M. Rueda

Oxidation of L-ascorbic acid on a gold electrode

Voltagrams of L-ascorbic acid on a gold electrode show the existence of two oxidation waves but no reduction wave. An analysis of the I—E curves was carried out together with the Tafel slopes and reaction orders. The products from the first oxidation were identified by paper chromatography. A scheme is proposed for the overall reaction of the first oxidation together with a reaction mechanism at potentials corresponding to the foot of the wave.

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.

Analysis of the interfacial admittance in the case of a two-step two-electron electrode reaction with a diffusing intermediate, with application to the reduction of pyrazine

A critical evaluation is presented of the analysis of impedance or admittance data in the case of an electrode reaction proceeding by two consecutive one-electron transfers with a stable, solution-soluble intermediate. It is shown that the expression for this case, as derived by Armstrong and Firman, has four limiting forms that have the same frequency dependence as the expression describing a single charge-transfer reaction (“pseudo-Randles behaviour”). Also, the conditions for another simplification, which leads to quarter circles in the complex plane diagram (“Amrstrongs equivalent circuit”), are considered. It follows that the special case, where the quarter circle is in the negative quadrant, cannot occur when the system is dc reversible or at equilibrium. For dc irreversible systems, this phenomenon can show up under rather narrow conditions. In addition, it is found that, if these limiting cases are not fulfilled, the general equation often has a frequency dependence that within the practically useful frequency range is hardly distinguishable from that of the single charge-transfer case (“apparent Randles behaviour”). Consequently, the presence of the consecutive mechanism has to be detected from the dc potential dependence of the admittance parameters rather than from the frequency dependence. Finally, it is shown that the general expression corresponds to an equivalent circuit consisting of five elements which, however, have no sensible physical meaning. These views are applied to the reduction of pyrazine at a DME from 1 M aqueous NaClO4 solutions of pH 4.8, 5.4 and 6.1. Although only one wave is observed in the dc polarogram, the potential dependence of the transfer resistance and the Warburg coefficient (found as parameters in a case of “apparent Randles behaviour”) indicates two consecutive one-electron transfers. The standard rate constants and the transfer coefficients of these two electron transfers are calculated, as well as the rate constant of a homogeneous chemical reaction following the second electron transfer. It is concluded that slow protonation steps largely determine the kinetics of the electron transfers. The reactants are found to be weakly adsorbed in contrast with the strong adsorption found earlier in strongly acidic solutions.

The reduction of Cr(III) in concentrated aqueous electrolytes at a DME: The influence of anions

Abstract The reduction of Cr(III) at a dropping mercury electrode is studied in the presence of highly concentrated NaCl, NaI and Na2SO4 solutions. Thermodynamic analyses of the interface in the supporting electrolyte solutions are also performed in order to account for the double-layer contributions to the reaction rates. The results can be rationalized better fi the relative surface excesses are corrected for the amount of adsorbed water and if the discharge plane is situated outside the OHP. The reactivities in NaI solutions are higher than those in NaClO4 at the same concentration but they are equal when compared at the same water activity. Lower rate constants are observed in NaCl and Na2SO4 solutions, which are explained on the basis of ion-pair associations in the bulk of the solution.

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.

The coupling of the double-layer admittance and the faradaic admittance of a stepwise electrode reaction complicated by adsorption of the reactants with experimental verification on the reduction of pyrazine

Abstract The reduction of pyrazine at the dropping mercury electrode has been studied by dc polarography and impedance measurements at two pH values. At pH 0.5 the dc polarogram shows two one-electron waves, while at pH 2.8 these waves merge into one two-electron wave. Nevertheless, in the later case also the charge transfer proceeds in two steps, with a stable intermediate. Contrary to previous studies, evidence is given that the electrode process is complicated by adsorption of the species involved in the reaction mechanism. Expressions are derived for the interfacial admittance, assuming that charge transfer proceeds by two consecutive reversible electron-transfer steps, and that the reactant, the intermediate and the product are absorbed. It is shown that this theory is applicable to the experimental results obtained for the pyrazine reduction, and that most probably indeed adsorption of all species occurs. In addition, it is concluded that the final reduction product is involved in an irreversible homogeneous reaction, most probably hydrolysis.

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.

The reduction of Cr3+ in NaClO4 solutions

Abstract The reduction of Cr3+ at the dropping mercury electrode is studied as a function of both electrode potential and NaClO4 concentration. The ionic surface excesses of the supporting electrolyte components are also determined in order to assess the double layer contribution to the observed rate response. A quantitative explanation of our results is obtained by placing the discharge plane in the diffuse layer at 0.28 nm from the OHP and by introducing a dehydration step preceding the electron transfer.

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.