Channel electrode cells

Design and fabrication of channel electrode cells and the associated flow system 

We first consider the construction of cells designed to obey the Levich equation [eqn. (27)]. This dictates that the flow conforms to a laminar regime, which firstly implies that obstructions and rough edges have been eliminated from the path of the solution, and secondly places an upper limit on the solution velocity in accordance with eqn. (6) such that the Reynolds number 2 x 103. 

The associated counter and reference electrodes can either be included in the cell body or plumbed into the flow system separately. The cell designed by Meyer et al. [105] and used by Aoki et al. [104] incorporates both a platinum counter electrode and a silver pseudo-reference electrode within the channel unit. The cell shown in Fig. 31(a) contains a silver-silver chloride reference electrode in one of the ducts at the end of the channel. For a.c. impedance measurements, where ohmic drop must be minimised, this is 

One of the advantages of flow-through electrodes over other types of hydrodynamic electrodes is that they involve no moving parts. They are therefore ideal for use in conjunction with both spectroscopic and microscopic techniques. Whilst both channel and tubular electrodes can be used in 

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Fig. 31. (a) Perspex channel electrode cell. A, Channel unit B, cover plate C, rubber block D, metal plate E, working electrode F, reference electrode G, silicone rubber gasket. From ref. 70. (b) Silica channel electrode cell (unassembled) showing the cover plate with electrode and lead-out wire and the channel unit. 

Fig. 33. Schematic representation of pumped-recirculating flow system employed by Matsuda and co-workers (from ref. 104). A, Channel electrode cell B, flow meter C, thermometer D, solution reservoir E, thermostat F, degas inlet G, degas outlet H, pump.

Fig. 34. Schematic representation of a gravity-fed flow system. A, Solution reservoir B, PTFE tubing C, PVC argon jacket D, thermostat E, channel electrode cell (including reference electrode) F, counter electrode G, calibrated capillary.

As with the channel electrode cell, and following from the previous work undertaken using the ex situ tube electrode arrangement, the EPR signal (S) is a function of the diffusion-limited current (i) and the volume flow rate. 

It has been seen that the other in situ hydrodynamic cells discussed above, the channel electrode cell and the tube electrode cell, both exhibit EPR signal (S) characteristics dependent upon the ratio However, in the case of the wall-jet electrode under steady state conditions, in which the entire cell downstream of the working electrode is filled with radicals, the rate of loss of radicals from the cell is proportional to Vf, while the rate of their formation is proportional to i, thereby anticipating... 

FIGURE 24. The transport-limited current (/um) for the reduction of fluorescein at the Compton-Coles channel electrode cell. The solid lines show the predicted flow rate (V) (cm s" ) behavior for simple one- and two-electron reductions. 

If the reaction proceeds via steps (a), (b), and (c), then we have an ECE process. The sequence (a), (b), (d) corresponds to a DISP reaction. Within the latter scheme there are two further possibilities, depending on whether step (b) or (d) is rate determining. In the former case, we have a DISPl process, since it is a (pseudo-) first-order reaction (in buffer), while in the latter case, we have a DISP2 process, since it is second-order. Conventional electrochemical methods readily recognize DISP2 processes but, with only a few exceptions (double potential-step chronoamperometry and possibly microelectrodes ), they cannot be used to discriminate between ECE and DISPl. It emerges that a combination of ESR transient and electrochemical data from the channel electrode cell can make this distinction. 

The above example shows several virtues of electrochemical ESR. Firstly, the existence of a radical intermediate is indicated, then its chemical identity (including state of protonation) is revealed and its lifetime and the order of its kinetic decay measured. Finally, because the channel electrode cell behaves as a satisfactory hydrodynamic electrode, the full mechanism of the electrolytic reaction is elucidated. 

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