IMAGING LOCAL MASS-TRANSFER RATES WITHIN AN IMPINGING JET AND STUDIES OF FAST HETEROGENEOUS ELECTRON-TRANSFER KINETICS USING THE MICROJET ELECTRODE

The microjet electrode (MJE) is a recently introduced hydrodynamic electrode in which a jet of solution impinges on a disc ultramicroelectrode (UME) under conditions of well defined, variable and high mass-transfer rates. It is shown that these properties make MJE voltammetry a powerful technique for characterising rapid electron-transfer kinetics under steady-state conditions. The MJE approach is illustrated through studies of the hexacyanoferrate(II/III) couple in aqueous strontium nitrate solutions (0.2 mol dm(-3)). The standard rate constant is found to be 0.76 +/- 0.11 cm s(-1) and the transfer coefficient is in the range 0.35-0.55. Complementary mass-transfer imaging experiments, in which the transport-limited current at the UME is monitored as a function of the nozzle position, as a jet of solution is scanned over the electrode surface in a raster pattern, are shown to provide useful information on the positioning requirements of the MJE and a unique insight into the nature of mass transfer from an impinging jet. The spatial variation of local mass transfer deduced from these experiments is found to be in qualitative agreement with the theoretical predictions for the fluid velocity profile of the impinging jet under laminar flow conditions.