Solid Triple-Phase Boundary

Aug 11, 2009 - Optimized conditions comprise a 13 mm diameter ultrasonic horn (24 kHz) with 15 W/cm2 power output at a distance of 15 mm from a 2.83 c...
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J. Phys. Chem. C 2009, 113, 15629–15633

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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 electrode surface in an aqueous electrolyte environment. Voltammetric methods are employed to investigate the effect of ultrasound on (i) the mass transport in the aqueous phase, (ii) the mass transport in the DCE-aqueous two-phase system, and (iii) the triple-phase boundary anion extraction reaction coupled to oxidation of n-butylferrocene (nBuFc) in the organic phase. Optimized conditions comprise a 13 mm diameter ultrasonic horn (24 kHz) with 15 W/cm2 power output at a distance of 15 mm from a 2.83 cm2 glassy carbon working electrode in 32 cm3 of aqueous solution. Mass transport in the aqueous phase is probed for the reduction of hexaammineruthenium(III) chloride in aqueous 0.1 M KCl supporting electrolyte, and an increase in mass transport induced by the DCE droplets is observed. Triple-phase-boundary ion transfer reactions are studied for the oxidation of nBuFc in DCE in the presence of aqueous 0.1 M NaBPh4, KPF6, NaClO4, and phosphate buffer pH 1. The hydrophobicity of the transferring electrolyte anion is observed to shift the electrochemical response according to the standard transfer potential. For phosphate electrolyte media, rather than phosphate transfer, 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 DCE phase is demonstrated; however, triple-phase-boundary processes in the absence of intentionally added electrolyte in the organic phase are feasible. 1. Introduction Electrochemically driven ion transfer at liquid/liquid/solid triple-phase boundaries has been achieved effectively with microdroplet arrays,1-3 with single droplets,4 in mesoporous structures,5 for punctured droplets,6 and more recently with flowing two-phase systems.7 The advantage of the two-phase electrochemical process over single-phase processes is the fact that supporting electrolyte in the organic phase can be avoided and products generated with a minimum of purification and separation effort. Minimization of the amount of intentionally added electrolyte in single-phase electrosynthetic processes has also been investigated using narrow flow channels and paired electrode reaction conditions.8,9 Microdroplet arrays provide a powerful analytical tool for the study of simultaneous electron- and ion-transfer reactions,10,11 but only very small amounts of material are electrolyzed. This makes practical electrolysis processes impossible (e.g., for bulk ion extraction or for electrosynthetic reactions). MacDonald et al. introduced a dynamic triple-phase boundary concept involving a laminar dual-phase flow system12 where the triple-phase boundary is moving across the surface of a boron-doped diamond or platinum electrode. However, microfluidic devices also are difficult to scale up and are experimentally challenging during operation. In this study a new and more practical method is introduced on the basis of the concept of “ultrasonic mobilization” of the triple-phase boundary. The new method takes advantage of the mechanical agitation effects introduced by 24 kHz power ultrasound. Large droplets (up to 2000 µL) can be placed onto an electrode surface and the sonication effect employed to break up the larger droplet into mobile microdroplets for facile electrolysis. Figure 1 gives * To whom correspondence should be addressed. E-mail: F.Marken@ bath.ac.uk.

a schematic description of the formation of an extended and dynamic triple-phase boundary during electrolysis. The sonication power is chosen to result in a highly mobile twodimensional layer of microdroplets for the duration of electrolysis. Emulsification of the two-phase system is avoided. The triple-phase boundary redox reaction proceeds via a coupled electron transfer (A f A+) and anion transfer (B-(aq) f B-(org)) mechanism (see Figure 1). This process requires a water-immiscible and relatively dense organic solvent such as 1,2-dichloroethane (DCE). Ultrasound has been shown to promote processes in photochemistry,13 in synthetic chemistry,14 in phase-transfer catalysis,15 and in single-phase electrochemistry.16 In synthetic electrochemistry ultrasound has been applied to change product distributions17 and to make insoluble organic materials reactive in water.18 Water is an ideal reaction medium for many organic electrochemical reactions due to its conductivity, polarity, and facile separation by extraction. In single-phase or emulsion sonoelectrochemistry the use of an ultrasonic horn probe is very effective and different types of horn-electrode configurations have 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. In the presence of power ultrasound, the diffusion layer thickness can be dramatically reduced primarily by “acoustic streaming”22 but also by more localized “cavitation” effects.23 Cavitation effects have been exploited to avoid electrode blocking by solid residues or byproduct. The sono-electrochemistry methodology has been successfully applied, for example, for biphasic Kolbe coupling24 processes (the generation of hydrocarbons and esters from carboxylates). Usually, powerful ultrasound is required for the complete emulsification of the organic phase for effective biphasic electrolysis. However, here ultrasound is employed only gently to “mobilize” a triple-phase boundary redox system

10.1021/jp905068r CCC: $40.75  2009 American Chemical Society Published on Web 08/11/2009

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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 organic phase, an extended and dynamic triple-phase boundary enhances the anodic process.

without emulsification and without the detrimental electrode surface 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-butylferrocene (nBuFc, 98%, Alfa Aesar), potassium hexafluororphosphate (Sigma Aldrich, 99.9+%), tetrabutylammonium hexafluorophosphate (Fluka analytical reagent, g99.0%), potassium tetraphenylborate (Sigma Aldrich, g99.5%), and phosphoric acid (Sigma Aldrich ACS reagent, 85 wt %) were obtained commercially and used without further purification. Filtered and demineralized water was taken from a Millipore water purification system with not less than 18 MΩ cm resistivity. 2.2. Instrumentation. Voltammetric measurements were conducted 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 (glassy carbon from Alfa Aesar, type I). In some experiments other types of graphite plate electrodes (basal plane pyrocarbon and graphite 2120PT from Le Carbone Ltd.) of 2.83 cm2 geometric area were employed for comparison. A platinum counter electrode and a saturated calomel reference electrode (SCE, REF401, Radiometer) were placed into 32 cm3 aqueous electrolyte solution. All experiments were conducted at 22 ( 2 °C. An ultrasound processor was used fitted with a 13 mm diameter glass horn (Hielscher UP 200G, 24 kHz, 200 W, maximum ultrasound intensity 30 W cm-2, calibrated on the basis of the thermal effect of ultrasound absorption in aqueous media25). During experiments the sonication time and intensity were chosen to minimize temperature effects within the solution. Figure 2 shows a schematic drawing of the electrochemical cell with the saturated calomel reference and a platinum wire counter electrode in the side-arms. The glassy carbon disk working electrode forms the bottom of the electrochemical cell with a rubber ring seal. The cell was filled with 32 cm3 aqueous electrolyte solution and then 50-5000 µL organic DCE phase was deposited with a syringe onto the working electrode. Next, the glass ultrasonic horn was inserted with a defined distance to the working electrode (15 mm) for ultrasound mobilization of the organic phase droplet on the surface of the working electrode. 3. Results and Discussion 3.1. Effect of Ultrasound on Mass Transport: Reduction of Ru(NH3)63+ in Aqueous 0.1 M KCl. The one-electron reduction of Ru(NH3)63+ (see eq 1) is employed to quantify ultrasound effects on mass transport in aqueous and aqueous/

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

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 sufficiently negative applied potentials. 2+ Ru(NH3)3+ 6 (aq) + e a Ru(NH3)6 (aq)

(1)

Figure 3A shows typical voltammetric responses for an increasing level of ultrasound intensity. The shape of these steady-state voltammograms is dominated by (i) an onset of the reduction at ca. -0.1 V vs SCE, (ii) a noise component in particular under full mass transport control due to turbulent flow of liquid at the electrode surface, (iii) a “drawn-out” current increase due to some resistivity (ca. 100 Ω, caused in part by the glassy carbon material) and high currents, and (iv) an increase in limiting current with ultrasound intensity. The appearance of “noise” is commonly observed in sonovoltammetry27 and usually explained in terms of hydrodynamic modulation effects such as turbulent eddies and interfacial cavitation. A plot of the mass transport controlled limiting current as a function of sonication power (see Figure 3B) suggests an approximately linear dependency. The same methodology was then applied for the reduction of 10 mM Ru(NH3)63+ in aqueous 0.1 M KCl in the presence of an electrochemically inactive droplet of DCE placed onto the glassy carbon electrode surface. During sonication the

Liquid/Liquid/Solid Triple-Phase Boundary Redox Systems

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15631 the active electrode surface with an insulating layer of DCE (a smaller contact area between aqueous phase and glassy carbon electrode surface). 3.2. Effect of Ultrasound on Mass Transport: The Oxidation of nBuFc in DCE. The surface mobilization of DCE droplets allows an extended and dynamic triple-phase boundary reaction zone to be formed. In order to probe anion transfer processes associated with triple-phase boundary oxidation processes, here the one-electron nBuFc redox system28 is employed (see eq 2). A solution of 10 mM nBuFc in DCE organic phase in contact to aqueous 0.1 M NaClO4 is investigated.

Figure 3. (A) Cyclic voltammograms (scan rate 5 mV s-1) for the reduction of 2 mM Ru(NH3)63+ in aqueous 0.1 M KCl at a glassy carbon working 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 transport controlled limiting current versus sonication power. (C) Plot of the mass transport controlled limiting current versus droplet size (for 10 mM Ru(NH3)63+ in aqueous 0.1 M KCl, sonication power of 15 W cm-2, distance 15 mm, varying organic DCE droplet volume).

droplet is “mobilized” and depending on the sonication power divided into smaller droplets. The shape of the resulting twophase voltammetric traces is consistent with those shown in Figure 3A. The effect of increasing sonication power also suggested an approximately linear increase in mass transport with the DCE droplets present (up to ca. 15 W cm-2; beyond this sonication power the organic phase was removed from the electrode surface and dispersed irreversibly into the bulk solution; the high density of DCE is believed to be responsible for the stable mobilization of droplets at low sonication power, see Figure 3B). Mass transport effects observed under these conditions are believed to reflect coupled hydrodynamic flow in the aqueous and organic phase. Perhaps surprisingly, the initial effect of the DCE addition is to increase current responses. For a sonication power of 15 W cm-2 a maximum in mass transport limiting current is observed with a 200 µL droplet of DCE (see Figure 3C) which is almost double of the mass transport limited current observed in the absence of DCE. Therefore, for a small volume of the organic phase present at the electrode surface there is a clear increase in mass transport limiting current and only for droplets of 500 µL or more a decrease in current occurs. The current enhancement effect is possibly due to increased streaming effects brought about by the presence of surface-immobilized droplets of lower viscosity. The decrease in limiting current for higher DCE volume is approximately linear and likely to be caused by blocking of

Figure 4A shows a typical cyclic voltammogram and the onset of nBuFc oxidation occurs at ca. 0.2 V vs SCE. The voltammetric response is drawn out due to resistivity effects (vide infra, ca. 1000 Ω) which arises from the low conductivity in the organic DCE phase. Voltammograms obtained under these conditions also exhibit non-steady-state effects such as new cathodic currents due to partial bulk conversion upon completion of the potential cycle (see Figure 4A). A mass transport controlled limiting current is observed at ca. 0.6 V vs SCE and this was investigated as a function of sonication power (see Figure 4B). An increase in oxidation current is observed up to a power of ca. 15 W cm-2. It is likely that more effective mobilization of organic droplets with increased sonication power is causing the increase in current. Both (i) breaking up of the droplet into smaller droplets to provide a more extended triple phase boundary reaction zone and (ii) the agitation effects within the aqueous and organic phases contribute to the increase in currents. Beyond 15 W cm-2 sonication power dispersion of the organic phase occurs and the process becomes ineffective. This trend in electrochemical behavior is confirmed by a visually observed bulk emulsion formation at elevated sonication power. The optimal power for organic droplet electrolysis is ca. 15 W cm-2 for this electrode and solvent system. The effect of the electrode material is demonstrated in Figure 4A. A systematic increase in the mass transport limited oxidation current for nBuFc in DCE is observed when going from glassy carbon (Figure 4Aiv) to basal plane pyrocarbon (Figure 4Aiii) and finally to graphite (2120PT from Le Carbone Ltd., Figure 4Aii). The hydrophobicity and roughness of these electrode material are believed to contribute to the effective sonomobilization of the organic phase at the electrode surface. The effect of the nBuFc concentration on the electrochemical response is shown in Figure 4C. An approximately linear increase in mass transport controlled limiting current with concentration is observed up to ca. 10 mM nBuFc (see dashed line) and deviation occurs at higher concentrations. The deviation at higher concentration is likely to be caused by resistivity effects and incomplete mass transport control. It is interesting to explore the effect of the droplet volume on the triple-phase boundary oxidation response. At a constant sonication power of 15 W cm-2 at a horn-to-electrode distance of 15 mm there is an initial increase of current followed by a plateau and a decrease at high DCE droplet volume (see Figure 4D). Effective electrolysis can be achieved for a droplet volume between 100 and 2500 µL. In the absence of ultrasound the surface remains blocked and electrolysis is not possible. The

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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 0.1 M NaClO4 with sonication (15 W cm-2 power, distance 15 mm). The electrode materials are (i) glassy carbon, (ii) graphite 2120PT, (iii) basal plane pyrolytic graphite, and (iv) glassy carbon. (B) Plot of the mass transport controlled limiting currents versus the sonication power (at a threshold of ca. 15 W cm-2 the droplets become emulsified and disperse). (C) Logarithmic plot of the mass transport controlled limiting currents (recorded at 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 in a 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 nBuFc in 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 approximate standard transfer potential for the transferring anion versus the half wave potential (Emid, determined approximately from the point in the voltammogram where 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 buffer pH 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 a 200 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.

approximately constant current at around 500 µL droplet volume suggests reproducible and optimized conditions. 3.3. Effect of the Nature of the Electrolyte on the Triple Phase Boundary Process: Oxidation of nBuFc in DCE. The identity of the transferring anion is of critical importance to the potential dependence and mechanism of the triple-phase

boundary oxidation process. Hydrophobic anions transfer more readily due to a greater affinity to the organic DCE phase (or a lower hydration energy) corresponding to a more negative standard transfer potential. The electrolyte anions studied were perchlorate (ClO4-), hexafluorophosphate (PF6-), tetraphenylborate (BPh4-), and phosphate buffer pH 1 solution. The mech-

Liquid/Liquid/Solid Triple-Phase Boundary Redox Systems anism for the hydrophobic anions (ClO4-, PF6-, and BPh4-) is confirmed to be the transfer into the DCE droplet as the nBuFc is oxidized (see Figure 1). The characteristic shift in the halfwave potential (Emid, obtained as the potential where half of the mass transport limited current is reached) is clearly shown in Figure 5A. Figure 5B shows a plot summarizing the experimental Emid values versus the standard potential of ion transfer.29,30 This plot shows that for the hydrophobic anions (ClO4-, PF6-, and BPh4-) there is a good linear correlation (see dashed line with slope 1.0). In the case of phosphate buffer pH 1 a different mechanism dominates on the basis of voltammograms shown in Figure 5C for conditions with and without ultrasound. In this case the phosphate anion is too hydrophilic and instead of it transferring into the DCE to balance the positive charge of the ferricenium cation, it is the ferricenium cation which is believed to transfer into the bulk aqueous phase. The current for this process is considerably lower (see Figure 5A) and the potential for the transfer deviates from the behavior observed for hydrophobic anions (see Figure 5B). Ferricenium is unstable in aqueous media and rapidly decomposed into iron(III) phosphate which is 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 for the decrease in the limiting current due to the deposit partially blocking the electrode surface. The peak feature at ca. 0.23 V vs SCE in Figure 5C corresponds to the iron(II/III) phosphate redox process.31 In the presence of ultrasound the process is not observed probably due to in situ removal of the deposit. Throughout this investigation a resistance effect has been apparent due to the absence of intentionally added supporting electrolyte in the organic phase. Figure 5D demonstrates the effect of adding 0.1 M NBu4PF6 into the organic phase on the oxidation 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 voltammetric response with supporting electrolyte in the organic phase is considerably less drawn out (less resistance) and the mass transport controlled plateau current is increased (a wider reaction zone from the triple-phase boundary into the droplet is formed). However, avoiding the use of intentionally added electrolyte in the organic phase has important advantages (lower cost, easier work up and purification of products, etc.). Data presented in this work suggest that electrolysis in surface mobilized DCE without intentionally added electrolyte is possible. 4. Conclusions It has been shown that triple-phase boundary processes for droplets of organic DCE on a glassy carbon electrode surface can be enhanced by power ultrasound. Surface mobilization of droplets can be achieved without emulsification. This effect allows an extended and dynamic triple-phase boundary reaction zone to be formed with the added benefit of agitation within the organic phase inducing improved transport during interfacial ion transfer. The resulting procedure will allow a wider range

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15633 of electrosynthetic or ion-extraction reactions to be carried out under 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.; Schro¨der, U.; Gulaboski, R. Electrochemistry of immobilized 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. B 2001, 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) Schro¨der, 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; p 217. (31) McKenzie, K. J.; Marken, F. Pure Appl. Chem. 2001, 73, 1885.

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