Sonovoltammetry

Sonochemistry is concerned with the effect of ultrasonic waves on chemical reactivity (Mason, 1991) and is an area of rapidly growing importance in a diversity of applications. Ultrasound has a frequency above that which is audible 

Ultrasound is known for its capacity to promote heterogeneous reactions (Ley and Low, 1989) mainly through greatly increased mass transport, interfacial cleaning and thermal effects. In addition, homogeneous chemical reactions have been reported to be modified (Suslick et al, 1983 Luche, 1990 Colarusso and Serpone, 1996) for example the sonochemical generation of radical species in aqueous media is important in environmental detoxification (Kotronarou et al, 1991 Serpone et al, 1994). 

Recent experimental advances have allowed the interpretation of sonoelectrochemical phenomena from a physical and mechanistic standpoint. In particular, the principles underlying voltammetric experiments conducted in the presence of ultrasound are now much better appreciated, and situations where the incorporation of ultrasound is beneficial have been identified. Fundamental aspects of sonoelectrochemistry have been reviewed (Compton et al, 1997a,b). In this section we focus almost exclusively on sonovoltammetry. 

Section 2 described a number of general concepts surrounding the experimental design of a general voltammetric experiment this section focuses upon how 

Ultrasonic baths will be familiar from their everyday use in the laboratory where they are commonly used for cleaning surfaces and to aid dissolution. A bath essentially comprises a number of transducers of fixed frequency, commonly 20-100 kHz, attached beneath the physical exterior of the bath unit. Baths typically deliver ultrasonic intensities between 1 and 10 W cm to the reaction medium. For sonovoltammetry (or sonoelectrosynthesis) the bath may be filled with distilled water and a conventional electrochemical cell is placed inside the bath at a fixed position (Walton et al, 1995) so that the cell is electrically isolated from the sound source. Alternatively, the internal metal casing of the bath can be coated so that the full volume is available to use as an electrochemical cell (Huck, 1987). For both arrangements results can be highly sensitive to positioning and/or cell geometry effects. 

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Akkermans R P, Wu M, Bain C D, Fidel-Suerez M and Compton R G 1998 Electroanalysis of ascorbic acid a comparative study of laser ablation voltammetry and sonovoltammetry E/eofroana/ys/s 10 613... 

Walton D J, Phull S S, Chyla A, Lorimer J P, Mason T J, Burke L D, Murphy M, Compton R G, Ekiund J C and Page S D 1995 Sonovoltammetry at platinum electrodes surface phenomena and mass transport processes J. Appl. Electrochem. 25 1083... 

Compton RG, Eklund JC, Page SD et al (1994) Voltammetry in the presence of ultrasound. Sonovoltammetry and surface effects. J Phys Chem 98 12410-12414... 

Cooper EL, Coury LA jr (1998) Mass transport in sonovoltammetry with evidence of hydrodynamic modulation from ultrasound. J Electrochem Soc 145 1994—1999... 

Esclapez MD, Saez V, Milan-Yanez D et al (2010) Sonochemical treatment of water poulled with trichloroacetic acid from sonovoltammetry to pre-pilot plant scale. Ultrason Sonochem 17 1010-1020... 

If (95) is used to estimate values for the diffusion layer thickness obtained for sonovoltammetry in acetonitrile, values of the order of a few micrometres are obtained - much smaller than encountered in conventional voltammetry under silent (stationary) conditions unless either potential scan rates of hundreds of mVs, or more, are employed or alternatively steady-state measurements are made with microelectrodes with one or more dimensions of the micrometre scale (Compton et al., 1996b). 

These kinetic results are interesting in that they are consistent with the physical reality of the thinned diffusion-layer model introduced above. Moreover it is evident that sonovoltammetry enables fast rate constants to be measured under steady-state conditions at conventionally dimensioned electrodes otherwise these would only be accessible via transient measurements such as fast-scan cyclic voltammetry or using steady-state microelectrode methodology. 

Mass transport effects are further considered in due course after pausing to describe an alternative electrode geometry for sonovoltammetry. 

Fig. 40 Three experimental approaches to sonovoltammetry (a) face-on, (b) side-on, and (c) sonotrode geometries.

Figure 40 summarizes the different geometries employed in sonovoltammetry including the side-on approach (Eklund et al., 1996). In this latter case a flow over a flat plate model gave good agreement with experiment assuming solution velocities of ca. 100 cm s" were obtained in solution. These were attributed to acoustic streaming (Marken et al., 1996a), as shown in Fig. 41. These observations prompt a further consideration of mass transport effects.

SONOVOLTAMMETRY MASS TRANSPORT EFFECTS - FURTHER ASPECTS... 

A major future application of sonovoltammetry may well lie in the field of electroanalysis where the ability to maintain electrode activity in dirty or otherwise passivating media may extend the range of applicability of such procedures. Reports of the benefits of insonation in anodic and adsorptive stripping voltammetry are just beginning to appear (Marken et al., 1997a Matysik et al., 1997 Agra-Gutierrez and Compton, 1998). 

For sonovoltammetry, for a typical diffusion coefficient (1 X 10 cm s ) the diffusion layer is reduced to around 1-10 /xm, corresponding to a typical ultrasound power range of 10-60 W cm". Using a simple diffusion-layer model, the time-scale may be calculated from tg = S ID. 

The simplest way to assist electrochemical techniques with US is by using a bath to immerse the electrochemical cell, as proposed by Lorimer et al. [154] (see Fig. 8.15A). These authors used a three-compartment thermostated voltammetric cell consisting of a platinum flag (the counter electrode), a saturated calomel electrode (the reference electrode) and a rotating disc (the working electrode). Although an ultrasonic bath affords less accurate control of US irradiation, it affords a tenfold current increase in sonovoltammetry [167]. 

Figure 8.15. Thermostated electrochemical cells employed for sonovoltammetry. (A) With an ultrasonic bath. (B) With an ultrasonic probe (Reproduced with permission of Elsevier, Refs. [133,154], respectively.)...

The methods for organio analytes are less common. Thus, sonovoltammetry has been used to determine dopamine in egg homogenate [142] riboflavine [186] and guanine and guanosine in aqueous solutions [188] and ascorbio acid in commercial fruit drinks [189]. The few existing examples testify to the limited applioability of sonovoltammetry to organio analytes. 

Indireot sonovoltammetry has enabled the determination of some speoies by their effeot on electrochemically initiated reactions. For instance, Cu(ll) can be determined by inhibiting the eleotrochemical reaction between A/,A/-diethyl-p-phenylenediamine and homocysteine through complexation of the latter [192]. 

Mass transport to microelectrodes 64 Microelectrodes and homogeneous kinetics 66 Microelectrodes and heterogeneous kinetics 68 Convective microelectrodes 69 Sonovoltammetry 69... 

Sonovoltammetry mass transport effects - further aspects 80 Electrode cleaning and activation 81 Electrode kinetics 82... 

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