Ultrasound Mobilization of Liquid/Liquid/Solid Triple-Phase Boundary Redox Systems

John D. Watkins, Steven D. Bull, and Frank Marken*Department of Chemistry, UniVersity of Bath, ClaVerton Down, Bath, BA2 7AY, U.K.

ReceiVed: May 29, 2009; ReVised Manuscript ReceiVed: July 20, 2009

Power ultrasound is used to “mobilize” droplets of 1,2-dichloroethane (DCE) on a glassy carbon electrodesurface in an aqueous electrolyte environment. Voltammetric methods are employed to investigate the effectof ultrasound on (i) the mass transport in the aqueous phase, (ii) the mass transport in the DCE-aqueoustwo-phase system, and (iii) the triple-phase boundary anion extraction reaction coupled to oxidation ofn-butylferrocene (nBuFc) in the organic phase. Optimized conditions comprise a 13 mm diameter ultrasonichorn (24 kHz) with 15 W/cm2 power output at a distance of 15 mm from a 2.83 cm2 glassy carbon workingelectrode in 32 cm3 of aqueous solution. Mass transport in the aqueous phase is probed for the reduction ofhexaammineruthenium(III) chloride in aqueous 0.1 M KCl supporting electrolyte, and an increase in masstransport induced by the DCE droplets is observed. Triple-phase-boundary ion transfer reactions are studiedfor the oxidation of nBuFc in DCE in the presence of aqueous 0.1 M NaBPh4, KPF6, NaClO4, and phosphatebuffer pH 1. The hydrophobicity of the transferring electrolyte anion is observed to shift the electrochemicalresponse according to the standard transfer potential. For phosphate electrolyte media, rather than phosphatetransfer, n-butylferricenium cation transfer into the aqueous phase and iron(III) phosphate formation occur.The beneficial effect of adding tetrabutylammonium hexafluorophosphate electrolyte into the organic DCEphase is demonstrated; however, triple-phase-boundary processes in the absence of intentionally addedelectrolyte in the organic phase are feasible.

1. Introduction

Electrochemically driven ion transfer at liquid/liquid/solidtriple-phase boundaries has been achieved effectively withmicrodroplet arrays,1-3 with single droplets,4 in mesoporousstructures,5 for punctured droplets,6 and more recently withflowing two-phase systems.7 The advantage of the two-phaseelectrochemical process over single-phase processes is the factthat supporting electrolyte in the organic phase can be avoidedand products generated with a minimum of purification andseparation effort. Minimization of the amount of intentionallyadded electrolyte in single-phase electrosynthetic processes hasalso been investigated using narrow flow channels and pairedelectrode reaction conditions.8,9

Microdroplet arrays provide a powerful analytical tool forthe study of simultaneous electron- and ion-transfer reactions,10,11

but only very small amounts of material are electrolyzed. Thismakes practical electrolysis processes impossible (e.g., for bulkion extraction or for electrosynthetic reactions). MacDonald etal. introduced a dynamic triple-phase boundary concept involv-ing a laminar dual-phase flow system12 where the triple-phaseboundary is moving across the surface of a boron-dopeddiamond or platinum electrode. However, microfluidic devicesalso are difficult to scale up and are experimentally challengingduring operation. In this study a new and more practical methodis introduced on the basis of the concept of “ultrasonicmobilization” of the triple-phase boundary.

The new method takes advantage of the mechanical agitationeffects introduced by 24 kHz power ultrasound. Large droplets(up to 2000 µL) can be placed onto an electrode surface andthe sonication effect employed to break up the larger dropletinto mobile microdroplets for facile electrolysis. Figure 1 gives

a schematic description of the formation of an extended anddynamic triple-phase boundary during electrolysis. The soni-cation power is chosen to result in a highly mobile two-dimensional layer of microdroplets for the duration of elec-trolysis. Emulsification of the two-phase system is avoided. Thetriple-phase boundary redox reaction proceeds via a coupledelectron transfer (A f A+) and anion transfer (B-(aq) fB-(org)) mechanism (see Figure 1). This process requires awater-immiscible and relatively dense organic solvent such as1,2-dichloroethane (DCE).

Ultrasound has been shown to promote processes in photo-chemistry,13 in synthetic chemistry,14 in phase-transfer cataly-sis,15 and in single-phase electrochemistry.16 In syntheticelectrochemistry ultrasound has been applied to change productdistributions17 and to make insoluble organic materials reactivein water.18 Water is an ideal reaction medium for many organicelectrochemical reactions due to its conductivity, polarity, andfacile separation by extraction. In single-phase or emulsion sono-electrochemistry the use of an ultrasonic horn probe is veryeffective and different types of horn-electrode configurationshave been exploreds(i) perpendicular to the electrode surface,19

(ii) parallel with the electrode surface,20 or (iii) as a sonotrode21

where the ultrasonic source is also the working electrode. Inthe presence of power ultrasound, the diffusion layer thicknesscan be dramatically reduced primarily by “acoustic streaming”22

but also by more localized “cavitation” effects.23 Cavitationeffects have been exploited to avoid electrode blocking by solidresidues or byproduct. The sono-electrochemistry methodologyhas been successfully applied, for example, for biphasic Kolbecoupling24 processes (the generation of hydrocarbons and estersfrom carboxylates). Usually, powerful ultrasound is requiredfor the complete emulsification of the organic phase for effectivebiphasic electrolysis. However, here ultrasound is employed onlygently to “mobilize” a triple-phase boundary redox system

* To whom correspondence should be addressed. E-mail: F.Marken@bath.ac.uk.

J. Phys. Chem. C 2009, 113, 15629–15633 15629

10.1021/jp905068r CCC: $40.75 2009 American Chemical SocietyPublished on Web 08/11/2009

without emulsification and without the detrimental electrodesurface erosion effects introduced by high-power ultrasound.

2. Experimental Section

2.1. Chemical Reagents. Sodium perchlorate (99%, Aldrich),hexaammineruthenium(III) chloride (analytical reagent grade,Alfa Aesar), DCE (Fluka, HPLC grade g99.8%), n-butylfer-rocene (nBuFc, 98%, Alfa Aesar), potassium hexafluororphos-phate (Sigma Aldrich, 99.9+%), tetrabutylammonium hexafluo-rophosphate (Fluka analytical reagent, g99.0%), potassiumtetraphenylborate (Sigma Aldrich, g99.5%), and phosphoricacid (Sigma Aldrich ACS reagent, 85 wt %) were obtainedcommercially and used without further purification. Filtered anddemineralized water was taken from a Millipore water purifica-tion system with not less than 18 MΩ cm resistivity.

2.2. Instrumentation. Voltammetric measurements wereconducted with a µ-Autolab III potentiostat system (Eco Chemie,Netherlands) in staircase voltammetry mode with a 2.83 cm2

geometric area glassy carbon disk working electrode (glassycarbon from Alfa Aesar, type I). In some experiments othertypes of graphite plate electrodes (basal plane pyrocarbon andgraphite 2120PT from Le Carbone Ltd.) of 2.83 cm2 geometricarea were employed for comparison. A platinum counterelectrode and a saturated calomel reference electrode (SCE,REF401, Radiometer) were placed into 32 cm3 aqueouselectrolyte solution. All experiments were conducted at 22 ( 2°C. An ultrasound processor was used fitted with a 13 mmdiameter glass horn (Hielscher UP 200G, 24 kHz, 200 W,maximum ultrasound intensity 30 W cm-2, calibrated on thebasis of the thermal effect of ultrasound absorption in aqueousmedia25). During experiments the sonication time and intensitywere chosen to minimize temperature effects within the solution.Figure 2 shows a schematic drawing of the electrochemical cellwith the saturated calomel reference and a platinum wire counterelectrode in the side-arms. The glassy carbon disk workingelectrode forms the bottom of the electrochemical cell with arubber ring seal. The cell was filled with 32 cm3 aqueouselectrolyte solution and then 50-5000 µL organic DCE phasewas deposited with a syringe onto the working electrode. Next,the glass ultrasonic horn was inserted with a defined distanceto the working electrode (15 mm) for ultrasound mobilizationof the organic phase droplet on the surface of the workingelectrode.

3. Results and Discussion

3.1. Effect of Ultrasound on Mass Transport: Reductionof Ru(NH3)6

3+ in Aqueous 0.1 M KCl. The one-electronreduction of Ru(NH3)6

3+ (see eq 1) is employed to quantifyultrasound effects on mass transport in aqueous and aqueous/

organic two-phase media. The reduction of 2 mM Ru(NH3)63+

in 0.1 M KCl results in well-defined voltammetric responses26

with a mass transport limited current plateau at sufficientlynegative applied potentials.

Figure 3A shows typical voltammetric responses for anincreasing level of ultrasound intensity. The shape of thesesteady-state voltammograms is dominated by (i) an onset ofthe reduction at ca. -0.1 V vs SCE, (ii) a noise component inparticular under full mass transport control due to turbulent flowof liquid at the electrode surface, (iii) a “drawn-out” currentincrease due to some resistivity (ca. 100 Ω, caused in part bythe glassy carbon material) and high currents, and (iv) anincrease in limiting current with ultrasound intensity. Theappearance of “noise” is commonly observed in sonovoltam-metry27 and usually explained in terms of hydrodynamicmodulation effects such as turbulent eddies and interfacialcavitation. A plot of the mass transport controlled limitingcurrent as a function of sonication power (see Figure 3B)suggests an approximately linear dependency.

The same methodology was then applied for the reductionof 10 mM Ru(NH3)6

3+ in aqueous 0.1 M KCl in the presenceof an electrochemically inactive droplet of DCE placed ontothe glassy carbon electrode surface. During sonication the

Figure 1. Schematic drawing showing the mobilization effect of low-power ultrasound on a surface droplet of DCE on a glassy carbon surface.The anodic triple-phase boundary process causes extraction of B- from the aqueous phase into the organic phase. Upon mobilization of the organicphase, an extended and dynamic triple-phase boundary enhances the anodic process.

Figure 2. Schematic diagram showing the electrode configuration andthe cell design used for positioning of the ultrasonic horn probeperpendicular to the electrode surface. The organic droplet phase (DCE)is located directly on the glassy carbon disk working electrode surface.

Ru(NH3)63+(aq) + e- a Ru(NH3)6

2+(aq) (1)

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droplet is “mobilized” and depending on the sonication powerdivided into smaller droplets. The shape of the resulting two-phase voltammetric traces is consistent with those shown inFigure 3A. The effect of increasing sonication power alsosuggested an approximately linear increase in mass transportwith the DCE droplets present (up to ca. 15 W cm-2; beyondthis sonication power the organic phase was removed from theelectrode surface and dispersed irreversibly into the bulksolution; the high density of DCE is believed to be responsiblefor the stable mobilization of droplets at low sonication power,see Figure 3B). Mass transport effects observed under theseconditions are believed to reflect coupled hydrodynamic flowin the aqueous and organic phase. Perhaps surprisingly, theinitial effect of the DCE addition is to increase currentresponses. For a sonication power of 15 W cm-2 a maximumin mass transport limiting current is observed with a 200 µLdroplet of DCE (see Figure 3C) which is almost double of themass transport limited current observed in the absence of DCE.Therefore, for a small volume of the organic phase present atthe electrode surface there is a clear increase in mass transportlimiting current and only for droplets of 500 µL or more adecrease in current occurs. The current enhancement effect ispossibly due to increased streaming effects brought about bythe presence of surface-immobilized droplets of lower viscosity.The decrease in limiting current for higher DCE volume isapproximately linear and likely to be caused by blocking of

the active electrode surface with an insulating layer of DCE (asmaller contact area between aqueous phase and glassy carbonelectrode surface).

3.2. Effect of Ultrasound on Mass Transport: The Oxida-tion of nBuFc in DCE. The surface mobilization of DCEdroplets allows an extended and dynamic triple-phase boundaryreaction zone to be formed. In order to probe anion transferprocesses associated with triple-phase boundary oxidationprocesses, here the one-electron nBuFc redox system28 isemployed (see eq 2). A solution of 10 mM nBuFc in DCEorganic phase in contact to aqueous 0.1 M NaClO4 is investi-gated.

Figure 4A shows a typical cyclic voltammogram and the onsetof nBuFc oxidation occurs at ca. 0.2 V vs SCE. The voltam-metric response is drawn out due to resistivity effects (vide infra,ca. 1000 Ω) which arises from the low conductivity in theorganic DCE phase. Voltammograms obtained under theseconditions also exhibit non-steady-state effects such as newcathodic currents due to partial bulk conversion upon completionof the potential cycle (see Figure 4A). A mass transportcontrolled limiting current is observed at ca. 0.6 V vs SCE andthis was investigated as a function of sonication power (seeFigure 4B). An increase in oxidation current is observed up toa power of ca. 15 W cm-2. It is likely that more effectivemobilization of organic droplets with increased sonication poweris causing the increase in current. Both (i) breaking up of thedroplet into smaller droplets to provide a more extended triplephase boundary reaction zone and (ii) the agitation effects withinthe aqueous and organic phases contribute to the increase incurrents. Beyond 15 W cm-2 sonication power dispersion ofthe organic phase occurs and the process becomes ineffective.This trend in electrochemical behavior is confirmed by a visuallyobserved bulk emulsion formation at elevated sonication power.The optimal power for organic droplet electrolysis is ca. 15 Wcm-2 for this electrode and solvent system.

The effect of the electrode material is demonstrated in Figure4A. A systematic increase in the mass transport limited oxidationcurrent for nBuFc in DCE is observed when going from glassycarbon (Figure 4Aiv) to basal plane pyrocarbon (Figure 4Aiii)and finally to graphite (2120PT from Le Carbone Ltd., Figure4Aii). The hydrophobicity and roughness of these electrodematerial are believed to contribute to the effective sono-mobilization of the organic phase at the electrode surface. Theeffect of the nBuFc concentration on the electrochemicalresponse is shown in Figure 4C. An approximately linearincrease in mass transport controlled limiting current withconcentration is observed up to ca. 10 mM nBuFc (see dashedline) and deviation occurs at higher concentrations. The devia-tion at higher concentration is likely to be caused by resistivityeffects and incomplete mass transport control.

It is interesting to explore the effect of the droplet volumeon the triple-phase boundary oxidation response. At a constantsonication power of 15 W cm-2 at a horn-to-electrode distanceof 15 mm there is an initial increase of current followed by aplateau and a decrease at high DCE droplet volume (see Figure4D). Effective electrolysis can be achieved for a droplet volumebetween 100 and 2500 µL. In the absence of ultrasoundthe surface remains blocked and electrolysis is not possible. The

Figure 3. (A) Cyclic voltammograms (scan rate 5 mV s-1) for thereduction of 2 mM Ru(NH3)6

3+ in aqueous 0.1 M KCl at a glassy carbonworking electrode with sonication from 15 mm distance and (i) 6, (ii)12, (iii) 18, and (iv) 30 W cm-2 power. (B) Plot of the mass transportcontrolled limiting current versus sonication power. (C) Plot of the masstransport controlled limiting current versus droplet size (for 10 mMRu(NH3)6

3+ in aqueous 0.1 M KCl, sonication power of 15 W cm-2,distance 15 mm, varying organic DCE droplet volume).

Liquid/Liquid/Solid Triple-Phase Boundary Redox Systems J. Phys. Chem. C, Vol. 113, No. 35, 2009 15631

approximately constant current at around 500 µL droplet volumesuggests reproducible and optimized conditions.

3.3. Effect of the Nature of the Electrolyte on the TriplePhase Boundary Process: Oxidation of nBuFc in DCE. Theidentity of the transferring anion is of critical importance tothe potential dependence and mechanism of the triple-phase

boundary oxidation process. Hydrophobic anions transfer morereadily due to a greater affinity to the organic DCE phase (or alower hydration energy) corresponding to a more negativestandard transfer potential. The electrolyte anions studied wereperchlorate (ClO4

-), hexafluorophosphate (PF6-), tetraphenylbo-

rate (BPh4-), and phosphate buffer pH 1 solution. The mech-

Figure 4. (A) Cyclic voltammograms (scan rate 5 mV s-1) for the oxidation of 10 mM nBuFc in (i) 500 µL and (ii-iv) 200 µL DCE/aqueous 0.1M NaClO4 with sonication (15 W cm-2 power, distance 15 mm). The electrode materials are (i) glassy carbon, (ii) graphite 2120PT, (iii) basalplane pyrolytic graphite, and (iv) glassy carbon. (B) Plot of the mass transport controlled limiting currents versus the sonication power (at a thresholdof ca. 15 W cm-2 the droplets become emulsified and disperse). (C) Logarithmic plot of the mass transport controlled limiting currents (recordedat a scan rate of 5 mV s-1 in the presence of aqueous 0.1 M KPF6, sonication power 15 W cm-2, 15 mm distance) versus nBuFc concentration ina 500 µL DCE droplet. (D) Plot of the mass transport controlled limiting currents (scan rate 5 mV s-1, 5 mM nBuFc, aqueous 0.1 M NaClO4,sonication power 15 W cm-2, 15 mm distance) versus the droplet volume.

Figure 5. (A) Cyclic voltammograms (scan rate 5 mV s-1, sonication power of 15 W cm-2, 15 mm distance) for the oxidation of 5 mM nBuFcin a 200 µL DCE droplet in aqueous 0.1 M (i) NaBPh4, (ii) KPF6, (iii) NaClO4, and (iv) phosphate buffer pH 1. (B) Plot of the approximatestandard transfer potential for the transferring anion versus the half wave potential (Emid, determined approximately from the point in the voltammogramwhere half the mass transport limited current is reached). (C) Cyclic voltammograms (scan rate 5 mV s-1, (i) obtained without ultrasound, (ii)sonication power of 15 W cm-2, 15 mm distance) for the oxidation of 5 mM nBuFc in a 200 µL DCE droplet in aqueous 0.1 M phosphate bufferpH 1. (D) Cyclic voltammograms (scan rate 5 mV s-1, sonication power of 15 W cm-2, 15 mm distance) for the oxidation of 5 mM nBuFc in a200 µL DCE droplet (i) with and (ii) without 0.1 M Bu4NPF6 supporting electrolyte in the organic phase in the presence of aqueous 0.1 M KPF6.

15632 J. Phys. Chem. C, Vol. 113, No. 35, 2009 Watkins et al.

anism for the hydrophobic anions (ClO4-, PF6

-, and BPh4-) is

confirmed to be the transfer into the DCE droplet as the nBuFcis oxidized (see Figure 1). The characteristic shift in the half-wave potential (Emid, obtained as the potential where half ofthe mass transport limited current is reached) is clearly shownin Figure 5A. Figure 5B shows a plot summarizing theexperimental Emid values versus the standard potential of iontransfer.29,30 This plot shows that for the hydrophobic anions(ClO4

-, PF6-, and BPh4

-) there is a good linear correlation (seedashed line with slope 1.0).

In the case of phosphate buffer pH 1 a different mechanismdominates on the basis of voltammograms shown in Figure 5Cfor conditions with and without ultrasound. In this case thephosphate anion is too hydrophilic and instead of it transferringinto the DCE to balance the positive charge of the ferriceniumcation, it is the ferricenium cation which is believed to transferinto the bulk aqueous phase. The current for this process isconsiderably lower (see Figure 5A) and the potential for thetransfer deviates from the behavior observed for hydrophobicanions (see Figure 5B). Ferricenium is unstable in aqueousmedia and rapidly decomposed into iron(III) phosphate whichis then detected (in the absence of ultrasound, see Figure 5C)as an electrochemically active deposit on the electrode surface.The formation of iron phosphate may also be responsible forthe decrease in the limiting current due to the deposit partiallyblocking the electrode surface. The peak feature at ca. 0.23 Vvs SCE in Figure 5C corresponds to the iron(II/III) phosphateredox process.31 In the presence of ultrasound the process isnot observed probably due to in situ removal of the deposit.

Throughout this investigation a resistance effect has beenapparent due to the absence of intentionally added supportingelectrolyte in the organic phase. Figure 5D demonstrates theeffect of adding 0.1 M NBu4PF6 into the organic phase on theoxidation of nBuFc (5 mM nBuFc, scan rate 5 mV s-1,sonication power 15 W cm-2, 15 mm distance, 200 µL DCE)in the presence of aqueous 0.1 M KPF6. The voltammetricresponse with supporting electrolyte in the organic phase isconsiderably less drawn out (less resistance) and the masstransport controlled plateau current is increased (a wider reactionzone from the triple-phase boundary into the droplet is formed).However, avoiding the use of intentionally added electrolyte inthe organic phase has important advantages (lower cost, easierwork up and purification of products, etc.). Data presented inthis work suggest that electrolysis in surface mobilized DCEwithout intentionally added electrolyte is possible.

4. Conclusions

It has been shown that triple-phase boundary processes fordroplets of organic DCE on a glassy carbon electrode surfacecan be enhanced by power ultrasound. Surface mobilization ofdroplets can be achieved without emulsification. This effectallows an extended and dynamic triple-phase boundary reactionzone to be formed with the added benefit of agitation withinthe organic phase inducing improved transport during interfacialion transfer. The resulting procedure will allow a wider range

of electrosynthetic or ion-extraction reactions to be carried outunder dynamic triple-phase boundary conditions.

References and Notes

(1) Banks, C. E.; Davies, T. J.; Evans, R. G.; Hignett, G.; Wain, A. J.;Lawrence, N. S.; Wadhawan, J. D.; Marken, F.; Compton, R. G. Phys. Chem.Chem. Phys. 2003, 5, 4053.

(2) Scholz, F.; Schroder, U.; Gulaboski, R. Electrochemistry ofimmobilized particles and droplets; Springer: Berlin, 2005.

(3) Rayner, D.; Fietkau, N.; Streeter, I.; Marken, F.; Buckley, B. R.;Page, P. C. B.; del Campo, J.; Mas, R.; Munoz, F. X.; Compton, R. G. J.Phys. Chem. C 2007, 111, 9992.

(4) Chen, J. Y.; Sato, M. J. Electroanal. Chem. 2004, 572, 153.(5) Ghanem, M. A.; Marken, F. Electrochem. Commun. 2005, 7, 1333.(6) Bak, E.; Donten, M.; Stojek, Z.; Scholz, F. Electrochem. Commun.

2007, 9, 386.(7) Macdonald, S. M.; Watkins, J. D.; Bull, S. D.; Davies, I. R.; Gu,

Y.; Yunus, K.; Fisher, A. C.; Page, P. C. B.; Chan, Y.; Elliott, C.; Marken,F. J. Phys. Org. Chem. 2009, 22, 52.

(8) Paddon, C. A.; Atobe, M.; Fuchigami, T.; He, P.; Watts, P.; Haswell,S. J.; Pritchard, G. J.; Bull, S. D.; Marken, F. J. Appl. Electrochem. 2006,36, 617.

(9) Horcajada, R.; Okajima, M.; Suga, S.; Yoshida, J. Chem. Commun.2005, 1303.

(10) Marken, F.; Webster, R. D.; Bull, S. D.; Davies, S. G. J.Electroanal. Chem. 1997, 437, 209.

(11) MacDonald, S. M.; Opallo, M.; Klamt, A.; Eckert, F.; Marken, F.Phys. Chem. Chem. Phys. 2008, 10, 3925.

(12) Macdonald, S. M.; Watkins, J. D.; Gu, Y.; Yunus, K.; Fisher, A. C.;Shul, G.; Opallo, M.; Marken, F. Electrochem. Commun. 2007, 9, 2105.

(13) Gaplovsky, A.; Gaplovsky, M.; Toma, S.; Luche, J. L. J. Org. Chem.2000, 65, 8444.

(14) Luche, J. L. Synthetic Organic Sonochemistry; Springer: Berlin,1998.

(15) Yadav, G. D. Top. Catal. 2004, 29, 145.(16) Compton, R. G.; Eklund, J. C.; Marken, F. Electroanal. 1997, 9,

509.(17) Marken, F.; Compton, R. G.; Davies, S. G.; Bull, S. D.; Thiemann,

T.; Sa e Melo, M. L.; Neves, A. C.; Castillo, J.; Jung, C. G.; Fontana, A.J. Chem. Soc. Perkin Trans. 2 1997, 10, 2055.

(18) Wadhawan, J. D.; Marken, F.; Compton, R. G. Pure Appl. Chem.2001, 73, 1947.

(19) Akkermans, R. P.; Wu, M.; Bain, C. D.; Fidel-Suarez, M.; Compton,R. G. Electroanalysis 1998, 10, 613.

(20) Eklund, J. C.; Marken, F.; Waller, D. N.; Compton, R. G.Electrochim. Acta 1996, 41, 1541.

(21) Marken, F.; Akkermans, R. P.; Compton, R. G. J. Electroanal.Chem. 1996, 415, 55.

(22) Goldfarb, D. L.; Corti, H. R.; Marken, F.; Compton, R. G. J. Phys.Chem. A 1998, 102, 8888.

(23) Maisonhaute, E.; White, P. C.; Compton, R. G. J. Phys. Chem. B2001, 105, 12087.

(24) Wadhawan, J. D.; Del Campo, F. J.; Compton, R. G.; Foord, J. S.;Marken, F.; Bull, S. D.; Davies, R. G.; Walton, D. J.; Ryley, S. J.Electroanal. Chem. 2001, 507, 135.

(25) Mason, T. J.; Lorimer, J. P.; Bates, D. M. Ultrasonics 1992, 30,40.

(26) Holt, K. B.; Del Campo, J.; Foord, J. S.; Compton, R. G.; Marken,F. J. Electroanal. Chem. 2001, 513, 94.

(27) Cooper, E. L.; Coury, L. A. J. Electrochem. Soc. 1998, 145, 1994.(28) Wadhawan, J. D.; Evans, R. G.; Compton, R. G. J. Electroanal.

Chem. 2002, 533, 71.(29) Schroder, U.; Wadhawan, J. D.; Evans, R. G.; Compton, R. G.;

Wood, B.; Walton, D. J.; France, R. R.; Marken, F.; Page, P. C. B.; Hayman,C. M. J. Phys. Chem. B 2002, 106, 8697.

(30) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997; p217.

(31) McKenzie, K. J.; Marken, F. Pure Appl. Chem. 2001, 73, 1885.

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