Anal. Chem. 1996, 68, 3966-3972

A Voltammetric Method for the Determination of Lead(II) at a Poly(4-vinylpyridine)/Mercury Film Electrode Jyh-Myng Zen* and Jyh-Way Wu

Department of Chemistry, National Chung-Hsing University, Taichung, Taiwan 402

A novel voltammetric method was developed for the determination of trace lead(II) in real samples by squarewave anodic stripping voltammetry at a poly(4-vinylpyridine)/mercury film electrode (PVP/MFE). By applying a preconcentration potential of -1.0 V vs Ag/AgCl in 0.025 M sulfuric acid, lead(II) can be converted into the anionic forms. The analytes are then preconcentrated onto the PVP/MFE by the ion-exchange effect of the PVP. The fairly good solubility of lead in mercury subsequently helps to increase the preconcentration effect. Various factors influencing the determination of lead by the proposed voltammetric method were thoroughly investigated in this study. The main advantages of the method are good resistance to interferences, easy detection without deoxygenating, and excellent electrode renewal. The analytical utility of the PVP/MFE using the proposed voltammetric method in the determination of lead is demonstrated by application to various water samples. There is a growing need for improved analytical methods for monitoring lead. Among the many methods developed, electrochemical stripping analysis has been widely used for this task.1-5 Nevertheless, improved electrochemical techniques are demanded to address the interference problems. There is a clear advantage for the selective accumulation of metals as their anionic complexes, because most species of analytical interferents exist as cations in sample solutions. Recently, our group has reported a poly(4-vinylpyridine)/gold film electrode (PVP/GFE) for the determination of trace mercury in real samples by square-wave anodic stripping voltammetry (SWASV).6 Mercury is preconcentrated as its anionic forms in chloride medium onto the PVP/ GFE by the ion-exchange effect of the PVP. The high solubility of mercury in gold subsequently helps to increase the preconcentration effect. In comparison with the conventional GFE, the PVP/GFE showed less interferences from surface-active compounds and common ions, increased sensitivity when used in conjunction with SWASV, and better mechanical stability of the gold film. Since the reduction of oxygen is a totally irreversible reaction, the SW wave form does not respond to it, thus, providing a means of carrying out the analysis without deoxygenating the (1) Boeckx, R. L. Anal. Chem. 1986, 58, 274A. (2) Wang, J. Stripping Analysis: Principles, Instrumentation and Application; VCH: Deerfield Beach, FL, 1985. (3) Boone, J.; Hearn, T.; Lewis, S. Clin. Chem. 1979, 25, 389. (4) Wang, J.; Lu, J.; Yarnitzky Anal. Chim. Acta 1993, 280, 61. (5) Zen, J.-M.; Huang, S.-Y. Anal. Chim. Acta 1994, 296, 77. (6) Zen, J.-M.; Chung, M.-J. Anal. Chem. 1995, 67, 3571.

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sample.7,8 Considering the similar idea that of the PVP/GFE, a poly(4-vinylpyridine)/mercury film electrode (PVP/MFE), was further developed for the determination of bismuth(III) and thallium(III).9,10 The same advantages as those of the PVP/GFE mentioned above were also achieved by the PVP/MFE. Overall, the PVP-modified electrodes are proved to be less subject to interferences from common ions than those of other polymermodified electrodes previously reported.6,9,10 Instead of developing a new PVP-modified electrode, we report here a novel voltammetric method using the PVP/MFE for the determination of lead. The key idea is to convert lead into anionic forms simply by applying a preconcentration potential negative enough to induce reactions that can generate hydroxide ions at the PVP/MFE. It is expected that the hydroxide ions produced at the electrode surface can then complex with lead(II) and inevitably convert lead(II) into anionic forms. Note that the hydrolysis of lead is rather complex and [Pb(OH)6]4- is one of the most likely species formed since the cumulative formation constant (log β6) of [Pb(OH)6]4- is 61.0.11 Subsequently, the anionic forms of lead can be accumulated onto the PVP/MFE by the ion-exchange effect of the PVP and eventually retained in the PVP/MFE as amalgam. In this paper, various factors influencing the determination of lead by the proposed voltammetric method were investigated. Typical interferences that can occur in water samples were discussed. The analytical utility of the proposed voltammetric method in the determination of lead at the PVP/ MFE was demonstrated by application to various water samples. EXPERIMENTAL SECTION Chemicals and Reagents. PVP (MW 50 000) solution in methanol containing ∼20 wt % polymer was obtained from the Aldrich Chemical Co. (Milwaukee, WI). Nafion perfluorinated ionexchange powder, 5 wt % solution in a mixture of lower aliphatic alcohols and 10% water, was also obtained from Aldrich. Standard solutions of mercury and lead (1000 mg/L, AAS grade) were brought from Merck. All 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. (7) Lovric, M.; Branica, M. J. Electroanal. Chem. 1987, 226, 239. (8) Lovric, M.; Komorsky-Lovric, S. J. Electroanal. Chem. 1988, 248, 239. (9) Zen, J.-M.; Chung, M.-J. Anal. Chim. Acta 1996, 320, 43. (10) Zen, J.-M.; Wu, J.-W. Electroanalysis, in press. (11) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill Inc.: New York, 1973. S0003-2700(96)00431-3 CCC: $12.00

© 1996 American Chemical Society

Apparatus. Electrochemistry was performed on a BAS CV50W electrochemical analyzer. A BAS Model VC-2 electrochemical cell was employed in these experiments. The three-electrode system consists of one of the following working electrodes: MFE, PVP-coated glassy carbon electrode (PVP/GCE), Nafion/GCE, and PVP/MFE, a Ag/AgCl reference electrode (Model RE-5, BAS), and a platinum wire auxiliary electrode. Procedure. The glassy carbon disk electrode (3-mm diameter, BAS) was polished with the BAS polishing kit, and the working electrodes were prepared generally following the procedures mentioned previously.6 The coating solution contained 20% (vs the pyridine moiety) of 1,5-dibromopentane as a cross-linking agent. Mercury was deposited onto the substrate by adding 10 ppm mercury to the 0.05 M H2SO4 and 0.1 M KCl supporting electrolyte medium at -0.5 V vs Ag/AgCl for the time required. The freshly prepared PVP/MFE was dipped into the stirred 0.025 M sulfuric acid solution containing lead at -1.0 V vs Ag/AgCl for the required preconcentration time. Quantitative determinations were then performed in the SW mode. The potential range was set from -0.2 to -0.7 V vs Ag/AgCl in the anodic direction for most cases. Unless stated otherwise, a medium containing 0.025 M sulfuric acid was used in the electrochemical experiments. Solutions and samples were detected without deoxygenating. The electrode was regenerated immediately after recording the voltammogram. The renewed electrode was then checked inthe supporting electrolyte to ascertain that it did not show any peak within the potential range before next measurement. Groundwater and electroplating waste solution were collected and prepared as reported previously.12 The standard addition method was used to evaluate the content of lead in the water samples. RESULTS AND DISCUSSION Electrochemical Behavior of Lead on the PVP/MFE. For the purpose of converting lead into anionic forms, the detection must proceed under suitable conditions to generate the hydroxide ions. It is expected that the hydroxide ions produced at the electrode surface can consequently complex with lead(II) and convert it into anionic forms. One of the most likely species formed is [Pb(OH)6]4- since the cumulative formation constant of [Pb(OH)6]4- is very large (log β6 ) 61.0).11 In this way, in the preconcentration step, the anionic forms of lead can be accumulated by the anion exchanger (PVP) of the PVP/MFE. To confirm the above expectation, two issues must be addressed. First, can lead cations really complex with hydroxide ions and convert into anionic forms? Second, under what reactions can the hydroxide ions be generated? To answer the first question, the response of the proposed method for the determination of 100 ppb lead by both the PVP/ GCE and the Nafion/GCE was studied. As shown in Figure 1A, the accumulation of lead is favored at more negative potentials for the PVP/GCE. In contrast, for the Nafion/GCE, the voltammetric response was found to decrease as the preconcentration potential was set more negative, as shown in Figure 1B. These results confirm that lead(II) cations are indeed complexed into anionic forms in the preconcentration step. As to the second question, two of the most possible reactions that can generate the hydroxide ions are listed below: (12) Zen, J.-M.; Lee, M.-L. Anal. Chem. 1993, 65, 3238.

Figure 1. SW voltammograms at variable preconcentration potential for 100 ppb lead in 0.025 M H2SO4 supporting electrolyte on (A) PVP/ GCE and (B) Nafion/GCE. SW parameters: modulation amplitude 50 mV, modulation frequency 200 Hz, and effective scan rate 800 mV/s.

2H2O + 2e- ) H2 + 2OHE° ) -0.8277 V vs NHE

(1)

SO42- + H2O + 2e- ) SO32- + 2OHE° ) -0.93 V vs NHE

(2)

If the water reduction reaction dominates the generation of the hydroxide ions, the hydrogen evolution at the electrode surface apparently will render the quantitative measurements difficult. This is indeed the case when the preconcentration potential is more negative than -1.2 V vs Ag/AgCl, as will be discussed in more detail. Whereas, there is no such problem for the sulfate reduction reaction. Furthermore, due to the anion-exchange property of the PVP, the concentration of sulfate ions in the PVP film is expected to be much higher than that in solution. It is therefore believed that the main source of the hydroxide ions could be the reduction reaction of the sulfate ions. To confirm the expectation, three different acids, H2SO4, HNO3, and HCl, were chosen as supporting electrolyte for the detection of lead by the PVP/GCE and the results obtained as shown in Figure 2A. As can be seen, the oxidation peak of lead can be detected only in the 0.01 M H2SO4 medium. To further demonstrate that the increase in lead response was indeed from the reduction of the sulfate ions, the same experiments were repeated by adding 0.01 M Na2SO4 into HNO3 and HCl mediums. Panels B and C of Figure 2 show the results obtained with and without the addition of Na2SO4. Again, the occurrence of the lead responses for both cases strongly indicates that the reduction reaction of sulfate ions is indeed the main source of the hydroxide ions. Since the PVP polymer can be protonated more completely in more acidic solution, the detection must be carried out in acidic solution to increase the anion-exchange capacity of the protonated PVP. This may raise another problem as to how a highly alkaline environment could be created in the vicinity of the electrode surface in addition to the neutralization of 0.025 M sulfuric acid. To answer this problem, the reduction reaction of the sulfate ions as shown previously must first be considered. As can be seen, 1 mol of sulfate ion is consumed with the production of 1 mol of sulfite ion and 2 mol of hydroxide ion in the sulfate reduction reaction. Because of the existence of the sulfite ions produced during the reduction of the sulfate ions, the neutralization of 0.025 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

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Figure 2. (A) SW voltammograms at PVP/GCE for 100 ppb lead with tp ) 60 s and pp ) -1.2 V vs Ag/AgCl in (a) 0.01 M H2SO4, (b) 0.01 M HCl, and (c) 0.01 M HNO3 supporting electrolyte. (B) Same as (A-b) with (b) and without (a) the addition of 0.01 M Na2SO4. (C) Same as (A-c) with (b) and without (a) the addition of 0.01 M Na2SO4. SW parameters as in Figure 1.

Figure 3. SW voltammograms at (A) GCE and PVP/GCE for 100 ppb lead with tp ) 180 s and pp ) -1.2 V vs Ag/AgCl and (B) MFE and PVP/MFE for 10 ppb lead with tp ) 60 s and pp ) -1.0 V vs Ag/AgCl in 0.025 M H2SO4 supporting electrolyte. SW parameters as in Figure 1.

M sulfuric acid is therefore unnecessary. In this way, most of the hydroxide ions produced in the vicinity of the electrode surface were left for the complexation with lead cations. Moreover, considering that the concentration of sulfate ions in the PVP film is much higher than that in solution due to the anion-exchange property of the PVP, a highly alkaline environment could thus be created in the vicinity of the electrode surface. To demonstrate the proper function of the PVP/MFE, the functions of both the PVP and the mercury films in the detection of lead were further studied. The voltammetric responses of 100 ppb lead preconcentrated at -1.2 V vs Ag/AgCl with 3-min preconcentration time at a bare GCE and the PVP/GCE are shown in Figure 3A. As can be seen, there is a clear advantage to using the PVP/GCE in the detection of lead. Obviously, the lead anions are efficiently incorporated into the PVP film. The same experiments were then repeated with both the MFE and the PVP/MFE for comparison. Note that the detection was preconcentrated at -1.0 V vs Ag/AgCl with a 1-min preconcentration time for 10 ppb 3968 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

lead. As shown in Figure 3B, the voltammetric response shows a much better performance at the PVP/MFE. Considering the 10 times lower concentration of lead and 3 times shorter preconcentration time, the signal obtained for the PVP/MFE (Figure 3B) is apparently much higher than that of the PVP/GCE (Figure 3A). These results further confirm the proper functions of the PVP/MFE; i.e., the lead anions are incorporated into the PVP film and then accumulated by mercury. The other possibility for the increased sensitivity on the PVP/ MFE, as compared to the MFE, is as follows: During electrolysis in acid solution, PVP only slightly decreases the diffusion transport of reducible lead to the electrode surface where reduction and amalgamation take place. Approximately the same amount of lead is amalgamated on the PVP/MFE as on the MFE. During stripping from a coated electrode, the lead ions formed remain close to the electrode surface due to the quiescent conditions created by the coating. This means that as the SW is applied the same ion can be re-reduced and re-oxidized several times, increasing sensitivity. On a MFE the hydrodynamic conditions are such that the lead ions formed leave the vicinity of the surface and cannot “re-used”. To rule out the possibility that the increase in sensitivity on the PVP/MFE was simply an effect of the fast SW technique, experiments involving linear potential ramps were also performed for comparison as shown in Figure 4. As can be seen, compared to the MFE, an increase in sensitivity was obtained with the PVP/MFE. The increase in sensitivity on the PVP/MFE was therefore not simply an effect of the fast SW technique. Besides, if the increase in sensitivity on the PVP/ MFE was simply an effect of the fast SW technique, the same advantage should also be able to apply to other metals. Figure 5 shows the same SWASV experiments as those in Figure 3A using PVP/GCE for copper and cadmium. As can be seen, in both cases, due to the repulsive force between the cations and the anionic PVP film, the peak currents were higher at the GCE than those at the PVP/GCE. The increase in sensitivity is therefore a unique property for lead, and the mechanism of converting to anionic forms cannot be applied to copper and cadmium. Again,

Figure 4. Linear scan voltammograms at (A) PVP/MFE and (B) MFE for 10 ppb lead with tp ) 60 s asnd pp ) -1.0 V vs Ag/AgCl in 0.025 M H2SO4 supporting electrolyte. Scan rate was 800 mV/s.

these results confirm that the lead anions are incorporated into the PVP film and then accumulated by mercury. Optimum Conditions for Analysis. To arrive at the optimum conditions for lead determination, there are two aspects that should be considered: the electrode and the detection. As to the electrode aspect, the principal factors governing the performance of the PVP/MFE are the thickness of the PVP film and the deposition of mercury. As to the detection aspect, the factors consist of the solution pH, the preconcentration time, the preconcentration potential, and the SW parameters. Electrode. The thickness of the PVP film directly controls the electrode performance. The optimum film thickness depends on both the diffusion processes of the lead anions in the film and the maximum lead loading that does not affect the adhesion of the film to the glassy carbon surface. The film thickness was varied by preparing the electrodes with different weight percents

of PVP in methanol at a 3000 rpm spin-coating rate. The coating solution also contained 20% (vs the pyridine moiety) 1,5-dibromopentane as a cross-linking agent. The electrode prepared with 0.4 wt % of PVP in methanol shows the best performance. Electrodes prepared with the optimum coating solution of 0.4 wt % of PVP in methanol at a 3000 rpm spin-coating rate were therefore used in all subsequent work. The amount of mercury plated depends on the deposition time. The peak current increases as the deposition time increases and reaches a maximum after ∼4 min. A mercury deposition time of 4 min was therefore used in most of the subsequent work. Detection. To increase the anion-exchange capacity of the protonated PVP, the pH effect should be evaluated first. The dependence of the peak current on the acidity of the analyte solutions was studied, and the results obtained are shown in Figure 6A. As can be seen, the modified electrode shows an optimum performance when pH is lower than 1.5. This is because that the PVP polymer can be protonated more completely in more acidic solution. Similar results were also obtained for other PVPmodified electrodes previously reported.1,2 An acid solution of 0.025 M H2SO4 was therefore used in the subsequent experiments. The effect of preconcentration potential on the SW response for lead is shown in Figure 6B. As can be seen, the peak current increases as the potential of the electrode becomes more negative between -0.5 and -1.2 V vs Ag/AgCl. This behavior is explained by the fact that the formation of the lead anions depends on the formation of the hydroxide ions by the water reduction reaction. As a result, the accumulation of lead is favored at more negative potentials. However, when the preconcentration potential is more netgative than -1.2 V, the bubbles created at the electrode surface render the experiments difficult. On the other hand, the peak current drops rapidly as the potential is more positive than -0.6 V. This is due to two factors. First, when the preconcentration potential is more positive than -0.7 V, almost no water reduction reaction occurred and hence little complexion reaction between lead and hydroxide ions. Second, as the preconcentration potential moves closer to the redox potential of the lead couples, more of the deposited lead is oxidized, causing a decrease in peak

Figure 5. SW voltammograms at GCE and PVP/GCE for 100 ppb copper and cadmium. Other conditions as in Figure 3A.

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Figure 6. Effect of (A) pH, (B) preconcentration potential, and (C) preconcentration time on the peak current of 10 ppb lead obtained at the PVP/MFE in 0.025 M H2SO4 supporting electrolyte. Other conditions as in Figure 3B.

current. A preconcentration potential of -1.0 V vs Ag/AgCl was therefore chosen in all subsequent work. The effect of preconcentration time on the SW response for lead is shown in Figure 6C. For 10 ppb lead, the peak current increases as the preconcentration time increases and starts to level off around 15 min. It takes even longer for the peak current to level off for a lower concentration of lead. This phenomenon is as expected and further confirms the ion-exchange process between the PVP and the lead anions. Therefore, to increase the sensitivity of detection, a longer time is needed for the lower concentration of lead. The SW parameters that were investigated were the frequency, the pulse height, and the pulse increment. The parameters are interrelated and affect the response. The response for lead increases with SW frequency, but at frequencies higher than 200 Hz, sloping background current renders the measurement difficult. Increase in the pulse height causes an increase in the lead peak up to 50 mV. The scan increment together with the frequency defines an effective scan rate; hence, increase of either the frequency or the pulse increment results in an increase in the effective scan rate. Overall, the best signal-to-background current characteristics can be obtained with the following instrument settings: modulation amplitude, 50 mV; modulation frequency, 200 Hz; effective scan rate, 800 mV/s. Analytical Characterizations. To characterize the reproducibility of the modified electrode, repetitive preconcentrationmeasurement-regeneration cycles were performed. The electrode actually was immediately regenerated after the stripping process of detection, which can be explained as follows. During the stripping process, the accumulated lead amalgam was oxidized into the lead cations. Due to the positive charge of the lead cations produced, they were subsequently forced out of the protonated PVP film by the electrostatic repulsion effect. The renewed electrode was then checked in the supporting electrolyte before the next measurement to ascertain that it did not show any peak within the potential range. As can be seen in Figure 7, the result of 10 successive measurements showed a very small relative standard deviation of 0.9% for 10 ppb lead with a preconcentration time of 1 min. The results indicate that the electrode renewal gives an excellent reproducible surface, and this is one of the main advantages of the proposed method. 3970 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

Figure 7. Ten repetitive preconcentration-measurement-regeneration cycles for 10 ppb lead obtained at the PVP/MFE. Other conditions as in Figure 3B.

Calibration data were obtained under the optimum experimental conditions given above. Figure 8 presents SW voltammograms for the PVP/MFE after having been in contact with lead for 1and 5-min preconcentration times, respectively. In all cases, a stripping response was observed at a potential near -0.49 V vs Ag/AgCl. The observed peak currents were then used for the construction of the calibration plot. The plots show a very linear behavior with slope (µA/ppb), intercept (µA), and correlation coefficient of 0.97, 2.04, and 0.9997 for a 1-min preconcentration time and 6.43, 6.76, and 0.9991 for a 5-min preconcentration time. The linear range for a 1-min preconcentration was from 0.3 to 60 ppb lead and the detection limit was 0.3 ppb (S/N ) 3). The sensitivity started to decrease when the concentration of lead was higher than 60 ppb. An even lower detection limit of 0.1 ppb was achieved for lead with the preconcentration time of 5 min but at the expense of reducing the linear range from 0.1 to 10 ppb. Various ions were examined concerning their interference with the determination of lead, as summarized in Table 1. For the MFE, elements that can be reduced to the elemental state on the mercury electrode and at the same time are soluble in mercury are likely interferents. Whereas, for the PVP/MFE, the number of species interacting in this manner is limited to those present

Table 2. Determination of Lead in Groundwater and Electroplating Wastewater

original detected value (ppb) spike (ppb) detected value after spike (ppb) recovery (%) real valuea (ppb)

groundwater

electroplating wastewater

2.26 ( 0.01 1.0 3.26 ( 0.06 99.6 2.32 ( 0.01

2.67 ( 0.03 1.0 3.62 ( 0.02 95.0 133.5 ( 1.5

a Real value is obtained by multiplying the detected value and the dilution factor. Number of samples assayed, 3.

Figure 8. SW voltammograms and calibration plots for (a) 2, (b) 4, (c) 6, (d) 8, and (e) 10 ppb lead obtained at the PVP/MFE in 0.025 M H2SO4 supporting electrolyte with (A) tp ) 1 min and (B) tp ) 5 min. Other conditions as in Figure 3B. Table 1. Influence of Other Ions on the Response of Leada ions Hg(II) Zn(II) Cd(II) Sn(IV) Cu(II) Bi(III) Tl(III) Triton X-100 a

concn excess (×) over lead

contribution (%)b

1000 1000 1000 10 100 10 100 10 100 10 100 1000

+7 +3 -2 -4 -33 -3 +17 -10 -39 -27 -5 -28

[Pb(II)] ) 10 ppb; tp ) 3 min. b ip(lead) ) 100%.

in the anionic form due to the presence of the PVP film. For 10 ppb lead with a 3-min preconcentration time, the results showed that over 1000-fold excess concentration of zinc(II), mercury(II), and cadmium(II) did not influence the lead response. Copper(II) and tin(IV) were found to slightly interfere at a 100-fold excess, and thallium(III) and bismuth(III) interfered at a 10-fold excess. Note that thallium(III), tin(IV), and copper(II) are generally considered as major interferences in the determination of lead in ASV measurements on MFE.4,5,13 Even more significant is the

Figure 9. Typical SW voltammograms of the groundwater and electroplating wastewater obtained at the PVP/MFE. (A) the original and spiked (1 ppb/spike) groundwater and (B) the original and spiked (1 ppb/spike) electroplating wastewater. Other conditions as in Figure 3B.

improved selectivity in the presence of a large excess of copper(II). The lead peak may be easily overlapped or even shielded by the peak of thallium(III), tin(IV), and copper(II) provided that the interferents are in great excess of the lead concentration in the sample investigated. However, these problems can actually be largely overcome by the proposed modified electrode. The interference effects caused by surface-active compounds in ASV by using the bare-type MFEs are well recognized.14 One of the functions of the PVP membrane coating on the modified electrode is to prevent the organic interferences from reaching the interface at which the deposition/stripping takes place. Triton X-100 was used to simulate the effect of a typical nonionic surfactant. The results show that the detection can tolerate the presence of Triton X-100 for at least up to 5 ppm with the PVP/ MFE. Compared to similar experiments carried out with the MFE,5 the tolerance was improved by a factor of 5. The analytical utility of the PVP/MFE was assessed by applying it to the determination of lead in groundwater and electroplating wastewater. The results summarized in Table 2 are those for the original and spiked water samples. Typical SW voltammograms for the original and spiked water samples are provided in Figure 9. As can be seen, the lead stripping peaks are clearly displayed for all spiked water samples. The recovery of the spiked lead was also observed to be high in the two different water samples. These results provide sufficient evidence of a high (13) van der Berg, C. M. G. Anal. Chim. Acta 1986, 215, 111. (14) Hoyer, B.; Florence, T. M.; Batley, G. E. Anal. Chem. 1987, 59, 1608.

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feasibility of the PVP/MFE employed for determining lead for different water samples. Note that the dilution factors used for detection were 1.025 and 50 for groundwater and electroplating wastewater, respectively. CONCLUSIONS The results show that the application of the PVP/MFE by the proposed voltammetric method in the determination of trace lead in real samples is very promising. The recovery of the spiked lead was observed to be good in both groundwater and electroplating wastewater. The PVP/MFE, together with the proposed voltammetric method, possesses good selectivity and can be regenerated immediately after detection. Furthermore, the detec-

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tion can be achieved without deoxygenating and the mechanical stability of the mercury film is also improved. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Council of the Republic of China under Grant NSC 86-2113-M-005-021. The authors also express their thanks to Dr. Chen-Wen Whang for valuable discussion. Received for review May 1, 1996. Accepted September 4, 1996.X AC960431T X

Abstract published in Advance ACS Abstracts, October 1, 1996.