration detection, with hydrodynamic voltammetry is reported. A two dimensional SWNT network electrode is combined with a one piece channel flow cell unit, fabricated by microstereolithography. This configuration provides well defined hydrodynamics over a wide range of volume flow rates (0.05–25 mL min−1). Limiting current measurements, from linear sweep voltammograms, are in good agreement with the channel electrode Levich equation, for the one electron oxidation of ferrocenylmethyl trimethylammonium (FcTMA+), over a wide concentration range, 1×10−8 M to 2.1×10−5 M, with a detection limit of 5 nM. At the highest flow rates, some influence of the slightly recessed electrode geometry arising from the SWNT electrode fabrication is noted. However, this can be accounted for by a full simulation of the hydrodynamics and solution of the resulting convection–diffusion equation. Application of this hydrodynamic configuration to the voltammetric detection of dopamine is also demonstrated. Keywords: Channel flow electrode, Hydrodynamic electrode, Single walled carbon nanotubes, Dopamine, Band electrode

The electrochemical behaviour of Zn(II) ion as example of C rev E irr system was studied at hanging mercury drop electrode in aqueous solution in the presence of 1M KCl using cyclic voltammetry and convolutive voltammetry methods. Digital simulation technique was used to test the chemical and electrochemical parameters of Zn(II) ion estimated experimentally as well as verifying the proposed electrode mechanism. Keywords: Electrochemical behaviour, C rev E irr system, cyclic voltammetry, convolutive Voltammetry, digital simulation. introduction The electroreduction of Zn(II) has been extensively studied in aqueous and non-aqueous solutions and found to be controlled by both diffusion and charge transfer. 1-6 At mercury electrode different mechanisms have been proposed for electrode reduction of metal cations, e.g., a simple electron transfer process 7,8 , stepwise electron transfer 9 , a CE or CEE mechanism. 10,11 Several literature reports a possible validity of a mechanism involving two one-electron transfer has been discussed.

The numerical method for the simulation of linear scan voltammetry on the rotating disk electrode is adjusted to the problem of irreversible redox reaction between the adsorbed catalyst and the dissolved reactant under transient conditions. The response consists of the wave and the maximum. The peak current depends on the scan rate in linear scan voltammetry, while the limiting current of the wave depends on the rate of rotation of the working electrode. The rate constant of catalytic reaction is determined from the kinetic current under steady-state conditions.

As a complement to a previous contribution from us, the mass transport mechanisms of the electroactive species to and from the electrode in an uncomplicated electrode mechanism are considered. The electrode process as a whole is discussed, with emphasis to its reversibility degree, as results from the relevant responses in controlled potential techniques, such as chronoamperometry and current sampling voltammetry, linear sweep and cyclic voltammetry, and in rotating disk voltammetry. The electrode process as a whole, composed by charge transfer and mass transport steps that concur to condition the current flowing, is analysed on the basis of the relative rates of the two steps, as well of the time window within which the process is observed. The so-called ‘boundary value problem’ for uncomplicated charge transfers with different reversibility degrees is outlined. Supplementary Material is available, in which the simulated concentration profiles for reduced and oxidised species reacting at an electrode, at which a triangular potential waveform is applied, are linked to the corresponding current densities.

Theory for cyclic square wave voltammetry of electrode reactions with chemical reactions preceding the electron transfer is presented. Theoretical voltammograms were calculated following systematic variation of empirical parameters to assess their impact on the shape of the voltammogram. From the trends obtained, diagnostic criteria for this mechanism were deduced. When properly applied, these criteria will enable non-experts in voltammetry to assign the electrode reaction mechanism and accurately measure reaction kinetics.

A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase-sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt-Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport-limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ‘reference species’ (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF-MJE arrangement allows quantitative concentration measurements, down to at least 5×10−7 M.

The voltammetry at three micro-mesh electrodes is explored. It is found that at sufficiently short experimental durations, the micro-mesh working electrode first behaves as an ensemble of microband electrodes, then follows the behaviour anticipated for an array of diffusion-independent micro-ring electrodes of the same perimeter as individual grid-squares within the mesh. During prolonged electrolysis, the micro-mesh electrode follows that behaviour anticipated theoretically for a cubically-packed partially-blocked electrode. Application of the micro-mesh electrode for the electrochemical determination of carbon dioxide in DMSO electrolyte solutions is further illustrated. Neste trabalho investigamos a voltametria em eletrodos do tipo micromesh. Em experimento de duração suficientemente curta, os eletrodods de trabalho micromesh se comportam primeiramente como uma rede de eletrodos de microbanda, seguido pelo comportamento esperado para um arranjo de eletrodos de micro-anel, independentes de difusão, com perímetro igual ao tamanho da grade. Durante eletrólise prolongada, o eletrodo apresentou o comportamento previsto para um eletrodo parcialmente bloqueado, com arranjo cúbico. A aplicação deste eletrodo micromesh na determinação eletroquímica de dióxido de carbono em soluções eletrolíticas de DMSO é descrita.

Hydrodynamic voltammetry is reported at the interface between two immiscible electrolyte solutions (ITIES), using a rotating diffusion cell configuration. The voltammetry arises from laminar flow, induced in the organic and aqueous phases of the ITIES separately. The ITIES has been stabilized by a polyester (PET) track-etched membrane material. The resultant voltammetry is shown to be in accord with the Koutecký-Levich equation for limiting currents at rotating electrodes. The technique is promising for the determination of reaction mechanisms and kinetic parameters for reactions involving immiscible liquid phases.

Rotating disk electrode voltammetry (RDEV) in conjunction with anodic stripping differential pulse voltammetry (ASDPV) with a special type of gold side disk electrode has been evaluated for the direct estimation of copper in natural water (i.e. river water and underground water) as well as in whole blood without any pretreatment. Copper is quantified with high reproducibility and reliability even below 5 ppb without any interference of the common ions or organic materials dissolved in the samples, i.e., natural water and blood. The voltammetric measurements were carried out using a conventional three electrodes cell (i.e., gold as working, platinum as auxiliary and Ag/AgCl as reference electrodes) and the acidified sample. The major problem due to hydrogen evolution at cathode during the deposition of copper ions specially in acidified blood sample is overcome with the help of special design of rotating side disk gold electrode. A new type of rotating side disk electrode was designed to avoid the problem of gas bubbles interference in the stripping voltammetry. The shape, position and the rotation speed of the side disk electrode were optimized as per the requirement in the mild acidic condition. A simultaneous estimation of the zinc, lead and copper is also performed in aqueous sample with KCl as supporting electrolyte. The special design of working electrode and the modified voltammetric conditions are enhanced the sensitivity and reproducibility of the copper estimation at low ppb level with negligible interference.

Cu-rich Cu-Ni alloys (containing 70–90% Cu) are shown to be excellent indicator electrode materials for voltammetric measurements at cathodic potentials. These electrodes have been characterised using a variety of methods: rotating disk voltammetry, amperometric measurements and sono-ring-disk voltammetry. The mechanism of electrochemical oxygen reduction at the Ni-30Cu alloy has been established. The results are compared with the surface study results.

Fast scan stripping voltammetry is applied to copper determination, using a rotating glassy carbon electrode as working electrode. Optimal conditions are determined by an experimental design where the current peak is chosen as response factor. The effects of different parameters (deposition potential, deposition time, electrode rotation rate and potential sweep) and their interdependence is optimized. A value of 2.8×10–10 M is obtained for the detection limit, with a relative standard deviation of 0.16 in four replicated assays. The method is applied to the determination of copper in tap water and in some chemicals (HClO4, HNO3, etc.).

Predictions have been made of how total withdrawal of supporting electrolyte affects the steady-state limiting current at a rotating disk electrode. Incorporating transport by diffusion, migration, and convection, the treatment is restricted to a three-ion system under the assumption that all the ions share the same diffusivity. The results predict qualitative, but not quantitative, agreement with similar studies relating to steady-state voltammetry at a hemispherical microelectrode. The experimental findings of Gao and White are discussed in the light of the new treatment.

The theory of normal pulse voltammetry (NPV) and cyclic potential sweep voltammetry (CV) of the amperometric ion-selective electrodes (AISEs) based on the ionophore-assisted transfer of ions across polarized organic solvent or oil ∣ water (O ∣ W) interfaces is discussed. The numerical solution of the resulting integral equations has been used to show how their voltammograms vary with the ratio of the concentration of analyte ion in the W phase, that is, the test solution, to that of ionophore in the O phase, that is, the ion-sensitive membrane. Generally, the current response of the AISEs at constant ionophore concentration is not linear with respect to the concentration of analyte ion when the ratio exceeds 0.01. In order to obtain a linear calibration curve in a wide range of analyte concentrations, up to a few molar of Na + ion, e.g. a new technique, called F-pulse amperometry, is proposed and experimentally verified, where a pulse voltage corresponding to the ‘foot-of-wave’ potential is applied and the current is sampled according to the NPV technique. When the concentration of analyte ion is very low, the stripping voltammetry technique is promising. Poly(oxyethylene)alkyl ether surfactants at ppb levels can be determined using the stripping voltammetry technique based on the electrochemical pre-concentration of the surfactants by ion-assisted transfer at the O-gel ∣ W interface.

An electrochemical sensor was fabricated by screen printing techniques. This sensor, coupled with stripping voltammetry, could provide a means for a relatively inexpensive on site detector for trace levels of heavy metals. The sensor consists of an electrode pair of graphite/carbon black working electrode and Ag/AgCl reference electrode printed onto a polycarbonate surface. Microscope observation of the working electrode shows considerable surface roughness. Cyclic voltammograms of ferricyanide and lead nitrate indicated that the electrochemical nature of the carbon working electrode and the reference electrode on the sensor were comparable to the performance of conventional glassy carbon and Ag/AgCl reference electrodes. Lead in the 10−6–10−9 M concentration range using linear scan anodic stripping voltammetry with identical deposition times resulted in a linear relationship between peak current and analyte concentration. Square-wave anodic stripping voltammetry is compared with alternative electroanalytical techniques and is shown to offer the advantage of rapid determination of lead in O2-containing solutions.

The preparation of a new carbon composite electrode, constructed from PVC–graphite compressed mixtures, is described. The performance as a working electrode for oxidative voltammetry was studied with cyclic voltammetry on hexacyanoferrate(III) in 1 M HCl. Catechol and hydroquinone were also used as model test systems. Well defined current potential curves are reported. Examples of applications in anodic stripping voltammetry (ASV), including the formation of mercury films are presented. The possible use of PVC–graphite electrodes placed in a typical wall-jet flow cell as amperometric detectors was also examined by employing catechol as the test analyte. The PVC–graphite composite exhibits favorable qualities as an electrode material, with respect to its mechanical resistance, very low cost, simple preparation and easy renewal of the electrode surface. The results show a satisfactory combination of good electrode kinetics, with small background current, and relatively reproducible signals.

This paper presents the relation between the temperature value and the time constant value for the voltammetry electrode. The problem in voltammetry measurements is the accuracy of the time constant value. Based on mathematical analysis and laboratory verification, it was shown that it is possible to calculate the time constant of the electrode by measuring the current. The current must equal to the diffusion limited current value. It was also shown how important in the voltammetry measurements is the accuracy of the time constant value, because this value determines the dynamic error of electrode.

International audience; Polarography with dropping mercury electrode has been widely used in electroanalysis. However, the method is less and less employed due to the toxicity of mercury. In this work, we have shown that it is possible to replace the dropping electrode by a working electrode array, allowing the renewal of the electrode surface and of the analytical solution during the analysis. This new concept has been demonstrated on copper analysis. Sampled current voltammetry has been carried out on an electrode array, giving rise to I vs. E curves with a limiting diffusion plateau. The principle can be extended to other electroanalytical methods as exemplified here with differential pulse voltammetry. Linear calibration curves have been obtained with both methods and a limit of detection of 2 × 10−5 mol L−1 has been reached for copper detection by differential pulse voltammetry.

We introduce a novel method for the direct extraction of kinetic data from experimental cyclic voltammetry using numerical simulation. This method is not specific to a particular model of electrode kinetics, such as the Butler-Volmer or Marcus-Hush models, and is general to all electrode geometries for which the current density is uniform across the surface. The method is demonstrated using both theoretically simulated voltammetry and experimental data for the reduction of 2-methyl-2-nitropropane at a mercury hemisphere electrode. For the latter system, excellent agreement with previously reported kinetic parameters is obtained. © 2012 Elsevier B.V. All rights reserved.

The analytical benefits obtained from electrode modification with conductive porous layers, a common practice in the development of electrochemical (bio)sensors, are examined for the main electroanalytical techniques: differential pulse, square wave and linear sweep voltammetry. The electrochemical response is analyzed in terms of the effect of the modification on the electrode kinetics and on the diffusion transport of the target species. With respect to the former, the gain in the current predicted by simple enhancement of the electrode kinetics is analyzed, pointing out the improvement of the analytical sensitivity predicted in each voltammetric technique. Regarding the contribution of the thin-layer diffusion of the electroactive species trapped between the layers, we show that this gives rise to the lowering of the peak potential and the narrowing of the voltammograms, which can be wrongly attributed to electrocatalytic properties of the modified surface. In order to discriminate between electrocatalytic and mass transport effects, diagnosis tests are presented for differential pulse and square wave voltammetry. © 2011 Elsevier Ltd.

Hydrodynamic chronocoulometry is theoretically developed such that the diffusion coefficient (D) and concentration (c * ) (or nc * , where n is the number of electrons involved in an electrode reaction) of an electroactive species can be simultaneously determined without previous knowledge of either. The present technique involves measuring charges vs time curves after the electrode potential is stepped from an initial value, where no current flows, to a value at which the electrode reaction of the reacting species is controlled by mass transfer under hydrodynamic voltammetry using a rotating disk electrode

Voltammetric and amperometric experiments conducted during continuous sonication with a Ti immersion macrotip (1 cm 2 area) positioned parallel to Pt working electrode surfaces are described. The effects of concentration, electrode area, temperature, kinematic viscosity, amplitude of vibration of the Ti sonicator tip, cell geometry and voltage scan rate on limiting currents are considered and compared to analogous measurements made at a rotating disk electrode. It is found that steady-state voltammograms may be obtained during sonication at scan rates up to 25 V/s. Hydrodynamic conditions are described in terms of the effective rotation rate which would be needed in rotating disk voltammetry to achieve similar transport rates

In this article, electrochemical determinations based on oxidation or reduction processes that require a catalyst to obtain a current proportional to analyte concentration are described. The scope is restricted to cases where the catalyst is immobilized at the electrode surface. In general, oxidation or reduction of a substance at an electrode surface can be the basis of an analytical method as long as the electron-transfer reaction occurs in the potential window between redox of the solvent, supporting electrolyte, and/or the electrode material. The need for a catalyst arises because only a small fraction of solutes that are predicted on the basis of standard potentials to undergo redox in an available potential window are electrolyzed in the absence of a catalyst at a sufficient rate to provide an analytically useful current. As described later, the common mode of promotion of the rate of electron transfer is by mediation. A problem that results when immobilized mediators are used is that the potential at which the current is produced is related to the mediator as well as to the analytes; hence, many practical applications of catalytic surfaces involve electrochemical detection after separation by methods such as high-performance liquid chromatography (HPLC). Keywords: electrochemical catalysis; mediation; chemically modified electrode; pulsed electrochemical detection; potentiostatic detection; hydrodynamic voltammetry

The theory of cyclic voltammetry, in hydrodynamic systems, is extended to EC processes in which the reversible charge transfer is followed by a first order, homogeneous, irreversible chemical reaction. Laplace transformation and its properties have been used to convert the system of convective-diffusion equations, representing the boundary value problem, to an integral equation which is solved numerically. With slight modification the theory has been applied to linear sweep voltammetry for which theoretical current-potential curves have been obtained and the effects of reaction rate constant, velocity and potential scan rate on the shape of curve are shown graphically. A simple procedure for the determination of the reaction rate constant is presented. Theoretically derived results have been verified experimentally by carrying out the oxidation of N, N' -dimethyl- p -phenylenediamine in the presence of OH − ions at the tubular platinum electrode.

(J. Phys. Chem. C (2012) 116:20 (10913-10916) DOI: 10.1021/jp2118696)

Linear sweep and cyclic voltammetry for ion transfer across a micro liquid/liquid interface supported at the tip of a micropipette has been investigated experimentally and analysed using a pseudo analytical solution. Asymmetric sweep cyclic voltammetry is found to be a useful electroanalytical technique for the determination of ionic species in solution.

An electroanalytical study of the oxidation process of pirimicarb at a glassy carbon electrode using different voltammetric techniques was carried out. Two media were used: aqueous Britton-Robinson buffer and acetonitrile (0.1 mol L−1 Bu4NClO4). Studies using linear sweep voltammetry (LSV) at a rotating disk electrode show that the limiting current becomes nondiffusion controlled at concentrations higher than 6.0 × 10−5 mol L−1. Cyclic voltammetry showed an irreversible electrode process. The best detection and determination limits were obtained by differential pulse voltammetry at a stationary electrode in an acetonitrile medium. The differential pulse mode in an aqueous medium was used to determine pirimicab in soil samples.

The sections in this article are Introduction Historical Background Potential Scan Voltammetries—Definitions Scope of the Chapter Theory of Potential Scan Voltammetries General Reversible Electron Transfer Kinetics of Electron Transfer Coupled Homogeneous Chemical Kinetics Adsorption and Surface-Bound Redox Species Nonlinear Diffusion Restricted Diffusion Simulation Practical Considerations Electrodes and Cells Solvents and Supporting Electrolytes Experimental Limitations Artifacts and how to Avoid Them Present Methodological Developments Voltammetry at Ultramicroelectrodes Fast Scan Techniques Voltammetry without Supporting Electrolyte Voltammetry Under Hydrodynamic Conditions Voltammetric Spectroelectrochemistry Derivative Voltammetry Semiintegral (“Convolution”) and Semiderivative Voltammetry Applications Typical Voltammograms—Classical Applications Selected Recent Examples Conclusion Acknowledgments

Publisher Summary This chapter discusses the potential of voltammetric techniques for trace element analysis of body fluids. Voltammetry is a major branch of electroanalysis in which a potential is imposed upon an electrochemical cell and the resulting current is measured. The electrochemical cell contains the sample solution into which the working, reference, and counter electrodes are immersed. The potential of the working electrode is varied in some systematic manner to cause electroactive chemical species to be reduced or oxidized. The current flowing is a direct measure of the rate of this redox reaction. The resulting current-potential curve, known as voltammogram, exhibits a peak-shaped, or sigmoidal, response over the potential region where the analyte undergoes the electron-transfer reaction. New voltammetric approaches based upon flow injection analysis and metal-chelate adsorptive accumulation addresses properly the speed and scope limitations of voltammetry, and enhances its competitive power for clinical measurements. The characteristics of the most commonly applied voltammetric methods for measurements of trace metals are discussed, together with their applicability to analysis of clinical samples, modes of operation and pretreatment procedures.

Theory is presented which predicts the linear sweep voltammetry behavior at the wall-jet electrode for a reversible couple. The scan rate and electrode geometry dependences are established, and hence the requirements for the measurement of true "steady state" hydrodynamic voltammograms are defined. Theory is found to be in good agreement with experiments conducted on the oxidation of the ferrocyanide anion in aqueous solution. © 1992 American Chemical Society.

Publisher Summary Electrochemical techniques are a category of methods used for the study of the composition and properties of a medium based on electrochemical reactions occurring at the electrode-solution interface. This chapter reviews the principles and applications of most important electrochemical techniques in soil science. The basic principle of electrochemical techniques is to convert one chemical parameter of a medium into an electrical parameter by an appropriate device, usually an electrode or an electrode pair, and then to measure this electrical parameter using an instrument. As in the case of electrical measurements, in electrochemical techniques four parameters can be utilized. The electrode potential may be a function of the composition of the solution. Techniques based on the measurement of this potential are called potentiometry. When an external voltage is applied to an electrode, an electrical current may flow through the circuit. Techniques utilizing this current-voltage relationship are called voltammetry. One can also measure the electrical resistance (conductance) between two electrodes in the medium, and such a technique is called conductometry. In all these techniques, the electrode is the site at which electrochemical reactions take place and is the key element in electrochemical analyses. Therefore, in the discussion the electrode potential at the electrode- solution interface when no current flows is first examined, and then the effect of current on electrode potential, electrode polarization, is discussed.

Publisher Summary This chapter presents an overview of in vivo voltammetry technique. Voltammetry is a means of measuring the concentration of compounds based on their redox characteristics. In voltammetry, electrons are added to or removed from compounds by electric fields. This field is generated by application of voltage across an electrode/solution interface. At a critical voltage—dependent on the compound, electrode, and electrolyte—the chemical present at the electrode surface surrenders its electrons to the electrode, resulting in a small flow of current. If the polarity of the field is reversed, the electrons can be returned from the electrode to the oxidized compound, reverting it to its original state. In vivo voltammetry is a technique allowing repetitive measurements of transmitter release/metabolism to be made in vivo without the need to extract samples or sacrifice the animal. The high spatial and temporal resolution of the methodology allow data to be obtained in tiny nuclei or subregions. Fast cyclic voltammetry, and differential pulse voltammetry, are complementary techniques, both using carbon fiber microelectrodes and generating information about related aspects of neuronal function.

Microelectrodes permit steady-state voltammograms to be recorded more simply than older methods. Only electrodes that are small in all their linear dimensions, so that diffusion is sufficiently convergent, achieve steady states. The time required to approach a steady state closely is about 6A/D for an inlaid disc microelectrode of area A, D being the diffusion coefficient of the reactant. The plateau of the voltammogram reflects A and D, as well as the reactant concentration, so that these parameters are measureable experimentally. Additionally, the heterogeneous rate constant, the transfer coefficient and the standard potential may be accessible from the shape of the steady-state voltammogram.

Alternating current response at a rotating disk electrode to a small a.c. potential wave superimposed on the slowly changing d.c. potential is theoretically treated for an quasireversible and an irreversible redox-electrode process. Expressions for alternating current-d.c. potential curves are given for three types of a.c. signals, i.e., sinusoidal, rectangular and triangular waves. It is shown that the effect of the coupling of the diffusion layers due to hydrodynamics and a.c. polarization can be expressed in terms of a parameter which is defined by the ratio of the frequency of a.c. wave to that of the rotation of the electrode and Schmidt number.

The behavior of lead as an electrode material for electroanalytical purposes was studied in 1.0 M sulfuric acid. The useful negative range of potential was approximately −0.57 to −1.20 V vs. SCE for conventional voltammetry at a rotating disk electrode. For the sinusoidal hydrodynamic modulation technique, the negative limit was about −1.25 V. Studies of the reduction of cadmium ion, thallium(I) ion, and p -nitroaniline showed that the behavior of a rotating lead-disk electrode was comparable to that expected on the basis of theory. Limits of detection are in the micromolar range.

The theory of linear potential sweep voltammetry for a simple redox system in the case of a space distributed redox modified electrode (redox polymer electrode, or adsorption of an electroactive substance in several layers) is established on the basis of the model discussed in Part I of this series. When the rate of the reaction of electron exchange inside the coating is rapid, the theory for a monolayer adsorption is directly applicable. When it is slow, the peaks have a particular shape, with a “tailing” of the current; their characteristics are discussed. The theory is applied to the study of the multilayer adsorption of benzo(c)-cinnoline.

Sinusoidal hydrodynamic modulation (SHM) experiments were performed at a rotating disk electrode to test the theory derived previously for mixed electron transfer and convective diffusion (1). The , Fe(III)/Fe(II), and Tl(III)/Tl(I) couples were studied because they represented systems ranging from fairly fast to very slow electron transfer. Agreement was obtained between theory and experiment for the modulated current and phase shift in the SHM experiment.

A new minicomputer-controlled anodic stripping voltammetry technique is described. The technique uses differential voltammetry at one electrode, and uses a rapid data-averaging algorithm to avoid the need for scan averaging; this allows excellent sensitivities and short analysis times, with adequate reproducibilities. Both linear-scan and staircase waveforms are discussed in conjunction with the technique, and the approximate Roe—Toni theory for linear-scan voltammetry is shown to apply. The system response is investigated for film thickness, scan rate, deposition time, electrode rotation rate, and metal concentration. Results were used to optimize the sensitivity of the technique. The metal ions Cd, Pb and Cu could be determined in solutions as dilute as 1–10 pg/ml-1 in 15 min. The viability of the technique in solutions showing a large background is also discussed, and a comparison is made with scan-averaged techniques.

The theory of linear sweep voltammetry at the tubular electrode has been developed for reversible electrode processes. The convective diffusion differential equations have been transformed to an integral equation which is solved numerically. The theoretical current—potential curves at different velocities of the solution have been calculated. Comparison with experiments using a tubular electrode shows satisfactory agreement.

Channel-flow hydrodynamic voltammetry was used to study the direct electron-transfer reactions of two metalloproteins, myoglobin and cytochrome c, under steady-state conditions at methyl viologen modified (MVM) gold foil electrodes. Utilization of a dual working electrode cell with this technique permitted determination of the heterogeneous electron-transfer kinetics for both the reduction and oxidation of myoglobin. The formal heterogeneous electron-transfer rate constants in pH 7.00 phosphate buffered solutions were found to be 8.9 (±1.5)×10−5 cm s−1 for the reduction, and 7.7 (±1.2)×10−5 cm s−1 for the reoxidation of myoglobin. The transfer coefficient values obtained were 0.21 (±0.01) for the reductive (α) and 0.82 (±0.01) for the oxidative (1−α) electrode reactions. Ionic strength and pH dependences were observed in these direct electron-transfer reactions. Collective current efficiency measurements in the myoglobin experiments indicated that an overall simple charge-transfer process occurred at the respective electrode interfaces. A formal rate constant of 3.4 (±0.2)×10−5 cm s−1 with a transfer coefficient of 0.25 (±0.01) for the reduction (α) of cytochrome c was obtained by this hydrodynamic technique. The use of channel-flow hydrodynamic voltammetry in characterizing an electrode reaction as well as an interpretation of the data presented are discussed.

A theoretical study of electron transfer, mediated by a redox polymer electrode, to a species in solution, is presented for the rotating disc voltammetric method. The transfer occurs at the film—solution interface, and the electron exchange between the redox polymer and the electrode is assumed to be reversible. The case of a reduction is considered. The voltammograms are similar to those obtained at a bare electrode. When the redox potential of the system in solution, E20, is more positive than that (E10) of the system in the redoxpolymer, the transfer is always possible, but it occurs at potentials at most 0.25 V more positive than E 10. When E20

A very thin ring electrode (VTRE) inlaid in an insulating glass plane has been fabricated and studied. Its inner radius was 4 mm and its width was 0.7 μm. Current‐potential and current‐time behavior for the reduction of ferricyanide in potassium chloride and iodine in excess iodide were studied at the rotating and the stationary very thin ring electrode. Comparison of the electrochemical data with theory indicated that the ring electrode was approximately 0.3 μm below the surface of the insulating plane. This conclusion was verified by means of scanning electron microscopy.