Characterization and Application of a Mercury Hemisphere Microjet

In particular, the use of the Hg hemisphere MJE for stripping analysis is shown to greatly increase the efficiency of the preconcentration step, compa...
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Anal. Chem. 1997, 69, 5045-5051

Characterization and Application of a Mercury Hemisphere Microjet Electrode Julie V. Macpherson* and Patrick R. Unwin*

Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K.

The microjet electrode (MJE) is a hydrodynamic electrode in which a fine jet of solution impinges from a nozzle on an ultramicroelectrode (UME) under conditions of variable and high mass transfer rates. An assessment is made as to whether this methodologyshitherto employed with solid disk electrodes alonescan be used in conjunction with hemispherical mercury electrodes grown on Pt UMEs. Mass transfer imaging experiments, in which the transport-limited current at the Hg UME is monitored as a function of nozzle position, demonstrate that local mass transfer rates from the impinging jet are similar to those measured earlier at disk electrodes. When the electrode and nozzle are configured to produce the maximum mass transfer rates, the transport-limited current-flow rate characteristics, at low to intermediate flow velocities, are shown to be well-defined and predictable, by analogy to the rotating hemisphere electrode. At higher flow rates, the electrode becomes physically unstable and eventually detaches from the Pt UME. Within the physically stable region, mass transfer coefficients up to 0.2 cm s-1 are readily attainable, making the device attractive for both electroanalysis and kinetic applications. In particular, the use of the Hg hemisphere MJE for stripping analysis is shown to greatly increase the efficiency of the preconcentration step, compared to Hg UMEs in stationary solution or alternative flow configurations. The advent of ultramicroelectrodes (UMEs)selectrodes with at least one dimension in the micrometer or submicrometer rangeshas had a major impact on both electroanalysis and the voltammetric investigation of electrode kinetics and coupled chemical reactions.1 In particular, since the steady-state diffusionlimited current density at a UME varies reciprocally with the characteristic electrode dimension,2 very high mass transfer rates can be attained by using small electrodes.3,4 Additionally, the reduced capacitative and resistive effects at UMEs allow large perturbation methods such as cyclic voltammetry and potential step chronoamperometry to be extended to the submicrosecond time domain.5 (1) For reviews, see, for example: (a) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 267. (b) Heinze, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268. (c) Montenegro, M. I.; Queiro´s, M. A.; Daschbach, J. L. Eds. Microelectrodes: Theory and Applications; NATO ASI Series E., Vol. 197; Kluwer: Dordrecht, the Netherlands, 1991. (2) Saito, Y. Rev. Polarogr. 1968, 15, 177. (3) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118. (4) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, A.; Lewis, A. Anal. Chem. 1997, 69, 1627. S0003-2700(97)00638-0 CCC: $14.00

© 1997 American Chemical Society

It has been recently demonstrated that the employment of UMEs in convective systems allows enhanced and variable mass transfer rates to be achieved, typically under steady-state conditions, provided that sufficiently high convective rates can be generated.6-11 Previous work from our group introduced a microjet electrode (MJE) in which a high-velocity jet of solution was fired through a fine nozzle positioned directly over a disk UME.6-8 The geometry was such that the electrode diameter was much less than the diameter of the nozzle, and consequently the device represented a radical miniaturization of the wall tube electrode (WTE). In addition to the MJE being characterized by well-defined and variable mass transfer rates, it was demonstrated that mass transfer to a 25 µm diameter Pt disk UME could be enhanced by over 2 orders of magnitude, with mean solution velocities up to 50 m s-1.7 Alternatively, Compton and co-workers have shown that enhanced mass transfer rates can be achieved at a microband electrode by pumping solution at high speed over an electrode in a channel geometry.9-11 This technique has been used to measure fast heterogeneous electron transfer kinetics10 and homogeneous chemical reactions coupled to electron transfer events.9,11 In a further approach, Birkin and Silva-Martinez12,13 observed high rates of mass transfer as a result of the ultrasonic irradiation of a solution in the direct vicinity of a UME. In this case, high-velocity microjets were formed due to the collapse of cavitation bubbles, which were detected individually at the electrode surface. At present, the above convective-diffusion techniques are limited largely to UMEs constructed from solid materials, such as platinum, gold, and carbon. Some ultrasonically enhanced mass transfer experiments have been carried out with mercury film electrodes14 and mercury-modified electrodes15 but only on the (5) (a) Howell, J. O.; Wightman, R. M. Anal. Chem. 1984, 56, 524. (b) Howell, J. O.; Kuhr, W. G.; Ensman, R. E.; Wightman, R. M. J. Electroanal. Chem. Interfacial Electrochem. 1986, 209, 77. (c) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; Save´ant, J. M. J. Electroanal. Chem. Interfacial Electrochem. 1988, 200, 371. (d) Andrieux, C. P.; Hapiot, P.; Save´ant, J. M. J. Phys. Chem. 1988, 248, 447. (e) Wipf, D. O.; Wightman, R. M. J. Phys. Chem. 1989, 93, 4286. (f) Hseuh, C. C.; Braijter-Toth, A. Anal. Chem. 1993, 65, 1570. (6) Macpherson, J. V.; Marcar, S.; Unwin, P. R. Anal. Chem. 1994, 66, 2175. (7) Macpherson, J. V.; Beeston, M. A.; Unwin, P. R. J. Chem. Soc., Faraday Trans. 1995, 91, 899. (8) Martin, R. D.; Unwin, P. R. J. Electroanal. Chem. 1995, 397, 325. (9) Rees. N. V.; Dryfe, R. W.; Cooper, J. A.; Coles, B. A.; Compton, R. G.; Davies, S.; McCarthy, T. D. J. Phys. Chem. 1995, 99, 7096. (10) Compton, R. G.; Fisher, A. C.; Wellington, R. G.; Dobson, P. J.; Leigh, P. A. J. Phys. Chem. 1995, 99, 14813. (11) Coles, B. A.; Dryfe, R. W.; Rees, N. V.; Compton, R. G.; Davies, S. G.; McCarthy, T. J. Electroanal. Chem. 1996, 411, 121. (12) Birkin, P. R.; Silva-Martinez, S. J. Chem. Soc., Chem. Commun. 1995, 1807. (13) Birkin, P. R.; Silva-Martinez, S. J. Electroanal. Chem. 1996, 416, 127. (14) Marken, F.; Rebbitt, T. O.; Booth, J.; Compton, R. G. Electroanalysis 1997, 9, 19.

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macroscale. Additionally, Sluyters and co-workers have employed a dropping mercury microelectrode to investigate heterogeneous electron transfer kinetics.16 An important advantage of mercury electrodes over conventional solid electrodes is the extended cathodic window in aqueous solution.17 Additionally, anodic stripping voltammetry (ASV) at mercury electrodes represents a powerful technique for trace heavy metal analysis.18,19 The sensitivity of ASV results from the ability to cathodically preconcentrate the metal ion of interest in mercury before voltammetrically stripping the metal back into solution in an anodic detection step. Mercury UMEs have received considerable attention for use with ASV,20-25 due to the high and reproducible diffusion rates associated with electrodes of this size. This attribute is responsible for increasing the efficiency of the preconcentration step compared to that of many macroelectrode methods, thus allowing measurements to be made with greater precision. In an effort to further increase detection limits, improve characteristic signalto-noise ratios, and minimize sample consumption, Hg UMEs have been employed in flowing solutions26-29 (both continuous and pulsed), typically with low volume flow rates. However, under the conditions reported hitherto, a low dependence of stripping peak current on solution flow rate has usually been observed,27,29 suggesting that mass transfer in these convective systems is still dominated by diffusion.30-32 Given the variable and high mass transfer rates demonstrated for the MJE with Pt disk UMEs, the general purpose of this paper is to extend MJE methodology to Hg UMEs with the following specific aims. First, mass transfer to a hemispherical Hg UME in a WTE arrangement will be characterized, and the stability of a liquid electrode under impinging high jet velocities will be investigated. Second, MJE methodology will be used in conjunction with ASV to demonstrate that mass transfer during the preconcentration step can be significantly enhanced compared to that of a Hg UME in quiescent solution. Finally, mass transfer imaging experiments are reported which provide a unique insight into the spatial variation of local mass transfer rates for an impinging jet at a hemispherical UME. These studies yield (15) Matysik, F.-M.; Matysik, S.; Brett, A. M. O.; Brett, C. M. A. Anal. Chem. 1997, 69, 1651. (16) (a) Baars, A.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1990, 283, 99. (b) Baars, A.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1994, 364, 189. (c) Baars, A.; Bijl, F. J. C.; SluytersRehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1996, 404, 149. (d) van Venrooji, T. G. J.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1996, 414, 61. (17) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 413. (18) Wang, J. Stripping Analysis: Principles, Instrumentation and Applications; VCH: Deerfield Beach, FL, 1985. (19) Barendrecht E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1967; Vol. 2, pp 53-109. (20) Wehmeyer, K. R.; Wightman, R. M. Anal. Chem. 1985, 57, 1989. (21) Golas, G.; Osteryoung, J. Anal. Chim. Acta 1986, 181, 1. (22) Golas, G.; Osteryoung, J. Anal. Chim. Acta 1986, 181, 211. (23) Baranski, A. S.; Quon, H. Anal. Chem. 1986, 58, 407. (24) Baranski, A. S. Anal. Chem. 1987, 59, 662. (25) Bond, A. M. Analyst 1994, 119, R1. (26) Tay, E. B.-T.; Khoo, S.-B.; Loh, S.-W. Analyst 1989, 114, 1039. (27) Matysik, F.-M.; Werner, G. Analyst 1993, 118, 1523. (28) Economou, A.; Fielden, P. R. Analyst 1994, 121, 1903. (29) Zhou, F.; Aronson, J. T.; Ruegnitz, M. Anal. Chem. 1997, 69, 728. (30) Matysik, F.-M.; Emons, H. Electroanalysis 1992, 4, 501. (31) Caudill, W. L.; Howell, J. O.; Wightman, R. M. Anal. Chem. 1982, 54, 2532. (32) Bixler, J. W.; Bond, A. M. Anal. Chem. 1986, 58, 2859.

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Figure 1. Video microscopy image of the mercury hemisphere microjet electrode. The electrode is located in the maximum mass transport region of the submerged impinging jet of solution.

particularly valuable information on the general positioning requirements of impinging jet amperometric detectors. EXPERIMENTAL SECTION Reagents and Chemicals. All solutions were prepared from Milli-Q (Millipore Corp.) reagent water. The mercury plating solution was prepared as described elsewhere33 and contained 0.01 mol dm-3 mercurous(I) nitrate (Fisons, A.R. grade), 0.1 mol dm-3 nitric acid (Aldrich, 0.996 N volumetric standard), and 0.5 mol dm-3 potassium nitrate (Fisons, A.R. grade). For mass transfer imaging experiments, the solution contained 0.005 mol dm-3 hexaamineruthenium(III) chloride (Ru(NH3)63+; Strem, 99%) and 0.5 mol dm-3 potassium nitrate. For mass transfer characterization experiments, Ru(NH3)63+ was used at a concentration of 0.001 mol dm-3 in 0.1 mol dm-3 potassium nitrate solutions. Stock solutions of 0.01 mol dm-3 Pb(NO3)2 (Aldrich, 99.999%) were prepared and diluted on the day of use, as appropriate, and used with 0.01 mol dm-3 potassium nitrate (Aldrich, 99.99%), which served as a background electrolyte. MJE Apparatus. The main features of the MJE were as described previously7 with some modifications. As shown in Figure 1, the MJE comprised a nozzle with an internal diameter, dn, in the region 80-120 µm. Through a connection to a threeaxis positioner, the nozzle could be moved with micrometer to submicrometer resolution. For all experiments, the height, H, of the nozzle from the surface of the 12.5 µm radius UME was set at 300 µm. Although this distance is less than the theoretical value required to establish the full hydrodynamic profile for a submerged jet impinging on a flat surface,34 we,6 along with other workers,34 have found only a very weak dependence of the mass transfer characteristics of an impinging jet on the nozzle-electrode separation for 1 < H/dn < 7. The MJE cell comprised a fully detachable Teflon base, a cylindrical glass body, and a Teflon lid, with a total volume of 25 cm3. The glass cell body contained an outlet pipe to prevent solution overflow in the cell and an optical window (30 mm diameter) so that video microscopy could be used to monitor the stability of the Hg UME during experiments. The video micro(33) Beeston, M. A. Ph.D. Thesis, University of Warwick, 1996. (34) Chin, D. T.; Tsang, C. H. J. Electrochem. Soc. 1978, 125, 1461 and references therein.

scope consisted of a zoom microscope with a CCD camera attachment which offered a maximum on-screen resolution of 2.2 µm/pixel. The camera was attached to a video recorder and video capture card (Model Win-TV Celebrity, Hauppage, NY), which allowed images to be transferred to a PC. The UME was positioned through the center of the cell base such that it sat well below the level of the solution outlet, ensuring that the hydrodynamics in the vicinity of the electrode were not affected by solution outflow. A tap in the cell base facilitated the ready change of solution in the MJE cell, when necessary. Capillary nozzles were fabricated by drawing borosilicate glass capillaries (Clark Electromedical, Reading, U.K.; 2.0 mm o.d., 1.6 mm i.d.) to a fine point with the aid of a Narishighe (Tokyo, Japan) PB7 micropipet puller. Flow of electrolyte (at rates in the range 1.5 × 10-3-6.7 × -2 10 cm3 s-1) through the nozzle of the MJE was achieved with either a simple gravity feed system or a Gilson (Villiers-Le-Bel, France) Model 305 HPLC pump equipped with a 25 W Ti pump head and a Model 806 manometric module. In the former case, the dependence of the electrolyte volume flow rate on reservoir height was calibrated by measuring the mass of electrolyte solution flowing through the nozzle in a fixed period of time (and then converting to the corresponding volume). All solutions were purged with argon (BOC, 99.99%) in order to remove oxygen from the microjet system during electrochemical measurements. The ingress of oxygen through the PTFE tubing of the flow system (Anachem, 1.5 mm bore) was prevented by the use of a PVC jacket through which a continuous stream of argon was passed. Instrumentation. Two types of positioning instrumentation were employed. Mass transfer characterization and ASV measurements were made with a MJE in which the position of the nozzle relative to the UME was controlled with micrometer resolution by mounting the nozzle on a Newport Corp. (Santa Clara, CA) Model 461 x, y, z stage. For mass transfer imaging experiments, a modified scanning electrochemical microscope (SECM)7 was used to scan the nozzle over the UME, with the solution flowing, in a series of unidirectional scans. The jet nozzle was typically scanned at a velocity of 25 µm s-1. Images comprised ∼21 lines, with data acquired at 101 equally spaced points per line, and the datum at each position was the mean of 100 current readings. With both systems, the positioning stages were mounted on a vibrationally isolated bench. Shielding of the cell, using a home-built Faraday cage, was only necessary for experiments employing the Hg UME in quiescent solution. Current-potential characteristics were recorded with either an Eco-Chemie (Utrecht, Holland) Autolab Electrochemical Workstation, incorporating a preamplifier (model ECD) for low current measurements, or a home-built current follower (gains 10-5-10-9 A V-1) used in conjunction with a purpose-made triangular wave/ pulse generator (Colburn Electronics, Coventry, U.K.). With the latter system, current-voltage data were either acquired with a National Instruments (Austin, TX) Lab-PC board or recorded directly on an x-y recorder (Model PL3, Lloyd Instruments, Southampton, U.K.). All voltammetric measurements were made in a two-electrode arrangement with an Ag wire serving as a quasireference electrode (AgQRE) and a Hg UME serving as the working electrode. Analysis of ASV stripping peaks was made using software supplied by Eco-Chemie. Procedure. All measurements were made at 25 ( 1 °C. Prior to both the mass transfer characterization and imaging experi-

ments, mercury was deposited on a Pt UME in situ in the MJE cell, through the diffusion-controlled reduction of mercurous ions. Under steady-state conditions, the diffusion-limited current at a hemispherical UME is 1.57 times that at a disk UME of the same radius. Oldham et al.35 have shown that steady-state conditions do not strictly apply to long-time mercury deposition at electrodes of the size employed in this investigation. We found, empirically, that by holding the potential of the electrode at -0.225 V for approximately 200 s, current increases between 1.5 and 1.6 of the initial transport-limited current produced effectively hemispherical electrodes. The characteristic radii of these Hg UMEs were verified to be 13.0 ( 0.5 µm by recording the steady-state diffusionlimited current for reduction of 0.001 mol dm-3 Ru(NH3)63+ in 0.1 mol dm-3 KNO3. The diffusion coefficient of Ru(NH3)63+ was determined to be 8.8 × 10-6 cm2 s-1 from the limiting steadystate current recorded at a 25 µm diameter Pt UME in this solution. After mercury deposition, the cell was drained of mercurous nitrate solution, rinsed with water, and refilled with the appropriate mediator solution. With the jet turned on, the nozzle was aligned in both the x and y axes, such that the current for the transportlimited reduction of the mediator was at its maximum value. Video microscopy greatly simplified this task and was also used to set the distance between the nozzle and the electrode surface. With the nozzle and UME aligned in the configuration of maximum transport-limited current, mass transfer characterization experiments involved measuring the limiting current as a function of solution flow rate. The procedure for imaging measurements involved setting the position of maximum mass transfer-limited current to the center of the scan image (coordinates 0 µm x scan, 0 µm y step). For all ASV measurements with flow, the gravity feed system was employed, and the jet was turned off just prior to recording the stripping peak. Retardation of the flow was instantaneous. RESULTS AND DISCUSSION Mass Transfer Imaging. A typical image for the transportlimited reduction of Ru(NH3)63+ at a Hg UME, held at -0.80 V vs AgQRE, as a function of nozzle displacement is shown in Figure 2. The data were obtained with a nozzle of diameter dn ) 85 µm. A moderate solution flow rate of 0.0110 cm3 s-1 was employed in the imaging experiment in order to minimize solution consumption during the course of the measurement. The transport-limited current, iMJE, has been normalized with respect to the diffusionlimited current in stationary solution, iHg UME ) 32.8 nA. Figure 2 shows clearly a ring of maximum current which surrounds a central circular area where the transport-limited current is slightly lower. Increasing the radial separation between the nozzle and the Hg UME outside this region causes the current to decrease. These characteristics for the Hg UME are akin to those found previously for a 25 µm diameter Pt UME, operating in a MJE environment,7 with similar volume flow rates. For the latter system, the spatial variation in mass transfer rate was qualitatively consistent with the hydrodynamics of a submerged impinging jet close to the planar surface with which it collides.34 The similarity of the results in Figure 2 and those determined earlier with a disk UME7 suggests that the hemispherical Hg UME probe does not significantly perturb the interfacial flow profile of (35) Colyer, C. L.; Luscombe, D.; Oldham, K. B. J. Electroanal. Chem. 1990, 283, 379.

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with which fixed nozzle/electrode detectors of this size need to be fabricated if they are to operate with high mass transfer rates. Mass Transport Characterization. The factors controlling the transport-limited current for a WTE are well established34 and apply equally well to a Pt disk MJE.6,7 It has also been shown that an analogy can be drawn between the mass transport characteristics of the uniformly accessible rotating disk electrode (RDE) and the WTE.37 Both approaches yield similar equations for the transport-limited current, except that a very weak dependence of the current on the nozzle-electrode separation is predicted by considering the full hydrodynamics of the WTE.34 A theoretical treatment for the WTE with a hemispherical electrode geometry is not available. In the light of earlier work,37 an initial starting point in characterizing mass transfer to the mercury hemisphere MJE is to ascertain whether an analogy can be drawn between this system and the rotating hemispherical electrode (RHE).38 The rate of mass transfer to the uniformly accessible RHE is given by38

Sh ) βRe1/2Sc1/3

(1)

where β is a constant with a value of 1.19. Sh, Re, and Sc are the Sherwood, Reynolds, and Schmidt numbers, respectively. For the MJE system, these are given by Figure 2. Variation of the transport-limited current at the Hg hemisphere MJE with the position of the nozzle (dn ) 85 µm) in the x,y plane, located at a distance of 300 µm from the UME surface. Data are presented as (a) a surface plot and (b) a spectral plot, to emphasize the existence of a minimum in the central circular region of maximum transport-limited current. A solution flow rate of 0.011 cm3 s-1 was employed. The gray scale key applies to both images a and b.

the impinging jet. At the center of the impinging jet, flow toward the surface is purely axial, but, upon moving outward in the radial direction, there is an increasingly important radial contribution to the velocity profile close to the surface.36 When the UME is moved in the radial direction away from the center of the jet, an enhancement in the mass transport-limited current is thus expected, as observed. As the UME is moved farther out in the radial direction, beyond the hypothetical stagnation region, the rate of convective mass transport to the UME is predicted to decrease. This effect is also observed for radial distances of about one nozzle radius from the center of the jet. It follows from Figure 2 that, for true alignment of the jet nozzle and Hg UME, their positions should be adjusted until the transport-limited current is at a minimum relative to all possible movements in the x and y directions. However, previous work with the Pt disk MJE demonstrated that, with the nozzle slightly offset in the region of maximum current, the mass transport characteristics were close to those predicted for a WTE.7 It follows from both these latter studies and the mass transfer profile displayed in Figure 2 that it is advisable to operate the Hg hemisphere MJE in the region of maximum transport-limited current to enhance the signal. This is also the simplest condition to achieve when the nozzle is controlled with manual micropositioners. The result in Figure 2 also illustrates the high precision (36) Schlichting, H. Boundary Layer Theory; McGraw-Hill: New York, 1960; pp 78-83.

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Sh ) kTdn/D

(2)

h /ν Re ) dnU

(3)

Sc ) ν/D

(4)

where kT is the mass transfer coefficient (cm s-1), D is the diffusion coefficient of the electroactive species, U h is the mean solution velocity (cm s-1), and ν is the kinematic viscosity of the solution. The transport-limited current at the MJE can be expressed as

iMJE ) kTnFAc*

(5)

where n is the number of electrons transferred per redox event, F is Faraday’s constant, A is the area of the electrode, and c* is the bulk concentration of the electroactive species. The diffusionlimited current which flows at a hemispherical mercury UME is

iHg UME ) 2πnaFDc*

(6)

where a is the electrode radius. Drawing an analogy between the RHE and the mercury hemisphere MJE, it follows from eqs 1-6 that the enhancement in mass transfer to a hemispherical Hg UME in the WTE configuration is given by

iMJE/iHg UME ) βU h 1/2adn-1/2D-1/3ν-1/6

(7)

and, therefore, iMJE/iHg UME should be proportional to the square root of volume flow rate, Vf, where (37) Albery, J. W.; Bruckenstein, S. J. Electroanal. Chem. 1983, 144, 105. (38) (a) Chin, D. T. J. Electrochem. Soc. 1971, 118, 1434. (b) Chin, D. T. J. Electrochem. Soc. 1971, 118, 1764.

Vf ) π(dn/2)2U h

(8)

Over the wide range of volume flow rates investigated, a very good linear relationship between iMJE/iHg UME and Vf1/2 was observed for mean solution velocities between 0.7 and 3.5 m s-1. For example, for the reduction of 0.001 mol dm-3 Ru(NH3)63+ at a hemispherical Hg UME, with a jet characterized by dn ) 105 µm, analysis of the slope of the line yielded a value of β ) 1.25 (with a correlation coefficent of 0.9993), which is in very good agreement with that predicted above. The maximum enhancement in the steady-state current observed, under these conditions, at the Hg UME (iMJE/iHg UME ≈30) corresponds to kT ) 0.21 cm s-1, where D for Ru(NH3)63+ is defined above. To the best of our knowledge, this represents the highest mass transfer rate that has been achieved at a Hg electrode under steady-state conditions. For comparison, this is equivalent to steady-state mass transfer at a Hg UME with an effective radius of 0.42 µm. Although thin mercury film rotating disk electrodes have been fabricated,39 they would have to rotate at a frequency of 20 000 Hz to achieve an equivalent rate of mass transport. This value is at least 2 orders of magnitude greater than the maximum attainable rotation speed of commercially available devices. The mechanical stability of the electrode, when subjected to a submerged impinging jet of solution at higher velocities, was investigated further. Figure 3 shows data of iMJE/iHg UME vs Vf1/2 for the reduction of Ru(NH3)63+ at a hemispherical Hg UME, for dn ) 85 µm, over a wider range of volume flow rates, delivered with an HPLC pump. At volume flow rates less than 0.017 cm3 s-1, the iMJE/iHg UME vs Vf1/2 behavior is consistent with the predicted response for a submerged impinging jet. However, at solution velocities above this value, the current enhancements increase sharply with Vf1/2, deviating significantly from the simple predictions above. Video microscopy revealed corresponding changes in the geometry of the Hg UME under these conditions, as shown schematically in Figure 4. The apparent increase in electrode area and the likely associated local turbulent flow probably account for the dramatic current increases observed. At flow rates greater than 6.2 × 10-2 cm3 s-1, the mercury hemisphere was found to detach from the surface of the Pt electrode. The collective data from 10 experiments employing nozzle diameters in the range 80-120 µm and volume flow rates in the range 0.0015-0.083 cm3 s-1 demonstrated that the Hg hemisphere MJE, operated in the configuration of maximum transport-limited current, was stable physically, with well-defined transport-limited current characteristics for mean solution velocities less than 3.5 m s-1. Preliminary experiments indicated, that when the Hg UME was located directly in the center of the impinging jet, the electrode was stable to slightly higher flow rates, but the transportlimited currents were lower than the predicted theory for all solution velocities employed. Although, at present, there is no model for mass transfer in the vicinity of an unstable Hg UME, the impressive enhancements in the transport-limited current (iMJE/iUME > 100) achieved under these conditions at relatively low volume flow rates represents a potentially interesting method for significantly increasing mass transfer to a Hg UME. (39) (a) Bruckenstein, S.; Nagai, T. Anal. Chem. 1961, 33, 1201. (b) Ramaley, L.; Brubaker, R. L.; Enke, C. G. Anal. Chem. 1963, 35, 1088. (c) Daly, P. J.; Page, D. J.; Compton, R. G. Anal. Chem. 1983, 55, 1191.

Figure 3. Transport-limited current-flow rate data for the reduction of 0.001 mol dm-3 Ru(NH3)63+ at a Hg hemisphere MJE in the volume flow rate range 0.008-0.058 cm3 s-1. The MJE system was characterized by dn ) 85 µm and H ) 300 µm.

Figure 4. Schematic showing the shape of the mercury hemisphere electrode in the MJE arrangement for mean solution velocities less than 3.5 m s-1 and (b) mean solution velocities greater than 3.5 m s-1. (a) The shape of the mercury surface remains relatively unaffected by solution flow, and a fit of the transport-limited current-flow rate data to theory for the analogous RHE is observed. (b) The Hg surface becomes distorted. Under these conditions the current-flow rate data do not fit the RHE theory but yield much larger current values.

ASV Using the Hg Hemisphere MJE. Figure 5 shows typical anodic stripping voltammograms obtained for the determination of 4.6 × 10-7 mol dm-3 Pb2+ at a Hg hemisphere MJE using preconcentration times of 60 (smallest stripping peak), 90, 120, and 150 s (largest stripping peak), respectively. The electrode was held at a potential of -1.0 V during the deposition step and, for all stripping measurements, was scanned from -1.0 to -0.3 V vs AgQRE at a scan rate, v, of 0.2 V s-1. The flow system was characterized by dn ) 106 µm and Vf ) 0.0185 cm3 s-1, which was well within the stable operating regime for the Hg hemisphere MJE. To illustrate the tremendous increases in the current signal that can be achieved using a Hg hemisphere MJE, Figure 5 also shows a typical stripping peak obtained at the Hg UME in the absence of flow, where diffusion is the only form of mass transport. A preconcentration time of 60 s was used, while the other stripping Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

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Figure 6. (i) Log ip vs log v (left-hand scale, (O)) and (ii) log Qflow vs log v (right-hand scale, (0)) data taken from anodic stripping voltammograms for the determination of 4.6 × 10-7 mol dm-3 Pb2+ at a Hg hemisphere MJE, characterized by a solution flow rate of 0.0185 cm3 s-1, dn ) 106 µm, and H ) 300 µm. In all cases, a deposition time of 60 s was used.

Figure 5. (i) Anodic stripping voltammograms of 4.6 × 10-7 mol dm-3 Pb2+ obtained at a Hg hemisphere MJE, subject to deposition times of (a) 60, (b) 90, (c) 120, and (d) 150 s at a solution flow rate of 0.0185 cm3 s-1, dn ) 106 µm, and H ) 300 µm. (ii) Anodic stripping voltammogram of 4.6 × 10-7 mol dm-3 Pb2+ obtained at a Hg hemisphere UME, subject to a deposition time of 60 s under no flow conditions. All measurements during the stripping step were made in stationary solution at a potential scan rate of 0.2 V s-1.

parameters were as above. A comparison of the magnitude of the stripping responses obtained with and without convective preconcentration clearly indicates the impressive enhancements in signal that can be achieved in the former case. Under both flow and quiescent deposition conditions, good correlation was found between the peak current, ip, and the deposition time, tdep. Linear regressions of ip vs tdep plots yielded a slope of 36.6 nA/min (Vf ) 0.0185 cm3 s-1) and 1.60 nA/min in the absence of flow. The increase in the magnitude of the stripping peak signals, measured as charge, Q, compared with those obtained in quiescent solution, i.e. Qflow/Qno flow, was 23.0 ( 1.0. This value is in very good agreement with the theoretically predicted value of 22.6 from eq 7, with40 DPb2+ ) 1.0 × 10-5 cm2 s-1 and a ) 13.5 µm. Figure 6 shows the log ip vs log v characteristics recorded under the same solution and flow conditions as employed for the data in Figure 5. For all measurements, the deposition time was 60 s. The slope of the log ip vs log v plot in Figure 6 is 0.96, and the charge passed during stripping is seen to be relatively independent of scan rate (as represented by the log Qflow vs log v data). The former result, in particular, suggests that removal of the Pb from the mercury during the anodic dissolution step occurs exhaustively due to the efficient radial diffusion within the Hg UME. This behavior is predicted theoretically19,41 and has been observed routinely for Hg UMEs in quiescent solution at low scan (40) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969; p 220. (41) (a) Roe, D. K.; Toni, J. E. A. Anal. Chem. 1965, 37, 1503. (b) Vries, W. T. J. Electroanal. Chem. 1965, 9, 448. (c) De Vries, W. T.; Van Dalen, E. J. Electroanal. Chem. 1967, 14, 315.

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rates.20,23,26 Given that flow is stopped just prior to the anodic stripping step, the results displayed in Figure 6 agree well with the predicted responses. The major source of error in ASV is considered to be the irreproducibility of mass transfer.19 Many earlier techniques suffered from ill-defined hydrodynamics,19 while more recent work with impinging jet detectors lacks the positioning capabilities required to accurately locate the substrate electrode in a hydrodynamically defined flow regime.26,31,32,42,43 In contrast, the Hg hemisphere MJE is characterized by well-defined and highly reproducible mass transport, making it suitable for analytical and kinetic applications. CONCLUSIONS Results presented in this paper demonstrate that high mass transfer rates can be obtained at a Hg hemisphere UME operating in a MJE (WTE) arrangement. Mass transfer imaging has provided useful information on the positioning requirements of the MJE and has shown that the Hg UME does not significantly perturb the mass transfer characteristics of the impinging jet at low to moderate flow rates. With the Hg hemisphere MJE located at a position of maximum transport-limited current, the resulting mass transfer characteristics are well-defined and correlate well with the predictions for the analogous rotating hemispherical electrode at mean solution velocities less than 3.5 m s-1. For solution velocities above this value, the Hg hemisphere UME is prone to instability and distortion before eventually detaching from the electrode surface. Nonetheless, the ability to deliver reproducible steady-state mass transfer coefficients up to 0.2 cm s-1 at low volume flow rates makes the device suitable for electroanalysis and electrochemical kinetic applications. The methodology has proven extremely useful in conjunction with ASV. The high enhancements in the mass transfer rate to the Hg UME significantly increase the efficiency of the preconcentration step compared to that of conventional UMEs in (42) Soucaze-Guillous, B.; Kutner, W. Electroanalysis 1997, 9, 32. (43) Bjo ¨refors, F.; Nyholm, L. Anal. Chim. Acta 1996, 325, 11.

quiescent solution and those employed in hydrodynamic systems subject to low flow rates.

Received for review June 18, 1997. Accepted September 23, 1997.X AC9706382

ACKNOWLEDGMENT We thank the EPSRC for support (GR/K97011).

X

Abstract published in Advance ACS Abstracts, November 1, 1997.

Analytical Chemistry, Vol. 69, No. 24, December 15, 1997

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