DUAL ACTIVATION - COUPLING ULTRASOUND TO ELECTROCHEMISTRY - AN OVERVIEW

By introducing a powerful immersion horn probe as a source of intense ultrasound into a thermostatted conventional three-electrode cell, dua l activation experiments by simultaneously passing a current and apply ing ultrasound may be undertaken. These sono-voltammetric experiments may be used in order to analyse and quantify the processes induced by ultrasound at the electrode/solution interface. Different effects have been described. First, by applying intense sound fields direct effect s of ultrasound on electrode surfaces such as depassivation and erosio n can be induced in cavitation events, violent collapses of oscillatin g bubbles. Second, the huge effect of ultrasound on the mass transport at the electrode surface detected by various voltammetric techniques may be described by the model of an extremely thinned diffusion layer of uniform accessibility. This experimentally verified model may then be used in voltammetric experiments in order to separate pure mass tra nsport from other effects induced by sound waves. Several working elec trode geometries have been employed and particularly the use of an ele ctrode embedded in the tip of the ultrasound transducer, a so-called s onotrode, allows extreme conditions to be studied. In aqueous media an d under these conditions voltammetry parallel to ''classical'' hydrody namic techniques based on the effect of ''acoustic streaming'' was obs erved. A wide range of systems including the reduction of a metallopro tein, cytochrome c, are described. In this overview the current state- of-the-art is critically reviewed and the information that has been de rived from sonovoltammetric measurements illustrated. (C) 1997 Elsevie r Science Ltd.