Current-potential relationship for steady-state electron transfer

2 Current-potential relationship for steady-state electron transfer 

Vielstich and co-workers [99] modified the analysis of Blaedel and Klatt [66] to derive the steady-state current-potential relationship at the microtubular electrode under turbulent flow conditions. Their analysis assumed that the electrode/cell/flow rate parameters were such that mass transport to the electrode could be considered to be controlled by the laminar sublayer. In terms of the parameter 

Whilst the kinetic parameters of an electron-transfer reaction can be obtained in an identical fashion under laminar conditions [where u is now given by eqn. (58)] as illustrated by Blaedel [66], it is evident that the dependence of u on the cube root of the solution velocity in the laminar case [eqn. (58)] compared with the -dependence under turbulent conditions [eqn. (166)], implies that faster electron-transfer reactions can be investigated via the latter route. This is best illustrated with a practical example. Using flow rates characterised by Reynolds numbers up to 2 x 105 at a tubular electrode 7 pm in length within a tubular cell of radius 5 mm, Vielstich and co-workers [99] were able to measure a and ke for the ferro-ferricyanide redox couple (at 33.5°C). Their experimental data, in terms of a plot of In ut vs. (E - Ee), is represented in Fig. 50. The slope of both of the linear 

The strength of the technique in this application is demonstrated by Fig. 51, which depicts the current-potential behaviour (calculated from the experimental data in Fig. 50) with reference to that of a reversible process. It is clear that the kinetic parameters of the ferrocyanide/ferricyanide couple, a typical reversible electrode reaction [163], can be measured. Vielstich and co-workers have suggested [99] that, with the condition I/IKV 0.95 (over a reasonable potential range), the technique can be used to measure ks values up to 5 cm s 1 with an accuracy of around 10%. This 

The approximate maximum measurable values of fe8 a comparison of various experimental methods 

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