EFFECTS OF ULTRASONICALLY INDUCED CAVITATION ON ELECTROCHEMICAL PROCESSES - CELL GEOMETRY AND FREQUENCY-EFFECTS

A 'sonoelectrochemical reactor' with an ultrasonic source and the elec trochemical equipment has been devised to monitor various ultrasonic p arameters (wave frequency, geometry and position of working electrode) . Methylviologen and ferricyanide reduction and pyrrole oxidation were studied in order to obtain informations on the modifications occurrin g in the course of the reactions. Studies conducted on ferricyanide re duction show that the voltammograms obtained under ultrasonic conditio ns at 20 or 500 kHz exhibit an enhancement of the diffusion process at the electrode surface where cavitation occurs. The best and most repr oductible limiting currents were obtained at 500 kHz when the electrod e is near the gas-liquid interface; the current density increases with ultrasonic power. At 20 kHz, the limiting current is higher when the electrode is close to the ultrasonic source and almost independent of the ultrasonic power. Coupling ultrasound with electrochemistry suppre sses the electrode passivation that occurs during the methylviologen e lectroreduction at a glassy carbon electrode. Ultrasound was also effi cient at hindering the electrodeposition of polypyrrole film at a plat inum electrode and cleaning the surface coated by electrodeposited pol ypyrrole film. In addition, it is clearly shown that ultrasonic irradi ation can modify the electrochemical reactivity at electrode-electroly te interface.