Rotated disc electrode construction

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. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

The rotating disc and rotating ring disc electrodes are now the most popular systems for kinetic and mechanistic studies particularly the rotating disc electrode, which combines an ease of construction and use with the ability to... 

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. 

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. ... 

This derivation makes a number of assumptions. Firstly, we assume that there is no disruption to the laminar flow pattern due to a finite disc surface, finite cell size, or eccentricity in disc rotation. To what extent design factors affect measured currents will be discussed further in the section on electrode construction. It is sufficient at this point to say that the criteria for negligible disruption can be met. 

However, the application of the Split Ring Disc electrode especially with hydro-dynamic modulation of its rotation speed (HMRRD) will be mentioned briefly. The construction of appropriate ring disc electrodes including a modified commercial disc rotator and the necessary electronic tripotentiostat have been already described in detail elsewhere [25]. The investigations of K.G. Heusler on passive iron in 0.5 M sulphuric acid have been already mentioned [18]. Another example is shown for Cu in alkaline solution [26]. Two half rings permit determination of the formation of two Cu species simultaneously. The transfer efficiency from the disc to the ring has been... 

If an unstable intermediate or product is formed at the disc, only a fraction will reach the ring, and the ratio i /i will be smaller than N. The extent by which the collection efficiency is decreased is a function of the rate of rotation. The dependence of N on to can be used to evaluate the lifetime (or rate of decomposition) of the unstable intermediate. As in any kinetic measurement of this type, one attempts to design the system for the fastest possible transition time from disc to ring, to allow detection of short-lived intermediates. The gap in commercial RRDEs is of the order of 0.01 cm, but electrodes with substantially narrower gaps have been built. We may be tempted to use modem techniques of microelectronics to construct an RRDE with a very small gap, say, 0.1 Xm. A closer examination of the hydrodynamics involved reveals that this may not work and, in fact, there is little or no advantage in reducing the gap below about 5 0.m. 

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