Electrochemistry formal potential

Voltammograms of a polythiophene film showing reasonably reversible electrochemistry of both types are shown in Fig. 2.M The formal potentials (average of the anodic and cathodic peak potentials) for p- and n-doping can provide useful estimates of the energies of the polymer s valence and conduction bands and its band gap35... 

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

Electrochemistry. The redox processes for porphyrazines 21, 25, 28, 29, the heteroleptic Zr (pz/porphyrin) 30 and 31 have been measured by cyclic voltammetry and the formal potentials are given in Table VII. The potentials are compared to the available data for the analogous porphyrin and pc complexes. In general, the electrochemical behavior of the pz sandwiches more closely mirror that observed for the phthalocyanines than the porphyrins. In particular, all of the porphyrazines have at least one ring-based oxidation, attributable to the formation of the bis Jt-radical cation for Lu(III) sandwiches and the formation of the 7T-radical cation for the Zr(IV) and Ce(IV) sandwiches. Additionally, all of the porphyrazines exhibit at least one ring-based reduction. 

It is very important to define criteria to unequivocally proof a direct ET pathway between an immobilized redox protein and an electrode surface. The first important prerequisite is the occurrence of the direct electrochemistry of the redox cofactor inside the protein in the absence of the substrate. Hence, a reversible redox wave in a cyclic voltammogram of the protein-integrated cofactor has to be visible with a formal potential which clearly shows that the protein structure is not... 

The next problem concerns sweep voltammetry, which involves applying a slowly varying linear waveform starting at a potential where no electrochemistry occurs. The potential is swept past the formal potential of the couple. Once again we consider the reduction of the species O confined within a thin layer [Eq. (15)]. 

Voltammetric detection of a solution species normally depends on that species being reduced or oxidized at the electrode. Frequently in electrochemistry electrode kinetics are such that the redox reaction occurs at more positive or negative potentials than predicted on the basis of thermodynamics. Modification of electrode surfaces with redox centers that can mediate charge transfer to the analyte, as depicted in Fig. 10.2, may reduce this overpotential to allow the analyte to be detected within the electrolyte potential window and to bring the redox potential into a region where electroactive interferences are reduced. In addition to shifting the operating formal potential of the analyte species, modified electrodes can also increase the rate of the reaction over that observed at unmodified electrode surfaces. 

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