The effect of ultrasonic frequency and intensity upon limiting currents at rotating disc and stationary electrodes

The limiting current from Fe(CN)^3-₆/Fe(CN)⁴₆ redox in dilute aqueous KCl at a platinum electrode appears to be little affected by the frequency of simultaneous ultrasonic irradiation in the range 20-800 kHz, and is not influenced by choice of bath or probe as sonic source, provided measurements are made at constant ultrasonic intensity. The limiting current is dependent on ultrasonic intensity at constant frequency. In contrast, the peak potential of the redox couple varies with ultrasonic frequency in the same range, but does not vary with ultrasonic intensity at constant frequency. Application of ultrasound to a rotating disc electrode provides an increase in limiting current across the frequency range, the magnitude of which varies with both concentration and rotation speed. The average maximum increase is some 1.5-fold which corresponds to an effective rotation rate of 100 000 rpm (1667 Hz) for a silent solution in order to achieve the same transport rates. It is shown that changes in macroscopic temperature throughout the experiment are insufficient to cause the observed enhanced diffusion.