Richard P. Akkermans

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, dual activation experiments by simultaneously passing a current and applying 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 effects of ultrasound on electrode surfaces such as depassivation and erosion can be induced in cavitation events, violent collapses of oscillating 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 transport from other effects induced by sound waves. Several working electrode geometries have been employed and particularly the use of an electrode embedded in the tip of the ultrasound transducer, a so-called sonotrode, allows extreme conditions to be studied. In aqueous media and under these conditions voltammetry parallel to “classical” hydrodynamic techniques based on the effect of “acoustic streaming” was observed. A wide range of systems including the reduction of a metalloprotein, cytochrome c, are described. In this overview the current state-of-the-art is critically reviewed and the information that has been derived from sonovoltammetric measurements illustrated.

Voltammetry in the presence of ultrasound: the limit of acoustic streaming induced diffusion layer thinning and the effect of solvent viscosity

Abstract Fundamental aspects and applications of power ultrasound in electrochemistry are presently in rapid development. However, from the many literature reports using various types of, sometimes not very well characterized, sonoelectrochemical reactor, only a fragmented, often contradictory picture of the associated physicochemical processes arises. In this study, a classification of different sonoelectrochemical cell types depending on the effect of insonation is given and experimental results for ‘face-on’, ‘side-on’, and sonotrode geometries are compared. In particular, a minimum decrease in the diffusion layer thinning or a maximum increase in the transport-limited current can respectively be observed as the ultrasound power is increased. The comparative mass transport characteristics and the effect of solvent viscosity allow turbulent acoustic streaming to be identified as a major physical process which governs the mass transport in undivided sonoelectrochemical cells employing an immersion horn transducer. The effect of solvent viscosity on the limit of diffusion layer thinning is investigated using the electrochemical reduction of tetracyanoquinodimethane dissolved in dichloromethane, acetonitrile, dimethylformamide, and dimethylsulfoxide. A reciprocal relationship between the kinematic viscosity v and the limiting value for the diffusion layer thickness δlim is observed.

Sono-Electroanalysis: Application to the Detection of Lead in Petrol

The quantitative detection of lead in petrol is shown to be possible by anodic stripping voltammetry in aqueous media under conditions of insonation-induced emulsification. An immersion horn probe is introduced into a thermostatted conventional three-electrode cell opposite a mercury plated platinum disk working electrode. Under ultrasonic emulsification of the sample, lead is preconcentrated as an amalgam on the Hg/Pt electrode surface via reduction at –1.0 V (vs. SCE). The large mass transport associated with power ultrasound makes this step highly efficient. Subsequently the lead is quantified by applying an anodic linear sweep of the potential from –1.0 V to –0.15 V (vs. SCE) so as to oxidize the Pb(0) to Pb(II). The area under the stripping peak gives a measure of the lead formed during the initial step. By use of standard microaddition of lead to the solution the system can be calibrated to give the total amount of lead present in the petrol sample. Experiments using samples of 4 star leaded petrol gave a total lead content of 380±40 mgL–1. This value was in quantitative agreement with that obtained by an independent laboratory using atomic absorption spectroscopy (AAS). In addition to the high mass transport and emulsification insonation offers the crucial benefits of first surface activation and cleaning, helping to prevent electrode fouling by the organic components of petrol and second the complete extraction of lead from the water-insoluble target phase.

A Comparison Between Pulsed Sonovoltammetry and Low Power Laser Activated Voltammetry for the Electroanalysis of Ascorbic Acid in a Commercial Fruit Drink

A comparison is made between two ‘dual activation’ electroanalytical techniques for the detection and measurement of vitamin C (L-ascorbic acid) in a commercially available fruit drink via its two electron oxidation at platinum electrodes. Glucose is found not to interfere with the analytical response. First in sonovoltammetry pulses of 25 Wcm−2 are applied and the current response characterized in the ‘pulse off’ period where a current plateau is attained. Second in laser activated voltammetry a 10 Hz pulsed laser (532 nm, ca. 240 Wcm−2 average intensity) is used to ‘burn’ surface adsorbed passivating species off the electrode. Both methods cause agitation of the solution in the bulk phase or at the electrode-solution interface and lead to regular renewal of the diffusion layer. The mass transport limited oxidation currents so obtained are found to scale with ascorbic acid concentration in media where electroanalysis without simultaneous ultrasonic or laser stimulation may be precluded due to electrode passivation. Application to the quantitative electroanalysis of ascorbic acid in the fruit drinks Ribena and ‘No added sugar’ Ribena is reported. The results obtained are in excellent agreement with those yielded by independent chemical and electrochemical methods.

Electroanalysis of Ascorbic Acid: A Comparative Study of Laser Ablation Voltammetry and Sonovoltammetry

The electroanalytical detection of L-ascorbic acid via its two electron oxidation in aqueous solution is the subject of a comparison of two voltammetric methodologies both of which rely in part on electrode abrasion. First in sonovoltammetry cavitational collapse at the electrode–solution interface can lead to electrode erosion and activation. Second in laser ablation voltammetry (LAV) a 10 Hz pulsed laser (532 nm, 0.7 mJ per pulse) is used to abrade the electrode. In both cases the concomitant agitation of the solution leads to regular refreshment of the diffusion layer so that at sufficiently extreme potentials sustained currents (transport limited currents) are observed which scale with the concentration of ascorbic acid present and permit quantitative electroanalysis. Comparison of the methods as applied to ascorbic acid shows that for LAV the laser light intensity can be adjusted for maximum ablation of surface adsorbed blocking species but with minimum damage to the platinum surface itself. In contrast the sonovoltammetry technique does not facilitate selective erosion of surface adsorbed species. Instead the mass transport, amount of cavitation and damage to the electrode are all interlinked with the intensity of ultrasound employed. Thus while the amount of cavitation at the electrode surface can be controlled the relative adsorbate/electrode abrasion cannot. The limiting currents under insonation were found to be substantially (ca. 15 times) larger than for LAV suggesting that the major benefit of sonovoltammetry is in terms of enhanced mass transport whereas LAV shows more selective cleaning activation. The development of a novel sono-LAV is therefore reported which retains the merits of both the separate experiments in isolation. Thus the cleaning potential of LAV is coupled with the mass transport enhancement of ultrasound. Application to the quantitative electroanalysis of ascorbic acid is reported.

The Use of Sonotrodes for Electroanalysis:Sono-ASV Detection of Lead in Aqueous Solution

Sonotrodes were made by implanting a working electrode into the tip of an ultrasonic horn. These were investigated for the use in ultrasound assisted anodic stripping voltammetry (ASV) of Pb2+ and Cu2+ in aqueous solutions to which Hg2+ was added to permit the formation of lead/copper amalgam in a plating step. Insonation not only allows this preconcentration step to take place under conditions of unusually high mass transport but also causes enrichment of the trace metals in the form of intermetallic compounds leading to sharp stripping responses. The effect of increasing insonation time on the oxidation peak sizes, shapes and positions was studied. Comparison was made with the voltammetry of Pb2+ and Cu2+ observed at platinum electrodes in the absence of Hg2+ where broad signals for both were seen. The nature of the deposition of mercury on platinum electrodes was studied by both voltammetry and atomic force microscopy (AFM). The growth of mercury droplets with time under ‘silent’ plating conditions was seen. In anodic stripping experiments using platinum sonotrodes the ratio of Hg2+ to Pb2+ in the codeposition was examined along with the total amount of charge deposited for a range of Pb2+ concentrations. Sharp anodic stripping responses were obtained down to a Pb2+ concentration level of 2 μgL−1 making sono-ASV a potentially valuable technique for the detection of Pb2+ in solution. In contrast glassy carbon sonotrodes were found to be unsatisfactory since the adhesion of mercury under insonation was poor.

Ultrasound in photoelectrochemistry: a new approach to the enhancement of the efficiency of semiconductor electrode processes.

A novel photoelectrochemical experiment which simultaneously allows the illumination of a TiO2 semiconductor electrode surface and the application of power ultrasound emission is described. The horn probe of an immersion horn transducer is modified by an oxide coated titanium tip and placed in a conventional three electrode electrochemical cell which allows light from a monochromated source to be focussed onto the electrode surface. Well-defined photocurrents are observed in aqueous media and for the photoinduced oxidation of water in acetonitrile and of 2.4-dichlorophenol in acetonitrile. The effect of ultrasound is to shift the observed photocurrent responses to more negative potentials and therefore to enhance the observed processes. Several possible interpretations associated with the complex effects induced by ultrasound are considered and a model based mainly on the extreme change of mass transport at the semiconductor/solution interface is suggested. Considerably enhanced performance for non-Nernstian processes, such as those observed in photoelectrochemical reactions at semiconductor electrodes, can be achieved in the presence of ultrasound.

Sono-electrosynthesis: electrode depassivation and trapping of insoluble redox products

The electrosynthesis of water-insoluble products such as leuco-Methylene Green from soluble precursors can be accomplished using an emulsion formed via insonation so that the organic phase constantly extracts the product and simultaneously prevents the electrode passivation which would occur in aqueous solution alone.