Wall tube electrode

A. Rotating ring—disc electrode. Wall-tube electrode. 

In the case of the wall-tube electrode, the electrode is smaller than the jet diameter. The original treatment was due to Frossling [47] who demonstrated that, close to the wall... 

Using these formulae, Albery and Bruckenstein have derived the limiting current at a wall-tube electrode [49]. 

Fig. 6. Wall tube electrode geometry—schematic streamlines. (From ref. 49.)...

The fundamentals of the electrochemical response at electrodes operating in a regime of forced convection, hydrodynamic electrodes, and the information that can be obtained have been reviewed [23, 24]. Some of these electrodes are good candidates for direct introduction into flow systems, in particular tube/channel electrodes and impinging jet (wall-jet and wall-tube) electrodes. Particular practical advantages of these flow-past hydrodynamic electrodes are that there is no reagent depletion while the sample plug passes the electrodes, and there is no build-up of unwanted intermediates or products. Recent advances in instrumentation also mean... 

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

Rotating electrodes may be inconvenient in some applications when further devices like spectrometers shall be coupled with them in hyphenated techniques or because of noise caused by the inherently necessary contact brushes. Consequently, attempts have been made to move the electrolyte solution instead in a controlled fashion. In the case of the wall-tube electrode, a jet of electrolyte solution from a nozzle with diameter d is directed towards a circular electrode with radius r (with d larger than the electrode diameter 2r) embedded at distance h in insulating material as depicted below (Fig. 7). 

Controlled Flow Methods for Electrochemical Measurements, Fig. 7 Scheme of the wall-tube electrode configuration... 

Klymenko OV, Gavaghan DJ, Harriman KE, Compton RG (2002) Finite element simulation of electrochemically reversible, quasireversible and irreversible linear sweep voltammetry at the wall tube electrode. J Electroanal Chem 531 25-31... 

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.

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]. 

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). 

Fig. 8.2c. Mass transfer at an impinging jet electrode. I, central core potential region II, established flow region III, stagnation region ( wall-tube region) IV, wall-jet region (from Ref. 13 with permission).

Rotating ring-disc electrode (RRDE) Wall-tube ring-disc electrode (WTRDE) ... 

Fig. 10.5(a). Transfer function, H, relating modulated current response to modulated flow rate for tube electrode (Reference [20]), rectangular electrode embedded in a wall (Reference [12]) and modulated RDE (amplitude only). The dimensionless modulation frequency see text) is the ratio of the time scale for diffusion across the concentration boundary layer to the timescale for modulation of the hydrodynamics. 

Gabrielli and Perrot [23] carried out in situ mass measurements in well-defined flowing electrolyte with an electrochemical quartz crystal microbaiance (EQCM) adapted to a submerged impinging jet cell (wall tube configuration). The authors employed this new device for the study of nickel electrodeposition and evaluation of the cathodic efficiency. Under the conditions of their experiment (nozzle diameter d = 7 mm disc electrode diameter de = 5 mm and nozzle-to-electrode distance H = 2d), the current that flows at the electrode increases with the square root of flow rate (0-10 cm3 s"1). It should be noted that this approach is much simpler to implement than the rotating EQCM, while keeping control of the convective-diffusion conditions. 

The electric field runs were conducted in a companion shock tube with ports for inserting a large cylindrical electrode see Figure 1). The electrode was a 5.4-cm diameter copper electrode mounted in a Plexiglas insulator. The assembly was installed fiush with the bottom wall. The electrode/tube apparatus operated much like an ionization chamber. A D.C. power supply provided continuous output from 100 to 4000 V, giving a field strength of up to 500 V/cm. 

Fig. 1 Electrode setups for CE-EC with fiber microelectrodes (a) end column, (b) on-column, (c) improved on-column, (d) wall tube, and (e) waU-jet detection.

Mass transfer to liquids in turbulent pipe flow has been studied by using tubes made from a slightly soluble solid and measuring the rate of dissolution of the solid for various liquid flow rates. An alternate technique is to make a portion of the tube wall an electrode and carry out an electrochemical reduction under conditions where the current is limited by the rate of mass transfer of the reacting ion to the wall. 

One of the simplest flow systems is the tube electrode with the electrode set into the wall of a tube. The solution flows by under laminar conditions. 

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