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A study of pencil-lead bismuth-film electrodes for the determination of trace metals by anodic stripping voltammetry
Analytica Chimica Acta 519 (2004) 167–172
A study of pencil-lead bismuth-film electrodes for the determination of trace metals by anodic stripping voltammetry D. Demetriades, A. Economou∗ , A. Voulgaropoulos Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 541-24 Thessaloni, Greece Received 21 November 2003; received in revised form 31 March 2004; accepted 5 May 2004
Abstract This work reports the utility of inexpensive and disposable pencil-lead graphite as a substrate for bismuth-film electrodes (BFEs) for the simultaneous determination of Cd(II), Pb(II) and Zn(II) by square-wave anodic stripping voltammetry (SWASV). The BFE was generated in situ by depositing simultaneously the bismuth film and the metal cations by reduction at −1.4 V on the pencil graphite substrate. Then, the preconcentrated metals were oxidised by scanning the potential of the electrode from −1.4 to 0 V using a square-wave waveform. The stripping current arising from the oxidation of each metal was related to the concentration of each metal in the sample. The parameters for the simultaneous determination of the three metals were investigated with the view to apply this type of voltammetric sensor to real samples containing traces of these metals. Using the selected conditions, the limits of detection were 0.3 g l−1 for Cd and 0.4 g l−1 for Zn and Pb at a preconcentration time of 10 min and these values could be further decreased by the use of Nafion-covered pencil-lead BFEs. Finally, the pencil-lead BFEs were successfully applied to the determination of Pb and Zn in tap water with results in satisfactory statistical agreement with atomic absorption spectroscopy. © 2004 Elsevier B.V. All rights reserved. Keywords: Square-wave anodic stripping voltammetry; Bismuth-film electrodes; Pencil-lead graphite; Nafion
1. Introduction Stripping analysis has proved a useful and versatile technique for the determination of trace metals in various samples of environmental, clinical and industrial origin [1]. Mercury, in the form of mercury-film electrodes (MFEs) or the hanging mercury-drop electrode (HMDE), has been the traditional working electrode material in anodic stripping voltammetry (ASV) owing to the advantageous analytical properties of mercury in the negative potential range. However, the general trend for more environmentally friendly analytical methods and the extreme toxicity of metallic mercury and mercury salts have led some countries to completely ban the use of mercury [2]. In the search for alternative electrode materials, bismuth-film electrodes (BFEs), consisting of a thin bismuth-film deposited on a suitable substrate, have been shown to offer comparable performance to MFEs in ASV [3,4]. The utility of bismuth as an ∗ Corresponding author. Tel.: +30-2310-997728; fax: +30-2310-997719. E-mail address: [email protected] (A. Economou).
0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.05.008
electrode is based on its ability to form cold “fused” alloys with different heavy metals [5]. Different materials have been used as substrates for BFEs, including glassy carbon [3,4], wax-impregnated graphite [6], noble metals [7], carbon paste [8,9], screen-printed inks [10] and carbon-fibres [3,7]. Pencil-lead graphite has been previously used as an electrode material in different applications of stripping analysis [11–14]. The main attractions of this material are its high electrical conductivity, fast and easy pretreatment, low-cost, wide availability, low content in trace metals and low background current. In this work, the use of pencil-lead graphite is proposed as an inexpensive and disposable substrate material for BFEs. In the course of this investigation, we performed a study of the parameters affecting the performance of pencil-lead BFEs (i.e. the supporting electrolyte, the Bi plating conditions, the deposition time, the parameters of the SW stripping waveform and the tolerance to surfactants) in order to achieve high sensitivity for the simultaneous trace determination of Zn, Cd and Pb. Additionally, we have investigated the possibility of exploring Nafion-covered pencil-lead BFEs in order to increase the detection sensitiv-
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ity and alleviate the interference from surfactants. Finally, we have applied the senor developed to the determination of Pb and Zn in tap water. 2. Experimental 2.1. Chemicals and reagents All the chemicals were of analytical grade and purchased from Merck (Darmstadt, Germany). De-ionised water was used throughout. Working metal ion solutions were prepared from 1000 mg l−1 atomic absorption standard solutions after appropriate dilution with de-ionised water. A 1 mol l−1 acetate buffer (pH 4.5) stock solution, prepared by mixing the appropriate amounts of glacial CH3 COOH and NaOH, was used to prepare solutions of the supporting electrolyte. A 50 mg l−1 Bi(III) stock solution was prepared in 0.1 mol l−1 acetate buffer. Nafion (5% (w/v) solution in a mixture of water and lower alcohols) was purchased from Aldrich (St. Louis, Missouri); more dilute Nafion solutions were prepared after dilution with absolute ethanol. A 1000 mg l−1 solution of Triton X-100 (BDH, Poole, England) was prepared in water. 2.2. Apparatus Voltammetric measurements were performed with a home-made potentiostat interfaced to a PC though a 6025E PCI multi-purpose interface card (National Instruments, Austin, TX). The experimental sequence was fully automated and controlled by the PC using a control application developed in LabVIEW 5.1 (National Instruments) as reported previously [15]. The voltammetric cell was a standard 50 ml glass vial (Metrohm, Switzerland) equipped with a Ag/AgCl reference electrode and a Pt counter electrode. A magnetic stirrer was used during the preconcentration and cleaning steps. The pencil-lead rods were Pilot HB 0.5 mm in diameter and 6 cm in length purchased from a local bookstore. The pencil-lead was inserted into a piece of Teflon tubing so that 1 cm of the rod was exposed at one end and the rod was glued in place with epoxy glue. Electrical connection was made on the exposed reverse side of the rod. Before use, the electrode was rinsed with 4 mol l−1 HNO3 , gently rubbed with a clean soft tissue and rinsed with water. For the AAS measurements of Pb and Zn, a Perkin Elmer 5100 AAS spectrometer (CT, USA) was used. Pb was determined by electrothermal AAS in ␣ Perkin Elmer 5100 ZL furnace module while Zn was determined by flame AAS. 3. Experimental procedure 3.1. Sample preparation Tap water was collected from taps in our laboratory. For the determination of Pb(II), 18 ml of the tap water and 2 ml
of the 1 mol l−1 acetate buffer were placed in the cell and the analysis was carried out as described below (except that the deposition potential was –1.2 V). For the determination of Zn(II), 5 ml of tap water, 2 ml of the 1 mol l−1 acetate buffer and 13 ml of de-ionised water were placed in the cell and the analysis was carried out as described below with a preconcentration time of 30 s. 3.2. Coating the pencil-lead electrode with Nafion The pencil-lead rod was immersed for 1 min in a 1% (w/v) Nafion solution and the electrode was exposed at room temperature for 5 min to allow the solvents to evaporate and this procedure was repeated one more time. Then, the polymeric membrane was cured with a hot air stream from a heat-gun for 1 min and left to cool to room temperature before being used. This heat treatment has been shown to improve the properties of the Nafion membrane [16]. 3.3. Measurement procedure In situ plated pencil-lead BFEs were prepared by spiking the sample with the required concentration of Bi(III) and simultaneously depositing Bi and the metals on the surface of the electrode at −1.4 V under stirring for a carefully defined period of time followed by a 10 s rest period. The voltammogram was recorded between −1.4 and 0 V by applying a square-wave waveform and the electrode was cleaned from residual metals and the bismuth film for 30 s at 0.0 V under stirring. In situ plated pencil-lead MFEs were prepared by spiking the sample with 1000 g l−1 Hg(II) and simultaneously depositing Hg and the metals on the surface of the electrode at −1.4 V under stirring for a carefully defined period of time followed by a 10 s rest period. The voltammogram was recorded between −1.4 and 0.6 V by applying a square-wave waveform and the electrode was cleaned from residual metals and the bismuth film for 30 s at 0.6 V under stirring.
4. Results and discussion 4.1. Comparison of pencil-lead BFE and MFE—effect of the supporting electrolyte A comparison between an MFE and a BFE, both plated in situ on pencil-lead, in a solution containing 20 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5) (Fig. 1) suggested that the sensitivity for Pb and Cd was comparable on the two electrodes while the MFE produced a higher signal than the BFE for Zn. On the pencil-lead BFE, a higher background current was obtained at more negative potentials suggesting that the hydrogen reduction overpotential was higher on this type of electrode. The positions of the Pb and Zn peaks were approximately the same on the
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Fig. 1. Comparative voltammograms of a solution containing 20 g l−1 each of Zn(II), Cd(II) and Pb(II) on in situ plated pencil-lead BFE and MFE. Supporting electrolyte: 0.1 mol l−1 acetate buffer (pH 4.5); deposition potential: −1.4 V; deposition time: 120 s; frequency 25 Hz; pulse height: 40 mV; step increment: 2 mV; Bi(III) concentration: 500 g l−1 .
two electrodes but the Cd peak was shifted to more negative values on the BFE; this shift improved the resolution between the Cd and Pb peaks on the BFE compared to the MFE. Different supporting electrolytes were tested in a solution containing 20 g l−1 each of Zn(II), Cd(II) and Pb(II) on an in situ plated pencil-lead BFE: 0.1 mol l−1 KCl, 0.1 mol l−1 HCl, 0.1 mol l−1 HNO3 , 0.1 mol l−1 H2 SO4 , 0.1 mol l−1 H3 PO4 and 0.1 mol l−1 acetate buffer (pH 4.5). In neutral solutions (0.1 mol l−1 KCl), no significant plating of bismuth occurred due to the hydrolysis of Bi(III) ions. In the acid solutions (0.1 mol l−1 of different acids) the hydrogen evolution current increased excessively at more negative potentials and the Zn peak was either completely obscured or was superimposed on a sloping background that made difficult the precise measurement of the Zn peak height. Therefore, the 0.1 mol l−1 acetate buffer (pH 4.5) was selected for further experiments. 4.2. Effect of the bismuth concentration and the preconcentration time Representative stripping voltammograms for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5) on in situ plated pencil-lead BFEs with different Bi(III) concentrations in the range 125–4000 g l−1 are shown in Fig. 2(a). In this case, the concentration of the Bi(III) solution controlled the thickness of the Bi film. Although, the thickness of the film did not affect the peak position of any metal, all of the peaks became wider at higher Bi(III) concentration. In addition, the heights of the Cd and Pb peaks displayed a clear dependence on the Bi film thickness as they decreased with increasing thickness of the bismuth film, especially the Pb peak. The height of the Zn peak was unaffected by the thickness of the bismuth film except at concentrations of Bi(III) >2000 g l−1 in which
Fig. 2. (a) Voltammograms of a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) on in situ plated pencil-lead BFE using different concentrations of Bi(III) in the range 125–4000 g l−1 . (b) Effect of the deposition time on the peak heights for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) on in situ pencil-lead plated BFE. Conditions as in Fig. 1. (䉬, Zn; 䊊, Pb; 䊉, Cd).
the Zn peak was completely suppressed. The background current at negative potentials, where the Zn peak appeared, increased with the amount of Bi deposited resulting in a more sloping baseline for the Zn peak at higher Bi(III) concentrations. The metal preconcentration time was studied in the range 0–360 s for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5). The peak heights of the stripping peaks for the three metals increased with increasing deposition times following a rectilinear relationship (Fig. 2(b)). 4.3. Effect of the parameters of the stripping waveform Among the stripping waveforms, the square-wave modulation combines high sensitivity with high speed and insensitivity to dissolved oxygen which allowed the analysis to be carried out in the presence of oxygen and avoided the time-consuming deoxygenation step. The SW parameters affecting the response were the SW frequency, the SW step increment and the SW pulse height and were investigated
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univariantly using a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5). The effect of the frequency was studied in the range 12.5–200 Hz and the effect of the step increment was investigated between 1 and 16 mV. The peak potentials shifted to the anodic direction with increasing frequency or step increment (presumably as the oxidation of the metals became less reversible at higher frequencies) while the peak heights for Pb and Cd and Zn peak increased with increasing frequency up to 50 Hz and step increment up to; 8 mV at higher values of frequency and step increment, the peak heights stabilised and, at the same time, became wider. The effect of the SW pulse height was studied in the range from 10 to 80 mV. The peak potentials were shifted to the cathodic direction and the peak heights, especially that of Zn, increased upon increase of the pulse height. However, the background deteriorated and the peaks became wider at higher pulse heights. 4.4. Calibration data Calibration was performed on BFEs for the simultaneous determination of Pb(II), Cd(II) and Zn(II) by ASV using the following conditions—deposition potential: −1.4 V; deposition time: 60–180 s; frequency: 50 Hz; pulse height: 40 mV; step increment: 4 mV. Four different concentration ranges were investigated: 1–10 g l−1 , 2–20 g l−1 , 5–50 g l−1 and 10–100 g l−1 . A series of stripping voltammograms in the range 2–24 g l−1 is shown in Fig. 3. The relative standard deviation was 3.2% for Pb, 3% for Cd and 2.6% for Zn at the 10 g l−1 level (n = 10) with a preconcentration time of 120 s. The limit of detection was calculated as 0.3 g l−1 for Cd and 0.4 g l−1 for Zn and Pb at a preconcentration time of 10 min. This level of sensitivity
Fig. 3. A series of voltammograms and corresponding calibration curve for increasing concentrations of Zn(II), Cd(II) and Pb(II) on an in situ plated pencil-lead BFE (䉬, Zn; 䊊, Pb; 䊉, Cd). From below: blank and 12 successive additions of 2 g l−1 of Zn(II), Cd(II) and Pb(II). Supporting electrolyte: 0.1 mol l−1 acetate buffer (pH 4.5); deposition potential: −1.4 V; deposition time: 180 s; frequency 50 Hz; pulse height: 40 mV; step increment: 4 mV.
compared favourably with BFEs plated on conventional carbon substrates [3,4,6–9]. 4.5. Effect of surface-active compounds—study of Nafion-covered pencil-lead BFEs BFEs, in common with MFEs, are prone to interference from surface-active compounds that can adsorb on the electrode and cause deactivation of its surface. Triton X-100 was selected as model compound to study the effect of surfactants on the stripping response of BFEs at low concentrations of metals. The effect of Triton X-100 on the stripping peak heights for Zn, Cd and Pb in a solution containing 10 g l−1 each of Cd(II), Pb(II) and Zn(II) in 0.1 mol l−1 acetate buffer (pH 4.5) was severe since the peaks of the metals were severely decreased in height even at Triton X-100 concentrations as low as 4 mg l−1 . A method previously employed in conjunction with MFEs in order to minimise the problem of surfactants is covering the electrode surface with a permselective membrane [17]; the commonest such polymer is Nafion which possesses several ideal properties as a permselective material. Moreover, the formation of a Nafion coating on the electrode surface is straightforward, easy and fast. In the case of the pencil-lead BFE, the Nafion coating was generated by dipping the electrode in a 1% solution of Nafion (dip-coating) and allowing the solvent to evaporate before the analysis. Moreover, it has been previously shown that a Nafion coating imparts higher sensitivity in ASV on MFEs when pulsed stripping techniques were used [18]. The sensitivity enhancement of the Nafion-covered electrodes has been attributed to the fact that the Nafion film helped confine the stripped metals close to the electrode surface where more efficient replating could occur during the reverse pulse. Also, some degree of non-faradaic preconcentration, owing to the ion-exchange properties of the Nafion, could partly account for this enhancement [18]. This effect was also observed in the present work involving SWASV on pencil-lead BFEs as illustrated in Fig. 4 that compares SWASV signals on a bare pencil-lead BFE (Fig. 4(a)) and a Nafion-covered pencil-lead BFE (Fig. 4(b)). It was observed that both the forward and the reverse SW peak currents (and, therefore, also the differential peak current) were considerably higher on the Nafion-covered pencil-lead BFE. This increase in sensitivity, combined with the simple formation of the Nafion coating, could extend the scope and utility of Nafion-covered pencil-lead BFEs. A comparison of the performance of the Nafion-covered pencil-lead BFE versus the bare pencil-lead BFE in the presence of 4 mg l−1 Triton X-100 is illustrated in Fig. 5. On the bare pencil-lead BFE, the Cd and Pb peaks were severely diminished in height while the Zn peak was completely suppressed (Fig. 5(a)). On the contrary, on the Nafion-covered pencil-lead BFE the Cd and Pb peaks were hardly affected and the Zn peak was diminished in height but was still well-defined (Fig. 5(b)). In general, it was observed that the Zn peak was the most affected by the pres-
D. Demetriades et al. / Analytica Chimica Acta 519 (2004) 167–172
Fig. 4. Comparative SWASV signals and the constituent differential, forward and reverse currents (Id , If and Ir , respectively) on (a) an in situ plated bare pencil-lead BFE, and (b) an in situ plated Nafion-covered pencil-lead BFE, in a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II). Conditions as in Fig. 2.
ence of Triton X-100 on both the Nafion-covered pencil-lead BFE and the bare pencil-lead BFE. Moreover, it was the Cd peak that benefited mostly from the application of the Nafion coating both in terms of sensitivity and tolerance to surfactants. 4.6. Stability of pencil-lead BFEs The pencil-lead rods used in this work required only minimal pretreatment before use that involved rinsing with 4 mol l–1 HNO3 , rubbing with a clean soft tissue and rinsing with water. This procedure was sufficient to refresh the electrode surface and has been shown to produce stable and reproducible BFEs. Indeed, only three graphite rods were employed for the majority of the experiments in this study. The fragility of these electrodes was their main drawback since it was easy to break the rods during handling but the method of construction shown in Fig. 1 ensured that this problem was minimised. 4.7. Application to tap water Pencil-lead BFEs were finally applied to the analysis of tap water. Representative stripping voltammograms for the
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Fig. 5. Comparative SWASV signals for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) on (a) an in situ plated bare pencil-lead BFE, and (b) an in situ plated Nafion-covered pencil-lead BFE with (1) with no surfactant added, and (2) in the presence of 4 mg l−1 Triton X-100. Conditions as in Fig. 2.
determination of Pb and Zn in tap water with the method of standard additions are illustrated in Fig. 6(a) and (b), respectively. In the case of the determination of Pb, a large Zn peak appeared due to the fact that Zn exists in large excess over Pb in tap water. In order to minimise this interference, the preconcentration potential was set to −1.2 V. In this case, in which higher sensitivity was required, the use of a Nafion-covered pencil-lead BFE was considered advantageous. In order to determine Zn in tap water, dilution of the sample and use of a shorter preconcentration time on a bare pencil-lead BFE were used (Fig. 6(b)). Comparative results between SWASV and AAS, shown in Table 1, suggest that there is satisfactory agreement between the two techniques. Table 1 Results for the determination of Zn(II) and Pb(II) in tap water by SWASV on pencil-lead BFEs ASV (g l−1 )
AAS (g l−1 )
Zna Pbb
267 ± 10 2.5 ± 0.05
251 ± 15 1.9 ± 0.1
a
Bare pencil-lead BFE. Nafion-covered pencil-lead BFE.
b
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performance to the more expensive glassy-carbon electrodes for the simultaneous determination of Zn(II), Cd(II) and Pb(II). In particular, it has been shown that the sensitivity and selectivity of the analysis could be further improved by covering the pencil-lead BFEs with a Nafion permselective membrane. Combined with the low toxicity of bismuth and their small size, pencil-lead BFEs offer great scope for applications in which compact instrumentation and low sample volumes are critical such as field measurements and on-site monitoring of heavy metals.
References
Fig. 6. (a) Voltammograms for the determination of Pb(II) in tap water on in situ plated Nafion-covered pencil-lead BFE. From below: sample and 3 standard additions of 1 g l−1 Pb(II) each. Conditions as in Fig. 4 with −1.2 V deposition potential. (b) Voltammograms for the determination of Zn(II) in tap water on in situ plated BFE. From below: sample and 3 standard additions of 25 g l−1 Zn(II) each. Conditions as in Fig. 2 with 30 s deposition time.
5. Conclusions In this work, the utility of pencil-lead graphite was investigated as a substrate for BFEs, which were then applied to trace metal analysis by SWASV. Pencil-lead, an inexpensive, widely available and disposable material requiring minimal pretreatment, has been shown to offer comparable
[1] J. Wang, Stripping Analysis, VCH, Deerfield Beach, FL, 1985 [2] E. Gustafsson, Water Air Soil Poll. 80 (1995) 99. [3] J. Wang, J.M. Lu, S.B. Hocevar, P.A.M. Farias, B. Ogorevc, Anal. Chem. 72 (2000) 3218. [4] J. Wang, J.M. Lu, U.A. Kirgoz, S.B. Hocevar, B. Ogorevc, Anal. Chim. Acta 434 (2001) 29. [5] G.G. Long, L.D. Freedman, G.O. Doak, Bismuth and bismuth alloys, in: Encyclopedia of Chemical Technology, Wiley, New York, 1978, pp. 912–937. [6] G. Kefala, A. Economou, A. Voulgaropoulos, M. Sofoniou, Talanta 61 (2003) 603. [7] S.B. Hocevar, B. Ogorevc, J. Wang, B. Pihlar, Electroanalysis 14 (2002) 1707. [8] A. Krolicka, R. Pauliukaite, I. Svancara, R. Metelka, A. Bobrowski, E. Norkus, K. Kalcher, K. Vytras, Electrochem. Comm. 4 (2002) 193. [9] K. Vytras, I. Svancara, R. Metelka, Electroanalysis 14 (2002) 1359. [10] J. Wang, J.M. Lu, S.B. Hocevar, B. Ogorevc, Electroanalysis 13 (2001) 13. [11] J. Wang, A.N. Kawde, E. Sahlin, Analyst 125 (2000) 5. [12] T. Kakizaki, K. Hasebe, Fresenius J. Anal. Chem. 360 (1998) 175. [13] A.M. Bond, P.J. Mahon, J. Schiewe, V. Vicente-Beckett, Anal. Chim. Acta 345 (1997) 67. [14] D. Blum, W. Leyffer, R. Holze, Electroanalysis 8 (1996) 296. [15] A. Economou, S.D. Bolis, C.E. Efstathiou, G. Volikakis, Anal. Chim. Acta 467 (2002) 179. [16] B. Hoyer, N. Jensen, Talanta 41 (1994) 449. [17] A. Economou, P.R. Fielden, Analyst 128 (2003) 205. [18] B. Hoyer, T.M. Florence, G.E. Batley, Anal. Chem. 59 (1987) 1608.
A study of pencil-lead bismuth-film electrodes for the determination of trace metals by anodic stripping voltammetry D. Demetriades, A. Economou∗ , A. Voulgaropoulos Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 541-24 Thessaloni, Greece Received 21 November 2003; received in revised form 31 March 2004; accepted 5 May 2004
Abstract This work reports the utility of inexpensive and disposable pencil-lead graphite as a substrate for bismuth-film electrodes (BFEs) for the simultaneous determination of Cd(II), Pb(II) and Zn(II) by square-wave anodic stripping voltammetry (SWASV). The BFE was generated in situ by depositing simultaneously the bismuth film and the metal cations by reduction at −1.4 V on the pencil graphite substrate. Then, the preconcentrated metals were oxidised by scanning the potential of the electrode from −1.4 to 0 V using a square-wave waveform. The stripping current arising from the oxidation of each metal was related to the concentration of each metal in the sample. The parameters for the simultaneous determination of the three metals were investigated with the view to apply this type of voltammetric sensor to real samples containing traces of these metals. Using the selected conditions, the limits of detection were 0.3 g l−1 for Cd and 0.4 g l−1 for Zn and Pb at a preconcentration time of 10 min and these values could be further decreased by the use of Nafion-covered pencil-lead BFEs. Finally, the pencil-lead BFEs were successfully applied to the determination of Pb and Zn in tap water with results in satisfactory statistical agreement with atomic absorption spectroscopy. © 2004 Elsevier B.V. All rights reserved. Keywords: Square-wave anodic stripping voltammetry; Bismuth-film electrodes; Pencil-lead graphite; Nafion
1. Introduction Stripping analysis has proved a useful and versatile technique for the determination of trace metals in various samples of environmental, clinical and industrial origin [1]. Mercury, in the form of mercury-film electrodes (MFEs) or the hanging mercury-drop electrode (HMDE), has been the traditional working electrode material in anodic stripping voltammetry (ASV) owing to the advantageous analytical properties of mercury in the negative potential range. However, the general trend for more environmentally friendly analytical methods and the extreme toxicity of metallic mercury and mercury salts have led some countries to completely ban the use of mercury [2]. In the search for alternative electrode materials, bismuth-film electrodes (BFEs), consisting of a thin bismuth-film deposited on a suitable substrate, have been shown to offer comparable performance to MFEs in ASV [3,4]. The utility of bismuth as an ∗ Corresponding author. Tel.: +30-2310-997728; fax: +30-2310-997719. E-mail address: [email protected] (A. Economou).
0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.05.008
electrode is based on its ability to form cold “fused” alloys with different heavy metals [5]. Different materials have been used as substrates for BFEs, including glassy carbon [3,4], wax-impregnated graphite [6], noble metals [7], carbon paste [8,9], screen-printed inks [10] and carbon-fibres [3,7]. Pencil-lead graphite has been previously used as an electrode material in different applications of stripping analysis [11–14]. The main attractions of this material are its high electrical conductivity, fast and easy pretreatment, low-cost, wide availability, low content in trace metals and low background current. In this work, the use of pencil-lead graphite is proposed as an inexpensive and disposable substrate material for BFEs. In the course of this investigation, we performed a study of the parameters affecting the performance of pencil-lead BFEs (i.e. the supporting electrolyte, the Bi plating conditions, the deposition time, the parameters of the SW stripping waveform and the tolerance to surfactants) in order to achieve high sensitivity for the simultaneous trace determination of Zn, Cd and Pb. Additionally, we have investigated the possibility of exploring Nafion-covered pencil-lead BFEs in order to increase the detection sensitiv-
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ity and alleviate the interference from surfactants. Finally, we have applied the senor developed to the determination of Pb and Zn in tap water. 2. Experimental 2.1. Chemicals and reagents All the chemicals were of analytical grade and purchased from Merck (Darmstadt, Germany). De-ionised water was used throughout. Working metal ion solutions were prepared from 1000 mg l−1 atomic absorption standard solutions after appropriate dilution with de-ionised water. A 1 mol l−1 acetate buffer (pH 4.5) stock solution, prepared by mixing the appropriate amounts of glacial CH3 COOH and NaOH, was used to prepare solutions of the supporting electrolyte. A 50 mg l−1 Bi(III) stock solution was prepared in 0.1 mol l−1 acetate buffer. Nafion (5% (w/v) solution in a mixture of water and lower alcohols) was purchased from Aldrich (St. Louis, Missouri); more dilute Nafion solutions were prepared after dilution with absolute ethanol. A 1000 mg l−1 solution of Triton X-100 (BDH, Poole, England) was prepared in water. 2.2. Apparatus Voltammetric measurements were performed with a home-made potentiostat interfaced to a PC though a 6025E PCI multi-purpose interface card (National Instruments, Austin, TX). The experimental sequence was fully automated and controlled by the PC using a control application developed in LabVIEW 5.1 (National Instruments) as reported previously [15]. The voltammetric cell was a standard 50 ml glass vial (Metrohm, Switzerland) equipped with a Ag/AgCl reference electrode and a Pt counter electrode. A magnetic stirrer was used during the preconcentration and cleaning steps. The pencil-lead rods were Pilot HB 0.5 mm in diameter and 6 cm in length purchased from a local bookstore. The pencil-lead was inserted into a piece of Teflon tubing so that 1 cm of the rod was exposed at one end and the rod was glued in place with epoxy glue. Electrical connection was made on the exposed reverse side of the rod. Before use, the electrode was rinsed with 4 mol l−1 HNO3 , gently rubbed with a clean soft tissue and rinsed with water. For the AAS measurements of Pb and Zn, a Perkin Elmer 5100 AAS spectrometer (CT, USA) was used. Pb was determined by electrothermal AAS in ␣ Perkin Elmer 5100 ZL furnace module while Zn was determined by flame AAS. 3. Experimental procedure 3.1. Sample preparation Tap water was collected from taps in our laboratory. For the determination of Pb(II), 18 ml of the tap water and 2 ml
of the 1 mol l−1 acetate buffer were placed in the cell and the analysis was carried out as described below (except that the deposition potential was –1.2 V). For the determination of Zn(II), 5 ml of tap water, 2 ml of the 1 mol l−1 acetate buffer and 13 ml of de-ionised water were placed in the cell and the analysis was carried out as described below with a preconcentration time of 30 s. 3.2. Coating the pencil-lead electrode with Nafion The pencil-lead rod was immersed for 1 min in a 1% (w/v) Nafion solution and the electrode was exposed at room temperature for 5 min to allow the solvents to evaporate and this procedure was repeated one more time. Then, the polymeric membrane was cured with a hot air stream from a heat-gun for 1 min and left to cool to room temperature before being used. This heat treatment has been shown to improve the properties of the Nafion membrane [16]. 3.3. Measurement procedure In situ plated pencil-lead BFEs were prepared by spiking the sample with the required concentration of Bi(III) and simultaneously depositing Bi and the metals on the surface of the electrode at −1.4 V under stirring for a carefully defined period of time followed by a 10 s rest period. The voltammogram was recorded between −1.4 and 0 V by applying a square-wave waveform and the electrode was cleaned from residual metals and the bismuth film for 30 s at 0.0 V under stirring. In situ plated pencil-lead MFEs were prepared by spiking the sample with 1000 g l−1 Hg(II) and simultaneously depositing Hg and the metals on the surface of the electrode at −1.4 V under stirring for a carefully defined period of time followed by a 10 s rest period. The voltammogram was recorded between −1.4 and 0.6 V by applying a square-wave waveform and the electrode was cleaned from residual metals and the bismuth film for 30 s at 0.6 V under stirring.
4. Results and discussion 4.1. Comparison of pencil-lead BFE and MFE—effect of the supporting electrolyte A comparison between an MFE and a BFE, both plated in situ on pencil-lead, in a solution containing 20 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5) (Fig. 1) suggested that the sensitivity for Pb and Cd was comparable on the two electrodes while the MFE produced a higher signal than the BFE for Zn. On the pencil-lead BFE, a higher background current was obtained at more negative potentials suggesting that the hydrogen reduction overpotential was higher on this type of electrode. The positions of the Pb and Zn peaks were approximately the same on the
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Fig. 1. Comparative voltammograms of a solution containing 20 g l−1 each of Zn(II), Cd(II) and Pb(II) on in situ plated pencil-lead BFE and MFE. Supporting electrolyte: 0.1 mol l−1 acetate buffer (pH 4.5); deposition potential: −1.4 V; deposition time: 120 s; frequency 25 Hz; pulse height: 40 mV; step increment: 2 mV; Bi(III) concentration: 500 g l−1 .
two electrodes but the Cd peak was shifted to more negative values on the BFE; this shift improved the resolution between the Cd and Pb peaks on the BFE compared to the MFE. Different supporting electrolytes were tested in a solution containing 20 g l−1 each of Zn(II), Cd(II) and Pb(II) on an in situ plated pencil-lead BFE: 0.1 mol l−1 KCl, 0.1 mol l−1 HCl, 0.1 mol l−1 HNO3 , 0.1 mol l−1 H2 SO4 , 0.1 mol l−1 H3 PO4 and 0.1 mol l−1 acetate buffer (pH 4.5). In neutral solutions (0.1 mol l−1 KCl), no significant plating of bismuth occurred due to the hydrolysis of Bi(III) ions. In the acid solutions (0.1 mol l−1 of different acids) the hydrogen evolution current increased excessively at more negative potentials and the Zn peak was either completely obscured or was superimposed on a sloping background that made difficult the precise measurement of the Zn peak height. Therefore, the 0.1 mol l−1 acetate buffer (pH 4.5) was selected for further experiments. 4.2. Effect of the bismuth concentration and the preconcentration time Representative stripping voltammograms for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5) on in situ plated pencil-lead BFEs with different Bi(III) concentrations in the range 125–4000 g l−1 are shown in Fig. 2(a). In this case, the concentration of the Bi(III) solution controlled the thickness of the Bi film. Although, the thickness of the film did not affect the peak position of any metal, all of the peaks became wider at higher Bi(III) concentration. In addition, the heights of the Cd and Pb peaks displayed a clear dependence on the Bi film thickness as they decreased with increasing thickness of the bismuth film, especially the Pb peak. The height of the Zn peak was unaffected by the thickness of the bismuth film except at concentrations of Bi(III) >2000 g l−1 in which
Fig. 2. (a) Voltammograms of a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) on in situ plated pencil-lead BFE using different concentrations of Bi(III) in the range 125–4000 g l−1 . (b) Effect of the deposition time on the peak heights for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) on in situ pencil-lead plated BFE. Conditions as in Fig. 1. (䉬, Zn; 䊊, Pb; 䊉, Cd).
the Zn peak was completely suppressed. The background current at negative potentials, where the Zn peak appeared, increased with the amount of Bi deposited resulting in a more sloping baseline for the Zn peak at higher Bi(III) concentrations. The metal preconcentration time was studied in the range 0–360 s for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5). The peak heights of the stripping peaks for the three metals increased with increasing deposition times following a rectilinear relationship (Fig. 2(b)). 4.3. Effect of the parameters of the stripping waveform Among the stripping waveforms, the square-wave modulation combines high sensitivity with high speed and insensitivity to dissolved oxygen which allowed the analysis to be carried out in the presence of oxygen and avoided the time-consuming deoxygenation step. The SW parameters affecting the response were the SW frequency, the SW step increment and the SW pulse height and were investigated
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univariantly using a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) in 0.1 mol l−1 acetate buffer (pH 4.5). The effect of the frequency was studied in the range 12.5–200 Hz and the effect of the step increment was investigated between 1 and 16 mV. The peak potentials shifted to the anodic direction with increasing frequency or step increment (presumably as the oxidation of the metals became less reversible at higher frequencies) while the peak heights for Pb and Cd and Zn peak increased with increasing frequency up to 50 Hz and step increment up to; 8 mV at higher values of frequency and step increment, the peak heights stabilised and, at the same time, became wider. The effect of the SW pulse height was studied in the range from 10 to 80 mV. The peak potentials were shifted to the cathodic direction and the peak heights, especially that of Zn, increased upon increase of the pulse height. However, the background deteriorated and the peaks became wider at higher pulse heights. 4.4. Calibration data Calibration was performed on BFEs for the simultaneous determination of Pb(II), Cd(II) and Zn(II) by ASV using the following conditions—deposition potential: −1.4 V; deposition time: 60–180 s; frequency: 50 Hz; pulse height: 40 mV; step increment: 4 mV. Four different concentration ranges were investigated: 1–10 g l−1 , 2–20 g l−1 , 5–50 g l−1 and 10–100 g l−1 . A series of stripping voltammograms in the range 2–24 g l−1 is shown in Fig. 3. The relative standard deviation was 3.2% for Pb, 3% for Cd and 2.6% for Zn at the 10 g l−1 level (n = 10) with a preconcentration time of 120 s. The limit of detection was calculated as 0.3 g l−1 for Cd and 0.4 g l−1 for Zn and Pb at a preconcentration time of 10 min. This level of sensitivity
Fig. 3. A series of voltammograms and corresponding calibration curve for increasing concentrations of Zn(II), Cd(II) and Pb(II) on an in situ plated pencil-lead BFE (䉬, Zn; 䊊, Pb; 䊉, Cd). From below: blank and 12 successive additions of 2 g l−1 of Zn(II), Cd(II) and Pb(II). Supporting electrolyte: 0.1 mol l−1 acetate buffer (pH 4.5); deposition potential: −1.4 V; deposition time: 180 s; frequency 50 Hz; pulse height: 40 mV; step increment: 4 mV.
compared favourably with BFEs plated on conventional carbon substrates [3,4,6–9]. 4.5. Effect of surface-active compounds—study of Nafion-covered pencil-lead BFEs BFEs, in common with MFEs, are prone to interference from surface-active compounds that can adsorb on the electrode and cause deactivation of its surface. Triton X-100 was selected as model compound to study the effect of surfactants on the stripping response of BFEs at low concentrations of metals. The effect of Triton X-100 on the stripping peak heights for Zn, Cd and Pb in a solution containing 10 g l−1 each of Cd(II), Pb(II) and Zn(II) in 0.1 mol l−1 acetate buffer (pH 4.5) was severe since the peaks of the metals were severely decreased in height even at Triton X-100 concentrations as low as 4 mg l−1 . A method previously employed in conjunction with MFEs in order to minimise the problem of surfactants is covering the electrode surface with a permselective membrane [17]; the commonest such polymer is Nafion which possesses several ideal properties as a permselective material. Moreover, the formation of a Nafion coating on the electrode surface is straightforward, easy and fast. In the case of the pencil-lead BFE, the Nafion coating was generated by dipping the electrode in a 1% solution of Nafion (dip-coating) and allowing the solvent to evaporate before the analysis. Moreover, it has been previously shown that a Nafion coating imparts higher sensitivity in ASV on MFEs when pulsed stripping techniques were used [18]. The sensitivity enhancement of the Nafion-covered electrodes has been attributed to the fact that the Nafion film helped confine the stripped metals close to the electrode surface where more efficient replating could occur during the reverse pulse. Also, some degree of non-faradaic preconcentration, owing to the ion-exchange properties of the Nafion, could partly account for this enhancement [18]. This effect was also observed in the present work involving SWASV on pencil-lead BFEs as illustrated in Fig. 4 that compares SWASV signals on a bare pencil-lead BFE (Fig. 4(a)) and a Nafion-covered pencil-lead BFE (Fig. 4(b)). It was observed that both the forward and the reverse SW peak currents (and, therefore, also the differential peak current) were considerably higher on the Nafion-covered pencil-lead BFE. This increase in sensitivity, combined with the simple formation of the Nafion coating, could extend the scope and utility of Nafion-covered pencil-lead BFEs. A comparison of the performance of the Nafion-covered pencil-lead BFE versus the bare pencil-lead BFE in the presence of 4 mg l−1 Triton X-100 is illustrated in Fig. 5. On the bare pencil-lead BFE, the Cd and Pb peaks were severely diminished in height while the Zn peak was completely suppressed (Fig. 5(a)). On the contrary, on the Nafion-covered pencil-lead BFE the Cd and Pb peaks were hardly affected and the Zn peak was diminished in height but was still well-defined (Fig. 5(b)). In general, it was observed that the Zn peak was the most affected by the pres-
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Fig. 4. Comparative SWASV signals and the constituent differential, forward and reverse currents (Id , If and Ir , respectively) on (a) an in situ plated bare pencil-lead BFE, and (b) an in situ plated Nafion-covered pencil-lead BFE, in a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II). Conditions as in Fig. 2.
ence of Triton X-100 on both the Nafion-covered pencil-lead BFE and the bare pencil-lead BFE. Moreover, it was the Cd peak that benefited mostly from the application of the Nafion coating both in terms of sensitivity and tolerance to surfactants. 4.6. Stability of pencil-lead BFEs The pencil-lead rods used in this work required only minimal pretreatment before use that involved rinsing with 4 mol l–1 HNO3 , rubbing with a clean soft tissue and rinsing with water. This procedure was sufficient to refresh the electrode surface and has been shown to produce stable and reproducible BFEs. Indeed, only three graphite rods were employed for the majority of the experiments in this study. The fragility of these electrodes was their main drawback since it was easy to break the rods during handling but the method of construction shown in Fig. 1 ensured that this problem was minimised. 4.7. Application to tap water Pencil-lead BFEs were finally applied to the analysis of tap water. Representative stripping voltammograms for the
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Fig. 5. Comparative SWASV signals for a solution containing 10 g l−1 each of Zn(II), Cd(II) and Pb(II) on (a) an in situ plated bare pencil-lead BFE, and (b) an in situ plated Nafion-covered pencil-lead BFE with (1) with no surfactant added, and (2) in the presence of 4 mg l−1 Triton X-100. Conditions as in Fig. 2.
determination of Pb and Zn in tap water with the method of standard additions are illustrated in Fig. 6(a) and (b), respectively. In the case of the determination of Pb, a large Zn peak appeared due to the fact that Zn exists in large excess over Pb in tap water. In order to minimise this interference, the preconcentration potential was set to −1.2 V. In this case, in which higher sensitivity was required, the use of a Nafion-covered pencil-lead BFE was considered advantageous. In order to determine Zn in tap water, dilution of the sample and use of a shorter preconcentration time on a bare pencil-lead BFE were used (Fig. 6(b)). Comparative results between SWASV and AAS, shown in Table 1, suggest that there is satisfactory agreement between the two techniques. Table 1 Results for the determination of Zn(II) and Pb(II) in tap water by SWASV on pencil-lead BFEs ASV (g l−1 )
AAS (g l−1 )
Zna Pbb
267 ± 10 2.5 ± 0.05
251 ± 15 1.9 ± 0.1
a
Bare pencil-lead BFE. Nafion-covered pencil-lead BFE.
b
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performance to the more expensive glassy-carbon electrodes for the simultaneous determination of Zn(II), Cd(II) and Pb(II). In particular, it has been shown that the sensitivity and selectivity of the analysis could be further improved by covering the pencil-lead BFEs with a Nafion permselective membrane. Combined with the low toxicity of bismuth and their small size, pencil-lead BFEs offer great scope for applications in which compact instrumentation and low sample volumes are critical such as field measurements and on-site monitoring of heavy metals.
References
Fig. 6. (a) Voltammograms for the determination of Pb(II) in tap water on in situ plated Nafion-covered pencil-lead BFE. From below: sample and 3 standard additions of 1 g l−1 Pb(II) each. Conditions as in Fig. 4 with −1.2 V deposition potential. (b) Voltammograms for the determination of Zn(II) in tap water on in situ plated BFE. From below: sample and 3 standard additions of 25 g l−1 Zn(II) each. Conditions as in Fig. 2 with 30 s deposition time.
5. Conclusions In this work, the utility of pencil-lead graphite was investigated as a substrate for BFEs, which were then applied to trace metal analysis by SWASV. Pencil-lead, an inexpensive, widely available and disposable material requiring minimal pretreatment, has been shown to offer comparable
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