Other hydrodynamic electrodes

Besides the RDE and DME, other uniformly accessible electrodes under laminar flow have been described. They include the rotating hemispherical and rotating cone electrodes, which were developed to obviate the problem of trapped gas bubbles at the centre of an RDE their use is not widespread. 

A summary of limiting current expressions at many hydrodynamic electrodes is given in Table 3. 

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Voltammetry at other hydrodynamic electrodes The particular features of this technique are (a) plate, conical and tubular electrodes in contact with the flowing solutions and (b) vibrating dme and streaming mercury electrodes. 

There are other equivalent ways of solving eqn. (17) for example, by double integration [3]. However, as will be shown in following sections, the approach outlined above is particularly valuable as it may be applied directly to other hydrodynamic electrodes. 

The method of resolution of (8.1) was indicated in Sections 5.7-5.9, showing as an example the calculation of the limiting current at the rotating disc electrode. In this chapter we discuss this and other hydrodynamic electrodes used in the study of electrode processes. The rotating disc electrode has probably been the hydrodynamic electrode... 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

Solid electrodes, including metals and semiconductors, have attracted attention since the 1950s and extensive research has been done by Vetter, Gerischer, Bockris, Conway, Parsons, Yeager, etc. The development of hydrodynamic electrodes other than the DME in the late fifties and sixties... 

We will first briefly consider some of the treatments of potential step and current step techniques at hydrodynamic electrodes. One should bear in mind, however, that this division is somewhat artificial owing to the implicit dependence of one on the other. We then treat a.c. voltammetric techniques, LSV, and finally consider hydrodynamic modulation. 

The potential response of the RDE to current steps has been treated analytically [3, 237, 251] and accurately by Hale using numerical integration [252] this enables the elucidation of kinetic parameters [185, 253]. A current density—transition time relationship at the RDE has been established which accounts for observed differences from the Sand equation [eqn. (218)] and which has been applied to EC reactions [254]. Other hydrodynamic solid electrodes have not been considered in detail, although reversible reactions at channel electrodes have been discussed [255, 256]. 

Other more complex mechanistic schemes are studied by a variety of techniques. Double hydrodynamic electrodes are particularly useful for investigating schemes involving two electron transfer steps, such as ECE and DISP schemes. Some of the applications of the different electrochemical techniques in the elucidation of these reactions are described in the following chapters. 

The form of the response is a succession of points following the same profile as a conventional voltammogram. However, since a pulse causes greater mass transport than a steady-state technique (hydrodynamic electrode), a reaction that appears reversible in the steady state can appear quasi-reversible with this technique. On the other hand, given the short timescale, effects due to coupled homogeneous reactions may not be observed. 

The theoretical solution to the equations for electrode processes nearly always has to involve approximations, not only for numerical but also for analytical solutions—such as, for example, the assumption that there is no convection within the diffusion layer of hydrodynamic electrodes. In other cases, of complex mechanism, it is not even possible to resolve the equations algebraically. There is another possibility for theoretical analysis, which is to simulate the electrode process digitally. 

The aims of this chapter are to suggest that the channel (or tubular) electrode meets the criteria, set out above, more successfully than the other available hydrodynamic electrodes and to illustrate, by way of practical examples, that the channel electrode and channel electrode methodology can be applied to the study of a diverse range of electrochemical and other interfacial phenomena. By way of introduction, we will firstly consider the channel electrode in the light of the preceding criteria before proceeding with more detailed discussion. 

Let us compare the channel electrode with other hydrodynamic systems in the context of the criteria suggested above. At this point, however, we eliminate the wall-jet electrode from any further discussion since theories for coupled electrode-solution reactions have yet to appear. 

In this section, we will provide a detailed analysis of the ECE mechanism (and its nuances) so as to examine the channel electrode, by way of practical example, under the "criteria for hydrodynamic electrodes set out in the introduction. A brief survey of the theoretical analyses of other mechanisms will follow. 

One of the advantages of flow-through electrodes over other types of hydrodynamic electrodes is that they involve no moving parts. They are therefore ideal for use in conjunction with both spectroscopic and microscopic techniques. Whilst both channel and tubular electrodes can be used in... 

Electrochemical systems where the mass transport of chemical species is due to diffusion and electromigration were studied in previous chapters. In the present chapter, we are going to consider the occurrence of the third mechanism of mass transfer in solution convection. Although the modelling of natural convection has experienced some progress in recent years [1], this is usually avoided in electrochemical measurements. On the other hand, convection applied by an external source forced convection) is employed in valuable and popular electrochemical methods in order to enhance the mass transport of species towards the electrode surface. Some of these hydrodynamic methods are based on electrodes that move with respect to the electroljAic solution, as with rotating electrodes [2], whereas in other hydrodynamic systems the electrolytic solution flows over a static electrode, as in waU-jet [3] and channel electrodes [4]. 

The successful determination of copper in beer, a complex system that precludes meaningful measurements under silent conditions, opened up the possibility for analysis in even more inaccessible media such as biological samples. Cavita-tional depassivation provides a remarkable enhancement in measured Faradaic currents whilst the increased mass transport due to acoustic streaming lowered the accumulation times below those required for other hydrodynamic voltammetric techniques such as rotating disk electrodes. 

When the oxygen sensor is immersed in a flowing or stirred solution of the analyte, oxygen diffuses through the membrane into the thin layer of electrolyte immediately adjacent to the disk cathode, where it diffuses to the electrode and is immediately reduced lo water. In contrast with a normal hydrodynamic electrode, two diffusion processes are involved — one through the membrane and the other through the solution between the membrane and the electrode surface. For a steady-state condition to be reached in a reasonable period (10 to 20 s), the thickness of the membrane and the electrolyte film must be 20 pm or less. Under these conditions, it is the rale of equilibration of the transfer of oxygen across the membrane that determines the steadv-stalc current that is reached. 

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