Quasireversible processes

It is not uncommon that in electron transfer processes one observes that at low scan rates the process behaves reversibly, whereas at high scan rates the process behaves irreversibly (such behaviour is more easily seen for processes that are not complicated by coupled reactions). Processes occurring in the transition zone between reversible and irreversible behaviour are called quasireversible. 

A quasireversible process occurs when the rate of the electron transfer Ox + ne — Red is of the same order of magnitude as the mass transport (concomitantly, the inverse reaction Ox + we — Red has a non-negligible rate). 

It is commonly assumed that an electron transfer behaves quasi-reversibly when the standard rate constant lies within the values expressed as a function of the highest and lowest scan rates v  

Since in cyclic voltammetry the potential scan rate commonly ranges from 0.02 V s-1 to 50 V s-1, it follows that for a quasireversible process  

This criterion is slightly different from that assessed in Section 1.4.3 on the reversibility or irreversibility of an electron transfer  

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Diagnostic Criteria to Identify a Quasireversible Process. A quasireversible process is characterized by determining either the thermodynamic parameter E° or the kinetic parameters a and k°. 

In acetonitrile solution, the Cu Cu1 reduction of [CunL]2+ appears as a quasireversible process, giving rise to a forward-backward peak-system with AEp = 114 mV, at a scan rate of 0.1 V s l. 

Fig. 3.6 Cathodic SWV curves for three quinone dyes and pigments lawson (1, a quasireversible process), alizarin lake (2, a reversible process) and cochineal red (3, a quasireversible process). Scans from open-circuit potential toward negative potentials. Insets the net, forward and backward current components are shown for alizarin lake and cochineal red (reprinted from [186] with permission)...

For the above study the usual value of the transfer coefficient a = 0.5 has been considered. With small a values, DDPV peaks are found to show a special shape under certain conditions. As can be seen in Fig. 4.17a, fora < 0.3 the DDPV curves corresponding to quasireversible processes with k° 10-3 cm s-1 present a striking splitting of the peak, with a sharper peak appearing at more anodic potentials. This phenomenon is promoted by small transfer constants and is more obvious for positive pulse heights (AE > 0, reverse mode, where the anodic peak is even greater than the cathodic one) and at planar electrodes, since it becomes less apparent as the electrode size is reduced (see Fig. 4.17b). The description of this phenomenon is of great interest since this could lead to erroneous interpretation of... 

Cyclic voltammograms of [Mn402(02CPh)7(bipy)2]+ show two quasireversible processes a reduction to Mn402(02CPh)7(bipy)2 at —0.16 V (versus ferrocene) and an oxidation to the Mn4(III,III,III,IV)... 

On the second cycle, the onset for copper deposition is shifted to about -0.3 V. Since the copper deposited during the first cycle is not completely stripped from the surface, the 0.2 V shift in the deposition peak indicates that a nucleation overpotential is required for the deposition of copper onto TiN. Subsequent scans are essentially equivalent to the second sweep and suggest that the that the deposition and dissolution of copper on TiN/Cu is a quasireversible process. Similar features have been reported for copper deposition from borate solutions [2]. 

It is clear that these latter expressions are considerably more complicated than the corresponding expressions applicable either to surface or semi-finite diffusion processes. The extension of this type of analysis to irreversible and quasireversible processes has been recently described.62... 

Feedback theory has been the basis for most quantitative SECM applications reported to date. Historically, the first theoretical treatment of the feedback response was the finite element simulation of a diffusion-controlled process by Kwak and Bard [1], but we will start from a more general formulation for a quasireversible process under non-steady-state conditions and then consider some important special cases. 

IT reactions with no complications associated with slow diffusion in the bottom phase. One should also notice that unlike previously studied ET processes at the ITIES, the rate of the reverse reaction cannot be neglected. The difference is that in the former experiments no ET equilibrium existed at the interface because only one (reduced) form of redox species was initially present in each liquid phase [15,23], In contrast, reaction (8.29) is initially at equilibrium and has to be treated as a quasireversible process [73c], Probing kinetics of simple IT reactions at a nonpolarizable ITIES under steady-state conditions with the pipette SECM tips should be as advantageous as analogous ET measurements [23], but the required theory has not been published to date. 

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