Anal. Chem. 1996, 68, 3290-3294
Epoxy-Graphite Tube Bulk-Modified with 2-Mercaptobenzothiazole as a Robust Electrode for the Preconcentration and Stripping Analysis of Hg(II) Miriam Rehana Khan and Soo Beng Khoo*
Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 0511
A novel method of fabricating a bulk-modified composite electrode is proposed. In this method, a preformed, porous graphite tube was impregnated with a modifier by thorough soaking in a solution containing the modifier. After drying, the modifier was sealed in place in the pores with epoxy. For the purpose of demonstrating the usefulness of the electrode, 2-mercaptobenzothiazole was chosen as the modifier and the modified electrode was used to preconcentrate mercury(II), followed by voltammetric stripping analysis. The fabricated electrode possessed good stability and had an extended lifetime. Precision for 10 consecutive differential pulse stripping experiments, for a 2 min preconcentration, gave relative standard deviations of 1.9% and 2.6% for Hg(II) concentrations of 1.00 × 10-6 and 1.00 × 10-8 M, respectively. Detection limit was estimated to be 3.0 × 10-9 M (S/N ) 3, 3 min preconcentration). A certified aqueous sample, USEPA WP 386, was employed for method testing. Chemically modified electrodes (CMEs) are useful analytical tools which offer high selectivity and sensitivity. Several methods have been used for electrode modification, including physical coating, covalent attachment, mixing of a modifier with carbon paste, and electropolymerization.1-3 However, with the majority of these surface modifications, deterioration and/or contamination usually occurs with usage and frequent cleaning and recoating may be necessary. This process may be tedious and time consuming. The exception to this is the modified carbon paste electrode which, because of its bulk-modified nature, can be simply resurfaced by removing a small amount of the paste. Recently, interest has been generated in the use of the bulk composite electrodes in electroanalysis. A bulk composite electrode can be defined as one whose material in the bulk consists of at least one conducting phase mixed with a nonconducting phase.4 Although carbon paste also falls under this broad definition, of interest here are those composites that have the same advantage as carbon paste of possessing rapidly and easily renewable surfaces but, in contrast to carbon paste, are mechanically robust and polishable and have prolonged lifetimes. To date, several types of such composites have been investigated.5-8 Bulk (1) Murray, R. W. Chemically Modified Electrodes In Electroanalytical Chemistry Bard, A. J., Ed.; Marcel Dekker: New York 1984; Vol. 13; p 191. (2) Abruna, H. D. Coord. Chem. Rev. 1988, 86, 135. (3) Kalcher, K. Electroanalysis 1990, 2, 419-433. (4) Tallman, D. E.; Petersen, S. L. Electroanalysis 1990, 2, 499-510. (5) Clem, R. G. Anal. Chem. 1975, 47, 1778-1784.
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incorporation of modifiers into composite electrodes yield chemically modified composite electrodes which have been employed in numerous analytical applications.9-11 Practically all the carbon-based chemically modified composite electrodes reported so far employed particulate graphite as the conducting phase. Fabrication usually involves thorough mixing of the graphite powder with the nonconducting phase (e.g., epoxy) and the modifier prior to casting into the required shape and size. Preformed graphite materials (e.g., rod, tube, etc.) have received little attention for such applications. These materials, due to their porous nature, are likely candidates for bulk modifications. Therefore, we report here a novel approach for fabrication of a mechanically robust, polishable, bulk-modified composite electrode. This electrode used a porous graphite tube whose pores were impregnated with a chemical modifier. After impregnation, the modifier was sealed in place by epoxy. For the purpose of this study, the model modifier chosen was 2-mercaptobenzothiazole and the analyte was the mercuric ion. The fabricated electrode was characterized voltammetrically and then investigated for the preconcentration and stripping analysis of Hg(II). An optimized procedure was then developed for the determination of Hg(II). EXPERIMENTAL SECTION Reagents. All chemicals used were of analytical reagent grade of better. Polyethylene bottles for storing stock solutions were soaked overnight in 10% nitric acid before use. Water was obtained from a Millipore Alpha Q water purification system (Millipore Corp.). A mercury(II) stock solution (0.125 M) was prepared by weighing the appropriate amount of the liquid metal (Aldrich, ACS reagent, 99.999%) into a volumetric flask and dissolving with a minimum amount of concentrated nitric acid (Merck, 65%) before making to the mark with water. 2-Mercaptobenzothiazole (MBTH) was recrystallized twice from ethanol and dried in the vacuum oven at 70 °C for 2 days. Electrode Fabrication. A graphite tube (3 mm o.d., 1 mm bore, 150 mm length, Matthey reagent, Johnson Matthey) was cut into shorter pieces of approximately 1 cm length. To a piece of these tubes, a 12 cm long, thick copper wire was inserted into (6) (7) (8) (9) (10)
Anderson, J. E.; Tallman, D. E. Anal. Chem. 1976, 48, 209-212. Wang, J. Anal. Chem. 1981, 53, 2280-2283. Davis, B. K.; Weber, S. G. Anal. Chem. 1975, 62, 1000-1003. Wring, S. A.; Hart, J. P. Anal. Chim. Acta 1990, 229, 63-70. Wang, J.; Golden, T.; Varughese, K.; El-Ryes, I. Anal. Chem. 1989, 61, 508-512. (11) Cespedes, F.; Martinez-Fabregas, E.; Bartroli, J.; Alegret, S. Anal. Chim. Acta 1993, 273, 409-417. S0003-2700(95)01098-5 CCC: $12.00
© 1996 American Chemical Society
one end so that about 2 mm extended into the tube. This was sealed in place by conducting silver epoxy. A two-component epoxy (EpoTek 353ND Epoxy Technology Inc.) was prepared by thorough mixing of the resin with the hardener (10 parts by weight resin to 1 part by weight hardener). Some of this was transferred to a small test tube. The graphite tube with the copper wire attached to one end was then placed in the test tube so that it was completely immersed in the epoxy. Prior to this, the bore of the graphite tube had been completely filled with epoxy by pushing this into the open end with a thin wire. After a 10 min immersion, the test tube was sonicated for 1 h. The EpoTek 353ND remained fairly liquid even after more than 1 h at room temperature. This epoxy is said to have excellent wicking characteristics and good chemical resistance. After sonication, the epoxy-treated graphite was inserted into one end of a glass tube (3.5 mm i.d., 6 mm o.d., 9 cm length) and completely sealed with epoxy. The assembly was put into the oven at 60 °C for 3 days for the epoxy to cure. After curing, the electrode was polished with silicon carbide paper to expose the graphite and then successively finer grades of abrasive paper to smooth it. Final polishing was with alumina (0.3 µm)-water slurry on polishing cloth. For the MBTH-modified electrodes, four 1 cm pieces of graphite tubes were individually weighed and put into capped bottles containing freshly prepared solutions (0.050, 0.10, 0.25, and 0.50 M) of MBTH in acetone, one to each. A Teflon-coated magnetic stirring bar was also put into each bottle. The bottles were then wrapped with aluminium foil to protect from light and placed on magnetic stirrers. Continuous stirring was carried out for 24 h to ensure complete permeation of the solution throughout the tubes. (Graphite tubes rather than rods were chosen for this work because it was thought that they allowed more efficient penetration of the MBTH, i.e., from the outside and also from within the bore. However, there is no reason why graphite rods could not achieve the same results.) After this, the tubes were taken out and each was given a quick rinse with a small amount of acetone to remove excess MBTH. The tubes were placed in the oven at 50 °C overnight to dry. After drying and cooling to room temperature in a desiccator, they were again individually weighed. Subsequently, modified electrodes were fabricated by following the same procedure as for the unmodified electrode. Apparatus. All electrochemical experiments were performed with a Princeton Applied Research Corp. Model 264A polarographic analyzer/stripping voltammeter (PARC, EG&G) coupled to a Graphtec WX 2400 X-Y recorder (Graphtec Corp.). The electrochemical cell was a homemade glass cell of about 5 mL capacity with the reference electrode (Ag/AgCl, saturated KCl) separated from the working and counter (platinum wire coil) electrodes by a salt bridge. A microprocessor-controlled pH meter was used for all pH measurements. All experiments were carried out at an ambient temperature of 25 ( 1 °C. Procedure. Manual polishing of the modified electrode was performed only at the beginning of each day. This was done by first polishing on silicon carbide paper No. 1500, followed by an alumina (0.3 µm)-water slurry on polishing cloth. After rinsing, the adhering alumina particles were removed by wiping on wetted filter paper (Whatman No. 540) and finally the electrode was sonicated for 2 min. After this initial pretreatment, cleaning of the electrode at the end of each experiment was done electrochemically.
Except for differences in experimental parameters and media, the analysis cycle for Hg(II), which consisted of three steps (viz. preconcentration, electroreduction, and anodic stripping) incorporating medium exchange, has been described elsewhere.12 The relevant experimental conditions in subsequent discussions will be specified when needed. However, it should be stated that, at the end of each stripping step, it was only necessary to hold the electrode at +0.3 V for 3 min followed by rinsing to clean the electrode. No further polishing was necessary. Preparation of Samples for Mercury Determination. For the purpose of testing the modified electrode for Hg(II) determination, a United States Environmental Protection Agency water pollution quality control sample12 (USEPA WP386) was used. This sample was diluted 100 times with acetate buffer (0.10 M, pH 4.3) prior to use. RESULTS AND DISCUSSION Voltammetry at an Epoxy-Impregnated Graphite Electrode (EIGE) and a MBTH-Modified Epoxy-Impregnated Graphite Electrode (MBTH-EIGE). The EIGE was tested with the ferricyanide ion (0.50 mM, in 1 M KCl, pH 3.0). The cyclic voltammograms (CVs) exhibited reversible behavior (∆Ep ) 65 mV at 100 mV s-1) and were almost identical with those found for the glassy carbon electrode. A plot of peak current versus square root of the scan rate up to 500 mV s-1 gave a straight line through the origin, and peak potentials remained essentially unchanged with these scan rates. One of the main problems with using epoxy to seal electrodes is the question of the inertness of the epoxy in acidic and basic media and also in organic solvents. We examined this by soaking the EIGE for several hours each in 1 M HCl and 1 M NaOH. After each soaking, the CVs for ferricyanide were obtained. In both cases, there were no significant changes in the shape, position, and peak currents compared to CVs obtained prior to soaking. The EIGE was not examined in organic solvents, but Davis and Weber8 using the same epoxy in fabricating a microcellular carbon foam-epoxy composite electrode found that it did not soften in common organic solvents, including dimethylformamide. These results indicate that the EIGE can be beneficially employed in voltammetric studies. Figure 1a shows that the CV at the unmodified EIGE, in deaerated 0.10 M HCl, exhibited only residual current between -0.45 and +0.35 V. The cathodic and anodic breakdown limits were -0.9 and +0.8 V, respectively. In the same medium, the MBTH-EIGE gave similar results except for a slightly higher charging current. For the MBTH-EIGE, if the potential scan on the positive side were extended to +1.0 V, an irreversible wave starting at +0.65 V with peak potential at about +0.80 V can be seen. This oxidation wave was attributed to immobilized MBTH. A solution of MBTH in 0.10 M HCl showed a similar irreversible oxidation wave at the glassy carbon electrode but with peak potential at about +0.65 V (not shown). There was no significant reduction wave for MBTH, either immobilized or in solution. Lakshminarayanan et al.13 found similar electrochemical behavior for MBTH. Figure 1b shows the CV obtained after the blank EIGE had been dipped into the acetate buffer containing 5.00 × 10-5 M Hg(II). It is obvious that no preconcentration of Hg(II) (12) Cai, Q.; Khoo, S. B. Anal. Chem. 1994, 66, 4543-4550. (13) Lakshminarayanan, V.; Kanan, R.; Rajagopalan, S. R. J. Electroanal. Chem. 1994, 364, 79-86.
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Figure 2. Differential pulse voltammograms at the MBTH-EIGE in 0.10 M HCl. For 1.75% (w/w) MBTH, (a) accumulate for 2 min in blank acetate buffer (0.10 M, pH 4.3) and (b) accumulate for 2 min in acetate buffer containing 5.00 × 10-7 M Hg(II). Ed ) -0.45 V; td ) 15 s; pulse amplitude 50 mV; scan rate 10 mV s-1.
stripping step
Figure 1. Cyclic voltammograms in 0.10 M HCl. (a) EIGE, accumulate for 1 min in blank acetate buffer (0.10 M, pH 4.3); (b) same as (a) but buffer solution contained 5.00 × 10-5 M Hg(II); (c) MBTHEIGE (1.75% w/w MBTH), accumulate for 1 min in the same acetate buffer containing 5.00 × 10-5 M Hg(II). Scan rate was 50 mV s-1 in all cases.
occurred. The small stripping peak at +0.17 V was due to some adsorbed Hg(II) which was not removed even after rinsing. In contrast, a strong stripping peak with peak potential at +0.17 V is clearly shown in Figure 1c at the MBTH-EIGE after immersion in the same Hg(II)-containing acetate buffer. The stripping peak was due to the reoxidation of accumulated mercury following its reduction at -0.45 V. On the reverse scan, a tiny peak at +0.10 V was seen in Figure 1c due to the reduction of some Hg(II). The stripping peak at +0.17 V was found to increase with increasing concentration of Hg(II) and longer accumulation time and also to vary with the loading of MBTH in the MBTH-EIGE. Thus, the incorporation of MBTH into the EIGE enabled the open circuit preconcentration of Hg(II). Experiments with the mercurous ion, Hg22+ revealed that this ion was not complexed by MBTH. The differential pulse voltammograms of the MBTH-EIGE are shown in Figure 2. As expected, no stripping peak can be seen when preconcentration was carried out in the blank acetate buffer (Figure 2a), whereas preconcentration in the acetate buffer containing Hg(II) gave rise to a sharp peak with peak potential at +0.12 V. Based on these and other experiments, the processes occurring during the various steps are proposed to be as follows:
preconcentration step MBTH(s) + Hg2+ ) Hg(MBT)2,(s) + 2H+ reduction step Hg(MBT)2,(s) + 2H+ + 2e- ) Hg(0)(s) + 2MBTH(s) 3292 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
Hg(0)(s) ) Hg2+ + 2ewhere subscript s denotes electrode surface. In the stripping step, the Hg2+ obtained was believed to go into solution phase. This was supported by a number of observations. In the CV of Figure 1c, on the reverse scan, only a tiny peak was observed for Hg2+ reduction implying that most of the Hg2+ has left the surface and diffused out into solution. At the end of each experiment, the electrode was cleaned simply by sitting at +0.30 V for 3 min. The fact that repeatable results could be obtained in a series of successive runs indicated that negligible Hg2+ was left on the surface. For confirmation, a separate experiment was performed whereby at the end of an analysis cycle the electrode was held at +0.30 V for 15 s to ensure that all the Hg(0) had been oxidized to Hg2+. The electrode was then removed, rinsed as usual, and allowed to go through the reduction and stripping steps (but without preconcentration). If there were any Hg2+ retained on the surface, a significant stripping peak would be observed, but in this case, no such peak was seen. Factors Affecting Preconcentration and Differential Pulse Anodic Stripping of Mercury. Effect of MBTH Loading. The effect of various loadings of MBTH ranging from 0% to 1.75% (w/w) was investigated. It should be noted that the amount of MBTH that could be incorporated into the graphite tube electrodes was limited by the solubility of MBTH in acetone (about 10 g/100 mL).14 The stripping peak height was found to increase with increasing MBTH content. Therefore, the highest content of 1.75% (w/w) MBTH was chosen for subsequent studies. The Influence of Stripping Medium. Alkaline media could not be used for stripping because MBTH is soluble in aqueous alkali.14 Aqueous solutions of HCl, HNO3, H2SO4, HClO4, CH3COONH4, KCl, KBr, KSCN (all at 0.10 M), and acetate buffer (0.10 M, pH 5.0) were tested as stripping media. For these experiments, preconcentrations were done in 0.10 M HCl containing 5.00 × 10-6 M Hg2+, for 2 min. Those media that did not complex Hg(II) (HNO3, H2SO4, HClO4, CH3COONH4, acetate buffer) strongly were (14) Ueno, K.; Imamura, T.; Cheng, K. L. Handbook of Organic Analytical Reagents 2nd ed.; CRC Press: Boca Raton, FL 1992; pp 483-485.
found to be less satisfactory. The mercury stripping peaks occurred at more positive potentials and the peak widths were larger, besides being less sensitive. For example, in 0.10 M HClO4, the stripping peak potential was at +0.41 V and peak width at half-height was 65 mV. At this potential, the stripping peak was close to the commencement of oxidation of the immobilized MBTH. This would hinder the process of electrochemical cleaning and have a deleterious effect on the electrode surface, leading to poorer performance subsequently. The strongly complexing media (chlorides, bromide, thiocyanate) generally produced more sensitive and sharper peaks. Further, the peaks occurred at less positive potentials, thereby eliminating the problems mentioned. The shift in oxidation potential to less positive values could be explained by stabilization of Hg(II) by complexation. Of the complexing media, 0.10 M HCl showed the highest sensitivity, with peak potential at +0.12 V and peak width at half-height of 45 mV. Various concentrations of HCl, from 0.010 to 1.00 M were studied. The stripping peak height for 5.00 × 10-6 M Hg2+ increased with increasing HCl concentration until 0.10 M, after which it remained constant up to 1.00 M. Therefore, 0.10 M HCl was chosen. Influence of Accumulation Medium. Alkaline media were not studied for the same reason as before and also because the Hg(MBT)2 complex is soluble in alkaline solution.15 HCl, H2SO4, HNO3 (all 0.10 M), 0.20 M acetate buffer (pH 2.4-5.9), 0.20 M sodium hydrogen phosphate-phosphoric acid buffer (pH 1.36.3), and 0.20 M citric acid-sodium hydroxide buffer (pH 1.86.0) were investigated as media for the preconcentration of Hg(II). Of these, the acetate buffer gave the best sensitivity besides providing a buffered environment. Studies of the variation of stripping peak height with pH of the acetate buffer indicated that the optimum pH range for Hg(II) preconcentration was from pH 3.8 to 4.8. This corresponds to the best buffer range for the acetate buffer. A probable explanation for this is that a buffered environment is conducive for the complexation reaction. Otherwise, an accumulation of hydrogen ions near the electrode surface will lower the pH and make the forward reaction in the preconcentration step (see earlier equation) less favorable, thereby decreasing the peak height. The decrease in peak height at the higher pH values (pH >4.8) was probably due to some hydrolysis of Hg(II)16 and increasing solubility of Hg(MBT)2. The concentration of the acetate medium from 0.050 to 1.00 M was also studied but did not have much effect on the stripping peak height. Thus, for all subsequent studies, acetate buffer (0.10 M) at a pH of 4.3 was chosen. Effect of Reduction Potential, Ed, and Reduction Time, td. It was found that at Ed less negative than -0.45 V the reduction of Hg(II) was not complete, as evidenced by the decrease in peak current (not shown here). From -0.45 to -0.55 V the stripping peak height was constant. However, at potentials more negative than -0.55 V the peak height decreased again, probably due to interference from some extraneous process. In the constant peak height region, reduction of Hg(II) was complete to give maximum sensitivity. Thus, Ed at -0.45 V was chosen as optimum. At this Ed, the reduction time required to achieve maximum reduction of accumulated Hg(II) was examined. For td from 0 to 10 s, the stripping peak current increased approximately linearly (15) Bobtelsky, M.; Jungreis, E. Anal. Chim. Acta 1955, 12, 562-571. (16) Baes, C. F. Jr.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York 1976; pp 301-312.
Figure 3. Stripping peak current versus accumulation time. Accumulation medium, acetate buffer (0.10 M, pH 4.3) containing (a) 1.00 × 10-7 M Hg(II), (b) 5.00 × 10-7 M Hg(II), or (c) 5.00 × 10-6 M Hg(II); stripping medium 0.10 M HCl. All other conditions same as for Figure 2.
and from 10 to 30 s, it was constant. Therefore, a td of 15 s was judged to be sufficient. Effect of Open Circuit Preconcentration Time, ta. The dependences of the stripping peak current on ta for three different Hg(II) concentrations are shown in Figure 3. It can be seen that the rate of mercury uptake was dependent on Hg(II) concentration, being faster at higher concentrations as evidenced by the steeper slopes. Similarly, the times required to attain equilibrium were shorter at higher Hg(II) concentrations, these being 3, 8, and 11 min for Hg(II) concentrations of 5.00 × 10-6, 5.00 × 10-7, and 1.00 × 10-7 M, respectively. Calibration. Under the optimized conditions, the differential pulse stripping peak height had a linear relationship with Hg(II) concentration. The calibration plot from 5.00 × 10-8 to 5.00 × 10-6 M Hg(II) for an accumulation time of 3 min was linear with regression equation ip (µA) ) 0.084 + 0.522C (µM) and a correlation coefficient of 0.999. Precision, Detection Limit, and Electrode Stability. The precisions in peak heights for 10 consecutive runs each of 1.00 × 10-6 and 1.00 × 10-8 M Hg(II) (ta ) 2 min) were 1.9% and 2.6% (relative standard deviations), respectively. The variations in peak heights on a day-to-day basis were also investigated. For a 1.00 × 10-8 M Hg(II) solution (ta ) 2 min), the average peak height for three successive runs was recorded each day, for 5 consecutive days. Over this 5 day period, the relative standard deviation of the average peak height was found to be 5.70%. For a signal-tonoise ratio of 3, the detection limit was estimated to be 3.0 × 10-9 M, for a 3 min accumulation. This limit could possibly be lowered if the accumulation time were increased. With regard to electrode stability, this work was completed within a period of 6 months from the time of fabrication of the MBTH-EIGE (1.75% w/w). When not in use, it was stored by wrapping in aluminium foil and putting in a box away from light. Since the completion of this work, two years had already elapsed. Recently, we took the electrode out from storage and obtained the stripping peak for a 1.00 × 10-6 M Hg(II) (ta ) 2 min) and compared it with the peak height, for the same solution under the same conditions, obtained about two and a half years earlier. The peak position and width were unchanged, and the peak height was 92% of that obtained earlier. Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
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Interferences. When the optimized procedure was applied to the determination of 5.00 × 10-7 M Hg(II), with an accumulation time of 2 min, no interferences were observed in the presence of 2.50 × 10-5 M each of Na+, K+, Cs+, Ca(II), Mg(II), Zn(II), Ni(II), Ba(II), Mn(II), Co(II), Cd(II), Pb(II), Cu(II), Be(II), Ge(II), Sn(II), V(II), Bi(III), Al(III), Cr(III), Fe(III), As(III), and Se(IV) and 5.00 × 10-7 M Ag(I). For Bi(III), an additional stripping peak was observed at -0.70 V. However, this Bi(III) peak did not interfere in any way with mercury stripping. Ag+ was also accumulated and gave a stripping peak which was about 50 mV more positive than the mercury peak such that the presence of 1.00 × 10-6 M or greater concentration of Ag+ gave rise to interference. For 5.00 × 10-7 M Hg(II) determination, 5.00 × 10-6 M Ag+ interference can be masked by using a mixture of (NH2)2CO (5.00 × 10-4 M), and Br- (5.00 × 10-4).17 The urea reduced the Ag+ to the metal while the Br- reinforced the masking process by complexing/precipitating any unreduced Ag+. Organic and other inorganic substances may also interfere by competitive complexation and masking of Hg(II). In the determination of 5.00 × 10-7 M Hg(II) (ta ) 2 min), no interference was observed after addition of 5.00 × 10-4 M each of KCl, sodium citrate, ammonium tartrate, KSCN, (NH2)2CO, (NH4)2C2O2, NH2OH‚HCl, and KBr. Comparison with Other Electrodes and Stripping Methods. The performance of the graphite tube-epoxy electrode for voltammetry has been established, and in this respect, it is comparable if not better than other graphite composite electrodes.5-8 Its method of preparation may have some advantage over mixing graphite powder with epoxy when a high proportion of graphite is needed for good conductivity.6 Here, the difficulty of achieving homogeneous mixing, arising from the stickiness of the epoxy, is especially relevant in the presence of the modifier. We believe that the present design is advantageous toward achieving a more uniform distribution of the modifier in the bulk of the electrode. Although we do not have any similar data on the long-term stability of chemically modified epoxy-graphite electrodes, our results as (17) Karlik, M.; Srank, Z. Sci. Pap. Prague Inst. Chem. Technol. Anal. Chem. 1981, H16, 55-64. (18) Cai, Q.; Khoo, S. B. Electroanalysis 1995, 7, 379-385. (19) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York 1969; pp 26-121. (20) Labuda, J.; Plaskan, V. Anal. Chim. Acta 1990, 288, 259-263. (21) Alexio, L. M.; Souza, M. de F. B.; Godinto, O. E. S.; Oliveira, Neto, G. de O.; Gushikem, Y. Anal. Chim. Acta 1993, 271, 143-148. (22) Navratilova, Z. Electroanalysis 1991, 3, 799-802. (23) Cha, S. K.; Ahn, B. K.; Huang, J. U.; Abruna, H. D. Anal. Chem. 1993, 65, 1564-1569. (24) Wang, J.; Bonakdar, M. Talanta 1988, 35, 277-280. (25) Cai, X.; Kalcher, K.; Lintschinger, J.; Neuhold, C. Mikrochim. Acta 1993, 112, 135-146. (26) Turyan, I.; Mandler, D. Nature 1993, 362, 703-704.
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discussed earlier are gratifying. However, it should be pointed out that lifetime and stability would be dependent on experimental conditions and the modifier and epoxy used. The obvious advantages of an epoxy-graphite composite electrode over a carbon paste electrode are its polishability and mechanical strength. Another advantage we have observed is that, from our experience, the carbon paste electrode almost always show a broad reduction wave18 at negative potentials which limits its usefulness at these potentials. This is due to oxygen absorbed in the paste or adsorbed on the carbon particles,19 which is not removable by solution deaeration. The present electrode does not suffer from this shortcoming. We have not been able to find any report in the literature on a chemically modified epoxy-graphite electrode specially targeted for Hg(II). Labuda and Plaskan20 bulk-modified graphite powder with diphenylcarbazone and compressed this mixture (15.2 MPa) to form an electrode for Hg(II) determination. Using differential pulse stripping voltammetry, a detection limit of 5 × 10-8 M Hg(II) was obtained for a 10 min preconcentration, with an rsd of 10%. In recent years there have been numerous studies using CMEs for the determination of Hg(II), the majority of them carbon paste electrodes employing differential pulse voltammetry. Of those referenced here,21-26 the detection limit ranged from 5 × 10-10 (5% rsd, ref 23) to 2 × 10-6 M (2 min, 10% rsd, cyclic voltammetry, ref 22), with the majority banding in the 10-8 M region. Perhaps the lowest detection limit reported was 1 × 10-12 M (5 min, 3.3% rsd) using a glassy carbon modified with 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (Kryptofix222).26 Thus, looking at these figures of merit, the present electrode (detection limit (3 × 10-9 M, 3 min, rsd (3%) compares favorably. Analysis. The developed method was tested using the prepared USEPA WP 386 sample (see Experimental Section). Analyses were carried out using the linear regression equation of a calibration plot. The experimental value obtained for the Hg(II) concentration (5.1 µg L-1, rsd 3.71%, n ) 6) was in good agreement with the certified value (5.0 µg L-1). ACKNOWLEDGMENT This work was supported by a grant from the National University of Singapore. M.R.K. thanks Mr. Cai Qiantao for helpful discussions. Received for review November 3, 1995. Accepted June 17, 1996.X AC951098G X
Abstract published in Advance ACS Abstracts, August 1, 1996.
