The Allendoerfer cell

Initial experiments by Allendoerfer et al. [53] involved a central conduc- 

Allendoerfer et al. estimated the surface area of the working electrode to be close to 22 cm2, significantly larger than previous designs, and found that the cell resistance was 13 Q when filled with 0.1 M tetrabutylammonium perchlorate (TBAP) in dimethylformamide, which compares well with the 

A modification of the Allendoerfer cell has been described by Ohya-Nish-iguchi et al. [56-58], whereby low-temperature studies were undertaken on the electrochemistry of 20 aromatic compounds. The low temperature was achieved by placing the electrolysis cell within a temperature control dewar. It should be noted, however, that, in order to simplify the apparatus, the low-temperature in-situ cell did not employ a reference electrode. 

In the study of short-lived radicals, the presence of efficient hydrodynamic flow is essential to sustain a constant supply of electroactive material to the electrode surface and hence ensure a steady flux of radicals. For this reason, Carroll adapted the in-situ cell of Allendoerfer to provide the capability of flow [54, 55]. The changes to the helical arrangement can be seen in Fig. 15. Solution is prevented from flowing in the central ESR inactive part of the cell by a complex series of baffles. The value of flow coupled to the Allendoerfer cell was shown by the electroreduction of nitromethane in aqueous conditions the radical anion of nitromethane was observed and this was shown to have a lifetime in the order of 10 ms. To data, this is the shortest radical lifetime observed. Calculations [56] suggested that such a cell should be capable of observing radicals with lifetimes of 10 5 s. 

FIGURE 14. A schematic diagram of the Allendoerfer cell cavity design. The TEqh cylindrical cavity (A) is shown in cutaway section. A metal cylinder (B) is located along the axis of the cavity within a silica sample tube (C). 

FIGURE 15. The ESR-visible part of the Allendoerfer cell is that between the wire helix and the quartz sample tube. The inner part of the helix is therefore free to house the reference and counter electrodes without interference with the ESR. 

The Allendoerfer cell described above has been modified by Ohya-Nish-iguchi and coworkers for operation at low temperatures. Their design was successfully used to record the ESR spectra of 20 aromatic radical ions at temperature as low as 

Richard G. Compton and Andrew M. Waller Reference electrode. 

FIGURE 16. The Allendoerfer-Carroll cell as modified to provide the capability for solution flow. 

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Figure 2.99 The Allendoerfer cell as modified by Carroll for the capability of solution flow over the electrode. From Comprehensive Chemical Kinetics, 29 (1989), 297.

Fig. 14. The ESR active and inactive parts of the Allendoerfer cell where the central conductor is the finely wound helix. (Reproduced from ref. 55 with permission.)...

The high sensitivity of the Allendoerfer cell makes it of great value in the detection of unstable radicals but, for the study of the kinetics and mechanism of radical decay, the use of a hydrodynamic flow is required. The use of a controlled, defined, and laminar flow of solution past the electrode allows the criteria of mechanism to be established from the solution of the appropriate convective diffusion equation. The uncertain hydrodynamics of earlier in-situ cells employing flow, e.g. Dohrmann [42-45] and Kastening [40, 41], makes such a computational process uncertain and difficult. Similarly, the complex flow between helical electrode surface and internal wall of the quartz cell in the Allendoerfer cell [54, 55] means that the nature of the flow cannot be predicted and so the convective diffusion equation cannot be readily written down, let alone solved Such problems are not experienced by the channel electrode [59], which has well-defined hydrodynamic properties. Compton and Coles [60] adopted the channel electrode as an in-situ ESR cell. 

The ESR performance of the coaxial cell was similar to that observed with the Allendoerfer cell. The insertion of the copper rod caused a reduction in the effective size of the cavity and hence the resonant frequency was observed to shift above that of the empty cylindrical cavity. With copper rods of... 

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