Introducing power ultrasound into homogeneous solutions has a considerable effect upon mass transport processes due to macroscopic streaming and microscopic interfacial cavitation events. In electrochemical systems, cavitational erosion permits electrode activation by continuously removing material present at the electrode surface. In biphasic media, sonication results in the formation of an in situ emulsion, initiated by cavitational events at the liquid|liquid interface, with droplet size less than 1mm2.
Ultrasonically Induced Cavitational Processes
Microjets resulting from cavitational bubble collapse propel fresh analyte to the electrode surface dramatically increasing the current (figure on the right). Advantages that can arise from the use of sonoelectrochemistry include (a) increase in mass transport and electrode sensitivity erosion/digestion of particulates which facilitates release of target analyte, (b) mixing of biphasic systems to promote emulsification and (c) ablation of the electrode surface to maintain active surface in highly fouling media
The next stage of development has been to incorporate the working electrode within the tip of the ultrasound horn. This can significantly simplify the electrode arrangement but more importantly it should maximise cavitational cleaning of the electrode surface. In doing so, particularly intractable samples such as sludges and polymer forming organics can be analysed.
The applicability of the sonotrode has been demonstrated through the detection of sulphur dioxide/sulphite with sub ppm detection levels attainable at relatively low ultrasound powers. The response obtained at the platinum sonotrode are compared with those obtained under conventional silent conditions.
Ultrasound can also be used to enhance the detection of organic compounds. In this instance the ultrasound field is maintained throughout the electrochemical measurement and results in a hydrodynamic profile. The electrochemical detection of the anti-inflammatory drug 5-aminosalicylic acid (5-ASA) can be significantly improved through exploiting the increased mass transport. The oxidation signal intensity is substantially increased thereby enabling the detection of low concentrations with the linear range (1 to 57 uM) adequate for assessing the free drug within physiological samples.
It is clear that ultrasound can provide numerous benefits to electroanalysis and is not simply limited to cleaning applications. It can be profitably exploited for the determination of a large range of metals, organics and inorganics.