Voltammetric elucidation

II. VOLTAMMETRIC ELUCIDATION OF THE CHARGE TRANSPORT PROCESS THROUGH A LIQUID MEMBRANE OR A BILAYER LIPID MEMBRANE IN THE PRESENCE OF SUFFICIENT ELECTROLYTES [10,17,18]... 

Maeda, K., S. Nagami, Y. Yoshida, H. Ohde, and S. Kihara, Voltammetric elucidation of the process of self-sustained potential oscillation observed with a Uquid membrane system composed of water containing cetyltrimethylammonium chloride I nitrobenzene containing picric acid I pure water, J Electmanal Chem, Vol. 496, (2001) p. 124. 

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

Finally, it should be stressed that the voltammetric concept and methods are very useful for the elucidation of electrochemical reactions (redox or ion transport reactions) at artificial membranes as well as biomembranes. 

In well-aerated soil, it is expected that all species will be in their highest oxidation states. However, this does not happen for reasons elucidated in previous chapters. In well-aerated soil, both ferrous and ferric iron can exist along with elemental iron.3 Zinc, copper, and especially manganese can apparently exist in a mixture of oxidation states simultaneously in soil. Add to this a multitude of organic species that are also capable of oxidation-reduction reactions and the result is truly a complex voltammetric system [12,13],... 

The comparison of the voltammetric characteristics of PWig and P2W18 is useful to highlight the peculiarities of the former complex. Figure 21 shows in superimposition the cyclic voltammograms (CVs) of the two complexes in a pH 0.3 sulfate medium. The potential domain was selected to avoid any deleterious derivatization of the electrode surface [28]. Furthermore, such domain is the most useful for elucidation of electrocatalytic processes. Here, the voltammetric pattern is restricted to the first three waves observed for PWig in this medium. In... 

Voltammetric current-potential curves are important in elucidating electrode processes. However, if the electrode process is complicated, they cannot provide enough information to interpret the process definitely. Moreover, they cannot give direct insight into what is happening on a microscopic or molecular level at the electrode surface. In order to overcome these problems, many characterization methods that combine voltammetry and non-electrochemical techniques have appeared in the last 20 years. Many review articles are available on combined characterization methods [10]. Only four examples are described below. For applications of these combined methods in non-aqueous solutions, see Chapter 9. 

K.C. Honeychurch, J.P. Hart and N. Kirsch, Voltammetric, chromatographic and mass spectral elucidation of the redox reactions of 1-hydro-xypyrene occurring at a screen-printed carbon electrode, Electrochim. Acta, 49 (2004) 1141-1149. 

On the other hand, the voltammograms observed with the O/W interface showed quite different features. As shown in Figure 8.4B, the positive-current peak was depressed significantly by lowering [Fc]o, suggesting the existence of a kinetically controlled process. Such a difference in the voltammetric behaviour between the ECSOW and O/W systems is due to the fact that no IT can take place in the ECSOW system. The voltammetric behaviour for the O/W interface shown in Figure 8.4B can be elucidated in terms of the IT mechanism described below. 

Finally, the combined voltammetric and on-line differential electrochemical mass spectrometry measnrements allow a quantitative approach of the ethanol oxidation reaction, giving the partial current efficiency for each product, the total number of exchanged electrons and the global product yields of the reaction. But, it is first necessary to elucidate the reaction mechanism in order to propose a coherent analysis of the DBMS results. In the example exposed previously, it is necessary to state on the reaction products in order to evaluate the data relative to acetic acid production which cannot be directly detected by DBMS measurements. However, experiments carried out at high ethanol concentration (0.5 mol L" ) confirmed the presence of the ethyl acetate ester characterized by the presence of fragments at m/z = 61, 73 and 88 at ratios typical of the ethyl acetate mass spectrum. " This ethyl acetate ester is formed by the following chemical reaction between the electrochemically formed acetic acid and ethanol (Bq. 29) and confirms the formation of acetic acid. 

Analogously with EC data obtained for cyclic voltammetric experiments, the ratio of the limiting current for the reduction of iodine in the presence and absence of iodate was compared to that derived theoretically for an EC mechanism. This analysis of the data allowed elucidation of the reaction mechanism and quantification of kinetic parameters. 

Several complexes offer two or more redox-active moieties that may undergo electron transfer at rather similar potentials. In such cases voltammetric or potentiometric techniques alone may not suffice to unambiguously assign individual redox processes or to elucidate the order of redox events within such molecules. If, however, the individual redox-active constituents generate oxidised or reduced forms with clearly distinguishable spectroscopic properties, a suitable combination of electrochemical and spectroscopic investigations can... 

Elucidation of the mechanism of the catalytic process is a relatively complicated task because of the variety of factors influencing the catalytic reaction. For instance, preconcentration of redox species in the porous films can result in an apparent surface excess, similar to adsorption. Here, semi-integral analysis of voltammetric curves can aid in separating diffusional and surface-confined components (Freund and Brajter-Toth, 1992). 

The data below were recorded from a series of cyclic voltammetric experiments designed to elucidate the mechanism of the electrode reaction involving reduction of a certain compound. Formulate a mechanism to explain the behavior of the diagnostic functions, then briefly rationalize as many of the trends in the data as you can in terms of your mechanism. The switching potential was held constant at —1.400 V V5-. SCE. 

In order to elucidate the features of parallel transports of types I and II, based on the voltammetric method and concept, voltammograms for parallel transports of types I and 11 were measured and compared with that for Perpendicular transport. ... 

In addition to the techniques previously mentioned, a wide variety of methods has been used to characterize the new inclusion compounds in solution and in the solid state, in both directly practice-oriented and theoretical papers, to elucidate the relationship between the relative strength of interaction and some surface parameters of the guest molecules. Complexes obtained in solution are frequently studied by phase-solubility, to obtain the stoichiometric ratio for the complex and an apparent stability constant [12-14], but spectral studies including UV, infrared, fluorescence, and NMR spectroscopy (see Section 10.3 and Chapter 9) can also be used for characterization [6, 15-17]. Inclusion compounds obtained in the sohd state are frequently characterized using infrared spectroscopy, X-ray diffraction (Chapter 7), scanning electron microscopy techniques [18, 19] (Section 10.6), differential scanning calorimetry (DSC) (Chapter 8) [20, 21], and/or fluorescence (Section 10.3) and voltammetric measurements (Section 10.5) [16, 22]. 

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