I. Navarro

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.

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.

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.

Impedance voltammetric analysis of a consecutive E−C−E mechanism with two diffusing intermediates with application to the reduction of nitromethane

Abstract The faradaic admittance for a consecutive E-C-E mechanism with two stable intermediates is derived with the assumption that the chemical reaction is unidirectional. Both cases where this reaction is homogeneous or heterogeneous are treated. The admittance derived assuming that the chemical step is slow in comparison with the frequency of the a.c. perturbation has the same frequency dependence as that derived by Armstrong and Firman for a consecutive E-E mechanism with a stable intermediate. The presence of the chemical step in the E-C-E mechanism appears in the altered potential dependence of the admittance data. Four cases are distinguished in which the general admittance expression reduces to the simpler Randles expression (pseudo-Randles cases). The theory is applied to the reduction of nitromethane in aqueous 1 M NaCl solutions of pH 2.4. Several methods of analysis are proposed. It is found that a simultaneous analysis of the frequency and potential dependence of the admittance data is to be preferred compared with the habitual procedure of frequency analysis at successive fixed potentials. The final reaction parameters obtained from the analysis are discussed in comparison with previous data obtained in basic media, which conform well to an E-Ce mechanism with only one stable intermediate.

Adsorption of pyrimidine at the mercury | aqueous solution interface

Abstract The adsorption of pyrimidine on a mercury electrode from 1 M KF solutions is studied by means of differential capacity, zero charge potential and maximum surface tension measurements. The data are analysed conforming to the Frumkin isotherm and to the Nikitas approach. A value for the size ratio parameter close to one is obtained. The interaction parameters depend on the electrical variable and indicate predominant adsorbate-solvent interactions at the potential of maximum adsorption. The maximum standard Gibbs energy of adsorption is indicative of physical rather than chemical adsorption. The comparison with other diazines, pyrazine and pyridazine, and with pyridine on the basis of the Esin-Markov effect and the electrosorption valency at low coverages allows some conclusions to be drawn about the relative roles played by the electrostatic interactions with the field and the electronic interactions with the metal, and the participation of H-bonds with the solvent.

Mechanism of electrodimerization of pyrimidine on mercury from acid solutions

Abstract The two reduction waves of pyrimidine in acid solutions are studied. They consist of two successive one-electron transfer steps coupled with dimerization of the intermediate. Classic and differential pulse (DP) polarography and cyclic and convolution voltammetry have been used. Analyses of the waves using several theoretical models allow the mechanism of the overall electrochemical process to be inferred. It is concluded that the dimerization is of the radical-radical type and takes place in the reaction layer. The second reduction is slow with a transfer coefficient close to 0.5, indicating control by the electron transfer.