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-convention al environment: the 1-octanol/water microemulsion. The capability of u ltrasound both to emulsify instantly a liquid/liquid system in the abs ence of stabilizing agents and to generate a high flux of material tow ards the electrode surface is shown to provide new tools for the contr ol of electrochemical redox processes. First, the electrochemical redu ction of Ru(NH3)(6)(3+) has been studied. This is known to be a fast a nd reversible process at a glassy carbon electrode in aqueous 0.1 M KC l even in the presence of ultrasound. The addition of 1-octanol does n ot significantly affect this redox process up to a very high 1-octanol content in the emulsion system, although a thin film or adsorbed laye r of 1-octanol is believed to be present at the electrode surface. Sur prisingly, the limiting current and voltammetric characteristics of th e reduction process remain nearly unaffected, until, at a critical rat io of ca. 50 vol% 1-octanol, the voltammetric response suddenly disapp ears, attributed to loss of conductivity in the bulk liquid. Second, a nd in contrast, the reduction and deposition of Pb2+ from aqueous 0.1 M HClO4 at a glassy carbon electrode is strongly affected even by smal l quantities of 1-octanol, again consistent with a thin layer of 1-oct anol being permanently present at the electrode/emulsion interface. Wi th increasing quantities of added 1-octanol the reduction process is g radually shifted several hundred mV to more negative potentials wherea s the anodic lead stripping response is first shifted to more positive potentials and finally disappears. Third, the reduction of cobalticin ium, CoCp2+ (Cp = nu(5)-C5H5), in aqueous 0.1 M KOH under silent condi tions results in the formation of nearly insoluble, neutral cobaltocen e which precipitates at the glassy carbon electrode surface. In the pr esence of 1-octanol the neutral product dissolves in the organic liqui d and allows voltammetric experiments to be conducted without loss of electrode activity due to blocking. The efficiency and the mechanism o f this product 'trapping' process in the presence of 1-octanol is disc ussed. (C) 1998 Elsevier Science Ltd. All rights reserved.