Sonoelectrochemically modified electrodes: ultrasound assisted electrode cleaning, conditioning, and product trapping in 1-octanol/water emulsion systems

Electrode processes for three types of water soluble redox systems in the presence of power ultrasound have been studied in a non-conventional environment: the 1-octanol/water microemulsion. The capability of ultrasound both to emulsify instantly a liquid/liquid system in the absence of stabilizing agents and to generate a high flux of material towards the electrode surface is shown to provide new tools for the control of electrochemical redox processes. First, the electrochemical reduction of Ru(NH3)(6)(3+) has been studied. This is known to be a fast and reversible process at a glassy carbon electrode in aqueous 0.1 M KCl even in the presence of ultrasound. The addition of 1-octanol does not significantly affect this redox process up to a very high 1-octanol content in the emulsion system, although a thin film or adsorbed layer of 1-octanol is believed to be present at the electrode surface. Surprisingly, the limiting current and voltammetric characteristics of the reduction process remain nearly unaffected, until, at a critical ratio of ca. 50 vol% 1-octanol, the voltammetric response suddenly disappears, attributed to loss of conductivity in the bulk liquid. Second, and in contrast, the reduction and deposition of Pb2+ from aqueous 0.1 M HClO4 at a glassy carbon electrode is strongly affected even by small quantities of 1-octanol, again consistent with a thin layer of 1-octanol being permanently present at the electrode/emulsion interface. With increasing quantities of added 1-octanol the reduction process is gradually shifted several hundred mV to more negative potentials whereas the anodic lead stripping response is first shifted to more positive potentials and finally disappears. Third, the reduction of cobalticinium, CoCp2+ (Cp = nu(5)-C5H5), in aqueous 0.1 M KOH under silent conditions results in the formation of nearly insoluble, neutral cobaltocene which precipitates at the glassy carbon electrode surface. In the presence of 1-octanol the neutral product dissolves in the organic liquid and allows voltammetric experiments to be conducted without loss of electrode activity due to blocking. The efficiency and the mechanism of this product 'trapping' process in the presence of 1-octanol is discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.