The application of microelectrodes to the study of fast homogenous chemical reactions

A new technique for the determination of the kinetics of fast homogeneous reactions coupled to electrode processes is presented. This technique simply requires the recording of the steady state current at very small electrodes, termed microelectrodes. At slow sweep rates (steady state conditions) the i-E curves obtained at such electrodes reach a plateau current as for conventional polarograms. In the case of a simple electron transfer this plateau current density is diffusion controlled and, as for a hemispherical electrode, the current density is inversely proportional to the electrode radius. In the case of couple homogeneous reactions kinetic data can be obtained from the plateau current. Single platinum fibre microelectrodes sealed in glass with radii from 25 pm to 0.5 Um have been prepared and used. The application of the technique has been illustrated with a number of systems for which reliable kinetic data are available, namely some first or pseudo-first order ece, disproportionation 1 and catalytic ec' processes as well as second order ece reactions. It is shown that for ece and disproportionation I mechanisms using the electrodes available rate constants up to 105 s-1 may be determined but the two mechanisms are indistinguishable. In the case of the ec' mechanism, under pseudo-first order conditions, the upper limit for the rate constant is not quantifiable; it is dependent on the system under study but will be greater than for transient techniques. Kinetic data for the above first order cases were obtained from experimental results assuming that there is a hemispherical diffusion field at the electrode. Good agreement for the rate constants determined in this way was found with the values determined by other techniques. A second order ece system has also been studied ; in this case no analytical solution is obtainable for the relevant diffusion equations and therefore simulation techniques were used to try to determine the rate constant. Finite difference and orthogonal collocation methods were both used. It was found that in order to explain the experimental data the diffusion to a finite disc electrode using two dimensional coordinates has to be considered instead of the approximation of diffusion to a hemispherical electrode.