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Voltammetric determination of isoprenaline in pharmaceutical preparations using a copper(II) hexacyanoferrate(III) modified carbon paste electrode
Microchemical Journal 78 (2004) 55 – 59 www.elsevier.com/locate/microc
Voltammetric determination of isoprenaline in pharmaceutical preparations using a copper(II) hexacyanoferrate(III) modified carbon paste electrode Viviane G. Bonifa´cio a, Luiz H. Marcolino Jr. a, Marcos F.S. Teixeira b, Orlando Fatibello-Filho a,* a
Departamento de Quı´mica, Centro de Cieˆncias Exatas e de Tecnologia, Universidade Federal de Sa˜o Carlos, Caixa Postal 676, CEP 13560-970, Sa˜o Carlos, SP, Brazil b UNESP-Campus do Litoral Paulista, Unidade de Sa˜o Vicente, CEP 11330-900, Sa˜o Vicente, SP, Brazil Accepted 11 March 2004 Available online 10 May 2004
Abstract The electroanalytical determination of isoprenaline in pharmaceutical preparations of a homemade carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) was studied by cyclic voltammetry. Several parameters were studied for the optimization of the sensor such as electrode composition, electrolytic solution, pH effect, potential scan rate and interferences in potential. The optimum conditions were found in an electrode composition (in mass) of 15% CuHCF, 60% graphite and 25% mineral oil in 0.5 mol l1 acetate buffer solution at pH 6.0. The analytical curve for isoprenaline was linear in the concentration range from 1.96104 to 1.07103 mol l1 with a detection limit of 8.0105 mol l1. The relative standard deviation was 1.2% for 1.96104 mol l1 isoprenaline solution (n=5). The procedure was successfully applied to the determination of isoprenaline in pharmaceutical preparations; the CuHCF modified carbon paste electrode gave comparable results to those results obtained using a UV spectrophotometric method. D 2004 Elsevier B.V. All rights reserved. Keywords: Modified carbon paste electrode; Copper(II) hexacyanoferrate(III); Voltammetric determination; Isoprenaline; Pharmaceutical preparations
1. Introduction Isoprenaline or isoproterenol 4-[1-hydroxy-2-[(1-methylethyl)-amino]ethyl]-1,2-benzenediol is a catecholamine drug that is widely used in the treatment of allergic emergencies, status asthmatic, bronchial asthma, ventricular bradycardia, cardiac arrest, glaucoma and as styptic [1]. The cardiovascular effects of isoprenaline are compared with the epinephrine and norepinephrine, which can relax almost every kind of the smooth musculature that contains adrenergic nervous, but this effect is pronounced in the musculature of bronchus and also in the gastrointestinal tract. The isoprenaline is better absorbed when dispensed by inhalation [2]. Several techniques have been described in the literature for the determination of isoprenaline such as spectrophotometry [3 –7], spectrofluorimetry [8– 10], chemiluminescence * Corresponding author. E-mail address: [email protected] (O. Fatibello-Filho). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.03.010
[11 – 13] and gas chromatography [14]. Electrochemical procedures for the determination of isoprenaline have also been reported [15 – 17]. In recent years, the construction and application of modified electrode has received great attention owing the sensitivity and selectivity of the electrochemical techniques. The use of metal hexacyanoferrates to construct modified electrodes has been extensively studied [18 – 20]. Shankaran and Narayanan [20,21] determine sulfur dioxide and ascorbic acid by amperometry using a graphite electrode modified with (CuHCF). On the other hand, there are several metal hexacyanoferrate film modified electrodes reported in the literature [22 – 25]. In this paper, the construction and the analytical application of a carbon paste electrode modified with CuHCF are proposed for determining isoprenaline in pharmaceutical formulations. The influence of various parameters such as electrode composition, electrolyte, pH, potential scan rate and interference in potential was studied by cyclic voltammetry.
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2. Experimental 2.1. Apparatus The cyclic voltammetric measurements were carried out with an Autolab PGSTAT-30 (Eco Chimie) controlled by a PC using the GPES 4.9 software. The measurements were performed in a 10-ml glass cell, containing three electrodes: a modified carbon paste electrode as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum wire as auxiliary electrode. During the measurements, the solution in the cell (acetate buffer solution at pH 6.0) was not stirred. 2.2. Reagents and solutions All reagents were of analytical-reagent grade and all solutions were prepared using Millipore Milli-Q deionised water. The supporting electrolyte used for all experiments was a 0.5 mol l1 acetate buffer solution at pH 6.0. A stock solution 1.0102 mol l1 isoprenaline was prepared daily by dissolving isoprenaline (Aldrich) in 10 ml of water. Graphite powder (1– 2 Am particle size—Aldrich) and mineral oil (Aldrich) of high purity were used in the preparation of carbon paste. 2.3. Cu3[Fe(CN)6]2 preparation The preparation of Cu3[Fe(CN)6]2 was carried out using a procedure previously described [26] by mixing a 0.125 mol l1 potassium hexacyanoferrate(III) solution and a 0.375 mol l1 copper(II) sulfate solution with the molar ratio Cu/Fe equal to 3. The precipitate obtained was filtered in a sintered glass Gooch filter, washed with distilled water by several times and dried at 25 jC for 4 days. 2.4. Carbon paste electrode preparation The modified carbon paste electrode was prepared manually by carefully mixing the dispersed graphite powder with CuHCF at several mass ratios and subsequently adding of 250 mg of mineral oil (25% m/m). Those mixtures were mixed by magnetic agitation in a 50-ml beaker containing 20 ml of hexane. The final paste was obtained with the evaporation of this solvent. Of 1000 mg of prepared modified paste, about 200 mg of this paste was packed into the tip of a 1-ml insulin plastic syringe, which had its extremity cut (70 mm high and 3 mm internal diameter), and a copper wire was inserted instead of the plastic plunger to obtain the external electric contact. 2.5. Preparation and analysis of pharmaceutical samples Liquid formulations were appropriately diluted to 10 ml with 0.5 mol l1 acetate buffer solution. No other treatment
of the samples was required. The cyclic voltammograms were recorded by cycling the potential between 0.3 and +1.2 V at a scan rate of 20 mV s1. The percentage content of isoprenaline in these samples was determined by the standard addition method and compared with those results obtained using a UV spectrophotometric method at a wavelength of 280 nm.
3. Results and discussion 3.1. Supporting electrolyte effect Experimental results showed that Prussian blue and its analogues could allow alkali-metal cations to penetrate selectively the channels owing to their zeolitic property [27,28]. Recently, differential pulse voltammetric measurements with a carbon paste electrode modified with CuHCF in alkaline and earth-alkaline chloride solutions were studied [29]. The peak potentials ([FeII(CN)6]/[FeIII(CN)6]) of the modified electrode in the electrolyte solutions containing the alkali cations decrease with the increase of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the electrode. The peak potential was found linearly dependent with the ratio [charge]/[ionic radius] of the inserting metal ion. In this work, the voltammetric behavior of the carbon paste electrode modified with CuHCF was appraised in different supporting electrolytes such as LiCl, NaCl and KCl solutions at the same concentration (0.5 mol l1). The modified electrode showed one well-defined redox peak in the presence of NaCl solution, whereas in the presence of potassium ions the redox peak was broadened. For supporting electrolyte containing lithium ions, the peak currents decreased drastically. Therefore, we can confirm that there is a significant cation influence over the electrochemical properties of CuHCF. The influence of anions such as chloride and acetate over the electrochemical properties of CuHCF was also studied. In the presence of sodium acetate– acetic acid buffer solution, the peak potential was displaced to potential values smaller than in NaCl solution. In addition, a well-defined and reproducible cyclovoltammogram was obtained, which is not seen so clearly in sodium chloride. Therefore, the acetate buffer solution at pH 4.8 was chosen due to its better voltammetric profile and also to the shift of the redox peaks to more negative potentials than other electrolyte solutions. 3.2. Electrode composition effect The effect of the electrode composition on their voltammetric profiles was evaluated in acetate buffer solution at pH 4.8. Fig. 1 shows the influence of CuHCF amount varying from 10% to 35% (m/m) in the carbon paste on the analytical signal (anodic peak current). The peak current increased with increasing amount of CuHCF up to 30% (m/
V.G. Bonifa´cio et al. / Microchemical Journal 78 (2004) 55–59
Fig. 1. Anodic peak current in function of the CuHCF content in the carbon paste electrode, obtained in 0.5 mol l1 acetate buffer solution (pH 4.8), at scan rate 20 mV s1 and 25.0F0.2 jC.
m). For CuHCF amounts higher than 30% (m/m), the peak current decreased significantly. This occurs due to a decrease in the graphite content in the paste and consequent reduction of the conductive electrode area. Nevertheless, the electrode containing 15% (m/m) CuHCF, 60% (m/m) graphite and 25% (m/m) mineral oil was selected for further studies because it presented the best voltammetric profile with peak potential well-defined. 3.3. Oxidation of isoprenaline at the carbon paste electrode modified with CuHCF
57
that the modified electrode influences the oxidation process of isoprenaline. The mechanism of the isoprenaline oxidation can be proposed. The anodic peak at +0.813 V (peak A) and the cathodic peak (peak B) at +0.274 V corresponds to the [Fe(CN)6]3/ [Fe(CN)6]4 redox couple. The effect of the scan rate on the voltammetric response of a carbon paste electrode modified with CuHCF in a solution containing 1.96104 mol l1 isoprenaline was studied. The cyclic voltammograms revealed that the anodic peak currents increase and the peak potential shifts as the scan rate increases. For scan rates higher than 100 mV s1, the voltammetric profiles become distorted, which can be associated with the slow electron transfer rate between the graphite and CuHFC in the paste [30]. When peak current values were plotted against v1/2 (Fig. 3), the following linear relationship was obtained: Ipa ðAAÞ ¼ 59:6 þ 113:0v1=2 ðmV1=2 s1=2 Þ
r ¼ 0:9999
The change of peak current with the scan rate reveals a typical diffusion-controlled behavior and indicates that chemical reaction (Scheme 1) is the rate-determining step in the whole electrochemical reaction process [31]. The increment of the peak potential difference with the increase of the scan rate suggests that the electrode reaction is a quasi-reversible process. From these results, a scan rate of 20 mV s1 was chosen for the further studies. 3.4. Effect of pH
Fig. 2 shows the cyclic voltammograms obtained using an unmodified electrode in the presence of 1.96104 mol l1 isoprenaline (a) and the carbon paste electrode modified with CuHCF in the absence (b) and presence (c) of 1.96104 mol l1 isoprenaline. The increment of anodic current showed in the cyclic voltammogram (c) indicates
The electrochemical behavior of the carbon paste electrode modified with CuHCF (15% m/m) was studied in acetate buffer solution in a large pH range between 3.0 and 8.0 in a solution containing 1.96104 mol l1 isoprena-
Fig. 2. Cyclic voltammograms of oxidation of isoprenaline: (a) unmodified electrode containing 1.96104 mol l1 isoprenaline, (b) modified electrode in the absence of isoprenaline and (c) modified electrode in the presence of 1.96104 mol l1 isoprenaline, in acetate buffer (pH 4.8), at scan rate 20 mV s1 and 25.0F0.2 jC.
Fig. 3. Dependence of the anodic peak current of the modified electrode with square root of the scan rate in 0.5 mol l1 acetate buffer solution (pH 4.8) containing 1.96104 mol l1 isoprenaline, at 25.0F0.2 jC.
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Scheme 1. Proposed mechanism for the voltammetric response of the modified electrode.
line. The best analytical response was obtained at pH 6.0. Fig. 4 shows the effect of pH in the anodic peak current obtained with the modified electrode. The anodic peak current (DI/AA) was obtained by the difference of the currents in the presence and absence of isoprenaline. Besides that, the chemical oxidation of isoprenaline is generally easier in alkaline medium than in acidic medium [32], a decrease in anodic peak current above pH 6.0 is observed, which can be related with the copper(II) hydroxide formation in equilibrium with the CuHCF in the electrode surface. In addition, it was observed that the anodic peak current decreased gradually with the decrease of the pH, starting at 6.0, which is probably due to the influence of H+ on the chemical reactions kinetic between isoprenaline and CuHCF on the electrode surface (Scheme 1; chemical reaction). On the other hand, the change in the peak potential of the carbon paste electrode modified with CuHCF was not dependent on the pH (Scheme 1, electrode reaction). Therefore, the result indicates that a 0.5 mol l1 acetate buffer solution at pH 6.0 could be chosen for further studies. 3.5. Study of interference in potential The effect of excipient substances frequently found with isoprenaline in pharmaceutical formulations such as lactose, sucrose, fructose, starch, poly(ethylene glycol), sodi-
um chloride and magnesium stearate were evaluated using the modified electrode. None of these substances caused interference on the proposed method at the 1:1 concentration ratio. 3.6. Calibration graph, repeatability and lifetime of electrode Under the optimized conditions previously studied such as composition of the electrode (15% m/m), pH 6.0 and scan rate of 20 mV, the anodic peak current of the carbon paste electrode modified with CuHCF was rectilinear in the isoprenaline concentration from 1.96104 to 1.07103 mol l 1 (I pa (AA)=41.41+15.71 10 4 [isoproterenol]; r=0.9989). The detection limit (three times the standard deviation of blank/slope) was 8.0105 mol l1. The repeatability of the voltammetric measurements was evaluated making five successive cyclic voltammograms of the CuHCF modified carbon paste electrode in a 0.5 mol l1 acetate buffer solution at pH 6.0 containing 1.96104 mol l1 of isoprenaline. For this study, a relative standard deviation of 1.2% was obtained. The lifetime of the proposed electrode was at least 5 months (more than 100 determinations were performed using the same electrode surface and more than 600 determinations were done when the surface was renewed by pressing the plunger that forces an appropriate amount of paste out of the electrode tip). Then the surface is polished against a filter paper. 3.7. Analysis of pharmaceutical preparation The proposed method was applied to the determination of isoprenaline in two pharmaceuticals formulations. Table 1 presents the results obtained using an official UV spectroTable 1 Determination of isoprenaline in a pharmaceutical formulation using the modified carbon paste electrode with CuHCF compared with UV spectrophotometry [33] Sample Isoprenaline (mg ml1)
R1 % R2 %
Nominal value Spectrophotometry Voltammetry
Fig. 4. Influence of pH on the voltammetric response of the modified carbon paste electrode with 15% (m/m) CuHCF in acetate buffer solution containing 1.96104 mol l1 isoprenaline, at scan rate 20 mV s1 and 25.0F0.2 jC.
1 2
0.250 0.250
0.247F0.005 0.247F0.006
0.253F0.006 +2.4 0.256F0.008 +3.6
+1.0 +2.4
R1 %=relative error=voltammetric method versus spectrophotometric method. R2 %=relative error=voltammetric method versus nominal value.
V.G. Bonifa´cio et al. / Microchemical Journal 78 (2004) 55–59
photometric procedure [33] and the proposed voltammetric method. The good agreement between the results confirms the usefulness of the proposed procedure for isoprenaline analysis in pharmaceutical samples. 4. Conclusions The carbon paste electrode modified with copper(II) hexacyanoferrate(III) was prepared and applied for the voltammetric determination of isoprenaline in pharmaceutical preparation with good sensitivity and selectivity, with a simple and quick preparation. The cyclic voltammograms of the modified electrode show the presence of one pair of well-defined redox peaks. The optimum conditions for the analysis of isoprenaline were as follows: composition of the electrode of 15% (m/ m) CuHCF; 0.5 mol l1 acetate buffer solution, pH 6.0; and scan rate of 20 mV s1. The analytical curve for isoprenaline was linear in the concentration range from 1.96104 to 1.07103 mol l1 with a detection limit of 8.0105 mol l1. Acknowledgements The authors gratefully acknowledge financial support from Brazilian foundations (FAPESP, CAPES and CNPq). References [1] L.S. Goodman, A. Gilman, The Pharmacological Basis of Therapeutics, 9th ed., McGraw-Hill, New York, 1996, p. 105. [2] D. Voet, J.G. Voet, Biochemistry, Wiley, New York, 1995, p. 1268. [3] K.O. Lupetti, I.C. Vieira, O. Fatibello-Filho, Talanta 57 (2002) 135 – 143. [4] P. Solich, Ch.K. Polydorou, M.A. Koupparis, C.E. Efstathiou, J. Pharm. Biomed. Anal. 22 (2000) 781 – 789. [5] J.J.B. Nevado, J.M.L. Gallego, P.B. Laguna, Anal. Chim. Acta 300 (1995) 293 – 297. [6] H.A. El-Obeid, Anal. Lett. 17 (1984) 771 – 777. [7] R.M.V. Caman˜as, J.M.S. Mallols, J.R.T. Lapasio´, G. Ramis-Ramos, Analyst 120 (1995) 1767 – 1772.
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[8] H. Jeon, H. Nohta, H. Nagaoka, Y. Ohkura, Anal. Sci. 7 (1991) 257 – 262. [9] H. Nohta, T. Yukizawa, Y. Ohkura, M. Yoshimura, J. Ishida, M. Yamaguchi, Anal. Chim. Acta 344 (1997) 233 – 240. [10] G. Alberts, T. Lameris, A.H. van den Meiracker, A.J.M.I. ’t Veld, F. Boomsma, J. Chromatogr. B 730 (1999) 213 – 219. [11] G.J. Zhou, G.F. Zhang, H.Y. Chem, Anal. Chim. Acta 463 (2002) 257 – 263. [12] C. Zhang, J. Huang, Z. Zhang, M. Aizawa, Anal. Chim. Acta 374 (1998) 105 – 110. [13] A.A. Al-Warthan, S.A. Al-Tamrah, A. Al-Akel, Anal. Sci. 10 (1994) 449 – 452. [14] J.R. Watson, R.C. Lawrence, J. Pharm. Sci. 66 (1977) 560 – 564. [15] L. Elrod Jr., J.L. Schmit, J.A. Morley, J. Chromatogr. A 723 (1996) 235 – 241. [16] Y. Qu, L.F. Moons, F. Vandesande, J. Chromatogr. Biomed. Appl. 704 (1997) 351 – 358. [17] F. Mashige, Y. Matsushima, C. Miyata, R. Yamada, H. Kanazawa, I. Sakuma, N. Takai, N. Shinozuka, A. Ohkubo, K. Nakahara, Biomed. Chromatogr. 9 (1995) 221 – 225. [18] R.J. Mortimer, P.J.S. Barbeira, A.F.B. Sene, N.R. Stradiotto, Talanta 49 (1999) 271 – 275. [19] R. Garjonyte, A. Malinauskas, Sens. Actuators, B 46 (1998) 236 – 241. [20] D.R. Shankaran, S.S. Narayanan, Sens. Actuators B 55 (1999) 119 – 194. [21] D.R. Shankaran, S.S. Narayanan, Fresenius J. Anal. Chem. 364 (1999) 686 – 689. [22] S.M. Chen, J. Electroanal. Chem. 417 (1996) 145 – 153. [23] D.M. Zhou, H.X. Ju, H.Y. Chen, J. Electroanal. Chem. 408 (1996) 219 – 223. [24] I.L. Mattos, L. Gorton, T. Ruzgas, A.A. Karyakin, Anal. Sci. 16 (2000) 795 – 798. [25] C.X. Cai, K.H. Xue, S.M. Xu, J. Electroanal. Chem. 486 (2000) 111 – 118. [26] S. Ayrault, B. Jimenez, E. Granier, M. Fedoroff, D.J. Jones, C. LoosNeskovic, J. Solid State Chem. 141 (1998) 475 – 485. [27] A.A. Karyakin, Electroanalysis 13 (2001) 813 – 819. [28] A. Dostal, M. Hermes, F. Scholz, J. Electroanal. Chem. 415 (1996) 133 – 141. [29] R.S. Nunes, M.F.S. Teixeira, 54th Annual Meeting of the International Society of Electrochemistry, Sa˜o Pedro, Brazil, 2003, p. 206. [30] M.F.S. Teixeira, F.C. Moraes, O. Fatibello-Filho, N. Bocchi, Anal. Chim. Acta 443 (2001) 249 – 255. [31] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 1980, p. 224. [32] S.M. Chen, K.T. Peng, J. Electroanal. Chem. 547 (2003) 179 – 189. [33] United States Pharmacopoeia, XXIII, US Pharmacopoeial Convention, Rockville, MD, 1994.
Voltammetric determination of isoprenaline in pharmaceutical preparations using a copper(II) hexacyanoferrate(III) modified carbon paste electrode Viviane G. Bonifa´cio a, Luiz H. Marcolino Jr. a, Marcos F.S. Teixeira b, Orlando Fatibello-Filho a,* a
Departamento de Quı´mica, Centro de Cieˆncias Exatas e de Tecnologia, Universidade Federal de Sa˜o Carlos, Caixa Postal 676, CEP 13560-970, Sa˜o Carlos, SP, Brazil b UNESP-Campus do Litoral Paulista, Unidade de Sa˜o Vicente, CEP 11330-900, Sa˜o Vicente, SP, Brazil Accepted 11 March 2004 Available online 10 May 2004
Abstract The electroanalytical determination of isoprenaline in pharmaceutical preparations of a homemade carbon paste electrode modified with copper(II) hexacyanoferrate(III) (CuHCF) was studied by cyclic voltammetry. Several parameters were studied for the optimization of the sensor such as electrode composition, electrolytic solution, pH effect, potential scan rate and interferences in potential. The optimum conditions were found in an electrode composition (in mass) of 15% CuHCF, 60% graphite and 25% mineral oil in 0.5 mol l1 acetate buffer solution at pH 6.0. The analytical curve for isoprenaline was linear in the concentration range from 1.96104 to 1.07103 mol l1 with a detection limit of 8.0105 mol l1. The relative standard deviation was 1.2% for 1.96104 mol l1 isoprenaline solution (n=5). The procedure was successfully applied to the determination of isoprenaline in pharmaceutical preparations; the CuHCF modified carbon paste electrode gave comparable results to those results obtained using a UV spectrophotometric method. D 2004 Elsevier B.V. All rights reserved. Keywords: Modified carbon paste electrode; Copper(II) hexacyanoferrate(III); Voltammetric determination; Isoprenaline; Pharmaceutical preparations
1. Introduction Isoprenaline or isoproterenol 4-[1-hydroxy-2-[(1-methylethyl)-amino]ethyl]-1,2-benzenediol is a catecholamine drug that is widely used in the treatment of allergic emergencies, status asthmatic, bronchial asthma, ventricular bradycardia, cardiac arrest, glaucoma and as styptic [1]. The cardiovascular effects of isoprenaline are compared with the epinephrine and norepinephrine, which can relax almost every kind of the smooth musculature that contains adrenergic nervous, but this effect is pronounced in the musculature of bronchus and also in the gastrointestinal tract. The isoprenaline is better absorbed when dispensed by inhalation [2]. Several techniques have been described in the literature for the determination of isoprenaline such as spectrophotometry [3 –7], spectrofluorimetry [8– 10], chemiluminescence * Corresponding author. E-mail address: [email protected] (O. Fatibello-Filho). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.03.010
[11 – 13] and gas chromatography [14]. Electrochemical procedures for the determination of isoprenaline have also been reported [15 – 17]. In recent years, the construction and application of modified electrode has received great attention owing the sensitivity and selectivity of the electrochemical techniques. The use of metal hexacyanoferrates to construct modified electrodes has been extensively studied [18 – 20]. Shankaran and Narayanan [20,21] determine sulfur dioxide and ascorbic acid by amperometry using a graphite electrode modified with (CuHCF). On the other hand, there are several metal hexacyanoferrate film modified electrodes reported in the literature [22 – 25]. In this paper, the construction and the analytical application of a carbon paste electrode modified with CuHCF are proposed for determining isoprenaline in pharmaceutical formulations. The influence of various parameters such as electrode composition, electrolyte, pH, potential scan rate and interference in potential was studied by cyclic voltammetry.
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V.G. Bonifa´cio et al. / Microchemical Journal 78 (2004) 55–59
2. Experimental 2.1. Apparatus The cyclic voltammetric measurements were carried out with an Autolab PGSTAT-30 (Eco Chimie) controlled by a PC using the GPES 4.9 software. The measurements were performed in a 10-ml glass cell, containing three electrodes: a modified carbon paste electrode as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum wire as auxiliary electrode. During the measurements, the solution in the cell (acetate buffer solution at pH 6.0) was not stirred. 2.2. Reagents and solutions All reagents were of analytical-reagent grade and all solutions were prepared using Millipore Milli-Q deionised water. The supporting electrolyte used for all experiments was a 0.5 mol l1 acetate buffer solution at pH 6.0. A stock solution 1.0102 mol l1 isoprenaline was prepared daily by dissolving isoprenaline (Aldrich) in 10 ml of water. Graphite powder (1– 2 Am particle size—Aldrich) and mineral oil (Aldrich) of high purity were used in the preparation of carbon paste. 2.3. Cu3[Fe(CN)6]2 preparation The preparation of Cu3[Fe(CN)6]2 was carried out using a procedure previously described [26] by mixing a 0.125 mol l1 potassium hexacyanoferrate(III) solution and a 0.375 mol l1 copper(II) sulfate solution with the molar ratio Cu/Fe equal to 3. The precipitate obtained was filtered in a sintered glass Gooch filter, washed with distilled water by several times and dried at 25 jC for 4 days. 2.4. Carbon paste electrode preparation The modified carbon paste electrode was prepared manually by carefully mixing the dispersed graphite powder with CuHCF at several mass ratios and subsequently adding of 250 mg of mineral oil (25% m/m). Those mixtures were mixed by magnetic agitation in a 50-ml beaker containing 20 ml of hexane. The final paste was obtained with the evaporation of this solvent. Of 1000 mg of prepared modified paste, about 200 mg of this paste was packed into the tip of a 1-ml insulin plastic syringe, which had its extremity cut (70 mm high and 3 mm internal diameter), and a copper wire was inserted instead of the plastic plunger to obtain the external electric contact. 2.5. Preparation and analysis of pharmaceutical samples Liquid formulations were appropriately diluted to 10 ml with 0.5 mol l1 acetate buffer solution. No other treatment
of the samples was required. The cyclic voltammograms were recorded by cycling the potential between 0.3 and +1.2 V at a scan rate of 20 mV s1. The percentage content of isoprenaline in these samples was determined by the standard addition method and compared with those results obtained using a UV spectrophotometric method at a wavelength of 280 nm.
3. Results and discussion 3.1. Supporting electrolyte effect Experimental results showed that Prussian blue and its analogues could allow alkali-metal cations to penetrate selectively the channels owing to their zeolitic property [27,28]. Recently, differential pulse voltammetric measurements with a carbon paste electrode modified with CuHCF in alkaline and earth-alkaline chloride solutions were studied [29]. The peak potentials ([FeII(CN)6]/[FeIII(CN)6]) of the modified electrode in the electrolyte solutions containing the alkali cations decrease with the increase of the ionic radius, demonstrating that the counter-ions influence the voltammetric behavior of the electrode. The peak potential was found linearly dependent with the ratio [charge]/[ionic radius] of the inserting metal ion. In this work, the voltammetric behavior of the carbon paste electrode modified with CuHCF was appraised in different supporting electrolytes such as LiCl, NaCl and KCl solutions at the same concentration (0.5 mol l1). The modified electrode showed one well-defined redox peak in the presence of NaCl solution, whereas in the presence of potassium ions the redox peak was broadened. For supporting electrolyte containing lithium ions, the peak currents decreased drastically. Therefore, we can confirm that there is a significant cation influence over the electrochemical properties of CuHCF. The influence of anions such as chloride and acetate over the electrochemical properties of CuHCF was also studied. In the presence of sodium acetate– acetic acid buffer solution, the peak potential was displaced to potential values smaller than in NaCl solution. In addition, a well-defined and reproducible cyclovoltammogram was obtained, which is not seen so clearly in sodium chloride. Therefore, the acetate buffer solution at pH 4.8 was chosen due to its better voltammetric profile and also to the shift of the redox peaks to more negative potentials than other electrolyte solutions. 3.2. Electrode composition effect The effect of the electrode composition on their voltammetric profiles was evaluated in acetate buffer solution at pH 4.8. Fig. 1 shows the influence of CuHCF amount varying from 10% to 35% (m/m) in the carbon paste on the analytical signal (anodic peak current). The peak current increased with increasing amount of CuHCF up to 30% (m/
V.G. Bonifa´cio et al. / Microchemical Journal 78 (2004) 55–59
Fig. 1. Anodic peak current in function of the CuHCF content in the carbon paste electrode, obtained in 0.5 mol l1 acetate buffer solution (pH 4.8), at scan rate 20 mV s1 and 25.0F0.2 jC.
m). For CuHCF amounts higher than 30% (m/m), the peak current decreased significantly. This occurs due to a decrease in the graphite content in the paste and consequent reduction of the conductive electrode area. Nevertheless, the electrode containing 15% (m/m) CuHCF, 60% (m/m) graphite and 25% (m/m) mineral oil was selected for further studies because it presented the best voltammetric profile with peak potential well-defined. 3.3. Oxidation of isoprenaline at the carbon paste electrode modified with CuHCF
57
that the modified electrode influences the oxidation process of isoprenaline. The mechanism of the isoprenaline oxidation can be proposed. The anodic peak at +0.813 V (peak A) and the cathodic peak (peak B) at +0.274 V corresponds to the [Fe(CN)6]3/ [Fe(CN)6]4 redox couple. The effect of the scan rate on the voltammetric response of a carbon paste electrode modified with CuHCF in a solution containing 1.96104 mol l1 isoprenaline was studied. The cyclic voltammograms revealed that the anodic peak currents increase and the peak potential shifts as the scan rate increases. For scan rates higher than 100 mV s1, the voltammetric profiles become distorted, which can be associated with the slow electron transfer rate between the graphite and CuHFC in the paste [30]. When peak current values were plotted against v1/2 (Fig. 3), the following linear relationship was obtained: Ipa ðAAÞ ¼ 59:6 þ 113:0v1=2 ðmV1=2 s1=2 Þ
r ¼ 0:9999
The change of peak current with the scan rate reveals a typical diffusion-controlled behavior and indicates that chemical reaction (Scheme 1) is the rate-determining step in the whole electrochemical reaction process [31]. The increment of the peak potential difference with the increase of the scan rate suggests that the electrode reaction is a quasi-reversible process. From these results, a scan rate of 20 mV s1 was chosen for the further studies. 3.4. Effect of pH
Fig. 2 shows the cyclic voltammograms obtained using an unmodified electrode in the presence of 1.96104 mol l1 isoprenaline (a) and the carbon paste electrode modified with CuHCF in the absence (b) and presence (c) of 1.96104 mol l1 isoprenaline. The increment of anodic current showed in the cyclic voltammogram (c) indicates
The electrochemical behavior of the carbon paste electrode modified with CuHCF (15% m/m) was studied in acetate buffer solution in a large pH range between 3.0 and 8.0 in a solution containing 1.96104 mol l1 isoprena-
Fig. 2. Cyclic voltammograms of oxidation of isoprenaline: (a) unmodified electrode containing 1.96104 mol l1 isoprenaline, (b) modified electrode in the absence of isoprenaline and (c) modified electrode in the presence of 1.96104 mol l1 isoprenaline, in acetate buffer (pH 4.8), at scan rate 20 mV s1 and 25.0F0.2 jC.
Fig. 3. Dependence of the anodic peak current of the modified electrode with square root of the scan rate in 0.5 mol l1 acetate buffer solution (pH 4.8) containing 1.96104 mol l1 isoprenaline, at 25.0F0.2 jC.
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Scheme 1. Proposed mechanism for the voltammetric response of the modified electrode.
line. The best analytical response was obtained at pH 6.0. Fig. 4 shows the effect of pH in the anodic peak current obtained with the modified electrode. The anodic peak current (DI/AA) was obtained by the difference of the currents in the presence and absence of isoprenaline. Besides that, the chemical oxidation of isoprenaline is generally easier in alkaline medium than in acidic medium [32], a decrease in anodic peak current above pH 6.0 is observed, which can be related with the copper(II) hydroxide formation in equilibrium with the CuHCF in the electrode surface. In addition, it was observed that the anodic peak current decreased gradually with the decrease of the pH, starting at 6.0, which is probably due to the influence of H+ on the chemical reactions kinetic between isoprenaline and CuHCF on the electrode surface (Scheme 1; chemical reaction). On the other hand, the change in the peak potential of the carbon paste electrode modified with CuHCF was not dependent on the pH (Scheme 1, electrode reaction). Therefore, the result indicates that a 0.5 mol l1 acetate buffer solution at pH 6.0 could be chosen for further studies. 3.5. Study of interference in potential The effect of excipient substances frequently found with isoprenaline in pharmaceutical formulations such as lactose, sucrose, fructose, starch, poly(ethylene glycol), sodi-
um chloride and magnesium stearate were evaluated using the modified electrode. None of these substances caused interference on the proposed method at the 1:1 concentration ratio. 3.6. Calibration graph, repeatability and lifetime of electrode Under the optimized conditions previously studied such as composition of the electrode (15% m/m), pH 6.0 and scan rate of 20 mV, the anodic peak current of the carbon paste electrode modified with CuHCF was rectilinear in the isoprenaline concentration from 1.96104 to 1.07103 mol l 1 (I pa (AA)=41.41+15.71 10 4 [isoproterenol]; r=0.9989). The detection limit (three times the standard deviation of blank/slope) was 8.0105 mol l1. The repeatability of the voltammetric measurements was evaluated making five successive cyclic voltammograms of the CuHCF modified carbon paste electrode in a 0.5 mol l1 acetate buffer solution at pH 6.0 containing 1.96104 mol l1 of isoprenaline. For this study, a relative standard deviation of 1.2% was obtained. The lifetime of the proposed electrode was at least 5 months (more than 100 determinations were performed using the same electrode surface and more than 600 determinations were done when the surface was renewed by pressing the plunger that forces an appropriate amount of paste out of the electrode tip). Then the surface is polished against a filter paper. 3.7. Analysis of pharmaceutical preparation The proposed method was applied to the determination of isoprenaline in two pharmaceuticals formulations. Table 1 presents the results obtained using an official UV spectroTable 1 Determination of isoprenaline in a pharmaceutical formulation using the modified carbon paste electrode with CuHCF compared with UV spectrophotometry [33] Sample Isoprenaline (mg ml1)
R1 % R2 %
Nominal value Spectrophotometry Voltammetry
Fig. 4. Influence of pH on the voltammetric response of the modified carbon paste electrode with 15% (m/m) CuHCF in acetate buffer solution containing 1.96104 mol l1 isoprenaline, at scan rate 20 mV s1 and 25.0F0.2 jC.
1 2
0.250 0.250
0.247F0.005 0.247F0.006
0.253F0.006 +2.4 0.256F0.008 +3.6
+1.0 +2.4
R1 %=relative error=voltammetric method versus spectrophotometric method. R2 %=relative error=voltammetric method versus nominal value.
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photometric procedure [33] and the proposed voltammetric method. The good agreement between the results confirms the usefulness of the proposed procedure for isoprenaline analysis in pharmaceutical samples. 4. Conclusions The carbon paste electrode modified with copper(II) hexacyanoferrate(III) was prepared and applied for the voltammetric determination of isoprenaline in pharmaceutical preparation with good sensitivity and selectivity, with a simple and quick preparation. The cyclic voltammograms of the modified electrode show the presence of one pair of well-defined redox peaks. The optimum conditions for the analysis of isoprenaline were as follows: composition of the electrode of 15% (m/ m) CuHCF; 0.5 mol l1 acetate buffer solution, pH 6.0; and scan rate of 20 mV s1. The analytical curve for isoprenaline was linear in the concentration range from 1.96104 to 1.07103 mol l1 with a detection limit of 8.0105 mol l1. Acknowledgements The authors gratefully acknowledge financial support from Brazilian foundations (FAPESP, CAPES and CNPq). References [1] L.S. Goodman, A. Gilman, The Pharmacological Basis of Therapeutics, 9th ed., McGraw-Hill, New York, 1996, p. 105. [2] D. Voet, J.G. Voet, Biochemistry, Wiley, New York, 1995, p. 1268. [3] K.O. Lupetti, I.C. Vieira, O. Fatibello-Filho, Talanta 57 (2002) 135 – 143. [4] P. Solich, Ch.K. Polydorou, M.A. Koupparis, C.E. Efstathiou, J. Pharm. Biomed. Anal. 22 (2000) 781 – 789. [5] J.J.B. Nevado, J.M.L. Gallego, P.B. Laguna, Anal. Chim. Acta 300 (1995) 293 – 297. [6] H.A. El-Obeid, Anal. Lett. 17 (1984) 771 – 777. [7] R.M.V. Caman˜as, J.M.S. Mallols, J.R.T. Lapasio´, G. Ramis-Ramos, Analyst 120 (1995) 1767 – 1772.
59
[8] H. Jeon, H. Nohta, H. Nagaoka, Y. Ohkura, Anal. Sci. 7 (1991) 257 – 262. [9] H. Nohta, T. Yukizawa, Y. Ohkura, M. Yoshimura, J. Ishida, M. Yamaguchi, Anal. Chim. Acta 344 (1997) 233 – 240. [10] G. Alberts, T. Lameris, A.H. van den Meiracker, A.J.M.I. ’t Veld, F. Boomsma, J. Chromatogr. B 730 (1999) 213 – 219. [11] G.J. Zhou, G.F. Zhang, H.Y. Chem, Anal. Chim. Acta 463 (2002) 257 – 263. [12] C. Zhang, J. Huang, Z. Zhang, M. Aizawa, Anal. Chim. Acta 374 (1998) 105 – 110. [13] A.A. Al-Warthan, S.A. Al-Tamrah, A. Al-Akel, Anal. Sci. 10 (1994) 449 – 452. [14] J.R. Watson, R.C. Lawrence, J. Pharm. Sci. 66 (1977) 560 – 564. [15] L. Elrod Jr., J.L. Schmit, J.A. Morley, J. Chromatogr. A 723 (1996) 235 – 241. [16] Y. Qu, L.F. Moons, F. Vandesande, J. Chromatogr. Biomed. Appl. 704 (1997) 351 – 358. [17] F. Mashige, Y. Matsushima, C. Miyata, R. Yamada, H. Kanazawa, I. Sakuma, N. Takai, N. Shinozuka, A. Ohkubo, K. Nakahara, Biomed. Chromatogr. 9 (1995) 221 – 225. [18] R.J. Mortimer, P.J.S. Barbeira, A.F.B. Sene, N.R. Stradiotto, Talanta 49 (1999) 271 – 275. [19] R. Garjonyte, A. Malinauskas, Sens. Actuators, B 46 (1998) 236 – 241. [20] D.R. Shankaran, S.S. Narayanan, Sens. Actuators B 55 (1999) 119 – 194. [21] D.R. Shankaran, S.S. Narayanan, Fresenius J. Anal. Chem. 364 (1999) 686 – 689. [22] S.M. Chen, J. Electroanal. Chem. 417 (1996) 145 – 153. [23] D.M. Zhou, H.X. Ju, H.Y. Chen, J. Electroanal. Chem. 408 (1996) 219 – 223. [24] I.L. Mattos, L. Gorton, T. Ruzgas, A.A. Karyakin, Anal. Sci. 16 (2000) 795 – 798. [25] C.X. Cai, K.H. Xue, S.M. Xu, J. Electroanal. Chem. 486 (2000) 111 – 118. [26] S. Ayrault, B. Jimenez, E. Granier, M. Fedoroff, D.J. Jones, C. LoosNeskovic, J. Solid State Chem. 141 (1998) 475 – 485. [27] A.A. Karyakin, Electroanalysis 13 (2001) 813 – 819. [28] A. Dostal, M. Hermes, F. Scholz, J. Electroanal. Chem. 415 (1996) 133 – 141. [29] R.S. Nunes, M.F.S. Teixeira, 54th Annual Meeting of the International Society of Electrochemistry, Sa˜o Pedro, Brazil, 2003, p. 206. [30] M.F.S. Teixeira, F.C. Moraes, O. Fatibello-Filho, N. Bocchi, Anal. Chim. Acta 443 (2001) 249 – 255. [31] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 1980, p. 224. [32] S.M. Chen, K.T. Peng, J. Electroanal. Chem. 547 (2003) 179 – 189. [33] United States Pharmacopoeia, XXIII, US Pharmacopoeial Convention, Rockville, MD, 1994.