Wall jet electrodes

Tile wall jet electrode (WJE) has attracted considerable attention in analytical applications of voltammetry (see for example Brett et al., 1995,1996). In this electrode configuration, a high, fixed velocity jet of fluid is fired, through a nozzle of diameter, a, directly towards the middle of a disc electrode (radius = ri), whose centre coincides with that of the nozzle (Fig. 25). The solution thus impinges upon the electrode surface and is circulated outwards towards the extremities of the electrode surface, but the recirculated solution can never reach the electrode a second time. 

A reversible one-electron transfer process (19) is initially examined. For all forms of hydrodynamic electrode, material reaches the electrode via diffusion and convection. In the cases of the RDE and ChE under steady-state conditions, solutions to the mass transport equations are combined with the Nernst equation to obtain the reversible response shown in Fig. 26. A sigmoidal-shaped voltammogram is obtained, in contrast to the peak-shaped voltammetric response obtained in cyclic voltammetry. 

There are two critical parameters that are measured in a steady-state voltammogram  

Electrode Convective flow parameter Expression for limiting current, / i , 

RDE ChE WJE Angular rotational velocity, o Volume flow rate, V, Volume flow rate, Vt 0.62nFAD - [A o 0.925nFZ)2 [A]ow(V,4 /i2d)  

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A.A. Karyakin, E.E. Karyakina, and L. Gorton, The electrocatalytic activity of Prussian blue in hydrogen peroxide reduction studied using a wall-jet electrode with continuous flow. J. Electroanal. Chem. 456, 97-104 (1998). 

It can be concluded that the electrode reaction is the most effective at a wall-jet electrode, which means that under similar circumstances, the signal will be the highest at a wall-jet electrode. 

To learn what a wall-jet electrode is, and understand why it can be employed for electroanalyses despite operating with a turbulent flow. 

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

The wall-jet electrode has the general configuration shown in Figure 7.8(a). When using this system, the current is measured while a fine jet or spray of analyte solution is squirted under relatively high pressure towards the centre of the working... 

Figure 7.8 Schematic representation of a typical wall-jet electrode used for electroanalytical measurements (a) contact to Pt disc electrode (the shaded portion at the centre of the figure) (b) contact to ring electrode (c) AgCl Ag reference electrode (d) Pt tube counter electrode (e) cell inlet (f) cell body (made of an insulator such as Teflon), (b) A typical pattern of solution flow over the face of a wall-jet electrode, showing why splash back does not occur. Part (a) reproduced from Brett, C. M. A. and Brett, A. M. O., Electroanalysis, 1998, 1998, by permission of Oxford University Press.

Surely, with such a spray-type operation, it will be extremely difficult to attain laminar flow in the wall-jet electrode ... 

The wall-jet electrode is unusual as the solution flow is turbulent at all times. Since this turbulence occurs in terms of fine droplets of analyte... 

The limiting current at a wall-jet electrode is a function of the radius of the circular electrode, r, the rate of flow of solution, V/, the diameter a of the jet supplying the solution of analyte, the diffusion coefficient D of the analyte and the bulk concentration of analyte, Camiyte- hm also depends, in a complicated way, on the distance between the electrode and the nozzle, which we will denote here as k. 

The limiting current at the wall-jet electrode is given by the following equation ... 

Amounts of are being monitored at a wall-jet electrode. When a sample of concentration 3.23 pg cm" is squirted over the electrode, the limiting current is 152 pA. Keeping the flow rate and all other parameters constant, what is the concentration of a sample of Co when the limiting current is 214 pA Assume complete faradaic efficiency in both cases. 

What are the special advantages of employing a wall-jet electrode ... 

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. 

The article by Hitchman and Hill (above) is again useful here in that it also contains an introduction to the rotated ring-disc and wall-jet electrodes. 

Wall-jet electrode Electrochemical cell in which analyte solution is squirted at high pressure on to a flat circular electrode. 

Fig. 7.54. The wall-jet electrode schematic streamlines. (Reprinted with permission of Oxford University Press, from C. M. A. Brett and A. M. O. Brett, Electrochemistiy, Oxford University Press, 1993. p. 155.)...

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. 

Note that iL depends on Vf1/2 whereas, for the wall-jet electrode, it depends on Vf4. This equation only holds for 0.1 Mass transfer is more efficient than at an RDE however, the electrode has to be smaller. Nevertheless, in applications where it is difficult to fabricate a moving electrode (i.e. photoelectrochemical and semiconductor), it could be very valuable. From the theoretical point of view all that has to be done is replace by 0.98 Vf /r% in all the equations for a rotating disc or ring--disc electrode to obtain the wall-tube analogue. In particular, the steady-state collection efficiency, N0 [eqn. (41)], is the same not only in form but also in numerical value for the same radius ratios [50] (Table 2). 

Scheme of the wall-jet electrode as constructed in our laboratory with (1) solution tank, (2) vessel to control the solution flow rate, (3) pump, (4) flow-rate measuring device, (5) capillary, (6) measuring chamber, (7) counter electrode, (8) reference electrode, (9) working electrode, (10) overflow system to return solution to the solution tank. 

In this section, the aim is to explain how the previously discussed results can be implemented in an industrially acceptable setup, taking into account parameters such as long-term stability, selectivity, reproducibility, simplicity and low cost. Therefore, it was the intention of the authors not to use the rotating-disc electrode, but rather to implement a so-called wall-jet electrode, which possesses similar characteristics but with a long-term stability, simplicity and cost effectiveness that are much more favourable than for a rotating-disc electrode. The setup of the wall-jet electrode is discussed in Chapter 1, pages 19-21. 

Prior to use for analytical purposes, the developed wall-jet electrode should first be characterised and calibrated. This is described here, where the wall-jet disc electrode is optimised by making use of a reversible, one-electron exchanging, redox system ([Fe(CN)6]47[Fe(CN>,]3 ) in order to obtain the most favourable conditions for the determination of sodium dithionite, sulphite and indigo. 

In this section, the use of a wall-jet electrode (with optimal values for its parameters as described in section6.7.2) and the method to detect simultaneously sodium dithionite, sulphite and indigo (see section6.7.4) are evaluated as a function of reproducible dyeing processing. In order to evaluate this, a spectrophotometric method was used to measure the amount of dye absorbed and/or adsorbed by the dyed fabric. 

Fig. 16.3. Schematic streamlines at a wall-jet electrode illustrating the highly non-uniform accessibility of the electrode surface and that the electrode surface does not receive any solution from fluid recirculation (from Ref. [23]).

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