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-μm wall-tube microelectrode in a flow 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-μm-i.d. capillary outlet, flow 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 ∼0.001 and a flow rate of 200 μL/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 flow 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.