The Rotating Ring-Disc Electrode RRDE

The rotating ring-disc electrode (RRDE) is perhaps the most useftd extension of the idea of the rotating disc. The ring current has the same form as the disc current, namely, it is also proportional to the bulk concentration of the electroactive species and to the square root of the rotation rate  

2) Note that for the rotating-ring and rotating-ring-disc electrodes the common practice is to use the total currents, I, rather than the current densities, j. 

The great advantage of the RRDE is that it can be used to analyze short-lived intermediates in a steady-state measurement 

Since both electrodes are part of the same flow regime (evidently the convective flow of the liquid cannot distinguish between the disc electrode, the insulator and the ring), the equations for the limiting current at both the disc and the ring electrodes can be solved simultaneously. 

In order to operate an RRDE, one needs a four-electrode potentiostat, often referred to also as a bi-potentiostat . Now, we began this book with simple two-electrode cells and proceeded to the three-electrode configuration. How does the fourth electrode fit into this scheme. The best way to xmderstand this is to view both the ring and the disc as working electrodes, with common counter and reference electrodes. The important point is that the four-electrode potentiostat controls the potentials of the disc and of the ring with respect to the reference electrode independently and measures the current going through each of them separately. 

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Here we have to deal with three types (see Fig. 3.68), viz. (a) the rotating disc electrode (RDE), and (b) the rotating ring electrode (RRE) and the rotating ring-disc electrode (RRDE). The construction of the latter types suits all purposes, i.e., if the disc or the ring is not included in the electric circuit, it yields an RRE or an RDE, respectively, and if not an RRDE, where either the disc forms the cathode and the ring the anode, or the reverse. 

There arc many controllcd-convection techniques available but we will restrict our discussion to the two most commonly employed by the electrochemist the rotating disc electrode (RDE) and the rotating ring disc electrode (RRDE). 

To learn that the rotated ring-disc electrode (RRDE) is one of the most powerful analytical tools for following the kinetics of fast homogeneous reactions. 

The rotated ring-disc electrode (RRDE) has been shown to be an ideal tool for measuring the rate constants of very fast homogeneous reactions. In this method, we start with one reagent in the solution while the other is electrogenerated at the disc electrode, with the proportion of the latter that remains after reaction being monitored at the ring electrode. 

Albery, W. J. and Hitchman M. L., Ring-Disc Electrodes, Oxford University Press, Oxford, 1971. This now-classic book describes one of the most formidable tools in the arsenal of the electroanalyst, i.e. the rotated ring-disc electrode (RRDE). Its first two chapters are a clear and lucid introduction to the basic rotated disc electrode (RDE) and the multi-faceted problems of mass transport. Well worth a read, if only for the occasional dip into this field. 

The rotating ring—disc electrode (RRDE) is probably the most well-known and widely used double electrode. It was invented by Frumkin and Nekrasov [26] in 1959. The ring is concentric with the disc with an insulating gap between them. An approximate solution for the steady-state collection efficiency N0 was derived by Ivanov and Levich [27]. An exact analytical solution, making the assumption that radial diffusion can be neglected with respect to radial convection, was obtained by Albery and Bruckenstein [28, 29]. We follow a similar, but simplified, argument below. 

It is possible to investigate the mechanism using the rotating ring disc electrode (RRDE) technique. This has been described in detail by Albery and Hitchman [18]. Briefly the method is as follows. 

A photograph of the rotating ring-disc electrode (RRDE) is shown in Fig. 4.1 at the beginning of this chapter, and Fig. 4.17 also illustrates the essential features of the system. It is a view from below and shows the disc electrode surrounded by a concentric ring electrode but with a thin layer of insulating material between. 

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