The Rotating Disc Electrode RDE

There are many ways to increase the rate of mass transport by stirring. Moving the electrode in solution turns out to be more efficient, as a rtde, than moving the solution by gas bubbling, using a magnetic stirrer, and so on. 

One of the best methods of obtaining efficient mass transport in a highly reproducible manner is by the use of the rotating disc electrode. The RDE consists of a cylindrical metal rod embedded in a larger cylindrical plastic holder (usually Teflon, due to its great chemical stability and inertness). The electrode is cut and polished flush with its holder, so that only the bottom end of the metal cylinder is exposed to the solution. 

In hydrodynamics, the transition from laminar to turbulent flow is characterized by a dimensionless parameter called the Reynolds number, Re. This number is the product of a characteristic velocity, v, and a characteristic length I, divided by the kinematic viscosity, v. The latter is defined as the viscosity divided by the density, and has the dimensions cm s (the same as the dimensions of the diffusion coefficient, D). 

The characteristic velocity and length must be defined for each geometry separately, and in each case there is a critical value of the Reynolds number at which transition from laminar to turbulent flow takes place. For example, for flow 

In the case of the rotating disc, the characteristic velocity is the linear velocity at the outer edge of the disc, given by v = o r, where co is the angtdar velocity, expressed in radians per second. The characteristic length is taken as the radius of the disc and the critical Reynolds number is about 1 x 10. The condition for laminar flow is then  

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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). 

The rotated disc electrode (RDE) is one of the most commonly employed hydro-dynamic electrodes. Figure 7.1 shows a schematic representation of a typical RDE. The electrode itself is a flat, circular disc of metal, graphite or an other conductor, and has a radius of r its area A, therefore, is straightforwardly nr. The disc is embedded centrally into one flat end of a cylinder of an insulatory material such as Teflon or epoxy resin. Behind the face of the electrode is an... 

Related to the rotated disc electrode (RDE) is the rotated ring-disc electrfxle (RRDE). Such an electrode is illustrated in Figure 7.9 and is seen to be, in effect, a modified RDE, insofar as the central disc is surrounded with a concentric ring electrode. The gap between the ring and the disc is filled with an insulator such as Teflon or epoxy resin. The face of the RRDE is polished flat in order to prevent viscous drag, which is itself likely to cause the induction of eddy currents. 

Convection-based systems fall into two fundamental classes, namely those using a moving electrode in a fixed bulk solution (such as the rotated disc electrode (RDE)) and fixed electrodes with a moving solution (such as flow cells and channel electrodes, and the wall-jet electrode). These convective systems can only be usefully employed if the movement of the analyte solution is reproducible over the face of the electrode. In practice, we define reproducible by ensuring that the flow is laminar. Turbulent flow leads to irreproducible conditions such as the production of eddy currents and vortices and should be avoided whenever possible. 

As is thoroughly discussed in Chap. 2 of this volume, the convective diffusion conditions can be controlled under steady state conditions by use of hydrodynamic electrodes such as the rotating disc electrode (RDE), the wall-jet electrode, etc. In these cases, steady state convective diffusion is attained, becomes independent of time, and solution of the convective-diffusion differential equation for the particular electrochemical problem permits separation of transport and kinetics from the experimental data. 

The rotating disc electrode (RDE) is one of the most popular hydrodynamic electrodes due to its relatively easy fabrication, commercial availability and facile surface regeneration. Moreover, the corresponding simulation problem can be reduced to a single spatial dimension. 

This condition in terms of the system s chemical composition does not suffice to obtain quasi-steady states that are distinct from equilibrium states and that can be observed moreover under experiment for a reasonable time length. The rotating disc electrode (RDE), illustrated in figure 1.17, is an example of a device that can be used easily to obtain a quasi-steady state in the lab. The cylindrical shaft of this electrode is attached to an electric motor ensuring the rotation of the electrode around its axis. The only metallic part in contact with the electrolyte is the disc section. 

The activity of a catalyst toward the reduction of O2 can be evaluated by using the rotating disc electrode (RDE). The electrode rotates from several to 10,000 revolutions per minute (f). The thickness of the diffusion layer at the electrode surface is determined by the rotation rate of the electrode. 

Alternating current polarographic (ACP) limits are set by the frequencies generally applied (f= 10-2000 Hz). Similarly, the limitations of the rotating disc electrode (RDE) are given by the angular rotation rates (6-6000 rad s" ). The slowest chemical reactions may be followed in coulometric analysis (cf. subsection 6.1) lasting usually 10 min and... 

Convective Diffusion. From a practical point of view, convective diffusion can be achieved in two ways by moving the solution relative to the electrode, or moving the electrode relative to the solution. Of the systems developed to move the electrode, only one merits consideration here, the rotating disc electrode (RDE),... 

The rotating disc electrode (RDE) is the classical hydrodynamic electroanalytical technique used to limit the diffusion layer thickness. However, readers should also consider alternative controlled flow methods including the channel flow cell (38), the wall pipe and wall jet configurations (39). Forced convection has several advantages which include (1) the rapid establishment of a high rate of steady-state mass transport and (2) easily and reproducibly controlled convection over a wide range of mass transfer coefficients. There are also drawbacks (1) in many instances, the construction of electrodes and cells is not easy and (2) the theoretical treatment requires the determination of the solution flow velocity profiles (as functions of rotation rate, viscosities and densities) and of the electrochemical problem very few cases yield exact solutions. 

The exact solution of the convection-diffusion equations is complicated, because the theoretical treatments involve solving a hydrodynamic problem, that is, the determination of the solution flow velocity profile by using Navier-Stokes equation. For the calculation of a velocity profile, the solution viscosity, densities, rotation rate, or stirring rate, as well as the shape of the electrode should be considered. Exact solution has been derived for the rotating disc electrode (RDE) ... 

The difference between surface and bulk concentrations depends on the regularities of mass transport and manifests itself in the fact that the certain diffusion overvoltage arises in the system. Use of the rotating disc electrode (RDE) makes it possible to carry out the experiments under different intensities of forced convection and to control the intensity of diffusive mass transport. This provides the means to eliminate the diffusive part of the overvoltage promoting the determination of kinetic parameters. 

Tafel extrapolation of the rotating disc electrode (RDE) polarization curve of pure magnesium in an aerated 0.5 M Na2S04 (rotation speed 1500 rpm, potential scan rate 0.5mV/s) (corrected for the ohmic drop) (Ardelean et al., 1999). 

A further issue is the mechanical stability of both HMDE and MFE, which is in favour of the latter and, for instance, for hydrodynamic measurements with the rotating disc electrode (RDE, see Table 5.1 and reference (11)) or that in improvised shipboard laboratories, the reliable use of an HMDE with its dropping mechanism is hardly imaginable, perhaps, except for some special and purposely constructed automated analysers. ... 

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