Disc electrodes, 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. 

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. 

Different methods are proposed in the literature to construct disc electrodes, but the commonly used way is embedding a rod of the electrode material in an insulating material such as glass, polyamides, acrlyates, epoxy resins or polyvinylchloride. The electrodes used in the work described in this book were constructed by insulating the electrode material in epoxy resin (Fig. 1.7). First, a copper piece was made with a hole for the positioning of the electrode material rod. This was positioned in a PVC holder, which was then filled with epoxy resin. After curing for 48 h, the PVC mantel was removed and the excess of epoxy scoured away on emery paper. 

Electrodes can also be positioned in a wall-jet disc electrode configuration, a home prepared construction, schematically shown in Fig. 1.9. From a small bath (1) with a capacity of about 31, solution is taken by an EHEIM pump... 

In the Richmond laboratory, the single crystal disc electrode is mounted on the end of a hollow Kel-F shaft which extends outside of the electrochemical cell. The shaft is connected to a stepping motor and is interfaced to a computer. All parts of the electrochemical cell are constructed from either Kel-F, teflon, or high density polyethylene. The edges and back of the electrode are sealed from contact with the... 

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... 

The RRDE is constructed in such a way that both electrodes are in the same plane, and in close proximity to each other. The dimensions of the electrode are defined by three radii the radius of the disc (r ), the inner radius of the ring (r ) and its outer radius (r ), as shown in Fig. 12D(b). Since both electrodes are in the same flow regime (evidently the convective flow of the liquid cannot distinguish between the disc electrode, the insulator, and the ring electrode), the equations for the limiting current at both the disc and the ring electrode can be solved simultaneously. 

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. 

Lens components can also be constructed out of disc electrodes with circular apertures. Here the key fringing fields fan out from the apertures and it is straightforward to extend the arguments above to explain why these aperture lenses exhibit a focusing effect. Figure 3.5 shows a simulation of the focusing response of a three-element aperture lens. 

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. ... 

The H-type cell devised by Lingane and Laitinen and shown in Fig. 16.9 will be found satisfactory for many purposes a particular feature is the built-in reference electrode. Usually a saturated calomel electrode is employed, but if the presence of chloride ion is harmful a mercury(I) sulphate electrode (Hg/Hg2 S04 in potassium sulphate solution potential ca + 0.40 volts vs S.C.E.) may be used. It is usually designed to contain 10-50 mL of the sample solution in the left-hand compartment, but it can be constructed to accommodate a smaller volume down to 1 -2 mL. To avoid polarisation of the reference electrode the latter should be made of tubing at least 20 mm in diameter, but the dimensions of the solution compartment can be varied over wide limits. The compartments are separated by a cross-member filled with a 4 per cent agar-saturated potassium chloride gel, which is held in position by a medium-porosity sintered Pyrex glass disc (diameter at least 10 mm) placed as near the solution compartment as possible in order to facilitate de-aeration of the test solution. By clamping the cell so that the cross-member is vertical, the molten... 

One might note the striking similarity between Cases I and II. In both, a crucible failure allowed water to enter and mix with molten titanium. Steam (and hydrogen) formed and the pressure increased so as to bulge the crucible and rupture the safety discs. Tamping the water-metal mix by the fall of the electrode then caused a major explosion. No injuries resulted in the Case II incident because the vault walls provided protection. No data were available to allow an estimation of blast pressures, but as described, the vault construction maintained its integrity and the wave was forced to exit from the bottom. 

The extent of this mixing depends on the geometry of the ring relative to the disc. No can be maximized by constructing an RRDE with a large central disc and a narrow gap between the disc and ring electrodes. 

Several reticulated vitreous carbon (RVC)-plant tissue composite electrodes have also been reported where the open-cell structure of RVC serves as a template for the biocomponent used. One such sensor was constructed by press-fitting 100 pore/in RVC (2-mm thick x 3 or 6 mm OD) cylinders into a 6-mm diameter cavity of a thin-layer cell. The inner side of the disc was pressed into an edge of a copper wire, which provided electrical contact. 

The simplest way to construct these electrodes is to solder or glue with silver epoxy resin a cylindrical specimen of the electrode material to a stainless steel shaft. A mantle of epoxy resin or plastic is then made around the disc to the shape required. 

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