Studies in sonoelectrochemistry
The work described in this thesis employs 'dual activation' methodologies and in particular sonovoltammetry to study a variety of electrochemical systems. First, the effect of 20 kHz power ultrasound on the electrochemistry of simple redox systems in both water and acetonitrile is explored and characterised for different cell geometries. A simple Nernst diffusion layer model is validated. Second, the use of insonation for efficient pre-concentration of target species in anodic stripping voltammetry (ASV) is reported. 'Sonotrodes', where the electrodes are incorporated into the ultrasonic horn tips are shown to offer particularly high rates of mass transport and additionally cause ablation of mercury from plated platinum electrodes. The key benefits of sonication are shown to be electrode depassivation and extraction plus rapidity and ease of determination, coupled with the lack of any sample pre-treatment for the detection of lead in both wine and petrol by ASV. Lead levels determined by this method and calibrated by use of standard microaddition are in excellent agreement with those obtained 'blind' by atomic absorption spectroscopy performed at independent laboratories. Third, the novel use of ultrasonically formed emulsions is examined for both analytical and synthetic purposes. The possibility of extracting species both out of an organic phase for analysis and into an organic phase for the synthesis of water-insoluble redox products is realised. Fourth, the sonovoltammetric determination of ascorbic acid in both aqueous solution and the fruit drink Ribena®, is compared with results obtained by laser activated electroanalysis, where low level laser illumination of an electrode is employed to maintain a fresh electrode surface. Small amounts of laser-induced thermal convection at the electrode result in steady-state voltammetry. The level of agreement is excellent between the two methods and also agrees very well with independent chemical analysis. Fifth, laser activated voltammetry (LAV) is further explored and a simple mass transport model verified. Damage caused by high power laser ablation is evidenced by atomic force microscopy (AFM). Applications of the technique for depassivation are illustrated by the reduction of toluidine blue dye and oxidation of ferrocyanide in the presence of blood proteins. The technique is further employed under channel flow conditions to elucidate an unambiguous mechanism for aqueous iodide oxidation at platinum electrodes in the absence of the usual build up of bulk iodine (as evidenced by AFM). Finally, the reduction of methylene green dye at platinum electrodes is used to compare and contrast sonovoltammetry and LAV with thermal and microwave dual activation techniques. The build-up of the passivating reduced form of the dye on the electrode surface, again seen by AFM, is removed by sono-emulsion and laser activation but not by microwave heating. However, greater conductivity is observed at higher temperatures and an electron-hopping mechanism is postulated.