Coupled homogeneous electrode reactions methods

Many interesting processes occurring at the liquid/liquid interface involve coupled homogeneous chemical reactions. In principle, electrochemical methods used for probing complicated mechanisms at metal electrodes (61) can be employed at the ITIES. However, many of these techniques (e.g., rotating ring-disk electrode or fast-scan cyclic voltammetry) are hard to adapt to liquid/liquid measurements. Because of technical problems, few studies of multistep processes at the ITIES have been reported to date (1,62). 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

Chronoamperometry Chronoamperometry involves the study of the variation of the current response with time under potentiostatic control. Generally the working electrode is stepped from a potential at which there is no electrode reaction to one corresponding to the mass-transport-limited current, and the resulting current-time transient is recorded. In double-step chronoamperometry, a second step inverts the electrode reaction and this method is useful in analysing cases where the product of the initial electrode reaction is consumed in solution by a coupled homogeneous chemical reaction. 

Quantitative studies using LSV and CV can be carried out for both heterogeneous charge transfer kinetics and the kinetics of homogeneous chemical reactions coupled to charge transfer at electrodes. These methods should continue to play a major role in the study of electron transfer reactions. 

The intensive electrochemical studies of polycyclic systems, especially cyclic volta-metry (CV) are now at a stage which justifies naming cyclic voltametry an electrochemical spectroscopy as was suggested by Heinze 65). Early electrochemical studies referred only to the thermodynamic parameters while CV studies provide direct insight into the kinetics of electrode reactions. These include both heterogeneous and homogeneous electron-transfer steps, as well as chemical reactions which are coupled with the electrochemical process. The kinetic analysis enables the determination of reactive intermediates in the same sense as spectroscopic methods do. As already mentioned, electron transfer processes occur in both the electrochemical and metal reduction reactions. 

Besides its obvious application to preparative electrolysis, controlled-potential electrolysis (CPE) also can aid in mechanistic analysis of the electrode reaction. The treatment of coupled chemical reactions is simpler theoretically in CPE than in most other electrochemical methods, because the solution can be treated as being homogeneous, rather than having to account for concentration changes as a function of distance from the electrode. The mathematics are more straightforward. 

We have seen that the rate of an electrochemical process is affected by the rates at which reactants can be supplied to the electrode and products can be dispersed from it. Often the overall process is governed completely by the rates of mass transport and homogeneous chemical reaction. One can usually write the set of coupled differential equations describing the transformations and movements of material, but often they can be solved in closed form with difficulty or not at all. Numerical methods are frequently applied to the solution of such equations (1). 

Note that the homogeneous chemical reactions (C steps) coupled to the electrode process alter the concentration profiles of the electroactive species and therefore the electrochemical response of the system. Thus, electrochemical methods enable the characterisation of the chemical reaction in solution, that is, the determination of the mechanism as well as the rate and equilibrium constants. 

In the preceding three sections reaction mechanisms in which the homogeneous chemical reaction was coupled with the electrode process were discussed. This coupling enables exceptionally fast chemical reactions to be investigated and their rate constants determined. Nevertheless, voltammetric methods can also be exploited for kinetic studies on chemical reactions occurring independently of the electrode process in the bulk of the solution. For this purpose all voltammetric techniques can be used for which the dependence of voltammetric response on the concentration of one or more reactants is defined in a simple way. Various amperometric sensors are mostly applied, working at the potentials of limiting current. The response need not be a diffusion-controlled current. Kinetic currents within the diffusion-controlled zone can also be taken into account. 

Ruzit I (1985) Extension of the improved "heterogeneous equivalent" method to the digital simulation of the slow pseudo-first order homogeneous reactions coupled to the electrode reaction. 

Compton RG, PtUdngton MBG, Steam GM (1988) Mass transport in channel electrodes. The application of the backwards implicit method to electrode reactions (EC, ECE and DISP) involving coupled homogeneous kinetics. J Chem Soc Faraday Trans I 84 2155-2171... 

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