L. Lamberts

Fullerene film electrodes in aqueous solutions Part 1. Preparation and electrochemical characterization

Solution cast films of C60 on gold and glassy carbon substrates were studied in aqueous solutions as possible electrode materials. These films could be reduced in 1 M NaOH solutions to became conductive, possibly due to the formation of NaxC60. The reduced films were quite stable even on exposure to air, and their conductivity decreased only after a long time or when they were oxidized electrochemically. Both the reduction and the oxidation were completely irreversible. If oxidation is avoided, i.e. the potential is kept below +600 mV/SHE, these films can be used as electrodes, displaying individual properties as electrocatalysts; some electrochemical reactions were faster and others were slower than on bare gold or glassy carbon electrodes.

Fullerene film electrodes in aqueous solutions part 2. Effects of doping cations

Abstract Solution cast C 60 films were studied in aqueous solutions containing a variety of doping cations. Li + , Na + , K + , Rb + , Cs + , Ca 2+ , and Ba 2+ were used as dopants and it was established that the reduction of the C 60 films shifted in parallel with the hydration energy of these cations. The reduction peak potentials also changed with the concentration of the cations, and this was assumed to be the consequence of the formation of reduced films which acted as cation exchange membranes. The cation exchange character was also detected after the reduction, but only with smaller ions (Li + , Na + ), suggesting that, unlike the larger ions, they remain mobile in the films. The reduced C 60 films had quite different electrochemical activity and stability depending on the dopants. The most active and stable were those formed with K + and Rb + dopants. Some of the films (those doped with Li + , Na + and Ba 2+ ) could be oxidized electrochemically, while the others were probably oxidized only on their surfaces. In these cases an increased compactness and the formation of some protecting layers were assumed.

Square wave voltammetric determination of paraquat and diquat in aqueous solution

Abstract A rapid, sensitive method for the determination of paraquat and diquat in aqueous media using square wave voltammetry (SWV) is discussed on the basis of the analyte behaviour in cyclic voltammetry. Cation-radicals formed upon the one-electron reduction of the analyte(s) adsorb on the mercury electrode surface with both flat and vertical orientations. With low concentrations only the flat oriented deposit occurs and the adsorption becomes progressively diffusion-controlled, especially at high scan rates. SWV experiments were based on this diffusion-controlled adsorption process. Effects on square wave frequency f , staircase step height ΔE s , scanning direction, presence of surfactants and supporting electrolyte concentration were examined for the optimization of experimental conditions. The optimal parameters found were f = 100 Hz and ΔE s = 2 mV for scanning potentials from −0.85 to −0.25 V in the presence of 0.003% gelatin and with supporting electrolyte concentrations ranging from 0.02 to 0.05 M. Calibration curves were linear in the range 5 × 10 −8 to 1 × 10 −5 M with a detection limit of 1.5 × 10 −8 M. SWV is a sensitive technique, more rapid than all other described procedures for determining these pesticides, and allows analysis to be performed in the presence of an ionic matrix. The method was extended to the analysis of paraquat and diquat in potatoes after digestion in hot sulphuric acid, neutralization and solid phase extraction. The detection limit has been found to be 1 μg g −1 for both pesticides.

Use of a zeolite-modified electrode for the study of the methylviologen–sodium ion-exchange in zeolite Y

The electrochemical response of zeolite-modified carbon paste electrodes (ZMCPEs) has been exploited to plot the normalised ion-exchange isotherm of methylviologen and sodium in zeolite Y. Results observed from the proposed methodology agree well with those obtained using the conventional procedure. Selectivity was very high for the large organic divalent cation over sodium species, but the degree of exchange of methylviologen did not exceed 64%. Electrochemistry of ZMCPEs allows the in situ quantitative determination of methylviologen in the zeolite phase, without significant modification in the solution-phase concentrations, so that the ion-exchange isotherm can be plotted very rapidly and without any other chemical analysis.

Electrochemical behaviour of selenoorganic compounds—I. Dibenzo (b,d) selenopyrane and related compounds

The electrochemical properties of selenoxanthene, selenoxanthene-9-ol and selenoxanthone in non-aqueous and mixed media are investigated by voltammetry at rotating platinum and vitrous carbon disk electrodes, cyclic voltammetry, chronopotentiometry and constant potential coulometry. In “dry” acetonitrile selenoxanthene leads to selenoxanthylium cation. In the presence of water, selenoxanthene-9-ol is obtained. Further oxidation of selenoxanthene-9-ol gives the selenoxanthone which can be oxidized to the corresponding selenoxide. Influence of acids and bases is shown.

Electrochemical behaviour of seleno-organic compounds. Part 2 : Benzo(b) selenophene and dibenzo(b,d)selenophene

Abstract The electrochemical oxidation of benzo( b )selenophene and dibenzo( b,d )selenophene in non-aqueous media has been studied using platinum and glassy carbon electrodes. The selenoxide of dibenzo( b,d )selenophene has been isolated. A mechanism consistent with voltammetric measurements has been established. The electrochemical oxidation of dibenzo( b,d )thiophene was discussed and compared with its seleno analog.

Electrochemical study of dibenzo(c,e)-1,2-dithiin in acetonitrile medium

The electrochemical properties of dibenzo(c,e)-1,2-dithiin in acetonitrile solution are investigated by voltammetry at rotating platinum and vitrous carbon disk electrodes, and by cyclic voltammetry and constant potential coulometry. Attention is particularly devoted to the first charge transfer step and the subsequent chemical reactions. Evidence for an ECE process is given. Comparison of the chemical and electrochemical oxidation of dibenzo(c,e)-1,2-dithiin and related open-chain compounds is discussed.

Electrochemical oxidation of selenanthrene in acetonitrile at conventional electrodes and microelectrodes

Abstract The electrochemical oxidation of selenanthrene was studied by conventional and microelectrode techniques. The first signal was proved to follow an ECE mechanism: steady-state voltammetry at microelectrodes and cyclic voltammetry showed a monoelectronic transformation into the cation-radical, whereas controlled potential coulometry and electrolysis gave the corresponding 5-selenoxide with an exchange of 2F mol −1 . The electrochemical reversibility of the intermediary cation-radical was investigated and its stability compared with those of similar heterocyles. Further oxidation led to 5,10-selendioxide.

The electrochemical oxidation of thioselenanthrene in acetonitrile at conventional electrodes and microelectrodes

Abstract The electrochemical oxidation of thioselenanthrene (1c) in acetonitrile was studied by conventional and microelectrode techniques. Steady-state and cyclic voltammetry proved the occurrence of a DISP2 mechanism for the first oxidation signal. Controlled potential coulometry led to the formation of the corresponding selenoxide. The first electron transfer proceeds with a high heterogeneous rate constant (0.58 ± 0.10 cm s−1), whereas a pseudo-first-order apparent rate constant of 8.87 ± 1.11 s−1 was computed for the chemical step, giving a half-lifetime of 78 ± 10 ms for the cation-radical of 1c. In order to compare their cation-radical stabilities, similar measurements were also realized for phenoxathiine (1d) and phenoxaselenine (1e).

The electrochemical oxidation of dibenzo(c,e)-1,2-diselenine to its cation radical. A voltammetric study in acetonitrile at conventional electrodes and microelectrodes

Abstract The electrochemical oxidation of dibenzo(c,e)-1,2-diselenine ( 1a ) in dry acetonitrile was studied by conventional and microelectrode techniques. Steady-state voltammetry and cyclic voltammetry proved the occurrence of an electro-chemical-electronic (ECE)-type mechanism for the first oxidation signal. The chemical step involves the reaction of the cation radical with residual water. The first electron transfer proceeds with a heterogeneous rate constant of 0.14 ± 0.01 cm s −1 , whereas a pseudo-first-order kinetic rate constant of 20.7 ± 2.8 s −1 is computed for the chemical step, giving a half-life of 33 ± 4 ms for the cation radical of 1a . A comparison of these two rate constants with those of dibenzo(c,e)-1,2-dithiin ( 1b ), selenanthrene ( 2a ) and thianthrene ( 2b ) indicates that the electron transfer rate depends on the nature of the oxidized chalcogen, whereas the stabilization of the cation radical is also dependent on the geometry of the heterocycle.