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.

The strength of acids in alcohols as determined by steady-state voltammetry.

Steady-state voltammograms for reduction of acids of various strengths in alcohols with excess supporting electrolyte and without any supporting electrolyte can be used to infer charge type and strength of the acid on the basis of the phenomenon of migration. For strong and moderately weak acids (K(a)/[Formula: see text] > 10(-)(3)) in alcohols, the ratio of steady-state transport-limited current to diffusion-limited current, corrected appropriately for ion-ion interactions, the presence of ionic impurities, and changes in viscosity, for hydrogen ion reduction without supporting electrolyte and with excess supporting electrolyte equals 2. For acetic acid, which is very weak (K(a)/[Formula: see text] < 10(-)(6)), the value of the steady-state transport-limited current is, under the experimental conditions applied here, independent of supporting electrolyte concentration. In the case of a homogeneous acid-base equilibrium, a novel analytical procedure yields diffusion coefficients of both hydrogen ion and undissociated weak acid molecules from the diffusional and migrational currents. Limiting currents obtained in alcohols with excess supporting electrolyte and without supporting electrolyte are compared by means of an extended formula that incorporates the ionic strength dependence of diffusion coefficients.

Migration and diffusion coupled with a fast preceding reaction. Voltammetry at a microelectrode.

A mathematical model implemented by simulation is presented for voltammetry of a reversible couple that involves a fast preceding chemical reaction and mixed diffusional and migrational transport. The hydrogen couple, H(+)/H(2), fulfills the above criteria. For strong acids there is no preceding reaction, whereas for weak acids the preceding reaction is HA = H(+) + A(-). The computed voltammograms are compared with experimental voltammograms for the reduction of strong and weak acids at Pt microelectrodes with excess of and without supporting electrolyte. The key assumption in the calculations is that the flux of hydrogen ion is independent of the anion. This assumption is supported by the experimental fact that the wave heights in the absence of supporting electrolyte of several strong acids of equal concentration and with anions of various size are identical.

Conditions of strict voltammetric reversibility of the h(+)/h(2) couple at platinum electrodes.

Cyclic voltammetric curves obtained at Pt electrodes for the hydrogen couple, H(+)/H(2), fit very well the Shuman theory, as corrected, for reversible electrode processes of other than 1:1 stoichiometry. Good agreement was obtained for acid concentrations in the millimolar range and for normal scan rates, which minimize the effect of the adsorption peaks. An error in Shuman's equation for potential is corrected. Voltammograms obtained at Pt microelectrodes fit well the theoretical simulated data.