Intermediates in Electrode Reactions

The role of organic intermediates in electrode reactions was recently reviewed in some detail (Fleischmann and Fletcher, 1969 and 1971). 

The double-pulse potentiostatic method (Fig. 5.18C) is suitable for studying the products or intermediates in electrode reactions, formed in the A pulse by means of the B pulse. For example, if an electroactive substance is reduced in pulse A and if pulse B is sufficiently more positive than pulse A, then the product can be reoxidized. The shape of the I-t curve in pulse B can indicate, for example, the degree to which the unstable product of the electrode reaction is changed in a subsequent chemical reaction. 

This resulted in a need for appropriate characterization of the structural and electronic properties of electrode surfaces and detection of adsorbed intermediates in electrode reactions. 

VI. Involvement of Chemisorbed Intermediates in Electrode Reactions, and Methods of Analysis... 

The origins of SECM homogeneous kinetic measurements can be found in the earliest applications of ultramicroelectrodes (UMEs) to profile concentration gradients at macroscopic (millimeter-sized) electrodes (1,2). The held has since developed considerably, such that short-lived intermediates in electrode reactions can now readily be identified by SECM under steady-state conditions, which would be difficult to characterize by alternative transient UME methods, such as fast scan cyclic voltammetry (8). 

The success of the RDE method has stimulated the development of several other rotating configurations. The RRDE is perhaps the most useful extension of the idea of the RDE. The RRDE was first developed by Frumkin and Nekrasov to detect unstable intermediates in electrode reactions. The RRDE, Fig. 4, consists of a central disk electrode surrounded by... 

Various types of spectroscopy have been used very successfully for the determination of structure of molecules and intermediates in solution. However, the application of these techniques to electrode processes is limited by the fact that the latter take place at the surface and direct spectroscopy is clearly not possible since the electrode is opaque. There is one exception in that e.s.r. spectroscopy has been used for some time in identifying radicals as intermediates in electrode reactions. In order to do this an electrochemical cell must be constructed in the e.s.r. cavity. This has been done and some very interesting data on the mechanism of several organic electrode reactions resulted. However, the application of e.s.r. spectroscopy to the study of electrode processes is limited to free radical intermediates the concentration of which is fairly high, i.e. the life time is fairly long. 

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