MATHCAD – a Tool for Numerical Calculation of Square-Wave Voltammograms

An alternative approach for numerical calculation of the square-wave voltammograms using the mathematical programming package MATHCAD is presented. A quasi-reversible redox reaction is considered and a mathematical model is developed under conditions of the square-wave voltammetry (SWV). Application of the mathematical model in MATHCAD is discussed and the file used for numerical simulation is presented. The relationships between the properties of the SW response and the parameters of both the quasireversible redox reaction and the excitation signal are discussed

New aspects of the electrochemical-catalytic (EC’) mechanism in square-wave voltammetry

Several new theoretical aspects of the electrocatalytic (regenerative) EC’ mechanism under conditions of square-wave (SWV) and staircase cyclic voltammetry (SCV) are presented. Elaborating the effect of the rate of the catalytic reaction in the diffusion-controlled catalytic mechanism (diffusional EC’ mechanism) and surface catalytic mechanism (surface EC’ mechanism), we refer to several phenomena related to the shift of the position and the half-peak width of the net peak in square-wave voltammetry (SWV). If the rate of the catalytic reaction is much higher than the kinetics of the electrode reaction, a linear dependence between the peak potential of the simulated net SWV peaks and the logarithm of the catalytic parameter can be observed. The intercept of that linear dependence is a function of the kinetics of the electrode reaction. Based on this finding, we propose a new methodology to determine the electrode kinetics rate constant. The proposed approach relies on the variation of the concentration of the regenerative reagent. To the best of our knowledge, this is one of very few voltammetric approaches for electrode kinetic measurements not based on the time or potential variation in the experimental analyzes. In addition, we present a brief analysis of the catalytic mechanism under conditions of staircase cyclic voltammetry in order to emphasize the main differences between SCV and SWV

Electrode Kinetics from a Single Square-Wave Voltammogram

A novel methodological approach for processing electrochemical data measured under conditions of square-wave voltammetry (SWV) is proposed. The methodology takes advantage of the chronoamperometric characteristics of the electrochemical system observed in the course of the voltammetric experiment,which are commonly not considered in conventional SWV.The method requires processing of a single voltammogram only, measured under given set of the potential modulation parameters (i.e. SW frequency and height of the potential pulses). The core idea is to analyze the variation of the current with time, considering the last quarter time period of each potential pulse of the potential modulation, instead of measuring the single current value from the very end of the pulses. Hence, a single point current measurement is replaced with a multisampling current procedure, leading to a series of new SW voltammograms measured at different time increments of each potential pulse. The novel method is illustrated by the theoretical analysis of a quasireversible electrode reaction of a dissolved and surface confined redox couple, as well as a reversible electrode reaction preceded by a chemical equilibrium reaction (CE mechanism)

Determination of the standard Gibbs energies of transfer of cations across the nitrobenzene|water interface utilizing the reduction of iodine in an immobilized nitrobenzene droplet

When a nitrobenzene (NB) droplet containing iodine is attached to a graphite electrode and immersed into a chloride containing aqueous (AQ) solution, the electrochemical reduction of iodine is accompanied by a transfer of chloride ions from NB to water. These chloride ions enter the NB phase in a preceding partition between the AQ and the NB phases, supported by formation of I2Cl� ions in NB and accompanied by the transfer of stoichiometric amounts of cations. The overall electrode reaction is of CErev type, where C refers to the preceding chemical step forming I2Cl�, and Erev refers to the reversible reduction of iodine at the graphitejNB interface and the simultaneous transfer of chloride from NB to water. If the chloride concentration in NB is insufficient to compensate by leaving the NB the amount of electrochemically produced iodide, a second voltammetric signal occurs at more negative potentials due to the transfer of iodide from NB to water. The kinetics and thermodynamics of the preceding chemical step C, determine the voltammetric behaviour of the system in such way that the ratio of peak currents of the first and second signals depends linearly on the Gibbs energy of transfer of the co-partitioned cations. The method was validated for cations of known Gibbs energies of transfer, and it was applied to cations of amino acids

Probing the redox activity of T-lymphocytes deposited at electrode surfaces with voltammetric methods

Background: Reactive oxygen species and redox signaling play an important role in the regulation of many vital biological processes. However, they are also tightly connected with many pathological conditions. The detection and evaluation of these signaling events are very often accompanied with great difficulties. In this article, we describe the development of a novel electrochemically-based technique for monitoring the cellular redox state. Methods and results: T-cells were attached on the surface of a working electrode, which was modified with 2-palmitoylhydroquinone as a redox mediator. Using cyclic voltammetry, we were able to indirectly (via the redox mediator) monitor an electron transport from the cells towards the working electrode, which enabled us to evaluate the redox activity of the cells. Conclusions: This new technique is rather simple and sensitive and may be used in the future as a valid diagnostic procedure in various branches of biomedical science

Theoretical and experimental study of the surface redox reaction involving interactions between the adsorbed particles under conditions of square-wave voltammetry

A surface redox reaction of a strongly adsorbed redox couple involving interactions between the adsorbed particles was analyzed theoretically under conditions of square-wave voltammetry (SWV). If uniform interactions act between the adsorbed particles, the effect of interaction forces is represented through the product of the relative surface coverage and Frumkin interaction parameter �a. The kinetics of the redox reaction in the presence of uniform interactions are a function of the apparent rate constant defined as ks,app.=ks exp(−2a�), where ks is a real standard rate constant. The apparent reversibility of the redox reaction is determined solely by a dimensionless kinetic parameter defined as the ratio of the apparent rate constant and the signal frequency, �=ks,app/f. This dimensionless kinetic parameter unifies the effects of both the interaction forces and the charge transfer rate. The height of the dimensionless response depends parabolically on the kinetic parameter �, as a consequence of the phenomenon known as a ‘quasireversible maximum’. Based on this peculiarity, a method for estimating the real standard rate constant ks and Frumkin interaction parameter a is proposed. The theoretical results are compared qualitatively with the experimental SW voltammograms of probucole

Protein film voltammetry: electrochemical enzymatic spectroscopy. A review on recent progress

This review is focused on the basic principles, the main applications, and the theoretical models developed for various redox mechanisms in protein film voltammetry, with a special emphasis to square-wave voltammetry as a working technique. Special attention is paid to the thermodynamic and kinetic parameters of relevant enzymes studied in the last decade at various modified electrodes, and their use as a platform for the detection of reactive oxygen species is also discussed. A set of recurrent formulas for simulations of different redox mechanisms of lipophilic enzymes is supplied together with representative simulated voltammograms that illustrate the most relevant voltammetric features of proteins studied under conditions of square-wave voltammetry

Redox Chemistry of Ca-Transporter 2-Palmitoylhydroquinone in an Artificial Thin Organic Film Membrane

The redox chemistry of 2-palmitoylhydroquinone (H2Q), a recently introduced synthetic transmembrane Ca2+ transporter, was studied with cyclic and square-wave voltammetry in an artificial thin organic-film membrane sandwiched between a pyrolytic graphite electrode and an aqueous solution. The membrane has a micrometer dimension and consists of the water immiscible organic solvent nitrobenzene, which contains suitable electrolyte and H2Q as a redox active compound. The potential drop at the electrode/membrane interface is controlled by the potentiostat, whereas the potential drop at the membrane/water interface is dependent on the ClO4 - concentration, which is present in a large excess in both liquid phases. The redox transformation of H2Q at the electrode/membrane interface is accompanied by a corresponding ion-transfer reaction at the other side of the membrane. Proton transfer at the membrane/water interface is critical for the redox transformation of H2Q in the interior of the membrane, as a strong dependence of the voltammetric response on the pH of the aqueous medium was observed. H2Q undergoes two oxidation processes due to existence of two distinctive redox forms of H2Q. The electrochemical mechanism can be explained with two tautomer forms of H2Q formed by migration of a proton between the 1-hydroxyl group and the adjacent carbonyl group of the palmitoyl residue. Both tautomers undergo 2e/2H+ distinctive redox transformations to form the quinone form of the studied compound. In the presence of Ca2+ in the aqueous phase, voltammetric experiments confirmed the capability of both tautomers to form 1:1 complexes with Ca2+ and to extract it into the organic membrane. Upon the oxidation of the complexes, Ca2+ is expelled back to the aqueous phase. The studied compound exhibits very similar complexing affinity toward Mg2+, implying that it is not highly selective for transmembrane Ca2+ transport

Surface catalytic mechanism: theoretical study under conditions of differential square-wave voltammetry

Differential square-wave voltammetry (DSWV) is the most recent modification of square-wave voltammetry (SWV), developed to advance the performance of the technique for both analytical and kinetic applications. The differential current-measuring protocol in DSWV leads to improved voltammetric features of the forward and backward current components, particularly when slow, i.e., quasi-reversible or irreversible electrode reactions are studied. In the present theoretical work, the catalytic electrode mechanism of the surface bound redox species (surface ECʹ mechanism) is studied under the conditions of the new technique, where E denotes the electrode reaction and Cʹ refers to the irreversible follow-up regener�ative chemical reaction. The theoretical data presented provides a general overview of the ECʹ reaction scheme, implying some specific voltammetric features that can be exploited for the estimation of relevant physical parameters of the electrode reaction E and the regenerative chemical reaction Cʹ