Oxide systems, reversible potentials

The mixed-potential model demonstrated the importance of electrode potential in flotation systems. The mixed potential or rest potential of an electrode provides information to determine the identity of the reactions that take place at the mineral surface and the rates of these processes. One approach is to compare the measured rest potential with equilibrium potential for various processes derived from thermodynamic data. Allison et al. (1971,1972) considered that a necessary condition for the electrochemical formation of dithiolate at the mineral surface is that the measmed mixed potential arising from the reduction of oxygen and the oxidation of this collector at the surface must be anodic to the equilibrium potential for the thio ion/dithiolate couple. They correlated the rest potential of a range of sulphide minerals in different thio-collector solutions with the products extracted from the surface as shown in Table 1.2 and 1.3. It can be seen from these Tables that only those minerals exhibiting rest potential in excess of the thio ion/disulphide couple formed dithiolate as a major reaction product. Those minerals which had a rest potential below this value formed the metal collector compoimds, except covellite on which dixanthogen was formed even though the measured rest potential was below the reversible potential. Allison et al. (1972) attributed the behavior to the decomposition of cupric xanthate. 

As in Eq. (3.22), F is the Faraday constant, n is the number of electrons taking part in the reaction, but iq is a new quantity called the exchange current density. These rates have units of mol/cm s, so the exchange current density has units of A/cm. Typical values of io for some common oxidation and reduction reactions of various metals are shown in Table 3.4. Like reversible potentials, exchange current densities are influenced by temperature, surface roughness, and such factors as the ratio of oxidized and reduced species present in the system. Therefore, they must be determined experimentally. 

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

To impose the diffusion-controlled conversion of O to R as described earlier, the potential E impressed across the electrode-solution interface must be a value such that the ratio Cr/Cq is large. Table 3.1 shows the potentials that must be applied to the electrode to achieve various ratios of C /Cq for the case in which Eq R = 0. For practical purposes, C /C = 1000 is equivalent to reducing the concentration of O to zero at the electrode surface. According to Table 3.1, an applied potential of -177 mV (vs. E° ) for n = 1 (or -88.5 mV for n = 2) will achieve this ratio. Similar arguments apply to the selection of the final potential. On the reverse step, a small C /Cq is desired to cause diffusion-controlled oxidation of R. Impressed potentials of +177 mV beyond the E° for n = 1 (and +88.5 mV for n = 2) correspond to Cr/Cq = 10"3. These calculations are valid only for reversible systems. Larger potential excursions from E° are necessary for irreversible systems. Also, the effects of iR drop in both the electrode and solution must be considered and compensated for as described in Chapter 6. 

Taking account of the charge associated with peak IIIa, this implies that about 1 pC of the 5 pC of peak Ic measured in scan 20 is still due to gold oxide reduction, corresponding to 95% CoTSPc coverage. Therefore, it can be concluded that peaks Ic and IIIa in scan 20 are related to the same reversibly behaved redox system. Reversibility can be proven by the fact that the half-wave potentials for both waves are the same, and that the peak potential for the anodic wave (IIIa) does not shift with scan number. 

Often the first step in the electrochemical characterization of a compound is to ascertain its oxidation-reduction reversibility. In our opinion, cyclic voltammetry is the most convenient and reliable technique for this and related qualitative characterizations of a new system, although newer forms of pulse polarography may prove more suitable for quantitative determination of the electrochemical parameters. The discussion in Chapter 3 outlines the specific procedures and relationships. The next step in the characterization usually is the determination of the electron stoichiometry of the oxidation-reduction steps of the compound. Controlled-potential coulometry (discussed in Chapter 3) provides a rigorously quantitative means for such evaluations. 

For reversible oxidation-reduction systems (- reversibility) the adsorption wave is observed at more positive potentials than that of the diffusion-controlled process when the reduced form is adsorbed, at more negative potentials, when the oxidized form is adsorbed. For irreversible processes, the adsorption of both the oxidized or that of the reduced form can either facilitate or hinder reduction or oxidation. 

Luther s rule -> Luther studied the relation between the standard electrode -> potentials of metals that can exist in more than one oxidation state. For some electrochem-ically reversible systems (- reversibility) he showed theoretically and experimentally that for a metal Me and its ions Me+ and Me2+ the following relation holds (in contemporary nomenclature) for the -> Gibbs energies of the redox transitions ... 

Peters equation — Obsolete term for the - Nernst equation in the special case that the oxidized and reduced forms of a redox pair are both dissolved in a solution and a reversible potential is established at an inert metal electrode. Initially Nernst derived his equation for the system metal/metal ions, and it was Peters in the laboratory of -> Ostwald, F. W. who published the equation for the above described case [i]. The equation is also sometimes referred to as Peters-Nernst equation [ii]. 

The transition of electrons across the metal-solution interface will actually occur whenever there is an electron acceptor in the solution with energy levels of appropriate value. This transition will be true, for example, when the reversible electrode potential for the oxidizing system Eeq is more noble than Egq. hi this case electrons will make transitions predominately from the metal to the solution at a rate given by... 

When all the species concerned, viz., A, B, , X, Y, etc., are in their standard states, i.e., at unit activity, the potential is equal to the standard oxidation-reduction potential of the system. It is important to remember that in order that a stable reversible potential may be obtained, all the substances involved in the system must be present the actual potential will, according to equation (3), depend on their respective activities. 

Reversible Oxidation-Reduction Processes.— The fundamental principles concerned in the reduction of a reversible system at a cathode or in the oxidation at an unattackable anode have been already given in Chap. Xlll. If the potential of the cathode is made slightly more negative or that of the anode more positive than the reversible potential of the system, reduction or oxidation, respectively, will take place. As the current is increased there will be some polarization due to concentration changes, and eventually the limiting c.d. for the particular process will be attained any further increase will be accompanied by another reaction, e.g., evolution of hydrogen at a cathode or evolution of oxygen or chlorine at an anode. 

The understanding gained by considering the Evans diagrams allows us to measure the corrosion current in a straightforward manner. First we must realize that the corrosion potential is in fact the open-circuit potential of a system undergoing corrosion. It represents steady state, but not equilibrium. It resembles the reversible potential in that it can be very stable. Following a small perturbation, the system will return to the open-circuit corrosion potential just as it returns to the reversible potential. It differs from the equilibrium potential in that it does not follow the Nemst equation for any redox couple and there is both a net oxidation of one species and a net reduction of another. 

The MCFC because of its high operating temperature has higher efficiency (>50%o) and faster electrode kinetics than any other fuel cell system.At 650° C, almost a theoretical reversible potential is established at the interface with low electrode overpotentials, which does not require any noble metal catalysts. The CO does not poison the anode, because in the MCFC it is oxidized at the anode interface. 

Reduction potentials for the reversible reduction of metal ions to the corresponding metal in aqueous solution and in liquid NH3 are listed in Table 8.5. Note that the values follow the same general trend, but that the oxidizing abihty of each metal ion is solvent-dependent. Reduction potentials for oxidizing systems cannot be obtained in liquid NH3 owing to the ease with which the solvent is oxidized. 

Figure V-12 P-Ni(OH)21 P-NiOOH system. Variation of reversible potential with the oxidation state of Ni at 298.15 K and 7=0. Solid curve regular model experimental data [62CON/GIL] o [81BAR/RAN].

Reversible potentials of partially charged a- and (3-Ni(OH)2 electrodes have been measured up to an oxidation state of 2.5 over a range of KOH concentrations from 0.01 -10.0 M. Couples derived from the parent a- and (3-Ni(OH)2 systems can be distinguished by the relative change in KOH level on oxidation and reduction as well as the difference in the formal potentials with respect to Hg HgO KOH. 

Reversible potentials of partially charged a- and P-Ni(OH)2 electrodes have been measured. Experimental emf-oxidation state measurements for both the P P- and the a y-phase couples agree well with theoretically derived expressions. From considerations of the homogeneous emf-composition regions it is deduced that the oxidised species are dissociated in both the p P- and the a y-phase systems. 

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