Wall-jet ring-disc electrode

It is more difficult to manufacture these electrodes than the simple disc electrode since the ring must be exactly concentric with the disc. Additionally, in many applications the insulation gap must be thin (0.25 mm or less) as must the ring. For rotating ring—disc electrodes, typical dimensions are a disc radius of 3—4 mm and an outer ring radius of 4—5 mm. For wall-jet ring—disc electrodes, these dimensions can be approximately halved. 

Figure 8.11 shows the type of curve that is obtained, which allows the determination of the concentration of X. The rotating ring-disc electrode3 and the wall-jet ring-disc electrode in continuous flow4 (Fig. 14.3) and with sample injection into potassium bromide solution5 have been used—this last procedure reduces the amount of sample necessary and... 

Fig. 14.3 Diffusion-layer microtitration curves at the wall-jet ring disc electrode for titration of As(III) (X) with bromine (B) generated at the disc electrode from bromide. Solution 10-2 m KBr + 0.5 m H2S04. Analysis of the curve leads to [X] (from Ref. 4 with permission).

Albery W J and Brett C M A 1983 The wall-jet ring disc electrode. 1. Theory J. Electroanal. Chem. 148 201... 

Albery WJ, Brett CMA (1983) The wall-jet ring-disc electrode Part I. Theory. J Electroanal Chem 148 201-210... 

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). 

Experimental work has been published on a ring—disc electrode which is intermediate in geometry between the wall-jet and wall-tube configurations. Consequently, and as expected, intermediate collection efficiency values were measured [51]. 

Fig. 8.4. A wall-jet cell with ring-disc electrode. A, disc electrode contact B, ring electrode contact C, Ag/AgCl reference electrode D, platinum tube auxiliary electrode E, solution entry F, cell body in Kel-F (from Ref. 1 with...

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.

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. 

Forced convection (hydrodynamic) generator - collector systems are commonly employed in rotating ring-disc or wall-jet geometries or in channel flow cells to improve collection efficiencies. For macroscopic interelectrode gap systems hydrodynamic agitation can be employed to improve feedback, but for diffusion - dominated nano-gap electrode systems hydrodynamic convection effects usually remain insignificant, whereas heating can be used to enhance the rate of diffusion processes and therefore to improve feedback currents. 

The shape of the electrode may vary significantly from the disc type if the sensor is used as a detector in flow systems, e.g., ring or mbular-shaped metal sheets nevertheless the most common form is the wall-jet configuration (the effluent stream hits the surface with a perpendicular angle) with a disc shaped detector. 

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