Coupled ion/electron transfer

As the electrochemical reaction is confined to the boundaries of the thin film, the voltammetric response exhibits a quasireversible maximum. The position of the quasireversible maximum on the log frequency axis depends on the kinetics of the overall reaction at the thin-film electrode, i.e., reflecting the coupled electron-ion transfer (4.3). Analyzing the evolution of the quasireversible maximum measured with different redox probes and various transferring ions, it has been demonstrated... 

First we review processes that ordinarily accompany the redox switching of an electroactive polymer. The key step is coupled electron/ion transfer, which converts one or more reduced forms of the polymer to one or more oxidized forms of the polymer. Solvent and neutral species transfers and polymer structural changes (reconfigurations or coordination state changes) accompany the switching process under permselective conditions. Under nonpermselective conditions, salt transfer also occurs [12]. 

A key issue in the redox switching of electroactive polymers relates to the value of the potential at which coupled electron/ion transfer occurs. The importance of this issue is illustrated in Fig. 13.3. The six cubes in... 

Next we consider the case where a low overpotential exists at some point in the electrochemical switching process, as is necessarily the case in cyclic voltammetric or chronopotentiometric experiments. Now, four additional mechanisms become possible on the time scale of the electron/ion transfer process. This is so because the electron/ion transfer step, E, is no longer constrained to be the first. Consequently, either one or both of the chemical steps may precede the coupled electron/ion transfer. During the oxidation of R to Of, if solvation of Ra is more rapid than its reconfiguration, the two mechanisms, CEC and CC E may occur. If reconfiguration of R is more rapid than its solvation, two additional mechanisms, C EC and C CE, become possible. 

Figures 13.10 to 13.16 present a visual explanation for the pattern of mass- and current-potential curves that is obtained in this system. They are based on a cube-system in which the three elementary steps are coupled electron/ion transfer, solvation and polymer reconfiguration. Explanation of the phenomena observed in Fig. 13.9 requires the use of two cubes, joined at the bottom front edge of one and the top back edge of the other,...

Fig. 4.10 Coupled electron-ion transfer reaction controlled by the ion transfer kinetics. Dependence of A fp on log(c ) for log(s ) = —2.4. The other conditions of the simulations are the...

E. Voltammetric Evaluation of Coupling of Ion Transfer with Electron Transfer at the W/NB Interface... 

Fig. 13.5. 3D axes showing elementary steps associated with poly(thionine) redox switching in aqueous acetic acid buffer coupled electron/ion (proton) transfer, solvation and acetic... 

Voltammetric Evaluation of Coupling of Ion Transfer with Electron Transfer at the W/NB Interface. The selective transfer of Na" " from NB to W in the cell of Equation (29) can be understood by referring to the ion transfer and electron transfer polarograms shown in Figure 6.4. 

Electrons in the iron-sulfur clusters of NADH-Q oxidoreduetase are shuttled to coenzyme Q. The flow of two electrons from NADH to coenzyme Q through NADH-Q oxidoreduetase leads to the pumping offour hydrogen ions out of the matrix of the mitochondrion. The details of this process remain the subject of active investigation. However, the coupled electron- proton transfer reactions of Q are crucial. NADH binds to a site on the vertical arm and transfers its electrons to FMN. These electrons flow within the vertical unit to three 4Fe-4S centers and then to a bound Q. The reduction of Q to... 

In subsequent papers, the same authors developed the technique further. In particular, they showed that it was very well suited for the study of metallopor-phyrins [277]. However, when using different solvents such as 4-methylben-zonitrile, chloroform, or benzene, they also showed that the coupling between ion-transfer and electron-transfer reactions can render the quantitative analysis difficult [278-280]. 

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

MEMED has also been used to investigate the nature of coupled ion-transfer processes involved in spontaneous electron transfer at ITIES [80]. In this application, a key strength of MEMED is that all of the reactants and products involved in the reaction can be measured, as shown in Figs. 19 and 20. The redox reaction studied involved the oxidation of either ferrocene (Fc) or decamethylferrocene (DMFc) in a DCE phase (denoted by Fcdce) by either IrCle or Fe(CN)g in the aqueous phase (denoted by Ox ) ... 

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... 

The difference in magnitudes of the currents for ion transfers coupled with the electron transfer from DMFC in NB to FMN in W is responsible for that Na" " transfers from NB to W spontaneously in the system of Eq. (5), though K" " does not. 

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