Anal. Chem. 2005, 77, 1928-1930
Hydrodynamic Electrochemistry: Design for a High-Speed Rotating Disk Electrode Craig E. Banks,† Andrew O. Simm,† Roger Bowler,† Keith Dawes,‡ and Richard G. Compton*,†
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom, and Windsor Scientific Ltd., 264 Argyll Avenue, Slough Trading Estate, Slough, Berkshire, SL1 4HE, United Kingdom
We report a novel gas-driven high-speed rotating disk electrode (HSRDE). The HSRDE when immersed in an aqueous solution rotates at ∼650 Hz, generating laminar flow resulting in a diffusion layer, under steady-state conditions, of thickness ∼2 µm. The use of high-pressure gas to drive the rotator offers significant improvement in electrical noise as compared to conventional mechanically driven devices. The electroanalytical utility of the HSRDE was exemplified by the anodic stripping voltammetry of arsenic(III) at a gold working electrode. The charge under the arsenic stripping peak was found to increase by more than 1 order of magnitude under the enhanced mass transport regime at the HSRDE in comparison to that seen under quiescent conditions. Hydrodynamic methods are widely applied within electrochemistry1 and electroanalysis so that rotating disks,2,3 channels,4-6 walljets, and tubes7,8 are familiar electroanalytical tools. More recently, the use of insonation9 or the application of microwave radiation10,11 has been used to promote mass transport in the vicinity of the solid electrode to enhance analytical performance. In particular, the extreme increases in rates of mass transport associated with the latter two approaches have enabled macroelectrodes to be conferred with the dynamical properties of microelectrodes; that is to say, a diffusion layer of the order of micrometers can be routinely employed. While the benefits of insonation or microwave heating are well documented, the use of “traditional” electrodes such as the rotating disk is attractive because of the relative ease of application. Accordingly, in the paper we describe a gas-driven high-speed rotating disk electrode (HSRDE) rotating, which when submerged * To whom correspondence should be addressed. E-mail: Richard.Compton@ chemistry.ox.ac.uk. Tel: 01865 275413: Fax: 01865 275410. † Oxford University. ‡ Windsor Scientific Ltd. (1) Alden, J. A.; Hakoura, S.; Compton, R. G. Anal. Chem. 1999, 71, 806. (2) Riddiford, A. C. Adv. Electrochem. Eng. 1966, 4, 47. (3) Pleskov, Y. V.; Filinovskii, V. Y.; Editors, S. Studies in Soviet Science: The Rotating Disk Electrode.; Consultants Bureau: New York, 1976. (4) Cooper, J. A.; Compton, R. G. Electroanalysis 1998, 10, 141. (5) Compton, R. G.; Dryfe, R. A. W. Prog, React. Kinet. 1995, 20, 245. (6) Lee, H. J.; Fermin, D. J.; Corn, R. M.; Girault, H. H. Electrochem. Commun. 1999, 1, 190. (7) Macpherson, J. V.; Marcar, S.; Unwin, P. R. Anal. Chem. 1994, 66, 2175. (8) Gunasingham, H.; Fleet, B. Anal. Chem. 1983, 55, 1409. (9) Banks, C. E.; Compton, R. G. Analyst 2004, 129, 678. (10) Tsai, Y.-C.; Coles, B. A.; Compton, R. G.; Marken, F. J. Am. Chem. Soc. 2002, 124, 9784. (11) Compton, R. G.; Coles, B. A.; Marken, F. Chem. Commun. 1998, 23, 2595.
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in water rotates at 656 Hz and which generates a laminar flow. The mass-transfer coefficient is quantified and found to correspond to a diffusion layer thickness of ∼2 µm. The benefit of such a design as compared to conventional mechanical devices is noted in respect of the much high rotation speeds that are achievable and the reduction of electrochemical noise. Finally, proof-ofconcept of the use of the HSRDE in electroanalysis is exemplified by anodic stripping voltammetry of arsenic(III) at a gold electrode where the sensitivity is improved by more than 1 order of magnitude as compared to analogous measurements under quiescent conditions. EXPERIMENTAL SECTION All chemicals used were of analytical grade and used as received without any further purification. These were potassium ferrocyanide, (99+%, Aldrich) sodium (meta) arsenite(III) oxide, (toxic, handle with care, Fluka, +99%), and nitric acid (Aldrich, 70%, double distilled PPB grade with any trace metal impurities no more than parts per trillion as determined by ICPMS). All solutions were prepared with deionized water of resistivity not less than 18.2 MΩ cm (Vivendi Water Systems). Voltammetric measurements were carried out using a µAutolab II (ECO-Chemie) potentiostat. All measurements were conducted using a three-electrode configuration. A gold electrode (0.075-cm diameter, area 0.004 41 cm2) was fabricated in-house by sealing gold wire into PTFE. The counter electrode was a bright platinum wire, with a saturated calomel electrode (Radiometer, Copenhagen, Denmark) completing the circuit. The working electrode was polished with decreasing alumina sizes on a soft lapping pad. The high-speed rotating disk main casing was obtained from W&H (UK Limited, Hertfordshire, U.K.). The manufacturers quote a rotation speed of 5800 Hz in air without any shaft connected to the main compartment. A schematic diagram of the high-speed rotating disk is depicted in Figure 1. The distance from the base of the working electrode to the top is 6 cm, and the length of the body is 7 cm. The HSRDE operates by passing nitrogen (BOC, 99%) through the HSRDE main compartment (see Figure 1) at a constant feed pressure of 1 bar of nitrogen, which causes the vaned wheel to rotate, which in turn makes the shaft of the working electrode spin. RESULTS AND DISCUSSION First, the voltammetric response of a gold macroelectrode in a 1.47 mM ferrocyanide solution containing 0.1 M KCl was explored using the HSRDE. Cyclic voltammograms were recorded 10.1021/ac048259d CCC: $30.25
© 2005 American Chemical Society Published on Web 02/09/2005
potential obtained from the hydrodynamic voltammogram (both shown in Figure 2). Digisim was used to simulate the hydrodynamic response using the rotating disk option for a range of heterogeneous rate constants (1 × 10-3 - 1.0 cm s-1) using the rotation speed found experimentally (see below) and a literature value13 for the diffusion coefficient. Next, the reversible limit for the cyclic voltammetric response was then used to calculate the difference in the simulated voltammetry with the changing halfwave potential from varying the ko value. Comparison of the differences between simulation and experimental provided an estimation of the steady-state current yielding a heterogeneous rate constant, ko, value of 0.01 cm s-1, which is in agreement with a reported value of 0.02 cm s-1 in 0.1 M aqueous KCl.14 The limiting current, IL and diffusion layer thickness, δ, at a rotating disk are described via the following equations:15 Figure 1. Schematic representation of the gas-driven high-speed rotating disk.
IL ) 0.62nFAD2/3ω1/2υ - 1/6Cbulk
(1)
δ ) 1.61D1/3ω-1/2υ1/6
(2)
where IL is the limiting current, F is the Faraday constant, n is the number of electrons transferred, D is the diffusion coefficient of the eletroactive species, ω is the rotation speed (rad s-1), υ is the kinematic viscosity, δ is the diffusion layer thickness, and Cbulk is the bulk concentration of the electroactive species. Using the limiting current from Figure 2, and a kinematic viscosity of 8.8 × 10-3 cm2 s-1 15 and a literature value for the diffusion coefficient, the rotation speed was found to correspond to 4120 ((200) rad s-1 or 656 (( 20) Hz. Using eq 2, the diffusion layer thickness was found to be 2.1 ((0.2) µm. Next, the Reynolds number was calculated to see if the highspeed rotating disk electrode operated under laminar or turbulent conditions, noting that eqs 1 and 2 presume laminar conditions. The Reynolds number was estimated via the following:
Re ) ωr2/υ Figure 2. Voltammetric oxidation of 1.47 mM ferrocyanide in 0.1 M KCl at a gold electrode in stationary (B) and rotating modes (A). Both recorded at 15 mV s-1 vs SCE. For clarity, only the forward scan is shown.
with the electrode stationary over a range of scan rates with a formal potential of 0.18 ((0.01) V, which compare well with literature reports of 0.17 V. 12Analysis of the peak currents using the Randles-Sevcˇik equation yielded a diffusion coefficient of 7 × 10-6 cm2 s-1. Next, cyclic voltammograms were recorded with the high-speed rotating disk “on” during the voltammetric scan. Shown in Figure 2 is the voltammetric response recorded at 15 mV s-1 corresponding to effectively steady-state voltammetry. Note that a characteristic hydrodynamic voltammogram is seen with a well-defined transport limited current producing a current of 1.79 ((0.02) × 10-5 A. The heterogeneous rate constant, ko, for the electrochemical oxidation of ferrocyanide was estimated by first measuring the experimental difference between the midpoint of the cyclic voltammetric response, recorded at 15 mV s-1 to the half-wave (12) Moore, R. R.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 2677.
(3)
The critical Reynolds number separating laminar from turbulent flow has been described as being Rec ) 3 × 105.16,17 Using eq 3, the Reynolds number was found to be 658 ((32), which is below the Rec number, confirming that the HSRDE operates under a laminar flow regime. Next the mass transport coefficient, k, was calculated for the high-speed rotating disk electrode, which is described via the following equation:
k ) D/δ ) 0.62D2/3ω1/2υ-1/6
(4)
Using eq 4, the mass transport coefficient was found to be 3.5 × 10-2 cm s-1. For comparison the recently reported hand-held (13) Adams, R. N. Electrochemistry at Solid Electrodes; Monographs in Electroanalytical Chemistry and Electrochemistry; Dekker: New York, 1969. (14) Montenegro, M. I. Applications of Microelectrode in Kinetics; Elsevier Science B.V.: Amsterdam, 1994. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications.; Wiley: New York, 1980. (16) Hanna, O. T.; Sandall, O. C.; Ruiz-Ibanez, G. Chem. Eng. Sci. 1988, 43, 1407. (17) Deslouis, C.; Tribollet, B.; Viet, L. Electrochim. Acta 1980, 25, 1027.
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for kinetic or mechanistic studies, as well as for enhancing electroanalysis. It is to this latter direction we next turn. We next consider the possible analytical sensing opportunities offered by the HSRDE. A solution containing 1 µM arsenic(III) in 0.1 M nitric acid was first prepared. Using linear sweep voltammetry with a deposition time of 60 s (under quiescent conditions) at - 0.5 V (vs SCE), a stripping signal corresponding to the oxidation of zerovalent arsenic to arsenic(III) is observed at ∼+0.16 V (see Figure 3, thick line), which is in agreement with literature values of 0.15 V on gold.18 Next this was repeated with the high-speed rotating disk “on” during the accumulation period (Figure 3, dotted line). The charge under the stripping peak was found to be 1.42 × 10-7 C under quiescent conditions while for the HSRDE “on” during the accumulation step the charge under the peak corresponds to 2.26 ((0.15) × 10-6 C; an increase of a factor of ∼16. Clearly there is a significant enhancement in the magnitude of the arsenic stripping peak using the HSRDE, where a small signal is transformed to a large and easily quantifiable signature. Figure 3. Linear sweep voltammograms of 1 µM arsenic(III) in 0.1 M nitric acid solution obtained under quiescent conditions (thick line) and rotating mode (dotted line) during the accumulation. Parameters: - 0.5 V for 60 s followed by a potential sweep at 50 mV s-1 (vs SCE).
infrasonotrode18 shows a value of 0.49 × 10-2 cm s-1 with a diffusion layer thickness of 15.2 µm recorded using ferricyanide. In contrast, power ultrasound applied from a sonic horn in a face on arrangement produces a mass transport coefficient of 0.13 cm s-1 with the smallest corresponding diffusion layer ever recorded under steady-state conditions at a macroelectrode (0.7 µm, Ru(NH3)63+).19 It is clear that the HSRDE is competitive with insonation as a means of achieving high rates of mass transport (18) Simm, A. O.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 5051. (19) Marken, F.; Akkermans, R. P.; Compton, R. G. J. Electroanal. Chem. 1996, 415, 55. (20) Albery, W. J.; Hitchman, M. L. Ring-Disk Electrodes; Oxford Science Research Papers; Oxford University Press: Fair Lawn, NJ, 1971.
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CONCLUSIONS The high-speed rotating disk electrode offers a renaissance in the application of such generic types of electrodes in electroanalysis with very thin diffusion layers at the micrometer scale being readily obtained under sustained steady-state conditions. Moreover, is it evident from Figure 2 that the use of gas to drive the HSRDE produces an output much less noisy than alterative mechanical constructions.3,20 Work is in progress concerning the development of a variable-speed analogue. ACKNOWLEDGMENT C.E.B. thanks Windsor Scientific for additional financial support.
Received for review November 24, 2004. Accepted January 6, 2005. AC048259D
