Reversibility electrochemical

Xie Q, Arias F and Echegoyen L 1993 Electrochemically-reversible, single-electron oxidation of Cgg and... 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

In the previous section we saw how voltammetry can be used to determine the concentration of an analyte. Voltammetry also can be used to obtain additional information, including verifying electrochemical reversibility, determining the number of electrons transferred in a redox reaction, and determining equilibrium constants for coupled chemical reactions. Our discussion of these applications is limited to the use of voltammetric techniques that give limiting currents, although other voltammetric techniques also can be used to obtain the same information. 

Electrochemical Reversibility and Determination of m In deriving a relationship between 1/2 and the standard-state potential for a redox couple (11.41), we noted that the redox reaction must be reversible. How can we tell if a redox reaction is reversible from its voltammogram For a reversible reaction, equation 11.40 describes the voltammogram. 

Graphical determination of electrochemical reversibility, n, and half-wave potential in linear scan hydrodynamic voltammetry. 

The potential at which the current is one-half of its limiting value is called the half-wave potential, El/1. The half-wave potential (for electrochemically reversible couples) is related to the formal potential, E°, of the electroactive species according to... 

Oxygen electrode. In principle, a classical oxygen electrode in a liquid electrolyte would be possible if an electrode material were known on the surface of which the redox system 02/0H is electrochemically reversible however, Luther26 measured its standard potential from the following cell without a liquid junction ... 

Examples are tending to be more sophisticated and complex in form. For example, a dinuclear complex featuring a bridging phosphinate and phenolate in addition to peroxide (221) has been reported,966 as a model for phosphodiester systems. Apart from dicobalt(III) systems, a mixed-valence CoII,ni di-/i-superoxo complex (222) has been prepared.967 Transition between the three redox states CoII,n, Co11,111, and Co111111 is electrochemically reversible. 

In 1 [18a], the nine peripheral Fe(Cp)(MeC6H5)+ moieties are reduced simultaneously, indicating that such units are equivalent and independent. Since this reduction process is both chemically and electrochemically reversible, 1 can be considered as a reservoir of nine electrons [18].In2 [19],3 [20],and4 [21] only one oxidation process is observed, with a number of exchanged electrons equal... 

Jiang J., Beck F., Krohn H. Electrochemical reversibility of graphite oxide. J. Indian Chem. Soc. 1989 66 603-9. 

The thermodynamic redox potential of NAD+/NADH is —0.56 V vs SCE at neutral pH. The NADH cofactor itself is not a useful redox mediator because of the high overpotential and lack of electrochemical reversibility for the NADH/NAD+ redox process, and the interfering adsorption of the cofactor at electrode surfaces. 

Influence of Mass Transport on Charge Transfer. Electrochemically Reversible and Irreversible Processes... 

It must, however, be taken into account that the concept of electrochemical reversibility or irreversibility of an electron transfer is relative. In fact, to accelerate the redox processes one can act either on the mass transport (by stirring the solution) or on kRed and k0x (by changing the electrode potential, as seen in Section 4.1.1). 

As will become evident from an examination of the various voltammetric techniques, the electrochemical reversibility or irreversibility of a process influences the form of the relative current/potential curves. 

It is evident that (once the electrochemical reversibility of the process under examination has been checked, see the next section) the experimental measurement of the peak current, ip, allows one to calculate one of the parameters appearing in the equation. For instance, if the peak current ip at a certain scan rate v is measured, knowing the area of the electrode A, the diffusion coefficient D and the concentration C of the species under study, one can compute the number of electrons n involved in the redox change. On the other... 

The well known ferrocene/ferrocenium oxidation possesses the features typical of electrochemical reversibility. This foreshadows the substantial maintenance of the original molecular geometry on passing from decamethylferrocene to decamethylferrocenium. As a matter... 

At least from a theoretical viewpoint, several situations are possible depending on the extent of electrochemical reversibility of the electron transfer and on the reversibility or irreversibility of the chemical reaction following the electron transfer. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

It may happen that the use of different electrode materials leads to different degrees of reversibility in redox processes. This means that when one obtains voltammograms which depart significantly from electrochemical reversibility it is advisable to use different electrode materials in order to ascertain that the effect is really due to the slow rate of the electron transfer of the species under examination rather than from surface phenomena connected with the electrode material. 

The fact that either the peak-to-peak separation, AEp, somewhat departs from the value of 59 mV or the current function ipJvl/2 is not rigorously constant seems to contrast with the diagnostic criteria (illustrated in Chapter 2, Section 1.1.1) for an electrochemically reversible one-electron process. This can be largely attributed to the non-compensated resistance given by the dichloromethane solution, which is a low conducting solvent. 

As far as the AEP values are concerned, it cannot be ruled out that the molecular rearrangements that occur on passing from ferrocene to ferrocenium ion might also play a role, even if minimal, in their departure from the value of 59 mV, or in lowering the degree of electrochemical reversibility of the process. 

Firstly, let us discuss its electrochemical behaviour. As previously illustrated in Chapter 2, Figure 5, the anodic response in dichlorome-thane solution also shows features of chemical reversibility 0pCApa= 1)-The peak-to-peak separation (A p = 76mV) again indicates a slight deviation from the theoretical value of 59 mV expected for an electrochemically reversible one-electron process. 

In conclusion, also in the case of the [Fe( 5-C5H5)2]/[Fe( /5-C5H5)2]+ couple, the slight deviation from the electrochemical reversibility could be due not only to the lengthening of the Fe-C distances, but also to a different disposition of the cyclopentadienyl rings. 

In the absence of crystallographic data one cannot discuss in detail the structural variations triggered by these reduction processes, but their electrochemical reversibility, or quasireversibility, suggests that there are not significant structural rearrangements. 

The first reduction, which is likely centred on the Cr(III)— Cr(II) process, has substantial electrochemical reversibility (A2sp = 70 mV, at 0.2 V s-1), thus suggesting that no significant structural rearrangements accompany such redox step. Unfortunately, no further investigations have been carried out to clarify whether the successive reductions are centred on the metal or on the ligand. 

Fensaloph] undergoes a one-electron oxidation (E° = —0.28 V), whereas [Fein(salen)]+ undergoes a one-electron reduction (E° — -0.33 V). In both cases the Fe(III)/Fe(II) process is chemically and electrochemically reversible (Reverse)Ap(forward)= 1 EEV 60 mV> at 0.2 V s 1). Therefore, one may reasonably expect that the corresponding [Fem(saloph)]+ and [Feu(salen)] congeners maintain the initial planar geometry. 

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