Anal. Chem. 1995, 67,3571-3577
SquareDWave Voltammetric Stripping Analysis of Mercury(II)at a Poly(4=vinylpyridine)/GoldFilm Electrode Jyh=MyngZen* and Mudye Chung
Department of Chemistv, National Chung-Hsing University, Taichung, Taiwan 402,Republic of China
A novel pob(4-~inylpyridine)/gold film electrode (PIP/ GFE) was developed for the determination of trace mercury(II) in real samples by Osteryoung square-wave anodic stripping voltammetry. Mercury is preconcentrated, as the anionic forms in the chloride medium, onto the modified electrode by the ion-exchange effect of the PVP. The high solubility of mercury in gold also helps to increase the preconcentration effect. The preparation of the PVP/GFE is performed by h t spin-coatinga solution of the PVP polymer onto the electrode surface. Subsequently, gold is plated onto the electrode. Various factors influencing the determination of mercury@) were thoroughly investigated in this study. In comparisonwith the conventional gold film electrode, this modified electrode showed improved resistance to interferencesfirom surfaceactive compounds and common ions, especially for copper@), which is generally considered as a major interference in the determination of mercury(I1) on gold film electrode. The PVP/GFE also showed increased sensitivity and better mechanical stability of the gold film when used in conjunction with the square-wave voltammetric method. In addition, detection can be achieved without deomenation, and the electrode can be easily renewed. The analytical utility of the PVP/GFE is demonstrated by application to various water samples.
have already been developed for this purpose. Unfortunately, similar to the main disadvantage of using a mercury flm electrode in many applications,17J8the most common problem in applying the bare-type gold electrode in ASV is the interference effect caused by surface-active compounds and several metal i0ns.9-l~ A convenient way to improve the problem is to coat the working electrode with a permselective membrane to protect the surface from these interferences. Although many permselective membranes have been introduced, in practice, a compromise between the exclusion of the interference matter and the unhindered transport of the metal ions must be considered. Overall, the key issue in making an electrochemical analytical technique one of the conventional methods for detecting mercury in the category associated with high sensitivity, such as cold vapor atomic absorption spectrometry, cold vapor atomic fluorescence spectrometry, and neutron activation analysis, is the availability of a suitable working electrode. The modfication of electrode surfaces with polymer films has received considerable attention because of the many advantageous properties of polymers.1g However, most of the previous studies related to this topic were cation-oriented, and hence cation permselective membranes were particularly explored. We have also sought to exploit the advantages of polymer-modfied electrodes for the determination of cationic metals such as nickel, lead, copper, and c a d m i ~ m . 2 ~In- ~reality, ~ however, many species of analytical interest exist in the form of anions in sample solutions. Thus, anion exchangers are promising for the determination of It is well known that mercury is one of the most toxic metals, such analytes. This is the case for mercury00 in a solution and its determination in trace level quantities is very important acidified with hydrochloric acid. Recently, indeed, a carbon paste in environmental and biological analyses.'a2 Several previous electrode modified with a liquid anion exchanger (Amberlite LA2) studies concluded that gold is the preferable electrode material was reported for the voltammetric determination of m e r c u r y 0 for detecting mercury by anodic stripping voltammetry (ASV). For in a chloride medi~m.2~ Nevertheless, the addition of Amberlite example, gold disk or rod electr0des,3-~gold fiber electrodes,lOJ1 gold twin disk ele~trodes,'~J~ and gold film carbon e l e ~ t r o d e s ~ ~ - l ~LA2 apparently can provide an improvement only in the sensitivity aspect. As a result, a medium change procedure was needed to (1) Von Burg, R; Greenwood, M. R In Mercuy; Merian, E., Ed.; VCH: separate many components that may interfere with the detection. Weinheim, Germany, 1991; pp 1045-1088. Even so, this method still required a fairly long accumulation time (2) Niessner, R Trends Anal. Chem. 1991,10,310. (3) Vydra, F.; Stulikova, M.; Petak, P. J. Elecfroanal.Chem. 1972,40, 99. (4) Luong, L.; Vydra. F. J. Elecfroanal. Chem. 1974,50,379. ' (5) Ulrich, L.; Ruegsegger, P. Z. Anal. Chem. 1975,277, 349. (6) Fukai, R; Herynh-Ngoc, L. Anal. Chim. Acta 1976,83,375. (7) Taddia, M. Microchem. J. 1978,23,64. (8) Golimowski, J.; Gustavsson, I. Z. Anal. Chem. 1984,317,481. (9) Jagner, D.; Josefson, M.; h e n , K. Anal. Chim. Acta 1982,141, 147. 1. (10) Huang, H.; Jagner, D.; Renman, L. Anal. Chim.Acta 1987,201, (11) Huang, H.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987,201,269. (12) S i p s , L.; Numberg, H. W.; Valenta, P.; Branica, M. Anal. Chim. Acfa 1980, 115,25. (13) S i p s , L;Valenta, P.; Numberg, H. W.; Branica, M. J. Electroanal. Chem. 1977,77,263. (14) Allen, R T.; Johnson, D. C. Talanfa 1973,20,799.
0OO3-2700/95/0367-3571$9.00/0 0 1995 American Chemical Society
(15) Lo, J.-M.; Lee, J.-D. Anal. Chem. 1994,66, 1242. (16) Gil, E. P.; Ostapczuk, P. Anal. Chim. Acta 1994,293,55. (17) Brezonic, P. L.; Brauner, P. A; Stumm, W. WaferRes. 1976,10, 605. (18) Hoyer, B.; Florence, T. M.; Batley, G. E. Anal. Chem. 1987,59,1608. (19) Murray, R W. In Electroanalytical Chemisty; Bard, A J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (20) Zen, J.-M.; Lee, M.-L. Anal. Chem. 1993,65,3238. (21) Zen,J.-M.; Huang, S.-Y. Anal. Chim. Acta 1994,296,77. (22) Zen,J.-M.; Chi, N.-Y.; Hsu, F.-S.;Chung, M.-J. Analyst 1995,120,511. (23) Zen,J.-M.; Hsu, F.-S.; Chi, N.-Y.; Huang, S.-Y.; Chung, M.-J. Anal. Chim. Acfa 1995,310,407. (24) Cai, X;Kalcher, K; Diewald, W.; Neuhold, C.; Magee, R J. Fresenius' Z. Anal. Chem. 1993,345, 25.
Analytical Chemistry, Vol. 67, No. 19, October 1, 1995 3571
of 15 min to achieve a detection limit of 1 ppb. Several other carbon paste electrodes modified with zeolite,25crown diphenylcarbaz~ne,~~ 1,5diphenylcarbazide,28 or tetraphenylborate29 were also reported for the determination of mercury. Unfortunately, either these methods suffer from a fairly high detection limit25-27or the electrode surface cannot be easily regene~xted?~-~~ Apparently, more sensitive and selective chemically modified electrodes are still needed to be developed for the determination of mercury, due to its environmental and biological significance. Based on the same concept as our previous studies using polymer-mod5ed electrodesfor the detection of metal cati0ns,2~-~~ we expect that similar advantages should also be achieved if a suitable anion permselective membrane is used for the detection of mercury@) in the chloride medium. As part of our continuing interest in the analytical application of chemically modifled electrodes, we report here the use of a poly(4vinylpyridme)/gold film electrode (PVP/GFE) for the determination of mercuryar). Protonated and quatemized PVPs have been examined extensively as matrices for immobilization of negatively charged redox centers near electrodes since the first work by Oyama and The cross-linked PVP polymer is chemically inert, nonelectroactive, hydrophilic, and insoluble in water and thus possesses almost ideal properties for the preparation of chemically modified electrodes. In this paper, the preparation and regeneration of the PVPIGFE and the various factors influencing the determination of mercury(I0 are investigated. Typical interferences that can occur in water samples are discussed. The analytical utility of the PVP/GFE is demonstrated by application to various water samples. EXPERIMENTAL SECTION
Chemicals and Reagents. Poly(4vinylpyridine) (MW 50 OOO) solution in methanol containing -20 wt % polymer was obtained from Aldrich. GoldUII) standard solution (0.100 g, AAS grade) was purchased from RDH. All buffers and supporting electrolyte solutions were prepared from Merck Suprapur reagents. The standard metal solutions used in the interference studies were also obtained from Merck. All the other compounds (ACS certified reagent grade) were used without further purification. Aqueous solutions were prepared with doubly distilled deionized water. Apparatus. Electrochemistrywas performed on a BAS CV50W electrochemical analyzer. A BAS Model VC-2 electrochemical cell was employed in these experiments. The threeelectrode system consists of one of the following working electrodes, gold film electrode (GFE), PVP-coated glassy carbon electrode (PIT/ GCE) , and PVP/GFE; a Ag/AgCl reference electrode Model RE5, BAS); and a platinum wire auxiliary electrode. Electrode Preparation. The glassy carbon disk electrode (3 mm diameter, BAS) was polished on a polishing cloth sequentially with diamond of decreasing particle size (15, 3, 1, (25) Hernandez, P.; Ada, E.; Hernandez, L. Fesenius' 2.Anal. Chem. 1987,327, 676. (26) Wang, J.; Bonkadar, M. Talanta 1988,35,277. (27) Labuda, J.; Plaskon, V. Anal. Chim. Acta 1990,228,259. (28) Navaratilova, Z. Electroanalysis 1991,3,799. (29)Hua, C.; Smyth, M. R Talanta 1992,39,391. (30) Oyama, N.; Anson, F. C. J. Electroanal. Chem. 1980,127,640. (31) Doblhofer, IC; Braun, H.; Lange, R J. Electroanal. Chem. 1986,206, 93. (32) Lindholm, B. J. Electroanal. Chem. 1988,250, 341. (33) Oh, S.-M.; Faulkner, L. R. J. Electroanal. Chem. 1989,269,77.
3572 Analytical Chemistry, Vol. 67, No. 79, October 7, 7995
and 0.05 pm) to a shiny surface. It was then rinsed with deionized water and further cleaned ultrasonically in 1:l nitric acid and deionized water successively. The three kinds of working electrodes, GFE, PVP/GCE, and PVP/GFE, used in this work were prepared as follows. Gold was deposited onto a glassy carbon substrate by adding 50 ppm AuCL- to the 0.05 M HC1 and 0.05 M KC1 supporting electrolyte medium at -0.1 V vs Ag/ AgCl for either 3 or 6 min in the preparation of the GFE. For the PVP/GCE, 4 p L of the coating solution was first spin-coated onto the glassy carbon electrode surface at 3000 rpm. A 0.25 wt % solution of PVP in methanol was used as the spincoating solution. The solution also contained 7% (vs the pyridine moiety) of 1,s dibromopentane as a cross-linking agent. A uniform thin film was formed after about 3 min of spinning. The coated electrodeswere then heated in an oven at 90 "C for about 2 h in order to hasten the cross-linking process and to prevent dissolution of the films in aqueous solutions. Finally, for the preparation of the PVP/ GFE, gold was deposited onto the PVP/GCE under the same conditions as those used in the preparation of the GFE, except that the deposition time was varied as required. Procedure. The freshly prepared PVP/GFE was dipped into the stirred analyte solution containing mercury(I0 at +0.3 V vs Ag/AgCl for the required time for preconcentration. Quantitative determinations were then performed in the square-wave (SW) mode. The potential range was set from -0.2 to +0.7 V vs Ag/ AgCl in the anodic direction for most cases. Actually, after the deposition at +0.3 V, scanning the potential from -0.2 or from +0.3 to +0.7 V produces the same result. The reason for changing the potential in the analytical procedure from -0.2 to +0.7 V was only for the convenience of examining the interference effect. Unless stated otherwise, a medium containing 0.025 M sulfuric acid and 0.005 M KC1 was used in the electrochemical experiments. Solutions and samples were detected without deoxygenation. After the voltammogram was recorded, the electrode was regenerated with 0.1 M HzS04 at +0.7 V vs Ag/ AgCl for 100 s. The renewed electrode was then checked in the supporting electrolyte to ascertain that it did not show any peak within the potential range before the next measurement. Groundwater and electroplating waste solution were collected and prepared as reported previously.2o Seawater was collected from the harbor of Taichung, Taiwan. After being filtered through membrane filters (0.45pm), the sample was stored in the dark at 4 "C. Photooxidation of seawater was accomplished by exposing the sample to a 1-kW UV lamp for 12 h. The standard addition method was used to evaluate the content of mercury in the water samples. RESULTS AND DISCUSSION Electrochemical Behavior of Mer-11)
on the PVP/ GFE. For the purpose of complexing Hg(I0 into anionic forms, the detection was chosen to proceed in the chloride medium since the cumulative formation constants for HgCl3- and HgQ- are 1.2 x l O I 4 and 1.2 x 1015, re~pectively.3~In this way, in the preconcentration step of the electrochemical stripping determination of mercury(I0, the mercurate(I0 anions can be accumulated by the anion exchanger (WP) of the PVP/GFE. In order to confirm the above expectation, the responses of the proposed method for the determination of mercury@) in different (34) Dean, J. A Lange's Handbook of Chemistty; McGraw-Hill Inc.: New York, 1973.
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