Ion and Electron Transfer

The behavior that we observed for the iodide ion is typical for the transfer of a univalent ion. For multivalent ions the situation is more complicated. Depending on the system under consideration and on the electrode potential a multivalent ion can either be transferred in one step, or its charge is first reduced by an electron-transfer reaction. Table 9.1 summarizes the different behavior of ion-transfer and electron-transfer reactions. 

The electrodes used in conventional polarography and voltammetry are electronic conductors such as metals, carbons or semiconductors. In an electrode reaction, an electron transfer occurs at the electrode/solution interface. Recently, however, it has become possible to measure both ion transfer and electron transfer at the interface between two immiscible electrolyte solutions (ITIES) by means of polarography and voltammetry [16]. Typical examples of the immiscible liquid-liquid interface are water/nitrobenzene (NB) and water/l,2-dichloroethane (DCE). 

FIGURE 6.3. Time causes of ion transfer and electron transfer polarograms after W containing K and Fe CN)6 is brought into contact with NB containing TCNQ, TCNQ and Val. 

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. 

This presentation deals with ion-transfer and electron-transfer types of reactions. The electrode reactions in mixtures of water with solvents of lower Lewis basicity will be discussed first, followed by the presentation of such reactions in mixtures of water with solvents of higher donor properties. 

Electrochemical processes at the ITIES involve two basic types of elementary reactions ion transfer and electron tunneling across the liquid liquid boundary. Depending on the properties of the ionic species and the solvents, these two processes can be accompanied by a variety of phenomena such as solvent exchange, interfacial complexation, adsorption, photoexcitation, acid-base dissociation, etc. There are conceptual as well as practical... 

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]. 

Samec, Z., V. Marecek, J. Weber, and D. HomoUca, Charge transfer between two immiscible electrolyte solutions.Part 7. Convolution potential sweep votlammetry of Cs-i- ion transfer and electron transfer between ferrocene and hexacyanofeirate(II) ion across the water-nitrobenzene interface, J Electroanal Chem, Vol. 126, (1981) p. 105. 

With the help of such cell constructions, as described by Figures 2.6 and 2.7, assisted by calculations on the basis of thermodynamic data where measurements were not possible, the electromotive series of electrode reactions could be determined for ion transfer and electron transfer reactions relative to a particular reference electrode. In order to get some idea of the factors which control the position of an electrode reaction in such a scheme, an ion transfer reaction of type (2.23) and a redox reaction of type (2.27) shall be analyzed. 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

Proton Transfer and Electron Transfer Equilibria. The experimental determination used for the data discussed in the above subsections of Section IV.B were obtained from ion-molecule association (clustering) equilibria, for example equation 9. A vast amount of thermochemical data such as gas-phase acidities and basicities have been obtained by conventional gas-phase techniques from proton transfer equilibria,3,7-12-87d 87g while electron affinities88 and ionization energies89 have been obtained from electron transfer equilibria. 

Equilibria Where a Neutral Molecule Is Exchanged. The difficulties discussed above for proton transfer and electron transfer equilibria involving multiply charged ions are not present when neutral molecules (ligands) which are complexed to a given ion are exchanged. Equation 43 is a typical example ... 

Depending on the fabrication techniques and deposition parameters, the pH sensitive slope of IrOx electrodes varies from near-Nemstian (about 59 mV/pH) to super-Nemstian (about 70mV/pH or higher). Since the compounds in the oxide layers are possibly mixed in stoichiometry and oxidation states, most reported iridium oxide reactions use x, y in the chemical formulas, such as lr203 xH20 and IrOx(OH)y. Such mixed oxidation states in IrOx compounds may induce more H+ ion transfer per electron, which has been attributed to causing super-Nerstian pH responses [41],... 

In summary, the copper ion transfers an electron from the unsaturated substrate to the diazo-nium cation, and the newly formed diazonium radical quickly loses nitrogen. The aryl radical formed attacks the ethylenic bond within the active complexes that originated from aryldiazo-nium tetrachlorocuprate(II)-olefin or initial arydiazonium salt-catalyst-olefln associates and yields >C(Ar)-C < radical. The latter was detected by the spin-trap ESR spectroscopy. The formation of both the cation-radical [>C=C<] and radical >C(Ar)-C < as intermediates indicates that the reaction involves two catalytic cycles. In the other case, radical >C(Ar)-C < will not be formed, being consumed in the following reaction ... 

Bulk crystalline radical ion salts and electron donor-electron acceptor charge transfer complexes have been shown to have room temperature d.c. conductivities up to 500 Scm-1 [457, 720, 721]. Tetrathiafiilvalene (TTF), tetraselenoful-valene (TST), and bis-ethyldithiotetrathiafulvalene (BEDT-TTF) have been the most commonly used electron donors, while tetracyano p-quinodimethane (TCNQ) and nickel 4,5-dimercapto-l,3-dithiol-2-thione Ni(dmit)2 have been the most commonly utilized electron acceptors (see Table 8). Metallic behavior in charge transfer complexes is believed to originate in the facile electron movements in the partially filled bands and in the interaction of the electrons with the vibrations of the atomic lattice (phonons). Lowering the temperature causes fewer lattice vibrations and increases the intermolecular orbital overlap and, hence, the conductivity. The good correlation obtained between the position of the maximum of the charge transfer absorption band (proportional to... 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

For instance, the superoxide ion transfers one electron to quinones, nitro compounds, and transforms them into anion radicals (Sawyer Gibian 1979). Scheme 1-69 below illustrates the reversible transformation of quinone (Q) into semiquinone (SQ). 

Oxidation-reduction electrodes These electrodes consist of an inert metal in a solution containing ions that can undergo oxidation-reduction reactions. An example is a platinum wire immersed in a solution containing Fe2+ and Fe3+ ions. Transfer of electrons into the solution is accompanied by reduction of some Fe3+ to Fe2+. 

Area, M., Mirkin, M. V. and Bard, A. J. (1995), Polymer-films on electrodes. 26. Study of ion-transport and electron-transfer at polypyrrole films by scanning electrochemical microscopy. J. Phys. Chem., 99(14) 5040-5050. 

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