Electron Transfer at the Working Electrode

Kinetics of the Electron Transfer Reaction

Electron-transfer reactions can be either kinetically fast or slow. For a fast reaction most of an analyte will react at the working electrode's surface. This type of reaction is said to be reversible and no advantage will be gained by increasing the applied potential. For slow reactions not all of the analyte reaching the working electrode's surface will have time to react. To drive the electrolytic reaction at a faster rate a much higher potential or "overpotential" must be used. Slow reactions are essentially irreversible. The more irreversible a reaction, the greater the applied potential must be. This results in greater noise and a poorer limit of detection. It is usually found that organic species react more favorably on one particular working electrode material than another.

Summary

The correct selection of the working electrode material is dependent upon three major factors: the potential limit of the working electrode; the involvement of the working electrode in the electrolytic reaction; and the kinetics of the electron-transfer reaction. In general, carbon-based working electrodes are used for many analyses because they are readily available in most laboratories using HPLC-ECD. However, carbon-based electrodes are not always the best working electrode and can lead to poor sensitivity, selectivity and stability of the analytical method. Carbon-based working electrodes are good "general" working electrodes and offer a wide range of applied potentials (both negative and positive).

References

The following references are recommended for more in depth reading:

 Acworth, I.N., and Bowers, M. (1996). An introduction to HPLC-based electrochemical detection: from single electrode to multi-electrode arrays. In: Progress in HPLC, Volume 6. Acworth, I.N., Naoi, M., Parvez, S., and Parvez, S. (Eds.). VS Press, The Netherlands. In press.

Acworth, I.N., and Gamache, P.H. (1996). The coulometric electrode array: for use in HPLC analysis. Part 1. Theory. American Lab., 5, 33-37.

Svendsen, C.N. (1993). Multi-electrode array detectors in high-performance liquid chromatography: A new dimension in electrochemical analysis. Analyst 118, 123-129.

Rocklin, R.D. (1984). Working-electrode materials. L.C 2, 588-593.