Influence of mixed diffusional, migrational, and convective mass transport on 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 injection system under the conditions of low ionic strength has been examined experimentally for several redox systems such as ferrocene in methanol, undiluted methanol, and water in acetone. The examination involved the dependence of the current on the positioning of the microelectrode relative to a 760-mu m-i.d. capillary outlet, now rate, potential pulse time, and ratio between the concentrations of the supporting electrolyte and electroactive species (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 rate and the positioning of the microelectrode versus the capillary tip were reversed compared to the presence of excess supporting electrolyte. The current was thus found to decrease with increasing now rate while a local current maximum could be seen when the microelectrode was center-aligned with the capillary tip. The changes in the current are the results of local differences in convective transport. These differences alter the rate of migrational accumulation of counterions at the electrode surface. It is shown that results similar to those obtained for the excess supporting electrolyte case 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 appropriately.