Influence of mixed diffusional, migrational, and convective mass transporton the response of a wall-tube microelectrode in a flow injection system

The current response of a 10-mu m walt-tube microelectrode in a now injecti on system under the conditions of low ionic strength has been examined expe rimentally for several redox systems such as ferrocene in methanol, undilut ed methanol, and water in acetone. The examination involved the dependence of the current on the positioning of the microelectrode relative to a 760-m u m-i.d. capillary outlet, now rate, potential pulse time, and ratio betwee n the concentrations of the supporting electrolyte and electroactive specie s (C-el/C-redox) For C-el/C-redox ratios smaller than similar to 0.001 and a now rate of 200 mu L/min, the dependencies of the current on the flow rat e and the positioning of the microelectrode versus the capillary tip were r eversed compared to the presence of excess supporting electrolyte. The curr ent was thus found to decrease with increasing now rate while a local curre nt maximum could be seen when the microelectrode was center-aligned with th e capillary tip. The changes in the current are the results of local differ ences in convective transport. These differences alter the rate of migratio nal accumulation of counterions at the electrode surface. It is shown that results similar to those obtained for the excess supporting electrolyte cas e can be obtained for a low value of C-el/C-redox and a given flow rate if the electrode potential and time scale of the experiments are chosen approp riately.