Francisco Javier García Prieto

Impedance measurements with phospholipid-coated mercury electrodes

Abstract Electrochemical measurements of impedance at dioleoylphosphatidylcholine (DOPC) coated mercury electrodes have been applied to several experimental situations and carried out using different procedures. Firstly the DOPC and gramicidin-modified DOPC coated electrodes were studied in the absence of electroactive species in solution. Secondly the gramicidin-modified DOPC coated mercury electrode was studied in the presence of Tl + ions in solution. The procedures consisted of: (i) potential and frequency sweeps on a single adsorbed DOPC monolayer, (ii) potential and frequency sweeps on a single adsorbed DOPC layer including an electrochemical pre-treatment before every perturbation step and (iii) experiments each carried out at successive frequencies or potentials on successively formed adsorbed DOPC monolayers respectively. The guidelines to be followed in order to check the consistency of the results and to establish the most suitable experimental conditions are outlined in the paper. The results are compared to similar experiments on uncoated hanging mercury electrodes in order to explain the observed behaviour.

Channel electrode voltammetry and reversible electro-dimerisation processes. The reduction of the methyl viologen di-cation in aqueous solution

Abstract Theory is developed to account for the variation of the voltammetric half-wave potential with the rate of mass transport at a channel electrode for the case where an electrochemically reversible electron transfer is followed by the (partially) chemically reversible dimerisation of the electrode product. A working surface is presented which allows the analysis of experimental data and it is shown that when the homogeneous dimerisation kinetics are sufficiently rapid the half-wave potential becomes independent of the rate of mass transport but is shifted from the true formal potential. Experiments are reported on the reduction of the methyl viologen di-cation in aqueous solution. The resulting mono-cation is believed to undergo a reversible dimerisation forming a π-dimer, and the possibility of further aggregation is investigated. Measurements of the half-wave potential as a function of solution flow-rate and concentration are consistent with dimer formation and permit the inference of a value of −0.703 ± 0.003V (vs. SCE) for the one-electron reduction of the di-cation.

Impedance voltammetry of electro-dimerization mechanisms: Application to the reduction of the methyl viologen di-cation at mercury electrodes and aqueous solutions

Abstract The faradaic impedance for an electrode mechanism with a reversible homogeneous dimerization reaction following the electron transfer step is derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced by Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order chemical reaction. Two limiting cases can be distinguished in which the general expression reduces to the simpler Randles or pseudo-Randles expression. Under those conditions, the presence of the dimerization reaction can be inferred from the potential dependence of the impedance parameters. The theory is applied to the reduction of the methyl viologen di-cation at mercury electrodes in aqueous solution. The rate and the equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warburg coefficient, while the potential dependence of the irreversibility coefficient allows the calculation of the standard rate constant and the transfer coefficient for the electron transfer step.

Interfacial properties of hypoxanthine adsorbed at the mercuryelectrolyte interface

The adsorption of hypoxanthine on a mercury electrode from sulfate solutions 0.2 and 0.5 M at pH 2.0 and 5.0 is studied. Differential capacity, zero charge potential and maximum surface tension measurements are used to establish the characteristics of the dilute layer. Condensed film formation is reported for first time, detected from differential capacity data at high hypoxanthine concentrations in solutions at pH 5.0. The data for the dilute layer conform to a Frumkin isotherm, contrary to previous findings about a Langmuir isotherm. The data are also analysed following the Nikitas approach and a value for the size ratio parameter close to one is obtained. The discussion in terms of Esin-Markov effect and electrosorption valency in comparison with other aromatic compounds allows some conclusions to be drawn about the orientation of the molecule, the role played by electrostatic and π-electron interactions and the effect of intermolecular interactions.

Impedance study of thallous ion movement through gramicidin–dioleoylphosphatidylcholine self-assembled monolayers supported on mercury electrodes: the C–(C)–CE mechanism

Abstract A new mechanism (the C–(C)–CE mechanism) is proposed for Tl + reduction and Tl–amalgam oxidation on gramicidin-modified dioleoylphosphatidylcholine (DOPC) mercury and Tl–amalgam electrodes. The faradaic impedance equations are derived, and applied to the experimental results obtained in 0.1 M KCl solutions at a high gramicidin concentration. The mechanism includes two consecutive heterogeneous chemical steps with an intermediate, which decomposes in a parallel homogeneous chemical step. The two heterogeneous chemical steps are assumed to mimic the interaction of the ion with the channel mouth and the further translocation across the channel, respectively. The results fit the mechanism well, and the analysis provides the equilibrium constant for the first chemical step and the forward rate constant for the translocation step. The latter was found to depend on the drop in potential across the interface, with a transfer coefficient α =0.2, in both the reduction and the oxidation experiments. However, the translocation step standard rate constant provided by the oxidation experiment is lower than that by the reduction experiment, indicating some asymmetry in the half-channel behaviour.

Channel Microband Electrode Arrays for Mechanistic Electrochemistry. Two-Dimensional Voltammetry: Electrode Kinetics

The theory of “two-dimensional voltammetry” using arrays of gold channel microband electrodes ranging in size from the millimeter to the micrometer scale is extended to permit the deduction of electrochemical rate constants through the analysis of hydrodynamic voltammograms as a function of both electrode size and electrolyte flow rate (convection). The technique is applied to the reduction of p-chloranil and the oxidation of 2,3,7,8-tetramethoxythianthrene both in acetonitrile solution and electrochemical rate constants are reported.

Electrode processes with coupled chemistry. Heterogeneous or homogeneous chemical reaction? The reduction of nitromethane in basic aqueous solution

Abstract The reduction of nitromethane in aqueous solution (pH 8.3) at Au/Hg channel electrodes is suggested, by means of joint electrochemical ESR and voltammtric measurements, to follow an ECEEE mechanism in which the rate determining chemical step is a surface catalysed reaction. Theory characterising heterogeneous ECE (and ECEFE) processes at channel electrodes, studied voltammetrically and with in situ ESR detection, is reported.

Analysis of the faradaic admittance for an ECE mechanism in the case of non-Randles behaviour with frequency and its application to nitromethane reduction

Abstract The analysis of admittance and impedance data in the case of a particular ECE mechanism in which a stable intermediate is assumed to undergo a unidirectional chemical reaction followed by an infinitely fast electron transfer is evaluated critically. It is shown that situations can prevail which might be misinterpreted as “pseudo-Randles” behaviour with frequency. To avoid this risk some procedures are proposed for obtaining the kinetic parameters from the frequency dependence of the impedance data, using the complete equation deduced previously for this mechanism. These procedures are applied to the reduction of nitromethane on mercury at two extreme pH values: at pH 9 where a “pseudo-Randles” circuit holds and at pH 1 where the “non-Randles” behaviour requires the application of the new procedures. In the latter conditions the more complex potential dependence of the impedance parameters contradicts the mechanism proposed previously on the basis of polarographic data.

Impedance analysis of the reduction of pyrimidine at the dropping mercury electrode: Part 2. Parallel mechanisms

Abstract The rate equations for a two-electron reduction, also involving heterogeneous protonation steps, in which several pathways are followed in parallel have been derived using two procedures. The first is based on the “scheme of squares” to which the kinetic laws are applied. The second is a more intuitive method formulated in terms of equivalent resistances which allows the possible mechanisms to be studied easily in a more systematic way. The charge transfer resistance versus potential curves for pyrimidine reduction in the pH range 6.5–7.7 are analysed in terms of the rate equations deduced for several branched mechanisms. The influence of the potential and pH allows the pathways preferred by the system to be determined.

Impedance analysis of the reduction of pyrimidine at the dropping mercury electrode

Abstract The electrochemical reduction of the 3,4-NC bond of pyrimidine was studied in the pH range 6.5–7.7 within which only one d.c. polarographic wave is observed. The rate constants were obtained as a function of potential both from polarography and from impedance analysis. Because the latter method provides data over a wider potential range, the existence of a sequential mechanism with more than one determining step could be shown. In fact, the potential dependence of the rate constants can be explained by a sequential CEE mechanism. However, the CEE mechanism cannot account for the more involved R ct - E curves. These curves and the influence of the pH on the rate constants indicate the presence of a parallel mechanism, in which species with different degrees of protonation are being reduced simultaneously.