Channel electrode

Cooper J A and Compton R G 1998 Channel electrodes—a review Electroanalysis 10 141... 

To realize that with stationary electrodes, the relationship between the limiting current, /um, and the rate of solution flow, Vf, will depend on whether a flow cell or a channel electrode is employed. 

Flow Cells, Channel Electrodes and Wall-Jet Electrodes... 

In Section 7.2, we looked at electroanalytical systems where the electrode rotates while the bulk of the solution remained still. In this present section, we will reverse this experimental concept by considering the case where it is the solution which flows - this time past a stationary electrode. Here, we shall be looking at flow ceils and channel electrodes. The principal mode of mass transport in both cases is convection, since the solution moves relative to the electrode. 

There is no absolute distinction in the literature between flow cells and channel electrodes. We shall say here that a flow cell contains a tubular electrode (often termed an annulus), while a channel electrode system contains a flat (or occasionally curved) electrode. Figure 7.6 shows a typical flow cell with an annular electrode. In contrast, the channel electrode illustrated in Figure 7.7 is flat and embedded inside a rectangular cavity. 

For both flow cells and channel electrodes, we assume that the electrode is solid and absolutely immobile. Its surface is flush with the surrounding insulator in which it is embedded, thereby inhibiting the incidence of turbulent flow. In addition, the electrode is polished, again to prevent turbulence. 

Figure 7.7 Schematic representation of a typical channel electrode system used for elec-troanalytical measurements. As with the flow cell show in Figure 7.6, note how the counter electrode (CE) is positioned downstream in order to stop the products from the CE flowing over the working electrode (WE) the reference electrode is positioned over the WE.

For a channel electrode, the relationship between limiting current, flow rate and cell geometry is given by the following ... 

What are the advantages of using a flow cell or a channel electrode ... 

Flow cells or channel electrodes are ideal for the continuous monitoring of analyte, for example from an HPLC column or some other form of... 

Like the flow and channel electrodes described above, the wall-jet electrode is not a batch-mode system, so it can be employed as the basis for... 

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. 

Convection That form of mass transport in which the solution containing electroanalyte is moved natural convection occurs predominantly by heating of solution, while forced convection occurs by careful and deliberate movement of the solution, e.g. at a rotated disc electrode or by the controlled flow of analyte solution over a channel electrode. 

A variety of vanadocene-derived organometallic species has been studied by electrochemical means. CP2VCI2 has been studied under a variety of conditions [57]. In MeCN, the oxidation appears well behaved by CV, but rotating-disk and channel-electrode voltammetry studies show that the oxidized complex undergoes follow-up chemistry shown in Eqs (1-3) ... 

Fig. 1. Coordinate systems for common electrode geometries, (a) Cylindrical symmetry (i) ring—disc electrodes, (ii) tubular electrodes (b) Cartesian symmetry channel electrodes (c) spherical symmetry dropping mercury electrode.

C. Double tube electrode. Double channel electrode. 

Since the form of the dimensionless convective-diffusion equation for tube and channel electrodes is exactly the same as for rotating electrodes, we can immediately conclude that the steady-state collection efficiency, N0, under conditions of uniform surface concentration at the generator electrode (which corresponds to the limiting current at the generator or to any point on a reversible wave) is, once again... 

Double channel electrodes were first used by Gerischer et al. [38] and this expression for AT0 was obtained by Braun [39] and subsequently experimentally verified [40]. 

The wall-jet disc electrode is clearly not uniformly accessible (current density oc r s/4 ). Another important point is that iL depends on the three-quarter power of the flow rate it is more sensitive in this sense than rotating or tube/channel electrodes. 

CO = rotation speed (rad s-1) Vf = volume flow rate (cm3 s 1 ) rT = radius of wall-tube U = linear velocity of solution (cm s 1 ) 0 = angle between cone surface and rotation axis x = length of tubular/channel electrode w = width of channel electrode d — width of channel h = half-height of channel co = rotation speed of solution (rad s 1) co" = rotation speed of rotating disc (rad s-1). 

The construction of tubular electrodes may be divided into two basic types integral and demountable. Channel electrodes are only of the latter type. Final dimensions must satisfy the entry length criterion for Poiseuille flow (pp. 370 and 372). 

Demountable electrodes are of two kinds depending on whether tube or channel electrodes are employed. In the first case, the individually machined sections are bolted together round a former [128, 129] but, here, turbulence problems may arise owing to ridges at the joints between sections. In the second case, one face of the cell is made separately from the rest and the channel electrode(s) embedded in it or insulation cast round them. After machining and polishing, the cell is bolted together [38, 39]. 

For use with tubular and channel electrodes (Fig. 8), no extra special... 

Detector of double channel electrode (reactant produced at generator) Vf-1/3 (h2 d)1 3 X213 0.616 C (x, Edet) 157... 

EC reactions at tubular and channel electrodes have been considered [208]. An analytical solution is not possible due to the non-uniformly accessible nature of the electrode. However, an approximate equation for the half-wave potential can be written, for a reduction, as... 

The homogeneous reaction may be of first or second order in the latter case, X is a non-electroactive species. Both these cases have been studied at the rotating ring—disc electrode and the first-order case at a double channel electrode. 

The electrochemical impedance may be obtained from potentiostatic or galvanostatic experiments. Alternating current voltammetric techniques are well documented at the DME, as are various kinds of pulse techniques. The former has also been developed at rotating and tubular/channel electrodes. 

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