Redox species

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

Thus, it can basically be predicted under what conditions (pH, concentration of redox species) tire metal dissolution reaction (Fe Fe ) proceeds tliennodynamically. From a practical point of view, tire rate of tire reaction and tlierefore tire fate of tire oxidized species (Fe ) is extremely important tliey can eitlier be solvated, i.e., to fonn Fe (H20) complexes, and tlierefore be efficiently dissolved in tire solution, or tliey can react witli oxygen species of... 

The driving force for migration is established by the different electrochemical potentials (AU) that exist at the two interfaces of the oxide. In other words, the electrochemical potential at the outer interface is controlled by the dominant redox species present in the electrolyte (e.g. O2). 

The situation in figure C2.8.5(b) is different in that, in addition to the mechanism in figure C2.8.5(a), reduction of the redox species can occur at the counter-electrode. Thus, electron transfer tlirough the layer may not be needed, as film growth can occur with OH species present in the electrolyte involving a (field-aided) deprotonation of the film. The driving force is provided by the applied voltage, AU. 

In 1975, the fabrication of a chiral electrode by permanent attachment of amino acid residues to pendant groups on a graphite surface was reported At the same time, stimulated by the development of bonded phases on silica and aluminia surfaces the first example of derivatized metal surfaces for use as chemically modified electrodes was presented. A silanization technique was used for covalently binding redox species to hydroxy groups of SnOj or Pt surfaces. Before that time, some successful attemps to create electrode surfaces with deliberate chemical properties made use of specific adsorption techniques... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Table 16-6 Redox half reactions, pg°, pe, and calculated pe values from the redox species distributions psc for the zone 125-135 m in Saanich Inlet...

Emerson, S., Cranston, R., and Liss, P. (1979). Redox species in a reducing fjord Equilibrium and kinetic considerations, Deep-Sea Res. 26, 859-878. 

Fig. 7. A comparison of the FTIR and EPR redox titrations of the D. gigas hydro-genase. Top panel FTIR titration based on the height of the low-frequency band. For experimental conditions, see (65). Bottom panel Calculated potentials obtained from EPR-monitor titration and the TED model (80). The vertical axis represents the proportion of a redox species.

Monolayers of l-tert-bntyl-l,9-dihydrofullerene-60 on hydrophobized ITO glass exhibited three well-defined rednction waves at -0.55 V, -0.94 V, and -1.37 V (vs. satn-rated calomel electrode, SCE), with the first two stable to cycling [283]. Improved transfer ratios near nnity were reported. The peak splitting for the first two waves was 65-70 mV, mnch less than reported for the pnre C60-modified electrodes. The rednction and oxidation peak cnrrents were equal however, the peak currents were observed to be proportional to the sqnare root of the scan rate instead of being linear with the scan rate as normally expected for snrface-confined redox species. 

The prediction of which semiconductor materials are suitable as electrodes for a given photoelectrolysis reaction would be straightforward if the energy position of the bands at the electrode surface remained fixed with respect to the solution redox levels and independent of the redox species in the solution. Gerischer... 

Added stability in PEC can be attained through the use of non-aqueous solvents. Noufi et al. [68] systematically evaluated various non-aqueous ferro-ferricyanide electrolytes (DMF, acetonitrile, PC, alcohols) for use in stabilizing n-CdSe photoanodes. Selection of the solvent was discussed in terms of inherent stability provided, the rate of the redox reaction, the tendency toward specific adsorption of the redox species, and the formal potential of the redox couple with respect to the flat band potential (attainable open-circuit voltage). On the basis of these data, the methanol/Fe(CN)6 system (transparent below 2.6 eV) was chosen as providing complete stabilization of CdSe. Results were presented for cells of the type... 

FIG. 3 Comparison of potential scales for redox species in aqueous and DCE phases versus SHE. Redox potentials in DCE were experimentally measured against the couple Fc /Fc. The formal redox potential of this couple in DCE was obtained by evaluating the parameters in the thermodynamic cycle given by Eq. (7). 

Requirements for redox potential matching and high transfer energy have limited the choice for the aqueous redox species to [Fe(CN)6] [43,56,62,63,70,71,74,76,81,82,86],... 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

To extend the applicability of the SECM feedback mode for studying ET processes at ITIES, we have formulated a numerical model that fully treats diffusional mass transfer in the two phases [49]. The model relates to the specific case of an irreversible ET process at the ITIES, i.e., the situation where the potentials of the redox couples in the two phases are widely separated. A further model for the case of quasireversible ET kinetics at the ITIES is currently under development. For the case where the oxidized form of a redox species, Oxi, is electrolytically generated at the tip in phase 1 from the reduced species, Red], the reactions at the tip and the ITIES are ... 

With a fixed concentration of redox species in phase 2, the dependence of the ET rate constant, kf, on the driving force can be written as [48,70] ... 

FIG. 21 Plot of log ki2 vs. AEi/2 showing the dependence of ET rate on the driving force for the reaction between ZnPor and four aqueous reductants. The difference between the half-wave potentials for an aqueous redox species and ZnPor, AE-i/2 = AE° + A°0, where AE° is the difference in the formal potentials of the aqueous redox species and ZnPor and A° is the potential drop across the ITIES. The solid line is the expected behavior based on Marcus theory for X = 0.55 eV and a maximum rate constant of 50 cm s M . (Reprinted from Ref. 49. Copyright 1999 American Chemical Society.)... 

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