Polarographic Reversibility

At currents appreciably displaced from the exchange current, the rate of the reverse reaction of the process is negligible and the current is then essentially an exponential function of the potential. Consequently, the logarithm of the current varies linearly with the potential (Tafel behavior). 

Extrapolation of the linear (Tafel) region of the relation between log (current) and the potential to the condition 17 = 0 gives the exchange current density io- This presupposes that the potential corresponding to this condition (17 = 0) is known, i.e., that the reversible potential for the electrode reaction is known or can be determined, e.g., from appropriate thermodynamic data. For many organic electrode processes, however, the required thermodynamic data are not available so that Er cannot be calculated. 

Under these circumstances, the Tafel slope may be used to indicate another aspect of the degree of reversibility of the electrode reaction. Since a reversible reaction should occur essentially without significant overpotential, then not only should io be preferably large but the slope of the log [i]-E curve should be low hence, large values ( 118 mV) of the Tafel slope therefore reflect irreversiblity of the electrode process in another way. 

The magnitude of the exchange current and Tafel slope are useful indications of irreversibility in electrode processes but it is appropriate to consider three conditions that must be fulfilled before a redox system may be regarded as polarographically reversible  

For a diffusion-controlled reversible system, which does not involve semiquinone formation or dimerization of either the oxidized or the reduced forms, the correct shape of the polarographic current-potential curve is described by an equation derived by Heyrovsky and Ilkovic [cf. Eq. (70)]  

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Cinnolines are generally reducible [101] in two or more steps. The first step in the reduction of aryl- or alkyl-substituted cinnolines is the formation of 1,4-dihydrocinnolines. This reduction is not reversible. Only in the case of benzo[c]cinnoline is the reduction, a saturation of the nitrogen-nitrogen double bond the reaction resembles the reduction of azobenzene to hydrazobenzene and, like this system, is nearly polarographically reversible. 

The effect of adsorption on the polarographic electron transfer rates of cytochrome c is particularly evident with regard to the dependence of reaction overpotential on cytochrome c concentration. As long as the concentration of cytochrome c is kept below ca. 10-30 fiM in pH 6-7 electrolytes, its reduction is polarographically reversible. However, on raising... 

The direct electrochemical reduction and oxidation of bacteriorhodopsin, a bacterial photoreceptor protein, have recently been described. The redox site of this molecule is organic, being a conjugated double bond system and a Schiff base. As previously noted for other proteins, strong adsorption onto mercury is evident with bacteriorhodopsin. The adsorbed molecules undergo reduction-oxidation reactions near -0.8 V which appear to be polarographically reversible. An oxidation wave is observed at platinum at +0.80 V and has been ascribed to the chromophore. ... 

Ferrocene has been frequently recommended as a system by which to relate potential scales in different solvents because the solvation energies of ferricinium ions and ferrocene are believed to be smaller than for most other ions. It follows that the solvent contribution to the activation free energy should be relatively small in that system. However, no kinetic studies appear to have been made there ferrocene is known to be polarographically reversible but osmocene and rutheno-cene are sulSciently slow for the rate parameters to be determined polarographically and compared for different solvents. 

Except at high or medium pH, step (a) is a rapid protonation equilibrium. If, however, step (c) were rate controlling, i.e., if the rate constants were such that kc kb, k-b or ka, then step (b), as well as step (a), would be almost at equilibrium (this occurs, e.g., in some polarographically reversible systems see Section 8.2). Under these conditions, the surface coveraget 6k of the ketyl radical -COH CH2 ( iiT ) at the electrode surface could be represented approximately by... 

However, only a few organic compounds behave in a polarographically reversible manner although many may involve a reversible electron transfer step, this is often followed by irreversible chemical reactions. Irreversible processes are those for which the current is limited mainly by the kinetics of the process at the electrode surface and not by diffusion. The nature of such current-potential curves can be described by reference to Figure 6. If electrochemical equilibrium obtains at the electrode surface, then a reversible wave is obtained (curve a). The irreversible wave (curve b) is more drawn out, i.e., for a given current, say, /i or I2, a higher cathodic potential is required. 

In 1948 Kalousek introduced the commutator method to establish the reversibility of certain inorganic and organic redox processes. In this method, the electrode potential is alternately switched from a potential at which the reduction product is formed to that potential at which it is reoxidized (cf. cyclic voltammetry). The mean current is recorded against the increasing potential and, for a polarographically reversible process, the observed cathodic (reduction) current is equal but opposite to the anodic (oxidation) current. 

Consider on the other hand a similar reaction scheme where all steps are rapid and reversible. The mass transport step will be in equilibrium and the rate of electron transfer will be very rapid, so that a quasi-equilibrium (as was discussed previously) is maintained and step III, the so called chemical step , will also be in equilibrium. The whole process will be a quasi-equilibrium one. This quasi-equilibrium process is the polarographically reversible process. Presently we shall describe two ways by which reversibility can be determined and continue to describe some of the experimental aspects of polarography and voltammetry in stirred solutions, devoting a section to the various uses of the rotating disk electrode. 

Furthermore, we have specified the flavin redox states by the terms flavoquinone, flavosemiquinone and flavohydroquinone in view of the polarographic reversibility, i.e. the quinonoid character, of the oxido-reduction. These terms are definitely superior to oxidized flavin, flavin radical, fully reduced flavin which are most widely used in biochemistry. In this context, we have tried to outline chemical evidence for the existence and potential biological relevance of several dihydroflavin isomers. The terms fully reduced and certainly leucoflavin should be dropped definitively in favour of dihydroflavin . Leucoflavin is in fact not leuco (i.e. colourless) but even more deeply (though, of course, less intensely) coloured (79) than oxidized flavin. Hence, this term should be abandoned for 1,5-dihydroflavin , which is identical with the product of the reversible reduction of flavoquinone, namely flavohydroquinone. 

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