Glassy carbon electrodes which have been irradiated with 20-kHz ultrasound from a 475-W generator in dioxane are shown to exhibit enhanced heterogeneous electron-transfer rates for a variety of aqueous redox probes. When sonications are performed in water, however, no significant enhancement effects are observed. Several electroanalytical techniques with different time scales are employed along with scanning electron microscopy to characterize surfaces before and after ultrasonic modification in different solvents. Results indicate that surface roughness does not change appreciably after brief sonication in dioxane, although a small amount of surface pitting occurs.

No abstract

The anaerobic voltammetry of the Mo/Fe enzyme, sulfite oxidase (SO), is described for the mediators cytochrome c, [Ru(NH3)6]3+/2+, TMPD+/0, and [Co(bpy)3]3+/2+. Theory derived for steady-state voltammetric catalysis correctly predicts the observed concentration and scan-rate dependencies of the catalytic waves. The instances for which existing ECcat theories may be applied to two catalytic reactions coupled to an interfacial charge transfer are considered. The biomolecular rate constant for the reaction of [Co(bpy)3]3+ with reduced SO is calculated and determined to be approximately 5 X 10(4) L.mol-1.s-1. The appearance of catalytic prepeaks at low sulfite concentrations is noted and the shape of corresponding i/t curves from chronoamperometry is examined. The analytical implications of the novel time dependence of the catalytic current under these conditions are discussed.

Investigations show that continuous ultrasound produces modulated mass transport in sonovoltammetry. At masstransport-limited potentials, voltammetry in the presence of ultrasound shows near-steady-state behavior with large current output that oscillates about a stable, average value. The current signal consists of a time-independent component and a time-dependent component. As expected for hydrodynamically modulated mass transport, both components are proportional to bulk analyte concentration. We report chronoamperometric determinations of ferrocene during ultrasonic irradiation that have been analyzed using both the time-independent and time-dependent signal components. In both cases, the limit of detection was 4 x iO mol/L. This paper contains detailed investigations of the time-dependent current signal. The effect of electrolyte viscosity on sonoelectrochemical measurements demonstrates that both current signals arise from convective-mass-transport effects. Third-order, high-frequency cutoff filtering of the chronoamperometnc signal shows significant attenuation and smoothing of the time-dependent signal. To explain our results we propose a qualitative model of convective mass transport in sonovoltammetry, where the time-independent current arises primarily from acoustic streaming and the time-dependent component comes from a combination of field-induced fluid motion and cavitational effects.

Voltammetric and amperometric experiments conducted during continuous sonication with a Ti immersion macrotip (1 cm2 area) positioned parallel to Pt working electrode surfaces are described. The effects of concentration, electrode area, temperature, kinematic viscosity, amplitude of vibration of the Ti sonicator tip, cell geometry and voltage scan rate on limiting currents are considered and compared to analogous measurements made at a rotating disk electrode. It is found that steadystate voltammograms may be obtained during sonication at scan rates up to 25 V/s. Hydrodynamic conditions are described in terms of the effective rotation rate which would be needed in rotating disk voltammetry to achieve similar transport rates. A procedure for performing sonovoltammetry in a cell geometry-independent manner to probe the chemical effects of acoustic cavitation is discussed.