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A New Mercury-Free Sensor for Uranium Detection 873 Experimental A stock solution of 350 mM U(VI) was made by dissolution of analytical-grade UO2(NO3)2 ¥ 12H2O (Fluka) in a 0.1 M perchloric acid solution. Uranium solutions of concentra- tions ranging from 1 to 500 ppb (mg/L) were prepared daily by diluting the stock solution in 0.05 M CH3COONa buffer solution. Before adding the metal ion solution, the pH of the acetate solution was adjusted to a nominal value of 3, 4, 5, 6, or 7 with 1.0 M HNO3 and/or 0.1 M NaOH. All solutions were made with ultrapure Millipore water (18 MW cm). The modified carbon paste electrode was prepared by thoroughly mixing a 0.05 g quantity of Ac-Phos SAMMS with a 0.15 g quantity of CPO carbon paste (Bioanalytical Systems, Inc.). The preparation and characterization of Ac- Phos SAMMS are described elsewhere [16]. A 0.05g quantity of mineral oil (Aldrich Co.) was added to the carbon paste/Ac-Phos SAMMS mixture and mixed until obtaining a uniformly wetted paste. Unmodified carbon paste,preparedinthesamefashion,butwithoutaddingAc- Phos SAMMS, was packed into an 8-cm long cylindrical PTFE tube (inside cross sectional area of 0.08 cm2) with a copper piston providing an inner electrical contact. Then the carbon paste/Ac-Phos SAMMS mixture was packed into the end of the same tube. The electrode surface was polished on a weighing paper. When necessary, a new electrode surface was obtained by removing about 2±3mm of electrode material from the exposed end, adding freshly-made Ac- Phos SAMMS/carbon paste mixture, and polishing it. Square-wave voltammetry (SWV) experiments were performed on an electrochemical detector, Model CHI 660A (CH Instruments, Inc.) equipped with a three electrode system: a self-made carbon paste electrode modified with Ac-Phos SAMMS as the working electrode, a platinum wire as the auxiliary electrode, and a KCl saturated Ag/AgCl electrode as the reference electrode. All measurements were made at room temperature and under an atmospheric environment. Square-wave voltammetry was operated at a frequency of 100 Hz with a pulse- amplitude of 50 mV and a potential step height of 5 mV. The voltammetric detection procedure consisted of preconcentration, cathodic electrolysis, and stripping steps. During preconcentration step, the electrode was immersed (at 2 cm from the solution surface) in a 20-mL cell (2-cm inside diameter) containing 15 mL of metal ion solution at open circuit for a specified period of time (typically 3± 5 minutes, but varied from 1 to 10 minutes in the precon- centration time study). During preconcentration, the sol- ution was stirred at over 300 rpm using a magnetic stirring bar. The electrode was then removed, rinsed with DI water, and transferred to another 20-mL cell containing 15 mL of 0.2 M HNO3 as the supporting electrolyte solution. A negative potential ( 0.80 V) was applied to the electrode immediately after immersing it in the acid solution to initiate cathodic electrolysis. The stripping voltammetry was per- Electroanalysis 2004, 16, No. 10 formed in the same cell by sweeping square-wave potential toward positive direction (i.e., from 0.80 V to 0.40 V). Both cathodic electrolysis and stripping steps were done under quiescent conditions. No regeneration of the elec- trode was required. Each measurement was performed in duplicate, and the average values were reported. The percent relative standard deviations (%RSD) were nor- mally less than 5%. Acknowledgements This work was supported by the EMSP Program, U.S. Department of Energy (DOE). Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the U.S. DOE. The authors thank Christian D. Johnson for reviewing the manuscript. References [1]R.J.Lewis,Sr.,SAX×sDangerousPropertiesofIndustrial Materials, 10th ed., Wiley, New York 2000. [2] J. Wang, J. Lu, D. D. Larson, K. Olsen, Electroanalysis 1995, 7, 247. [3] J. Wang, J. Wang, J. Lu, K. Olsen, Anal. Chim. Acta 1994, 292, 91. [4] R. Djogic, M. Branica, Anal. Chim. Acta 1995, 305, 159. [5] M. Mlkar, M. Branica, Croat. Chem Acta 1987, 60, 325. [6] M. Mlakar, Anal. Chim. Acta 1993, 276, 367. [7] K. Cha, C. Park, S. Park, Talanta 2000, 52, 983. [8] J. Wang, J. Lu, J. Wang, D. Luo, B. Tian, Anal. Chim. Acta 1997, 354, 275. [9] C. M. G. van den Berg, Z. Q. Huang, Anal. Chim. Acta 1984, 164, 209. [10] C. M. G. van den Berg, M. Nimmo, Anal. Chem. 1987, 59, 924. [11] K. H. Lubert, M. Schnurrbusch, A. Thomas, Anal. Chim. Acta 1982, 144, 123. [12] J. Wang, J. Wang, B. Tian, M. Jiang, Anal. Chem. 1997, 69, 1657. [13] X. D. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu, K. Kemner, Science 1997, 276, 923. [14] X. B. Chen, X. D. Feng, J. Liu, G. E. Fryxell, M. Gong, Sep. Sci. Technol. 1999, 34, 1121. [15] Y. Lin, G. E. Fryxell, H. Wu, M. Engelhard, Environ. Sci. Technol. 2001, 35, 3962. [16] J. C. Birnbaum, B. Busche, Y. Lin, W. Shaw, G. E. Fryxell, Chem. Commun. 2002, 1374. [17] K. M. Michael, G. H. Rizvi, J. N. Mathur, S. C. Kapoor, A. Ramanujam, R. H. Iyer, Talanta 1997, 44, 2095; and refer- ences cited therein. [18] E. P. Horwitz, R. Chiarzia, in Separation Techniques in Nuclear Waste Management (Eds: T. E. Carleson, N. A. Chipman, C. M. Wai), CRC Press, Boca Raton 1995, ch. 2, pp. 3 ± 33. [19] E. P. Horwitz, W. W. Schulz, in Metal Ion Separation and Preconcentration: Progress and Opportunities, ACS Sympo- sium Series 716 (Eds: A. H. Bond, M. L. Dietz, R. D. Rogers), ACS, Washington, DC 1999, ch. 23, pp. 390 ± 400. [20] M. Paneli, H. Ouguenoune, F. David, A. Bolyos, Anal. Chim. Acta 1995, 304, 177. 1 2004 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimPDF Image | Carbon Paste Electrode Modified with Carbamoylphosphonic
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