Mass transport in sonovoltammetry with evidence of hydrodynamic modulation from ultrasound

Investigations show that continuous ultrasound produces modulated mass transport in sonovoltammetry. At mass-transport-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 10(-7) 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 chronoamperometric 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.