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Voltammetric determination of 4-nitrophenol at a sodium montmorillonite-anthraquinone chemically modified glassy carbon electrode
Talanta 54 (2001) 115– 123 www.elsevier.com/locate/talanta
Voltammetric determination of 4-nitrophenol at a sodium montmorillonite-anthraquinone chemically modified glassy carbon electrode Shengshui Hu a,*, Cuiling Xu a, Gaiping Wang b, Dafu Cui c a Department of Chemistry, Wuhan Uni6ersity, Wuhan 430072, PR China Faculty of Basic Medical Science, Xinxiang Medical College, Henan 453003, PR China c State Key Laboratory of Transducer Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, PR China b
Received 17 July 2000; received in revised form 9 November 2000; accepted 9 November 2000
Abstract A new method for the determination of 4-nitrophenol(4-NP) by differential pulse voltammetry (DPV) based on adsorptive stripping technique was described. Cyclic voltammetry (CV) and linear scan voltammetry (LSV) were used in a comparative investigation into the electrochemical reduction of 4-NP at a Na-montmorillonite(SWy-2) and anthraquione (AQ) modified glassy carbon electrode. With this chemically modified electrode, 4-NP was first irreversibly reduced from fNO2 to fNHOH at − 0.78 V. A couple of well-defined redox peaks at +0.22 V (vs. SCE) were responsible for a two-electron redox peak between fNHOH and fNO. Studies on the effect of pH on the peak height and peak potential were carried out over the pH range 2.0– 9.0 with the phosphate buffer solution. A pH of 3.4 was chosen as the optimum pH. The other experimental parameters, such as film thickness, accumulation time and potential etc. were optimized. Anodic peak currents were found to be linearly related to concentration of 4-NP over the range 0.3–45 mg l − 1, with a detection limit of 0.02 mg l − 1. The interference of organic and inorganic species on the voltammetric response have been studied. This modified electrode can be used to the determination of 4-NP in water samples. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Na-montmorillonite; Anthraquinone; 4-nitrophenol; Differential pulse voltammetry; Chemically modified electrode
1. Introduction Phenol and substituted phenols are known to be widespread as components in industrial and * Corresponding author. Tel.: + 86-27-87684573; fax: +8627-87882661. E-mail address: [email protected] (S. Hu).
natural waste. These compounds have toxic effect on humans, animals and plants, they give an undesirable taste and odor to drinking water, even in very low concentration. For these reasons, many of the phenols have been included in the environmental legislation. 4-Nitrophenol(4-NP) is one of the nitrophenols in the U.S. Environmental Protection Agency List of Priority Pollutants
0039-9140/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 6 5 8 - 5
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S. Hu et al. / Talanta 54 (2001) 115–123
[1,2]. 4-NP is a hazardous substance which can cause a high environmental impact due to its toxicity and persistence. The origin of contamination comes from manufacturing, chemical industry and agricultural practices [3]. However, organophosphorus pesticides yield nitrophenols as major degradation products [4]. Consequently, qualitative and quantitative detection of nitrophenols is a matter of concern for environmental control. The detection of nitrophenols is usually accomplished by means of chromatographic separation with spectrometry, such as mass spectrometry or flame ionization and ultraviolet detection [5–7], and with electrochemical detector [8 – 10]. The use of enzyme-linked immunosorbent assay (ELISA) has been reported [11]. Various electroanalytical techniques have been also applied to the determination of nitrophenols: polarography, cyclic voltammetry (CV), adsorptive stripping and differential pulse voltammetry (DPV) [12 – 17]. Voltammetric methods are particularly suited for the analysis of a wide variety of organic compounds. However, modified electrodes are being used frequently in the voltammetric determination of organic compounds because of their efficiency, the selectivity that can be obtained by varying the modifier and the sensitivity which is equivalent to that reached in anodic and cathodic stripping. Kauffmann [18] has reported that a carbon paste electrode modified with lipids and proteins (enzymes) has potential application in environmental analysis. In addition, clay (such as bentonite and zeolite) modified electrodes have been applied widely for the determination of organic compounds. Na-montmorillonite(SWy-2) exhibits similar properties to those of other clays, which are able to adsorb electroactive species for their direct determination. This article describes a differential pulse voltammetric method for the determination of 4-NP by using a SWy-2-AQ modified electrode. The modified electrode shows the remarkable accumulation effect of SWy-2 film for 4-NP and the high sensitivity by the DPV. It also displays good resistance to the interference from common ions such as cadmium and lead as well as the interference from some organic compounds.
2. Experimental
2.1. Chemicals and materials Na-montmorillonite(SWy-2) was obtained from Source Clay Minerals Repository, University of Missouri (Columbia, MO). Nitrophenols (Beijing Chemical Reagent Institute) and all other compounds were analytically grade and were used without further purification. Doubly distilled water was used to prepare all aqueous solution. The glassy carbon electrodes(GC, 3 mm in diameter) used in this study and microcloth pads were purchased from Bioanalytical Systems (West Lafayette, IN, USA). Supporting electrolyte used throughout this study was phosphate buffer solution, which was prepared from pH 3.4 HCl – NaH2PO4 solution, unless stated otherwise. Water samples were collected from East Lake of Wuhan, China.
2.2. Apparatus Differential pulse and linear scan voltammetry (LSV) were performed with use of an EG&G Princ. Potentiostat/Galvanostat Model 273A. Cyclic voltammograms measurements were made with an EG&G PARC Model 366 bi-potentiostat and recorded on Model 3086 X-Y recorder (Yokogawa Japan). The potential scan rate in all voltammetric measurements was 100 mV s − 1. A three-electrode system was employed with a saturated calomel electrode (SCE) as reference electrode, a platinum wire as counter electrode and a SWy-2-AQ modified electrode as working electrode. All potentials were reported versus SCE. Deaeration was performed by purging with nitrogen gas.
2.3. Preparation of the modified electrode The GCE was first hand polished on the microcloth pads, and rinsed thoroughly with doubly distilled water, and finally allowed to dry at room temperature. Then a SWy-2 coating was achieved by pipetting a desired volume of 2 mg ml − 1 SWy-2 on the GCE surface and dried at room temperature. The SWy-2-coating electrode was
S. Hu et al. / Talanta 54 (2001) 115–123
further modified by pipetting 10 ml of anthraquinone in desired concentration and dried at room temperature. Unless stated otherwise, the SWy-2-AQ chemically modified electrodes were used in all experiments.
2.4. Analytical procedure Supporting electrolyte (10 ml) was placed in the voltammetric cell and the required volume of standard 4-NP solution was added by micropipette. The solution was deaerated with nitrogen for 10 min, and accumulation was carried out at − 1.3 V for 3 min. After the accumulation period, the voltammetric curve was recorded. Potential window of +0.6 to − 1.0 V was for all the voltammetric measurements. The same procedure was carried out in sample analysis and all electrochemical experiments were carried out at room temperature.
117
3. Results and discussion
3.1. Electrochemical beha6ior of 4 -nitrophenol on chemically modified electrode Fig. 1 shows the continuous cyclic voltamograms of 3.6× 10 − 5 mol l − 1 4-NP on the SWy-2-AQ modified electrode in pH4.5 phosphate buffer. During the first cycle, one peak (A1) appears at −0.78 V on the cathodic sweep, and another one appears at +0.25 V (B2) on the anodic sweep. In the successive cycles, in addition to A1, one new reductive peak (A2) also appears at +0.17 V on the cathodic sweep. Thus, a couple of well-defined redox peaks are observed at around +0.2 V. A couple of peaks at about −0.5 V are redox peaks of anthraquinone, which already appear in phosphate buffer solution. It is interesting that the reversible redox couple (A2/ B2) increases at the expense of the irreversible peak (A1), which indicated that the product of 4-NP by irreversible reduction remained on or
Fig. 1. Continuous CV response of 3.6 ×10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode in pH 4.5 NaH2PO4 solution. ( — ) the first scan; (…) the fourth scan. Scan rate was 100 mV s − 1; switching potentials at +0.6 and − 1.0 V.
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Fig. 2. Linear scan voltammetric response of 3.6 × 10 − 5 mol l − 1 4-NP at the SWy-2-AQ chemically modified electrode ( —) and a bare GCE (…) in pH 3.4 phosphate buffer. Scan rate is 100 mV s − 1. Accumulation time and potential are 3 min and −1.3 V, respectively.
near the modified electrode surface and was oxidized on the anodic sweep. When the scan potential was changed in the range of +0.6 to − 0.4 V, the redox couple disappeared gradually. These interesting phenomena indicated that the development of A1 peak is responsible for the formation of A2 and B2 peaks. Heineman and Kissinger [19] reported that parathion at hanging mercury drop electrode (HMDE) gave the same behavior as that of 4-NP in our studies. Nicholson and Shain [20] also reported that p-nitrosophenol (fNO) could be transferred easily to p-hydroxylaminophenol (fNHOH) by the reversible two-electron oxidation/reduction with using stationary electrode polarography. Nevertheless, it is believed that the same electron transfer mechanism can be applied in this study: fNO2 + 4e− + 4H+ fNHOH + H2O fNHOH
l
(A1)
fNO + 2H+ +2e−
(A2/B2)
Fig. 2 shows the linear scan voltammograms of 4-NP at the SWy-2-AQ chemically modified electrode and a bare glassy carbon electrode. An enhanced peak was observed at the modified electrode, and it was as almost 3.5 times as the peak current measured at bare GCE. It can be explained that when fNHOH is formed, it remains fixed in the SWy-2-AQ modified membrane, whereas it diffuses away from the bare GCE. Experiments were carried out with differential pulse voltammetry for 4-NP at the SWy-2-AQ chemically modified electrodes. There is a symmetric peak as shown in Fig. 3, which indicated that DPV advantaged LSV for the detection of 4-NP.
3.2. Influence of the amount of SWy-2 and AQ As far as the electrode conditions are concerned, the amount of SWy-2 and AQ is a control factor of great interest. As expected, an increase
S. Hu et al. / Talanta 54 (2001) 115–123
in amount of SWy-2 and AQ gives a higher anodic peak current and the peak potential keeps constant. The highest ip is obtained with electrode modified with 10 ml of 2 mg ml − 1 SWy-2 and 10 ml of 10 − 3 mol l − 1 AQ. When increasing continuously the amount of SWy-2 and the concentration of AQ, anodic peak current decreases. As ion exchanger, increase in amount of SWy-2 obviously increases the film thickness, and thus, increases the ion exchange capacity. However, thick film may cause the mass transfer difficult resulting in the current response decrease. The higher AQ concentration is, the higher background is, which results in background covering the signal.
3.3. Influence of pH and buffer solution It is important for chemically modified electrode to choose buffer solution and the pH of buffer solution. A series of supporting electrolytes
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were tested. The peak height and shape were taken into consideration when choosing the supporting electrolytes. In 0.01 mol l − 1 borate buffer solution (pH 9.2), 0.01 mol l − 1 Na2SO4 solution and 5 × 10 − 3 mol l − 1 KCl solution, no redox peaks appeared. There were a small couple of redox peaks in pH 4.5 acetic acid/sodium acetate buffer. In pH 6.8 NaH2PO4 –Na2HPO4 buffer solution and pH 2.11 citrate buffer, high anodic peaks were observed. However, anodic peak was unstable in pH 2.11 citrate buffer, which caused the anodic peak potential positive going constantly. The optimum buffer solution for subsequent studies was phosphate buffer solution. The effect of pH on the electrode reaction was investigated by CV. Fig. 4 shows that the anodic peak current and potential were dependent on the pH. As can been seen, peak potential become more and more positive (Ep = 0.65V at pH 2.1) with decreasing pH. The anodic peak current
Fig. 3. Differential pulse voltammetric response of 3.6 ×10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode in pH 3.4 phosphate buffer. Conditions are the same as in Fig. 2.
S. Hu et al. / Talanta 54 (2001) 115–123
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Fig. 4. pH dependence of the anodic peak potential (…) and peak current ( — ) on Cyclic voltammetric response for 3.6 × 10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode. Scan rate: 100 mV s − 1.
decreased with decreasing pH in the range of 2.0 –3.0. Between pH 3.0 and 4.0, the peak current gives a maximum peak. A current decrease is observed when the pH of the solution is more than 7.0, and almost no anodic peak is observed at pH 8.0. This is most probably due to the hydrolysis of the adsorbed anthraquinones at high pH values [21 – 23], which might cause a dramatic loss in the electrode’s surface coverage. The optimum pH for further studies was pH 3.4. In order to ascertain the number of electrons involved in the oxidation of 4-NP at modified electrode, we determined the n value from cyclic voltammograms by using the Eq. (1): [24] n=
0.0565 Ep − Ep/2
(1)
The n was 1.88 at modified electrode in 3.6× 10 − 5 mol l − 1 4-NP (pH 3.4), which suggests that the number of the electrons corresponding to the redox process (A2/B2) is two. This process involves the loss of 2e− and 2H+, which is consistent with the electron transfer mechanism applied in this study.
3.4. Influence of accumulation time and potential It should be mentioned that it is important to choose accumulation time and potential when adsorption studies are undertaken, both parameters could influence the degree of adsorption of the reduced form of 4-NP (fNHOH). Bearing this in mind, the effect of accumulation time and potential on anodic peak current were investigated by LSV. Fig. 5 shows the dependence of the anodic peak current of fNHOH on the accumulation time for 3.6× 10 − 5 mol l − 1 4-NP. The peak current increases with the accumulation time in the range of 0.25 –3 min, which bears out the occurrence of adsorption accumulation of fNHOH on the surface of the modified electrode. However, about 3 min, further increase in accumulation time does not increase the amount of fNHOH at the electrode surface owing to surface saturation, and the peak current remains constant. This phenomenon is due to the cavity structure of SWy-2 that improves the capacity of the electrode to adsorb electroactive fNHOH.
S. Hu et al. / Talanta 54 (2001) 115–123
The effect of accumulation potential on the response for fNHOH is illustrated in Fig. 6. The figure shows that between − 0.9 and − 1.3 V, anodic peak current apparently increases as the accumulation potential becomes more negative. This behavior is expected because more negative potentials result in more complete reduction of the 4-NP. However, the peak current decreases rapidly as the potential becomes more negative than −1.3 V. A accumulation potential of − 1.3 V was chosen as optimum accumulation potential.
3.5. Analytical characterization Under the optimized conditions, a linear relationship exists between the peak current Ip and the concentration of 4-NP in the range 0.3 –45 mg l − 1 (r =0.9976) with a slope of 1.17 mA l mg − 1 in pH 3.4 phosphate buffer solution. The detection limit is as low as 0.02 mg l − 1. This modified electrode can be applied to determine 2-NP with
121
good results. In the mixture of 4-NP and 2-NP, the modified electrode only can determine the total amount of nitrophenols. The influences of several organic and inorganic species on the response of 4-NP have been tested. For organic species, solutions contained an interferent: mass ratio of 2:1 for 3.6× 10 − 5 mol l − 1 of 4-NP. All organic species containing nitro group interfered with the response of 4-NP, because all of them contain the same reductive group, such as dinitro benzene. Hydroquinone produced a pair of redox peaks in the range of +0.6 to −1.0 V, when the scan potential was changed in the range of + 0.6 to −0.4 V, the redox peaks did not disappear, which indicates that hydroquinone and nitrophenols have different redox mechanism. Because of the difference of redox potential of 4-NP and hydroquinone, 4-NP was also detected without interference of hydroquinone. Phenols, such as phenol and 1,3,5-trihydroxybenzene, had no redox peak in the range of +0.6 to −0.4 V,
Fig. 5. Effect of the accumulation time on peak height in pH 3.4 phosphate buffer solution containing 3.6 × 10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode. Scan rate:100 mVs − 1.
S. Hu et al. / Talanta 54 (2001) 115–123
122
Fig. 6. Effect of the accumulation potential on peak height in pH 3.4 phosphate buffer solution containing 3.6 × 10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode. Scan rate is 100 mV s − 1. Table 1 Determination of 4-nitrophenol in lake water
Lake water
4-NP 4-NP added(×10−5 M)
4-NP found (×10−5 M)
Recovery (%)
0 5.4 9.0 12.6
0 5.52 90.21 9.12 90.78 12.54 90.81
0 102.2% 101.3% 99.5%
which did not cause interference for the response of 4-NP. The possible interference of several inorganic species was also investigated. At the 1:1 ratio Pb(II), Cd(II) did not interfere the 4-NP response. At a 1000-fold excess, Zn(II), Mn(II), Mg(II) and Co(II) did not interfere, while Cu(II) interfered the 4-NP response. In the range of + 0.2 to − 0.2 V, Cu(II) produced a well-defined redox peak, the peak height of Cu(II) is higher than that of 4-NP. For this reason, the difference of the peak potential of 4-NP and Cu(II) is unable to obtain good
resolution, the peak for 4-nitrophenol was covered by the peak for Cu(II). This method was applied to the detection of 4-NP in lake water. No signal for 4-NP was observed when the natural water samples were analyzed. Thus, this method was applied to samples spiked with 4-NP at a certain concentration and the results were summarized in Table 1. The peaks of 4-NP were clearly displayed and the recovery of the spiked 4-NP was also obtained to be good in water samples. Apparently, the interference in lake water samples is almost negligible.
S. Hu et al. / Talanta 54 (2001) 115–123
Acknowledgements This research is supported by the State Key Laboratories of Transducer Technology, Chinese Academy of Science and the Natural Science Foundation of Hubei Province (99J060).
References [1] US Environmental Protection Agency, Fed. Regist., 44 (1979) 233. [2] US Environmental Protection Agency, Fed. Regist., 52 (1989) 131. [3] M.J. Christophersen, T.J. Cardwell, Anal. Chim. Acta 323 (1996) 39. [4] M. Castillo, R. Domingues, M.F. Alpendurada, D. Barcelo´, Anal. Chim. Acta 353 (1997) 133. [5] P.A. Realini, J. Chromatogr. Sci. 19 (1981) 124. [6] T.A. Berger, J.F. Deye, J. Chromatogr. Sci. 29 (1991) 54. [7] C. Brage, K. Sjo¨stro¨m, J. Chromatogr. 538 (1991) 303. [8] L. Michel, A. Zatka, Anal. Chim. Acta 105 (1979) 109. [9] A. Troja´nek, H.G. De Jong, Anal. Chim. Acta 141 (1982) 115. [10] J.J. Scanlon, P.A. Flaquer, G.W. Robinson, G.E. O’Brien, P.E. Sturrock, Anal. Chim. Acta 158 (1984) 169.
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[11] A. Oubin˜a, B. Ballesteros, R. Galve, D. Barcelo, M.-P. Marco, Anal. Chim. Acta 387 (1999) 255. [12] M. Zietek, Mikrochim. Acta 2 (1975) 463. [13] J. Barek, H. Ebertova´, V. Mejstrı´k, J. Zima, Collect. Czech. Chem. Commun. 59 (1994) 1761. [14] L. Herna´ndez, P. Herna´ndez, J. Vicente, Fresenius J. Anal. Chem. 345 (1993) 712. [15] I. Naranjo Rodrı´guez, J.A. Mun˜oz Leyva, J.L. Hidalgo Hidalgo de Cisneros, Anal. Chim. Acta 344 (1997) 167. [16] I. Naranjo Rodrı´guez, J.A. Mun˜oz Leyva, J.L. Hidalgo Hidalgo de Cisneros, Analyst 122 (1997) 601. [17] I. Naranjo Rodrı´guez, M. Barea Zamora, J.M. Barbera´ Salvador, J.A. Mun˜oz Leyva, M.P. Hernandez-Artiga, J.L. Hidalgo Hidalgo de Cisneros, Mikrochim. Acta 126 (1997) 87. [18] J.-M. Kauffmann, NATO ASI Ser. 38 (1997) 107. [19] W.R. Heineman, P.T. Kissinger, Amer. Lab. 11 (1982) 29. [20] R.S. Nicholson, I. Shain, Anal. Chem. 37 (1965) 190–195. [21] S. Rouhani, R. Rezaei, H. Sharghi, M. Shamsipur, G. Rounaghi, Microchem. J. 52 (1995) 22. [22] M. Shamsipur, J. Ghasemi, F. Tamaddon, H. Sharghi, Talanta 40 (1993) 697. [23] D. Almasifar, A. Forganiha, Z. Khojasteh, J. Ghasemi, H. Sharghi, M. Shamsipur, J. Chem. Eng. Data 42 (1997) 1212. [24] Z. Galus, Fundamentals of Electrochemical Analysis, Ellis Horwood, Chichester, 1976, p. 233.
Voltammetric determination of 4-nitrophenol at a sodium montmorillonite-anthraquinone chemically modified glassy carbon electrode Shengshui Hu a,*, Cuiling Xu a, Gaiping Wang b, Dafu Cui c a Department of Chemistry, Wuhan Uni6ersity, Wuhan 430072, PR China Faculty of Basic Medical Science, Xinxiang Medical College, Henan 453003, PR China c State Key Laboratory of Transducer Technology, Institute of Electronics, Chinese Academy of Sciences, Beijing 100080, PR China b
Received 17 July 2000; received in revised form 9 November 2000; accepted 9 November 2000
Abstract A new method for the determination of 4-nitrophenol(4-NP) by differential pulse voltammetry (DPV) based on adsorptive stripping technique was described. Cyclic voltammetry (CV) and linear scan voltammetry (LSV) were used in a comparative investigation into the electrochemical reduction of 4-NP at a Na-montmorillonite(SWy-2) and anthraquione (AQ) modified glassy carbon electrode. With this chemically modified electrode, 4-NP was first irreversibly reduced from fNO2 to fNHOH at − 0.78 V. A couple of well-defined redox peaks at +0.22 V (vs. SCE) were responsible for a two-electron redox peak between fNHOH and fNO. Studies on the effect of pH on the peak height and peak potential were carried out over the pH range 2.0– 9.0 with the phosphate buffer solution. A pH of 3.4 was chosen as the optimum pH. The other experimental parameters, such as film thickness, accumulation time and potential etc. were optimized. Anodic peak currents were found to be linearly related to concentration of 4-NP over the range 0.3–45 mg l − 1, with a detection limit of 0.02 mg l − 1. The interference of organic and inorganic species on the voltammetric response have been studied. This modified electrode can be used to the determination of 4-NP in water samples. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Na-montmorillonite; Anthraquinone; 4-nitrophenol; Differential pulse voltammetry; Chemically modified electrode
1. Introduction Phenol and substituted phenols are known to be widespread as components in industrial and * Corresponding author. Tel.: + 86-27-87684573; fax: +8627-87882661. E-mail address: [email protected] (S. Hu).
natural waste. These compounds have toxic effect on humans, animals and plants, they give an undesirable taste and odor to drinking water, even in very low concentration. For these reasons, many of the phenols have been included in the environmental legislation. 4-Nitrophenol(4-NP) is one of the nitrophenols in the U.S. Environmental Protection Agency List of Priority Pollutants
0039-9140/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 6 5 8 - 5
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S. Hu et al. / Talanta 54 (2001) 115–123
[1,2]. 4-NP is a hazardous substance which can cause a high environmental impact due to its toxicity and persistence. The origin of contamination comes from manufacturing, chemical industry and agricultural practices [3]. However, organophosphorus pesticides yield nitrophenols as major degradation products [4]. Consequently, qualitative and quantitative detection of nitrophenols is a matter of concern for environmental control. The detection of nitrophenols is usually accomplished by means of chromatographic separation with spectrometry, such as mass spectrometry or flame ionization and ultraviolet detection [5–7], and with electrochemical detector [8 – 10]. The use of enzyme-linked immunosorbent assay (ELISA) has been reported [11]. Various electroanalytical techniques have been also applied to the determination of nitrophenols: polarography, cyclic voltammetry (CV), adsorptive stripping and differential pulse voltammetry (DPV) [12 – 17]. Voltammetric methods are particularly suited for the analysis of a wide variety of organic compounds. However, modified electrodes are being used frequently in the voltammetric determination of organic compounds because of their efficiency, the selectivity that can be obtained by varying the modifier and the sensitivity which is equivalent to that reached in anodic and cathodic stripping. Kauffmann [18] has reported that a carbon paste electrode modified with lipids and proteins (enzymes) has potential application in environmental analysis. In addition, clay (such as bentonite and zeolite) modified electrodes have been applied widely for the determination of organic compounds. Na-montmorillonite(SWy-2) exhibits similar properties to those of other clays, which are able to adsorb electroactive species for their direct determination. This article describes a differential pulse voltammetric method for the determination of 4-NP by using a SWy-2-AQ modified electrode. The modified electrode shows the remarkable accumulation effect of SWy-2 film for 4-NP and the high sensitivity by the DPV. It also displays good resistance to the interference from common ions such as cadmium and lead as well as the interference from some organic compounds.
2. Experimental
2.1. Chemicals and materials Na-montmorillonite(SWy-2) was obtained from Source Clay Minerals Repository, University of Missouri (Columbia, MO). Nitrophenols (Beijing Chemical Reagent Institute) and all other compounds were analytically grade and were used without further purification. Doubly distilled water was used to prepare all aqueous solution. The glassy carbon electrodes(GC, 3 mm in diameter) used in this study and microcloth pads were purchased from Bioanalytical Systems (West Lafayette, IN, USA). Supporting electrolyte used throughout this study was phosphate buffer solution, which was prepared from pH 3.4 HCl – NaH2PO4 solution, unless stated otherwise. Water samples were collected from East Lake of Wuhan, China.
2.2. Apparatus Differential pulse and linear scan voltammetry (LSV) were performed with use of an EG&G Princ. Potentiostat/Galvanostat Model 273A. Cyclic voltammograms measurements were made with an EG&G PARC Model 366 bi-potentiostat and recorded on Model 3086 X-Y recorder (Yokogawa Japan). The potential scan rate in all voltammetric measurements was 100 mV s − 1. A three-electrode system was employed with a saturated calomel electrode (SCE) as reference electrode, a platinum wire as counter electrode and a SWy-2-AQ modified electrode as working electrode. All potentials were reported versus SCE. Deaeration was performed by purging with nitrogen gas.
2.3. Preparation of the modified electrode The GCE was first hand polished on the microcloth pads, and rinsed thoroughly with doubly distilled water, and finally allowed to dry at room temperature. Then a SWy-2 coating was achieved by pipetting a desired volume of 2 mg ml − 1 SWy-2 on the GCE surface and dried at room temperature. The SWy-2-coating electrode was
S. Hu et al. / Talanta 54 (2001) 115–123
further modified by pipetting 10 ml of anthraquinone in desired concentration and dried at room temperature. Unless stated otherwise, the SWy-2-AQ chemically modified electrodes were used in all experiments.
2.4. Analytical procedure Supporting electrolyte (10 ml) was placed in the voltammetric cell and the required volume of standard 4-NP solution was added by micropipette. The solution was deaerated with nitrogen for 10 min, and accumulation was carried out at − 1.3 V for 3 min. After the accumulation period, the voltammetric curve was recorded. Potential window of +0.6 to − 1.0 V was for all the voltammetric measurements. The same procedure was carried out in sample analysis and all electrochemical experiments were carried out at room temperature.
117
3. Results and discussion
3.1. Electrochemical beha6ior of 4 -nitrophenol on chemically modified electrode Fig. 1 shows the continuous cyclic voltamograms of 3.6× 10 − 5 mol l − 1 4-NP on the SWy-2-AQ modified electrode in pH4.5 phosphate buffer. During the first cycle, one peak (A1) appears at −0.78 V on the cathodic sweep, and another one appears at +0.25 V (B2) on the anodic sweep. In the successive cycles, in addition to A1, one new reductive peak (A2) also appears at +0.17 V on the cathodic sweep. Thus, a couple of well-defined redox peaks are observed at around +0.2 V. A couple of peaks at about −0.5 V are redox peaks of anthraquinone, which already appear in phosphate buffer solution. It is interesting that the reversible redox couple (A2/ B2) increases at the expense of the irreversible peak (A1), which indicated that the product of 4-NP by irreversible reduction remained on or
Fig. 1. Continuous CV response of 3.6 ×10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode in pH 4.5 NaH2PO4 solution. ( — ) the first scan; (…) the fourth scan. Scan rate was 100 mV s − 1; switching potentials at +0.6 and − 1.0 V.
S. Hu et al. / Talanta 54 (2001) 115–123
118
Fig. 2. Linear scan voltammetric response of 3.6 × 10 − 5 mol l − 1 4-NP at the SWy-2-AQ chemically modified electrode ( —) and a bare GCE (…) in pH 3.4 phosphate buffer. Scan rate is 100 mV s − 1. Accumulation time and potential are 3 min and −1.3 V, respectively.
near the modified electrode surface and was oxidized on the anodic sweep. When the scan potential was changed in the range of +0.6 to − 0.4 V, the redox couple disappeared gradually. These interesting phenomena indicated that the development of A1 peak is responsible for the formation of A2 and B2 peaks. Heineman and Kissinger [19] reported that parathion at hanging mercury drop electrode (HMDE) gave the same behavior as that of 4-NP in our studies. Nicholson and Shain [20] also reported that p-nitrosophenol (fNO) could be transferred easily to p-hydroxylaminophenol (fNHOH) by the reversible two-electron oxidation/reduction with using stationary electrode polarography. Nevertheless, it is believed that the same electron transfer mechanism can be applied in this study: fNO2 + 4e− + 4H+ fNHOH + H2O fNHOH
l
(A1)
fNO + 2H+ +2e−
(A2/B2)
Fig. 2 shows the linear scan voltammograms of 4-NP at the SWy-2-AQ chemically modified electrode and a bare glassy carbon electrode. An enhanced peak was observed at the modified electrode, and it was as almost 3.5 times as the peak current measured at bare GCE. It can be explained that when fNHOH is formed, it remains fixed in the SWy-2-AQ modified membrane, whereas it diffuses away from the bare GCE. Experiments were carried out with differential pulse voltammetry for 4-NP at the SWy-2-AQ chemically modified electrodes. There is a symmetric peak as shown in Fig. 3, which indicated that DPV advantaged LSV for the detection of 4-NP.
3.2. Influence of the amount of SWy-2 and AQ As far as the electrode conditions are concerned, the amount of SWy-2 and AQ is a control factor of great interest. As expected, an increase
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in amount of SWy-2 and AQ gives a higher anodic peak current and the peak potential keeps constant. The highest ip is obtained with electrode modified with 10 ml of 2 mg ml − 1 SWy-2 and 10 ml of 10 − 3 mol l − 1 AQ. When increasing continuously the amount of SWy-2 and the concentration of AQ, anodic peak current decreases. As ion exchanger, increase in amount of SWy-2 obviously increases the film thickness, and thus, increases the ion exchange capacity. However, thick film may cause the mass transfer difficult resulting in the current response decrease. The higher AQ concentration is, the higher background is, which results in background covering the signal.
3.3. Influence of pH and buffer solution It is important for chemically modified electrode to choose buffer solution and the pH of buffer solution. A series of supporting electrolytes
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were tested. The peak height and shape were taken into consideration when choosing the supporting electrolytes. In 0.01 mol l − 1 borate buffer solution (pH 9.2), 0.01 mol l − 1 Na2SO4 solution and 5 × 10 − 3 mol l − 1 KCl solution, no redox peaks appeared. There were a small couple of redox peaks in pH 4.5 acetic acid/sodium acetate buffer. In pH 6.8 NaH2PO4 –Na2HPO4 buffer solution and pH 2.11 citrate buffer, high anodic peaks were observed. However, anodic peak was unstable in pH 2.11 citrate buffer, which caused the anodic peak potential positive going constantly. The optimum buffer solution for subsequent studies was phosphate buffer solution. The effect of pH on the electrode reaction was investigated by CV. Fig. 4 shows that the anodic peak current and potential were dependent on the pH. As can been seen, peak potential become more and more positive (Ep = 0.65V at pH 2.1) with decreasing pH. The anodic peak current
Fig. 3. Differential pulse voltammetric response of 3.6 ×10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode in pH 3.4 phosphate buffer. Conditions are the same as in Fig. 2.
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Fig. 4. pH dependence of the anodic peak potential (…) and peak current ( — ) on Cyclic voltammetric response for 3.6 × 10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode. Scan rate: 100 mV s − 1.
decreased with decreasing pH in the range of 2.0 –3.0. Between pH 3.0 and 4.0, the peak current gives a maximum peak. A current decrease is observed when the pH of the solution is more than 7.0, and almost no anodic peak is observed at pH 8.0. This is most probably due to the hydrolysis of the adsorbed anthraquinones at high pH values [21 – 23], which might cause a dramatic loss in the electrode’s surface coverage. The optimum pH for further studies was pH 3.4. In order to ascertain the number of electrons involved in the oxidation of 4-NP at modified electrode, we determined the n value from cyclic voltammograms by using the Eq. (1): [24] n=
0.0565 Ep − Ep/2
(1)
The n was 1.88 at modified electrode in 3.6× 10 − 5 mol l − 1 4-NP (pH 3.4), which suggests that the number of the electrons corresponding to the redox process (A2/B2) is two. This process involves the loss of 2e− and 2H+, which is consistent with the electron transfer mechanism applied in this study.
3.4. Influence of accumulation time and potential It should be mentioned that it is important to choose accumulation time and potential when adsorption studies are undertaken, both parameters could influence the degree of adsorption of the reduced form of 4-NP (fNHOH). Bearing this in mind, the effect of accumulation time and potential on anodic peak current were investigated by LSV. Fig. 5 shows the dependence of the anodic peak current of fNHOH on the accumulation time for 3.6× 10 − 5 mol l − 1 4-NP. The peak current increases with the accumulation time in the range of 0.25 –3 min, which bears out the occurrence of adsorption accumulation of fNHOH on the surface of the modified electrode. However, about 3 min, further increase in accumulation time does not increase the amount of fNHOH at the electrode surface owing to surface saturation, and the peak current remains constant. This phenomenon is due to the cavity structure of SWy-2 that improves the capacity of the electrode to adsorb electroactive fNHOH.
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The effect of accumulation potential on the response for fNHOH is illustrated in Fig. 6. The figure shows that between − 0.9 and − 1.3 V, anodic peak current apparently increases as the accumulation potential becomes more negative. This behavior is expected because more negative potentials result in more complete reduction of the 4-NP. However, the peak current decreases rapidly as the potential becomes more negative than −1.3 V. A accumulation potential of − 1.3 V was chosen as optimum accumulation potential.
3.5. Analytical characterization Under the optimized conditions, a linear relationship exists between the peak current Ip and the concentration of 4-NP in the range 0.3 –45 mg l − 1 (r =0.9976) with a slope of 1.17 mA l mg − 1 in pH 3.4 phosphate buffer solution. The detection limit is as low as 0.02 mg l − 1. This modified electrode can be applied to determine 2-NP with
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good results. In the mixture of 4-NP and 2-NP, the modified electrode only can determine the total amount of nitrophenols. The influences of several organic and inorganic species on the response of 4-NP have been tested. For organic species, solutions contained an interferent: mass ratio of 2:1 for 3.6× 10 − 5 mol l − 1 of 4-NP. All organic species containing nitro group interfered with the response of 4-NP, because all of them contain the same reductive group, such as dinitro benzene. Hydroquinone produced a pair of redox peaks in the range of +0.6 to −1.0 V, when the scan potential was changed in the range of + 0.6 to −0.4 V, the redox peaks did not disappear, which indicates that hydroquinone and nitrophenols have different redox mechanism. Because of the difference of redox potential of 4-NP and hydroquinone, 4-NP was also detected without interference of hydroquinone. Phenols, such as phenol and 1,3,5-trihydroxybenzene, had no redox peak in the range of +0.6 to −0.4 V,
Fig. 5. Effect of the accumulation time on peak height in pH 3.4 phosphate buffer solution containing 3.6 × 10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode. Scan rate:100 mVs − 1.
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Fig. 6. Effect of the accumulation potential on peak height in pH 3.4 phosphate buffer solution containing 3.6 × 10 − 5 mol l − 1 4-NP at SWy-2-AQ chemically modified electrode. Scan rate is 100 mV s − 1. Table 1 Determination of 4-nitrophenol in lake water
Lake water
4-NP 4-NP added(×10−5 M)
4-NP found (×10−5 M)
Recovery (%)
0 5.4 9.0 12.6
0 5.52 90.21 9.12 90.78 12.54 90.81
0 102.2% 101.3% 99.5%
which did not cause interference for the response of 4-NP. The possible interference of several inorganic species was also investigated. At the 1:1 ratio Pb(II), Cd(II) did not interfere the 4-NP response. At a 1000-fold excess, Zn(II), Mn(II), Mg(II) and Co(II) did not interfere, while Cu(II) interfered the 4-NP response. In the range of + 0.2 to − 0.2 V, Cu(II) produced a well-defined redox peak, the peak height of Cu(II) is higher than that of 4-NP. For this reason, the difference of the peak potential of 4-NP and Cu(II) is unable to obtain good
resolution, the peak for 4-nitrophenol was covered by the peak for Cu(II). This method was applied to the detection of 4-NP in lake water. No signal for 4-NP was observed when the natural water samples were analyzed. Thus, this method was applied to samples spiked with 4-NP at a certain concentration and the results were summarized in Table 1. The peaks of 4-NP were clearly displayed and the recovery of the spiked 4-NP was also obtained to be good in water samples. Apparently, the interference in lake water samples is almost negligible.
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Acknowledgements This research is supported by the State Key Laboratories of Transducer Technology, Chinese Academy of Science and the Natural Science Foundation of Hubei Province (99J060).
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