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

The microjet electrode (MJE) is a recently introduced hydrodynamic ele ctrode in which a jet of solution impinges on a disc ultramicroelectro de (UME) under conditions of well defined, variable and high mass-tran sfer rates. It is shown that these properties make MJE voltammetry a p owerful 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 n itrate 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 r ange 0.35-0.55. Complementary mass-transfer imaging experiments, in wh ich the transport-limited current at the UME is monitored as a functio n of the nozzle position, as a jet of solution is scanned over the ele ctrode surface in a raster pattern, are shown to provide useful inform ation on the positioning requirements of the MJE and a unique insight into the nature of mass transfer from an impinging jet. The spatial va riation of local mass transfer deduced from these experiments is found to be in qualitative agreement with the theoretical predictions for t he fluid velocity profile of the impinging jet under laminar flow cond itions.