Anal. Chem. 2000, 72, 936-942
Voltammetric and Reference Microelectrodes with Integrated Microchannels for Flow through Microvoltammetry. 1. The Microcell O. C. Keller and J. Buffle*
CABE, Department of Inorganic, Analytical and Applied Chemistry, Sciences II, Geneva University, 30 Quai E. Ansermet, 1211 Geneva 4, Switzerland
This paper describes the construction of 2 microsensor units for on-line voltammetric detection inside a cylindrical microcell (a working microsensor unit and a reference and auxiliary microsensor unit), for application to heavymetal analysis in complex media such as natural waters. Both microsensor units include a channel for the solution renewal in the microcell after analysis. The working microsensor, a Hg-plated Ir microelectrode, is protected against fouling with an agarose gel including a hydrophobic chromatographic phase (C18). The fabrication steps and the quality tests related to long-term use and reliability, as well as to precision, are described. The application of the protective gel layer against fouling by hydrophobic or surface active small molecules is of general application, reliable, and very efficient. The reference and auxiliary unit is composed by an iridium oxide based mini reference electrode, an auxiliary Pt electrode, and a circulation channel. It is built to enable its use inside a 700-µm-diameter tubing connected to a hollow fiber supported liquid membrane lumen (volume, 5-10 µL) for heavy-metal analysis. However, it can be used in any other microanalytical system. The reference electrode is sufficiently stable for voltammetric applications (1-2 mV drift /day), and its lifetime is more than one year. The Ti/IrO2 core is immersed in a pH-buffered agarose gel, to guarantee potential stability even when the electrode is immersed in variable pH solutions. In environmental as well as in biomedical analysis, the knowledge of speciation and, in particular, the free-metal concentration is essential.1 In this respect, voltammetric techniques are potentially very useful, especially microelectrodes.2-8 * To whom correspondence should be addressed. Phone: 41-22-702. Fax: 4122-702-6069. E-mail:
[email protected]. (1) Buffle, J. Complexation reactions in aquatic systems. An analytical approach; Ellis Horwood: Chichester, U.K., 1988. (2) Wightmann, R. M.; Wipf, D. O. Electroanalytical chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15. (3) Microelectrodes: Theory and applications; Montenegro, M. I., Queiros, M. A., Dashbach, J. L., Eds. NATO ASI Series; Kluver Academic: Dordrecht, The Netherlands, 1991. (4) Special issue on microelectrodes. Bard, J., Ed. Electroanalysis 1990, 2, 175. (5) De Vitre, R. R.; Buffle, J.; Tsacopoulos, M. Anal. Chim. Acta 1991, 249, 419. (6) Kounaves, S. P.; Deng, W. J. Electroanal. Chem. 1991, 301, 77. (7) Kounaves, S. P.; Deng, W. Anal. Chem. 1993, 65, 375.
936 Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
New development in trace-metal analytical systems should consider the following criteria: (1) Fouling. Often environmental and biomedical samples are rich in colloidal particles and organic matter. For long term applications, the chosen sensor must then be protected against fouling. To date, few efficient protective systems have been reported; mainly membrane-based techniques have been used.9,10-12 (2) Reliability. Environmental analysis (for example for lake or groudwater monitoring) often requires recording the long-term evolution of parameters. The sensor must therefore deliver reproducible results over a long period of time. (3) Contamination in test metal or losses by adsorption. The best experimental approach to avoid contamination of the sample, as well as loss by adsorption on storage and handling of vessels is on-line analysis.13 For field or in situ measurements, this implies building an on-line mini or micro analytical system to minimize sample and reagent consumption. These two papers intend to report a novel analytical microsystem combining a separation and preconcentration step with a voltammetric detection step for on-line heavy-metal analysis. The separation step is based on the supported liquid membrane technique (SLM),14-15 and the detection is based on voltammetry and the use of a Hg-plated iridium microelectrode8 combined with an iridium oxide reference electrode. Iridium was chosen as a substrate for the working electrode, because of its low solubility in Hg (no formation of intermetallic compounds) and its good wettability by Hg.16-20 (8) Tercier, M. L.; Perthasarathy, N.; Buffle, J. Electroanalysis 1995, 7 (1), 55. (9) Tercier, M. L.; Buffle, J. Anal. Chem. 1996, 68 (20), 3670. (10) Alstadt, P.; Dewald, H. D. Anal. Chem. 1993, 65, 922. (11) Vidal, J. C.; Vinao, R. B.; Castillo, J. R. Electroanalysis 1992, 4, 653. (12) Morrison, G. M.; Florence, T. M. Electroanalysis 1989, 1, 458. (13) Luque De Castro, M. D. Flow injection Analysis Principles and applications; Ellis Horwood Series in Analytical Chemistry; Ellis Horwood: Chichester, U.K., 1987. (14) Bloch, R.; Finkielstein, A.; Kedem, O.; Vofsi, D. Ind. Eng. Chem. Process Des. Dev. 1967, 6 (2), 231-37. (15) Izatt, R. M.; Bruening, R. L.; Bradshaw, J. S.; Lamb, J. D.; Christensen, J. J. Pure Appl. Chem. 1988, 60 (4), 453. (16) Kounaves, S. P.; Buffle, J. Electrochem. Soc. 1986, 133 (12), 2495. (17) Kounaves, S. P.; Buffle, J. J. Electroanal. Chem. 1987, 216, 53. (18) Kounaves, S. P. Study of the formation of true mercury films for electroanalytical applications to speciation. Development of an iridium based electrode. Ph.D. Thesis, University of Geneva, Switzerland. (19) Kounaves, S. P.; Buffle, J. J. Electroanal. Chem. 1988, 239, 113. (20) Wechter, C.; Osteryoung, J. Anal. Chem. 1989, 61, 2092. 10.1021/ac9905064 CCC: $19.00
© 2000 American Chemical Society Published on Web 02/02/2000
To avoid chloride leaching, and consequent Hg layer deterioration on the working electrode, a calomel reference electrode was avoided and an iridium oxyde reference electrode was used instead. Both electrodes will include a circulation channel for solution renewal in the cell (lumen of a hollow fiber supported liquid membrane) after analysis. The present paper (part 1) will focus on the working and reference flow-through microelectrodes and their reliability for environmental flow-through analysis. Part 2 will describe the functioning of the whole system (SLM coupled to electrochemical detection). EXPERIMENTAL SECTION All reagents used were of puriss. pro anal. reagent grade, and all solutions were freshly prepared before use with Milli-Q water (Millipore). Agarose LGL was purchased from Biofinex Bulle, Switzerland, and C18-EC Chromabond (particle diameter, 5 µm) was purchased from Macherey et Nagel, Duren, Germany. Iridium wire (99.9% pure, 124 µm in diameter) and silver wire (99.9% pure, 200 µm in diameter) were purchased, respectively, from Goodfellow (Cambridge, U.K.) and Johnson and Mattey (Brandenberger/Zurich). Ordinary glass capillaries with external diameters of 9 and 3 mm and internal diameters of 7.7 and 1.1 mm have been pulled in order to obtain the double channel glass capillaries formed by an external capillary with a 3-mm external diameter and a 2.6-mm internal diameter and internal capillaries with a 1.2mm external diameter and a 0.4-mm internal diameter. A capillary puller machine purchased from RFR Corp. (The Netherlands) has been used for this purpose. Double-channel capillaries were pulled a second time, with a homemade vertical puller8 to obtain a long taper-shaped (1-2 cm) capillary having a 1-mm external diameter tip. Iridium and silver wire electroetching has been performed with a homemade alternative voltage generator (50 Hz AC frequency; peak-peak voltage, 40 V).8 The capillary tips were sealed in a homemade ceramic oven.8 Electrode-tip polishing has been achieved thanks to a homemade polishing machine.8 Polishing pads (Struers; SiC C5, SiC 2400, SiC 4000) were purchased from Merck (Darmstadt, Germany) and DP Stick-P KITON Diamond polishing paste and DP Blue DEPTI lubricant Blue for final polishing on DP-Mol DEKON pad were both Struers’ products (Merck). Silver wire soldered in glass capillary was dissolved electrochemically with a PRG5 polarograph purchased from Tacussel France. Ir wires were soldered to copper wires with a butane welding blowpipe purchased from Weller Pyropen, Japan. The soldering was covered with Epoxy resin (Fast Araldite, Ciba Geigy, Basel, Switzerland) to ensure mechanical stabilization. A Leitz Diavert optical inverted microscope and a Laborlux normal optical microscope were used for looking, respectively, in the directions normal and perpendicular to the Hg-plated Irbased microelectrodes. A computer-assisted Cypress CS 1090 polarograph was used for all electrochemical measurements. Reference electrodes were either Ag/AgCl/sat. KCl/1 M NaNO3 (Metrohm A.G., Switzerland) or homemade microreference Ti/ IrO2/Agarose-MES 1 M pH ) 6. For the auxilliary and reference electrodes, Pt and Ti (respectively 0.2 mm and 0.5 mm diameter, 99.9% purity) wires were purchased from Johnson and Mattey, (Brandenberger Zurich).
IrCl3‚xH2O was of technical purity and was purchased from Fluka (Buchs, Switzerland). IrCl3 was transformed into oxide in a Phoenix HTS/1000/0017 (Chapeltown, England) oven. Potential measurements were performed with a Tacussel-Aries 20000 Voltmeter (France) and a Metrohm Labograph 586 recorder (Switzerland). FABRICATION AND CHARACTERIZATION OF THE WORKING AND REFERENCE FLOW-THROUGH MICROELECTRODES Working Flow-Through Microelectrodes. A scheme of the working flow-through microelectrode as well as the combined reference-auxiliary flow-through microelectrode and the respective flow-through cell is given in Figure 1. Details of fabrication, as well as the reason of choices of the working and sacrificial metals are given in the Supporting Information; major points are emphazised below. (i) Working as well as reference-auxiliary combined units must include a circulation channel for flow-through analysis. (ii) The working electrode must be reliable; in other words, a perfect glass sealing of the electrode material (Ir wire) must be guaranteed, as well as a perfect connection between the Ir and connection wire (Cu). (iii) Circulation channel must be prepared without perturbing the working electrode performance. It can be obtained by using a double-channel capillary and introducing a sacrificial wire in one channel and the Ir wire in the other channel, prior to tip sealing. Sacrificial wire is removed later on by electrochemical dissolution. (iv) To avoid fouling the working electrode in natural media applications, a protective membrane must be applied on its surface. For this purpose, a C18 (octadecyl silica) containing agarose membrane (a few hundred microns thickness) was applied on the electrode surface. Reference Flow-Through Electrode. Since the flow-through electrodes will be used in a very small analysis cell (a few microliters), the reference flow-through electrode must not leach chloride, which would deteriorate the Hg film on the working electrode. So calomel must be avoided. Even calomel electrodes with “anti-leaching” membranes (Nafion,21-22 polyvinyl acetate,23-24 polyacrylamide hydrogels,25 and glass26) cannot be used in our system, since chloride leaching is always present. Reference Field Effect Transistors (REFET) were also avoided because of electrical noise when not used with an ISFET as a working electrode.27-29 Metal-oxide-based pH electrodes can be used as reference electrodes when kept in buffered solutions. We adopted this solution for our experiments. (21) Moussy, F.; Jakeway, S.; Harrison, D. J.; Rajotte, R. V. Anal. Chem. 1994, 66 (22), 3882. (22) Moussy, F.; Harrison, D. J. Anal. Chem. 1994, 66 (5), 674. (23) Rehm, D.; McEnroe, E.; Diamond, D. Anal. Proc. incl. Anal. Commun. 1995, 32, 319. (24) Diamond, D.; McEnroe, E.; McCorrick, M.; Lewenstam, A. Electroanalysis 1994, 6, 962. (25) Arquint, P.; Koudelka-Hep, M.; de Rooij, N. F.; Bu ¨ hler, H.; Morf, W. E. J. Electroanal. Chem. 1994, 378, 177. (26) Brehier, P. C.; Belford, R. E. Anal. Proc. incl. Anal. Commun. 1995, 32, 323. (27) Comte, P. A.; Janata, J. Anal. Chim. Acta 1978, 101, 247. (28) Yano, M.; Shimada, K. U.S. Patent 4,269,682, 1978. (29) Lisdat, F.; Moritz, W. Sens. Actuators, B 1993, 15, 228.
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Figure 1. Complete System. (A) reference electrode, (B) auxiliary electrode, (C) working electrode (Ir), (D) reference electrode buffered gel, (E) channel, (F) HFSLM, (G) source solution, (H) strip solution, (I) gel of the Hg-plated Ir microelectrode.
Sb,30-31 Pd,30-32 Ru,30,33 and Ir30,33-38 oxides have been used as pH electrodes (for a review, see ref 30). Metal oxide film fabrication procedures are mostly based on thermal oxidation,32,35 thermal metal salts’ decomposition37,39 on a metal substrate, sputtering,33 or electrochemical oxidation.32,38,40-42 Such electrodes can be used as reference electrodes when immersed in pH buffered solutions. The chosen metal for our reference electrode was iridium, because its oxide is chemically stable, very insoluble in aqueous media, and has a low impedance.34 To obtain long-lived reference electrodes, a thick-film (1 µm) oxide formation technique was used: thermal salt (IrCl3) decomposition39 on Ti substrate. The fabrication by this procedure of a capillary-shaped combined (30) Glab, S.; Hulaniki, A.; Edwall, G.; Ingman, F. Crit. Rev. Anal. Chem. 1989, 21 (1), 29. (31) Roberts, E. J.; Fenwick, F. J. Am. Chem. Soc. 1982, 50, 2125. (32) Kinoshita, E.; Ingman, F.; Edwall, G. Electrochimica Acta 1986, 31(1), 29. (33) Kreider, M.; Tarlov, J.; Cline, J. P. Sens. Actuators, B 1995, 28, 167. (34) Katsube, T.; Lauks, I.; Zemel, J. N. Sens. Actuators 1982, 2, 399. (35) Papeschi, G.; Bordi, S.; Beni, C.; Ventura, L. Biochim. Biophys. Acta 1976, 453, 192. (36) Bordi, S.; Cara, M.; Papeschi, G. Anal. Chem. 1984, 56, 357. (37) Ardizzone, S.; Carugati, A.; Trasatti, S. J. Electroanal. Chem. 1981, 126, 287. (38) Gottesfeld, S.; McIntyre, J. D. E. J. Electrochem. Soc. 1979, 126, 742. (39) Galizzoli, D.; Tantardini, F.; Trasatti, S. J. Appl. Electrochem. 1974, 4, 57. (40) deRooij, N. F.; Bergveld, P. Iridium/Anodic Iridium oxide film electrode as pH sensor. In Monitoring of vital parameters during extracorporal circulation. Proceedings of International Conference, Nijmegen, 1980; S. Karger: Basel, Switzerland, 1981; pp 156-165. (41) Gossner, K.; Mizera, E. J. Electroanal. Chem. 1981, 125, 347. (42) Ullmann, M. Geneva University, personal communication, 1994.
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Table 1. Potential-pH and Potential-Time Response for Different Ti/IrO2 Buffered Agarose Gel Electrodes electrode no.b dE/dpHa (mV) dE/dt (mV/h)
1 -4.2 0.2
2 -0.5 0.2
3 -0.4 0.35
4 -0.8 0.2
5 -0.8 0.1
a Measured dE/dpH valid in tested pH range (1-12). Potential was measured vs a Ag/AgCl/sat KCl/1 M NaNO3 Electrode. b Electrodes 1-5 have been fabricated with the same Ti/IrO2 core. The same gel and buffer concentrations were used, so the five electrodes should present no differences.
auxiliary-reference electrode including a microchannel is described in the Supporting Information. RESULTS Performance of the Reference Electrode and the Combined Reference-Auxiliary Microchannel Sensor Unit. To optimize the reference electrode’s quality, we studied the influence of different IrCl3‚xH2O solution concentrations and ages on electrode stability and pH response (Table 1 of the Supporting Information) The potential-pH response has been measured for both the Ti/IrO2 core and the reference electrode of the whole microsystem (Ti/TiIrO2 core + buffered gel) in order to check the Nernstian behavior in the absence of gel (on the basis of eq 1) and E-pH invariance in the presence of gel (Table 1).
2IrO2 + 2H+ + 2e- f Ir2O3 + H2O
(1)
Figure 2. Potential change of the reference electrode as a function of pH. 9: IrO2 reference electrode without gel (only core). b: IrO2 reference electrode with gel including 1 M MES pH ) 6 (complete system). The IrO2 core was no. 5 of Table 1 of the Supporting Information.
According to eq 1 the ideal slope of potential E vs pH should be dE/dpH ) -59 mV/pH at 25 °C.34,37 The more concentrated the IrCl3‚xH2O solution is, the closer the electrode slope is to the Nernstian value (Table 1 of Supporting Information). For a concentration of 40 g/L, a Nernstian behavior is obtained over a wide pH range (1-11), see Figure 2). Since, for our purposes, the potential should be independent of pH variations, the Ti/IrO2 electrode has been dipped in a buffered (1 M MES pH 6) agarose gel (Supporting Information). With a buffered gel, E is almost independent of pH, with an average value of dE/dpH ) -0.6 mV. This value is fully acceptable for voltammetric measurements in the HFSLM conditions where maximum pH change from one solution to the other is less than 2.5 pH units. As far as potential stability is concerned, a mean potential drift of 0.04 mV/h has been measured (Supporting Information) over 250 h. Such a drift is fully acceptable for practical purposes. Performance of the Combined Working Hg-Plated Microelectrode and Microchannel. A Hg film was plated on the iridium surface of the microelectrode-combined channel from a deoxygenated 6 mM mercury acetate, 1.5 M acetic acid buffer (pH ) 4.5) solution, by applying -850 mV vs IrO2 (pH ) 6). Acetic acid buffer was chosen in order to avoid mercury hydroxide formation (Supporting Information). Figure 3 shows examples of SWASV calibration peaks for 10-240 nM Pb(II) and Cd(II) in 0.01 MES pH ) 6 degassed solutions, obtained with a combined Irchannel microsensor without gel protection. The voltammetric peaks are symmetrical throughout the entire calibration range. The corresponding calibration curves are linear for both metals (standard deviation on the slopes for Pb and Cd are 1.6 and 1.9%, respectively). Mercury-film stability has been tested as a function of time. After film formation, the electrode was immersed permanently in the same solution (MES 0.01 M, pH ) 6, [Cd] ) 350 nM, [Pb] ) 385 nM) degassed under nitrogen to avoid Hg-film oxidation. Five sets of 15 measurements were performed during about 70 h (Figure 4a). It was found that mercury film is stable for at least 70 h and SWASV currents are reproducible for both metals: 3% standard deviation on 75 measurements (5 series). In optimal conditions (efficient degassing), the same mercury film can be used for at least 3 days. When the electrode is stored in non
Figure 3. SWASV calibration for Pb(II) and Cd(II) with a channelcombined Mercury plated Ir microelectrode. [Pb(II)], [Cd(II)] ) 10250 nM, MES 0.01 M, pH ) 6. Electrochemical conditions were as follows: 30-s precleaning at -150 mV, 180-s deposition at -1350 mV. Other electrochemical parameters were as reported in Figure 5. All potentials were vs Ag/AgCl/KClsat/NaNO3.
Figure 4. (a) Mercury-film stability as a function of time in a N2 degassed solution. Experimental conditions were as follows: [Pb(II)] ) 380nM, [Cd(II)])350nM, MES 0.01M, pH ) 6. Other conditions were as reported in Figure 3. (b) Mercury-film stability as a function of time for an electrode stored in a nondegassed solution. Experimental conditions were as follows: [Pb(II)] ) 380nM, [Cd(II)] ) 350nM, MES 0.01M, pH ) 6. Other conditions were as reported in Figure 3.
degassed solution, mercury-film lifetime is shorter, and less reproducible SWASV measurements are obtained (Figure 4b). Performance and Results of the Protective Gel Membrane on the Combined Hg-Plated Ir Microelectrode and Microchannel. The voltammetric peak of Pb can be drastically perturbed by small concentrations of surface-active agents, in particular the carrier, lauric acid, and organic solvent used in SLM (Figure 5a). Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
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Figure 5. (a) SWASV of Pb(II) measurements in the HFSLM lumen. Signal evolution as a function of time since the insertion of the unprotected electrode in the HFSLM. b 0 min, 9 120 min, 2 150 min. 375 nM Pb(II), 0.01 M Na4P2O7, and 0.01 M MES. Electrochemical conditions: 30-s precleaning at -550 mV, 180-s deposition at -1550 mV, 25 mV pulse amplitude, 8 mV step height, and 50 Hz frequency. All potentials were vs Ti/IrO2/Agarose LGL 4.5%, MES 1 M, pH ) 7. (b) Effect of dipping the combined Ir-channel microsensor for 10 s in the SLM organic phase, on the SWASV signal. Microsensor tip is protected with a 1-mm-thick membrane containing 8% C18 in 1.8% agarose LGL solution ) 1 M MES buffer, pH)7. 2: before electrode tip dipping in organic solution. 9: just after dipping of the electrode in the organic solution. b: 800 min after electrode tip dipping in the organic solution. +: after second tip dipping in the organic solution. Conditions as reported in Figure 3.
Gel efficiency, for avoiding such problems in conditions corresponding to electrode utilization in a SLM flow-through system, has been tested by dipping the electrode tip protected with an agarose gel membrane (see composition below) for 10 s in the organic phase composing the SLM (1/1 mixture of toluene/ phenylhexane containing 0.1 M 22DD and 0.1 M lauric acid), rinsing it, and performing SWASV measurements every 10 min in a 400 nM Pb(II), 0.01 M MES pH ) 7 test solution. This test with direct contact with the organic phase is much more drastic than in most practical cases, in particular that of HFSLM (hollow fiber supported liquid membrane) operation (Part 2). If the membrane conveniently protects the electrode surface under these conditions, it can be expected to be very efficient under the HFSLM conditions. Preliminary experiments have shown that 4.5% agarose gel alone is not a sufficient protection against the very hydrophobic solvent and metal carrier of the SLM.43 SWASV 940
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measurements have then been performed as mentioned above with 0, 1.5, 3, and 8% of the C18 derivatized silica particles in the agarose gel. The best results are obtained with 8% C18 in the gel: after electrode tip dipping in the organic phase, SWASV peak heights are constant for at least 800 min (Figure 5b). No peak deformation is observed, but a baseline shift is present. On the basis of these results, the combined iridium-channel microsensor has been systematically protected with a 500-µm-thick membrane including 4.5% agarose LGL, 1 M MES pH ) 6, and 8% C18. Pb and Cd Diffusion Coefficients Through the Protective Gel Layer. The solution in the HFSLM lumen will contain 0.02 M sodium pyrophosphate at pH ) 7 (Part 2). Pb(II) and Cd(II) are therefore under the form of pyrophosphate complexes 62PbP2O72-, Pb(P2O7)62 , CdP2O7 , Cd(P2O7)2 , whose complexPb ation constants are, respectively; logβ1 )6.4, logβ2Pb)9.4, logβ1Cd)4, logβ2Cd)6.3. Logarithms of acid-base constants of pyrophosphate are 2.5, 2.7, 6, and 8.3. Using these constants, it can be shown that, in our experimental conditions, ML2.ML.M. Consequently, the only diffusing species through the membrane are ML2 complexes. We measured Cd(P2O7)6and Pb(P2O7)6diffusion coef2 2 ficients in water, 4.5% agarose LGL gel, and C18 containing 4.5% agarose gel, to quantify the eventual slowing down of diffusion caused by agarose and/or C18. For this purpose, we used a diffusion cell composed by two 100-mL compartments, separated by a 8 cm2 membrane. One hundred-micrometer-thick 0.45-µmpore-size hydrophilic Schleicher and Schuell membrane was used for diffusion in water. A microelectrode covered with 500-µm-thick agarose gel with or without C18 was used in other cases; both compartments were stirred with a magnetic stirrer at about 350 rpm. We first compared the diffusion constants for Cd(P2O7)62 and Pb(P2O7)6complexes in water using 0.02 M pyrophos2 phate at pH ) 7 as supporting electrolyte in both compartments (experiment 2) with the diffusion constants of correponding free metal ion using 0.1 M NaNO3 as supporting electrolyte in both compartments (experiment 1). Pb and Cd concentrations, C, in the receiving solution were analyzed by flame AAS as a function of time. Diffusion coefficients of Cdaq2+ and Pbaq2+ (experiment 1) and Cd(P2O7)62 and Pb 6(P2O7)2 (experiment 2) complexes were obtained from eq 1.
F)
∆C V ∂C )D A ∂t l
(1a)
with F being the metal flux through the membrane [mol‚s-1‚cm-2], A the membrane surface area [cm2], V the receiving compartment volume [cm3], ∂C/∂t the slope of the linear part of the plot of metal concentration vs time in the receiving compartment [mol‚l-1‚s-1], D the metal or complex diffusion coefficients [cm2 s-1], ∆C the difference Csource - Creceiving [mol‚L-1], and l the membrane thickness [cm]. Because the membrane thicknesses used were large (>100 µm) and the solutions well-stirred, contribution of diffusion in the source and strip solutions was neglected. (43) Keller, O. Ph.D. Thesis no. 2957, University of Geneva, Switzerland, 1997.
Table 2. Summary of Diffusion Coefficients for Cd(P2O7)26- and Pb(P2O7)26- in Water, in Conditioned and Nonconditioned 4.5% LGL and LGL 4.5%/C18 8% Membrane with Various Thicknesses experiment
determination method
membrane conditioning
diffusion medium
l in µm
a b c d e f g
diffusion cell/AAS µsensor µsensor µsensor µsensor µsensor µsensor
1h 24 h 1h 1h 1h 24 h
waterb LGL 4.5% as in b LGL 4.5% + 8% C18 as in d as in d as in d
100 370 as in b 740 800 mean d, e 800
DPbPyrIN × 106a DPbPyrOUT × 106 DCdPyrIN × 106 DCdPyrOUT × 106 (cm2/s) (cm2/s) (cm2/s) (cm2/s) 5.63 ( 0.3 1.8 ( 0.3 1.13 ( 0.14 5.03 ( 0.28 4.1 ( 0.5 4.45 ( 0.8 3.54 ( 0.19
1.92 ( 0.12 1.25 ( 0.12 2.95 ( 0.28 2.3 ( 0.19 2.67 ( 0.55 3.08 ( 0.1
4.8 ( 0.27 1.69 ( 0.22 1.2 ( 0.15 3.87 ( 0.37 3.42 ( 0.5 3.58 ( 0.65 3.31 ( 0.05
1.97 ( 0.14 1.15 ( 0.13 2.7 ( 0.26 2.16 ( 0.14 2.49 ( 0.45 2.8 ( 0.07
a CdPyr Indicates Cd(P2O7) 6-, and PbPyr indicates Pb(P2O7) 6-. IN Indicates diffusion in the membrane, and OUT indicates diffusion out of 2 2 the membrane. b Measurements were performed with a diffusion cell, through the hydrophilic membrane S/S NY13, with pore size ) 0.45 µm. Membrane conditioninig was 3 h in 0.02 M, pH ) 7 sodium pyrophosphate solution.
Results are given as diffusion coefficient ratios between free metal ion and pyrophosphate metal complex through the diffusion cell:
DCd2+ DCd(P2O7)62 DPd2+ DPd(P2O7)62
) 1.44 ( 0.04
(2)
) 1.47 ( 0.04
(3)
The two diffusion coefficient ratios are very close to each other. This is not surprising, as Pb(II) and Cd(II) have the same charge and similar ionic radi. Using the diffusion coefficients for Pb2+ aq 44 and Cd2+ ) (4.8 aq reported in the literature, one gets DCd(P2O7)62 ) (5.6 ( 0.3) × 10-6 ( 0.3) × 10-6 cm2s-1 and DPb(P2O7)62 cm2s-1. The Cd(P2O7)6and Pb(P2O7)6diffusion coefficients 2 2 through agarose gel (with or without C18) were then determined with a gel-coated Hg-plated combined Ir-channel microsensor.43 The electrode was left at rest in a degassed 0.02 M pyrophosphate solution (pH ) 7) for 60 min, to equilibrate the gel with the pyrophosphate. Spikes of 7 µM Pb(II) and 7 µM Cd(II) were then introduced into the pyrophosphate solution, and SWASV peaks were recorded as a function of time elapsed since metal spiking (1 measurement every 2 min). After attaining equilibrium in the membrane (constant SWASV peaks), the electrode was dipped in a degassed 0.02 M pH ) 7 pyrophosphate solution without any Pb(II) and Cd(II), to observe the reverse process, i.e., metal diffusing out of the membrane. The results are shown in Figure 6 for gel without C18. From Figure 6 and eq 4,9 diffusion coefficients for the two complexes in the gel can be obtained (Table 2).
ln
Csol - C Dt ) 0.2306 - 2.452 2 Csol - Cm l
(4)
with Csol being the metal concentration (mol.L-1) in the bulk of the solution, C the metal concentration (mol‚L-1) at the electrode surface at time t, Cm the initial metal concentration (mol‚L-1) in the membrane (t ) 0), D the diffusion coefficient through the (44) Von Stackelberg, M.; Pilgram, M.; Toome, V. Z. Elektrochem. 1953, 57, 342.
Figure 6. Diffusion of Cd(P2O7)2 and Pb(P2O7)26- in (b: Pb, 9: Cd) and out (1: Pb, 2: Cd) of a 370-µm-thick 4.5% LGL agarose membrane. The membrane was preconditioned 1 h in 0.02 M pyrophoysphate solution, pH ) 7. SWASW conditions were as follows: 20-s precleaning at -550 mV, 10-s deposition at -1550 mV, and other parameters as reported in Figure 4 (all potentials referred to the Ti/IrO2/Agarose-MES 1 M pH ) 6 electrode). Il is the current intensity when the gel is equilibrated with the external solution.
membrane (cm2 s-1), l the membrane thickness (cm), and t the time (s) elapsed after a concentration change in the bulk of the solution. The same experimental procedure has been used to determine the diffusion coefficients of the complexes in 4.5% LGL agarose membrane containing 8% of C18. An example of the observed curves i/il as a function of time elapsed after metal spiking or removal is given in Figure 7 where il is the current at equilibrium and i is the current at time t. The results of these studies are summarized in Table 2. (1) Diffusion in LGL 4.5% agarose membrane. (i) diffusion coefficients of Pb and Cd through the membrane decrease with increasing conditioning time. This might be explained by gel conformation change in relation with hydration. (ii) Statistically similar diffusion-coefficient values are observed for diffusion in and out of the membrane for the two metals, suggesting that they do not react irreversibly with the agarose gel. (iii) DCd(P2O7)6and 2 DPb(P2O7)6are similar in water and in the gel. This is consist2 ent with the fact that Cd2+ and Pb2+ have similar sizes and supports the fact that they do not react chemically with agarose. Indeed, Pb(II) forms stronger complexes than Cd(II), and different values of effective D would be observed. (iv) The diffusion coefficients of cadmium and lead pyrophosphate complexes in the membrane are significantly less than the corresponding coefficients in water, which is also observed for Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
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protected Hg-coated Ir microelectrode, gave satisfactory results in the SWASV technique, with respect to peak potential and intensity stability. A current of 5.4 ( 0.1 nA (( 2%) was observed over 6h. The potential was stable at -620 ( 0.5 mV during the same period.
6Figure 7. Cd(P2O7)62 and Pb(P2O7)2 diffusion curves in (b: Pb, 9: Cd) and out (1: Pb, 2: Cd) of a 740-µm-thick 4.5% LGL agarose/ 8% C18 membrane. Membrane was conditioned 1 h in 0.02 M pyrophoysphate solution, pH ) 7. SWASW conditions were: 20-s precleaning at -550 mV, 10-s deposition at -1550 mV, other parameters as reported in Figure 3 (all potentials referred to the Ti/ IrO2/Agarose-MES 1 M pH ) 6 electrode).
other cations and anions9 and is probably due to physical hindering effects. (2) Diffusion in 4.5% agarose/8% C18 membrane. (i) The coefficients for diffusion in and out of the membrane for Pb and Cd are larger with C18 than without it. This is likely due to the fact that C18 coated silica particles influence the gel structure and create preferential paths in the gel. (ii) The diffusion coefficients in the membrane decrease with conditioning time, which may be related to the gel structure, as they do in the absence of C18silica particles. (iii) The differences in diffusion coefficients for diffusion in and out of the membrane at short conditioning times might suggest that Pb(II) and Cd(II) may adsorb and desorb on C18 coated silica particles only slowly. However, diffusion coefficient values are similar at long preconditioning times, i.e., times which should have no effect on Pb(II) or Cd(II) adsorption. It is therefore more likely that the difference in diffusion coefficients at short preconditioning times are due to differences in gel structure, in particular the existence of preferential paths at short times which disappear when conditioning time increases. For measuring the diffusion of metals entering the gel, Cd(II) and Pb(II) stay in the gel for about 30 min. For measuring the coefficient of diffusion out of the gel, the experiments take about 1-2 h after gelation. Gel structure may change during that period, explaining the difference in the diffusion coefficient. The aforementioned results suggest that an agarose gel layer including C18 coated particles may be used reliably to efficiently protect voltammetric microelectrodes against fouling by hydrophobic or surface-active small molecules. Care should be taken, however, to prepare the gel and condition it correctly, to get reliable results. Application of the Complete-Flow-Through Microcell to SWASV Measurements. The performance of the complete-flowthrough microcell, including the reference, the auxiliary, and the gel-protected Hg-coated Ir microelectrodes, has been cheeked by measuring Pb(II) in SWASV mode with a working flow-through microelectrode. SWASV measurements have been performed in the HFSLM lumen containing a 375 nM Pb(II), 0.01 M Na4P2O7, and 0.01 M MES, pH ) 6 solution. SWASV peaks are recorded as a function of time each 30 min. The complete-flow-through microcell, including the reference, the auxiliary, and the gel942 Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
CONCLUSIONS (a) We described the fabrication and performance of a successful microunit including a gel-protected Hg-plated iridium microelectrode combined with a microchannel for on-line voltammetric measurements of heavy metals. Such electrodes are miniaturized (φtip ) 700 µm, φcanal ) 160 µm, φIr ) 10-20 µm), rugged, long-living (>1 year), and reliable (standard deviation in SWASV < 3%). Iridium disk surface has been mercury-electroplated from nonacidic mercury solution; the obtained mercury film is stable at least 3 days in degassed solutions. We developed a protective gel membrane, with a channel fabricated through the membrane, to allow solution circulation through the electrode. The membrane efficiently protects the mercury film against contamination by organic solvent or surfaceactive molecules. Such a protective layer guarantees correct electrode behavior in very drastic conditions (electrode dipping in the SLM organic phase) and is of general application. (b) We described the fabrication of a successful unit including reference and auxiliary electrodes and a channel. The total size of the system is 15 cm in length, 4 mm in diameter, and 0.8 mm in tip diameter. The fabricated reference electrode is free of chloride which allows its application to voltammetry with mercuryfilm electrodes even in very small volumes of test solutions (
