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Direct simultaneous determination of Pb(II) and Cu(II) in biodiesel by anodic stripping voltammetry at a mercury-film electrode using microemulsions
Fuel 103 (2013) 1164–1167
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Direct simultaneous determination of Pb(II) and Cu(II) in biodiesel by anodic stripping voltammetry at a mercury-film electrode using microemulsions Lorena C. Martiniano a,b, Vivia R. Abrantes a, Sakae Y. Neto a, Edmar P. Marques a,b, Teresa C.O. Fonseca c, Leonardo L. Paim d, Antônio G. Souza b, Nelson R. Stradiotto d, Ricardo Q. Aucélio e, Glene H.R. Cavalcante a, Aldaléa L.B. Marques a,b,⇑ a
Department of Chemical Technology, LPQA/LAPQAP-UFMA, São Luís, MA, Brazil Department of Chemistry, CCEN-UFPB, João Pessoa, PB, Brazil c DIQUIM–CENPES/PETRÓLEO BRASILEIRO S/A-PETROBRAS, Rio de Janeiro, RJ, Brazil d Institute of Chemistry, UNESP, 14801-970, Araraquara, SP, Brazil e Institute of Chemistry, PUC-RIO, Rio de Janeiro, RJ, Brazil b
a r t i c l e
i n f o
Article history: Received 20 April 2012 Received in revised form 30 June 2012 Accepted 2 July 2012 Available online 20 July 2012 Keywords: Biodiesel microemulsion Mercury film electrode Anodic stripping voltammetry
a b s t r a c t A mercury film electrode was used to determine direct and simultaneously Pb(II) (at 410 mV) and Cu(II) (at 100 mV) in biodiesel by anodic stripping voltammetry. A linear response was obtained for Pb(II) and Cu(II) in the 2.00 108–1.00 107 mol L1 concentration range and detection limits were 2.91 109 mol L1 and 4.69 109 mol L1 for Pb(II) and Cu(II), respectively, with recovery around of 100.0%. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel can be used in diesel engines as pure fuel (B100), or as blended (BX) at any level with petroleum diesel with some changes in the hoses and gaskets of the engines. B5, for example, is the mixture of 5% of biodiesel and 95% of petroleum diesel. In Brazil, since 2010, all diesel fuel contains 5% of B100 [1]. In the United States, B20 is recognized as an alternative diesel fuel. Governments around the world, such as the US, Brazil, China, and India have set goals for increased biofuel use and research into processing of raw materials that minimize competition with food and water [2]. As pure biofuel or blending stock, biodiesel must follow the specifications in according the standards ASTM D6751 [3] and EN 14214 [4], in order to attend the maximum allowable concentrations of contaminants in pure (B100) and finished product. Other chemical physical properties are also required, considering a safe and satisfactory engine operation [1,3] besides the quality of biodiesel. Quality control of biodiesel fuel on metal content is important as they may change the motor performance and may be a
⇑ Corresponding author at: Department of Chemical Technology, LPQA/LAPQAPUFMA, São Luís, MA, Brazil. Tel.: +55 98 33018684. E-mail address: [email protected] (A.L.B. Marques). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.07.002
pollution source, with consequences to the human health [5,6]. Besides the possible presence in the biofuels, lead and cooper are among the most common environmental contaminants and important for industry. Electroanalytical methods are commonly used and the stripping voltammetry (SV) has been proven as one of the most efficient electroanalytical techniques for metal determination [7–10]. Studies have been conducted in order to improve the samples treatment, most using spectroscopic techniques [11,12] and few studies has showed the use of microemulsions in determinations of metals in fuels [13–15] by stripping voltammetry. These studies have shown that microemulsions are a satisfactory media for performing electrochemical measurements, especially in biodiesel, due to the difficulty of direct analysis, because of its volatility, corrosiveness and immiscibility with water. In the present work a simple and direct procedure is presented to determine simultaneously Pb(II) and Cu(II) in microemulsion of biodiesel, based on the differential pulse anodic stripping voltammetry (DPASV) at a mercury film electrode (MFE) prepared in situ. Additionally, the advantage of using MFEs are reported, especially regarding of very small quantity of mercury in the preparation of modified electrodes, providing mechanical stability and a larger surface/volume ratio [16,17].
L.C. Martiniano et al. / Fuel 103 (2013) 1164–1167
2. Materials and methods 2.1. Apparatus A three-electrode system of a working electrode, Ag/AgCl (KCl saturated) reference electrode and Pt counter electrodes was used throughout the study. The working electrode was a MFE prepared by in situ procedure on glassy carbon (GC) electrode surface. Voltammograms of DPASV were obtained by using a Potentiostat BAS voltammetric analyzer CV-50W model. Ultrapure water prepared from the Nanopure D4741 model (Ultrapure Water System), was used throughout the study. 2.2. Reagents and solutions Pb(II) and Cu(II) standard stock solutions were prepared from Titrisol standard, 1000 mg L1 (Merck). The other standard solutions were prepared by placing 31.8 lL of Cu(II) (1000 mg L1) and 103.7 lL of Pb(II) (1000 mg L1) into a 50.0 mL volumetric flask to obtain an aqueous solution whose final concentration of both ions was 1.00 105 mol L1. The microemulsion was prepared with the following reagents: 4.00 mL of nitric acid (PA and suprapure) 1.00 102 mol L1 aqueous solution with 1.50 mL of biodiesel, leaving under stirring for 5 min [14]. Then, 8.00 mL of propan-1-ol, under mild agitation for about 10 min, were added. This microemulsion had an apparent pH around 1.5. For all electrochemical experiments, 10.00 mL of this mixture was added into the electrochemical cell and deaerated with nitrogen for 20 min. For the optimization of the operational parameters of the equipment, we also used 10.00 mL of the same microemulsioned mixture but without biodiesel. 2.3. MFE preparation The GC electrode surface was, firstly, adequately polished (alumina 0.3 lm) and cleaned, washing it with purified water and putting it into an ultrasonic bath for 15 min. The electrochemical deposition of mercury film was performed by adding 60 lL of standard solution of Hg(II) 1000 mg L1 diluted to a final volume of 10.0 mL with the electrolyte solution, resulting in a concentration solution of Hg(II) ions 3.00 105 mol L1. After this step the solution was deaerated with nitrogen for 10 min to remove molecular oxygen and then a deposition potential of 900 mV was applied to the system for 10 min before analyte deposition. 2.4. Analysis of Pb(II) and Cu(II) in biodiesel The experiments were realized in same solution of MFE preparation. MFE was applied to metallic standard solution and samples
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of biodiesel in an electrochemical cell with 10.00 mL of microemulsion solution with apparent pH 1.5. The measurements of metallic ions in biodiesel were performed by differential pulse anodic stripping voltammetry (DPASV). In each series of analysis, a new MFE was prepared in order to get a better reproducibility.
3. Results and discussion For the experiments of the MFE response in presence of Pb(II) and Cu(II) the electrode was first immersed in an electrochemical cell containing 9.00 mL of blank and 60 lL of Hg(II). After mercury deposition, 1.00 mL of biodiesel microemulsion was added and a deposition potential of 700 mV for 5 min was applied, sweeping the potential until +100 mV. The influence of various parameters were considered as most important, such as, time of pre-concentration (tpre), deposition potential (Edep), potential scan rate (t), amplitude of application of potential pulses (Amp) and composition of the microemulsions for the simultaneous determination of Pb(II) and Cu(II), using an in situ MFE preparation. This study was carried out in the same electrolytic mixture used for the metal determination in biodiesel microemulsion. All studied parameters were observed to affect the electroanalytical performance of the DPASV measurements referent to the stripping peaks of Pb(II) (around 400 mV) and Cu(II) (around 100 mV). These optimization studies were performed with Pb(II) and Cu(II) ion concentration of 9.00 108 mol L1 in the supporting electrolyte mixture. According to the optimization of the parameters of DPASV, the values chosen for tpre, Edep, Amp and t were 5 min, 700 mV, 50 mV and 20 mV s1, respectively, because they meet the expectations in terms of resolution and sensitivity for the system and the experimental conditions in question. The same behavior obtained before for biodiesel microemulsion was observed for all parameters in this medium, in absence of biodiesel. 3.1. Stability of the electrode Subsequent to the optimization studies seeking for better experimental conditions, a feasibility trial on the stability, sensitivity and reproducibility of MFE, adequate for the simultaneous determination of metals in biodiesel microemulsions was carried out. The aim was to find out the ideal analytical conditions to determine Pb(II) and Cu(II) in this medium. In this study, the mercury film was found to be stable even after 12 cyclic voltammetric scans, because the electrode, after the formation of the mercury film, was used only for a same series of measurements. On the other hand, it was not removed from the medium where it has been prepared, being the sample added to this supporting electrolyte. Another new fresh surface was formed in a very simple way
Fig. 1. Voltammograms (A) and standard addition curves (B) for biodiesel microemulsion sample containing standard additions between 2.00 108 and 9.00 107 mol L1 of Pb(II) and Cu(II). Edep: 700 mV, Amplitude: 50 mV; t: 20 mV s1; tpre: 300 s.
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L.C. Martiniano et al. / Fuel 103 (2013) 1164–1167
Table 1 Figures of merit for metal determination in biodiesel microemulsion considering 95% of confidential level for n = 3. Evaluated parameters 00
E Linear concentration range LOD R Coefficient of variation for metal determination in biodiesel microemulsion
Pb(II)
Cu(II)
420 mV 2.00 108–1.00 107 mol L1 2.91 109 mol L1 0.993 19.4%
105 mV 2.00 108–1.00 107 mol L1 4.69 109 mol L1 0.996 18.0%
Table 2 Recovery tests for Pb(II) and Cu(II). Determination in biodiesel microemulsion samples. Added value of Pb(II) and Cu(II) (mol L1)
3.00 108 6.00 108 9.00 108 1.20 107 1.50 107
Recovered value (mol L1)
Recovered percentage (%)
Pb(II)
Cu(II)
Pb(II)
Cu(II)
2.85 108 5.55 108 8.92 108 1.17 107 1.41 107
3.14 108 6.22 108 9.26 108 1.31 107 1.65 107
95.0 92.5 99.1 97.5 94.0
104.7 103.7 102.9 109.2 110.0
(as presented in Section 2), where mercury film was electrodeposited and made ready for the second measurement series.
3.2. Determination of metals in biodiesel samples The voltammetric experiments and corresponding analytical curves for the simultaneous determinations of Pb(II) and Cu(II) were carried out in the electrolyte solution by DPASV with MFE prepared in situ. In these conditions, after scanning, we observed two peaks at potentials near 420 mV and 105 mV, attributed to Pb(II) and Cu(II), respectively. With the addition of increasing aliquots of standard solution of these metal ions, we observed clearly the linear increase of the peaks current intensity, however, there is also a shift of positions of these peaks, mainly of copper for the anodic region. The same peaks and behavior were observed when biodiesel microemulsion aliquot were added to the electrolyte solution, as showed in Fig. 1A. All measurements were made in triplicate, whose averages were plotted to get the analytical curves. On the other hand, it was observed that above this region of concentration there is a deviation from the linearity. The linear response obtained for Pb(II) and Cu(II) in both, electrolyte solution and biodiesel microemulsion, showed the same behavior. For the analytical curve in the electrolyte solution, two lines passing very close to the origin were obtained for the two ions, on the studied conditions, in the 2.00– 10.00 108 mol L1 concentration range (not showed). When the method was applied to the biodiesel samples, using the standard addition method (Fig. 1B), the linear regression for Cu(II) presented the equation: y = 1.84 106 + 79.19 x with a correlation coefficient (R) of 0.996. The equation for Pb(II) is y = 1.87 106 + 86.00 x, with a correlation coefficient of 0.993. Good linearity was obtained for all situations with R P 0.993. The standard additions method produced similar slopes for each analyte, indicating compensation of possible matrix effect. The limit of detection was defined as three times the standard deviation of ten measurements of the blank divided by the slope of the calibration curve and calculated for the biodiesel samples (3 SD/b), considering a 5 min of preconcentration time, and the values found were 2.91 109 mol L1 for Pb(II) and 4.69 109 mol L1 for Cu(II). The precision was evaluated through the repeatability, presenting values of variation coefficient of 19.4% and 18.0% for Pb(II) and Cu(II), respectively. Table 1 shows the results for the application to samples of biodiesel.
Recovery test was carried out with a microemulsion biodiesel sample fortified with metallic analytes and the values found were close to 100.0%, ranging from 92.5% to 99.1% for Pb(II) and 102.9 to 110.0% for Cu(II) (Table 2 ), with average values of (95.6 ± 2.7)% and (106.1 ± 3.3)% for Pb(II) and Cu(II), respectively. According to the results, the proposed method was successfully applied for the simultaneous determination of Pb(II) and Cu(II) in biodiesel microemulsion samples. 4. Conclusion The use of microemulsions has proved to be a very interesting way to overcome problems associated with the high viscosity of biodiesel and the required ionic strength to carried out electrochemical measurement on this organic medium. The results obtained by differential pulse anodic stripping voltammetry presented satisfactory values for accuracy and precision and the procedure was successfully employed in biodiesel samples. The simple, improved and optimized procedure proposed for the direct and simultaneous quantification of Cu(II) and Pb(II) at trace level in biodiesel, displays well-defined wave form peaks and showed good results in terms of analytical performance, indicating a very good alternative to determine these metals. Acknowledgements This study (Research Project EMEPETRO /No. 3548/05) was supported by Grants from PETROBRAS (Contrato 0050.0018742.06.04), FINEP (Conv. 01.05.0883.00), CNPq (Projeto Casadinho, Processo 620089/2008-3) and CAPES (Project 211/2007-PROCAD), and doctoral Fellowship for Lorena C. Martiniano from FAPEMA (Fundação de Amparo a Pesquisa do Estado do Maranhão). References [1] Conselho nacional de Política energética – CNPE, resolution N° 6 (09.16.2009). DOU Sect. 1; 2009. p. 16. [2] Grosser ZA, Davidowski LJ, Wee P. The analysis of biodiesel for inorganic contaminants, including sulfur, by ICP-OES. Shelton: PerkinElmer Inc.; 2009. [3] ASTM D 6751: standard specification for biodiesel fuel blend stock (B100) for middle distillate fuels. [4] EN 14214: automotive fuels – fatty acid methyl esters (FAME) for diesel engines – requirements and test methods. [5] Speight JG. Handbook of petroleum analysis. New York: John Wiley & Sons; 2001.
L.C. Martiniano et al. / Fuel 103 (2013) 1164–1167 [6] Fontenele APG, Pedrotti JJ, Fornaro A. Evaluation of trace metals and major ions concentrations in rainwater in downtown São Paulo city. Quím Nova 2009;32:839–44. [7] Rodrigues JA, Rodrigues CM, Almeida PJ, Valente IM, Gonçalves LM, Compton RG. Increased sensitivity of anodic stripping voltammetry at the hanging mercury drop electrode by ultracathodic deposition. Anal Chim Acta 2011;701:152–6. [8] Abbasi S, Bahiraei A, Abbasai F. A highly sensitive method for simultaneous determination of ultra trace levels of copper and cadmium in food and water samples with luminol as a chelating agent by adsorptive stripping voltammetry. Food Chem 2011;129:1274–80. [9] Mahesar SA, Sherazi STH, Niaz A, Bhanger MI, Uddin S, Rauf A. Simultaneous assessment of zinc, cadmium, lead and copper in poultry feeds by differential pulse anodic stripping voltammetry. Food Chem Toxicol 2010;48:2357–60. [10] Jakmunee J, Junsomboon J. Determination of cadmium, lead, copper and zinc in the acetic acid extract of glazed ceramic surfaces by anodic stripping voltammetric method. Talanta 2008;77:172–5. [11] Ghisi M, Chaves ES, Quadros DP, Marques EP, Curtius AJ, Marques ALB. Simple method for the determination of Cu and Fe by electrothermal atomic absorption spectrometry in biodiesel treated with tetramethylammonium hydroxide. Microchem J 2011;98:62–5.
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[12] Aucélio RQ, Doyle A, Pi Zzomo BS, Tristão MLB, Campos RC. Electrothermal atomic absorption spectrometric method for the determination of vanadium in diesel and asphaltene prepared as detergentless microemulsions. Microchem J 2004;78:21–6. [13] Mendonça CRB, Bica CID, Piatnicki CMS. Water in soybean oil microemulsions as medium for electrochemical measurements. J Braz Chem Soc 2003;14:628–36. [14] Cardoso CE, Pacheco WF, Sarubi R, Ribeiro MLN, Farias PAM, Aucélio RQ. Voltammetric determination of copper and lead in gasoline using sample preparation as microemulsions. Anal Sci 2007;23:1065–9. [15] Miguel EM, Pacheco WF, Cunha ALM, Farias PAM, Aucélio RQ. Bismuth film anodic stripping voltammetry for the determination of lead in kerosene: a metrological study. Electroanalysis 2010;22:1505–10. [16] Vyskocil V, Barek J. Mercury electrodes – possibilities and limitations in environmental electroanalysis. Critical Rev Anal Chem 2009;39:173–88. [17] Sherigara BS, Shivaraj Y, Mascarenhas RJ, Satpati AK. Simultaneous determination of lead, copper and cadmium onto mercury film supported on wax impregnated carbon paste electrode: assessment of quantification procedures by anodic stripping voltammetry. Electrochim Acta 2007;52:3137–42.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Direct simultaneous determination of Pb(II) and Cu(II) in biodiesel by anodic stripping voltammetry at a mercury-film electrode using microemulsions Lorena C. Martiniano a,b, Vivia R. Abrantes a, Sakae Y. Neto a, Edmar P. Marques a,b, Teresa C.O. Fonseca c, Leonardo L. Paim d, Antônio G. Souza b, Nelson R. Stradiotto d, Ricardo Q. Aucélio e, Glene H.R. Cavalcante a, Aldaléa L.B. Marques a,b,⇑ a
Department of Chemical Technology, LPQA/LAPQAP-UFMA, São Luís, MA, Brazil Department of Chemistry, CCEN-UFPB, João Pessoa, PB, Brazil c DIQUIM–CENPES/PETRÓLEO BRASILEIRO S/A-PETROBRAS, Rio de Janeiro, RJ, Brazil d Institute of Chemistry, UNESP, 14801-970, Araraquara, SP, Brazil e Institute of Chemistry, PUC-RIO, Rio de Janeiro, RJ, Brazil b
a r t i c l e
i n f o
Article history: Received 20 April 2012 Received in revised form 30 June 2012 Accepted 2 July 2012 Available online 20 July 2012 Keywords: Biodiesel microemulsion Mercury film electrode Anodic stripping voltammetry
a b s t r a c t A mercury film electrode was used to determine direct and simultaneously Pb(II) (at 410 mV) and Cu(II) (at 100 mV) in biodiesel by anodic stripping voltammetry. A linear response was obtained for Pb(II) and Cu(II) in the 2.00 108–1.00 107 mol L1 concentration range and detection limits were 2.91 109 mol L1 and 4.69 109 mol L1 for Pb(II) and Cu(II), respectively, with recovery around of 100.0%. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Biodiesel can be used in diesel engines as pure fuel (B100), or as blended (BX) at any level with petroleum diesel with some changes in the hoses and gaskets of the engines. B5, for example, is the mixture of 5% of biodiesel and 95% of petroleum diesel. In Brazil, since 2010, all diesel fuel contains 5% of B100 [1]. In the United States, B20 is recognized as an alternative diesel fuel. Governments around the world, such as the US, Brazil, China, and India have set goals for increased biofuel use and research into processing of raw materials that minimize competition with food and water [2]. As pure biofuel or blending stock, biodiesel must follow the specifications in according the standards ASTM D6751 [3] and EN 14214 [4], in order to attend the maximum allowable concentrations of contaminants in pure (B100) and finished product. Other chemical physical properties are also required, considering a safe and satisfactory engine operation [1,3] besides the quality of biodiesel. Quality control of biodiesel fuel on metal content is important as they may change the motor performance and may be a
⇑ Corresponding author at: Department of Chemical Technology, LPQA/LAPQAPUFMA, São Luís, MA, Brazil. Tel.: +55 98 33018684. E-mail address: [email protected] (A.L.B. Marques). 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.07.002
pollution source, with consequences to the human health [5,6]. Besides the possible presence in the biofuels, lead and cooper are among the most common environmental contaminants and important for industry. Electroanalytical methods are commonly used and the stripping voltammetry (SV) has been proven as one of the most efficient electroanalytical techniques for metal determination [7–10]. Studies have been conducted in order to improve the samples treatment, most using spectroscopic techniques [11,12] and few studies has showed the use of microemulsions in determinations of metals in fuels [13–15] by stripping voltammetry. These studies have shown that microemulsions are a satisfactory media for performing electrochemical measurements, especially in biodiesel, due to the difficulty of direct analysis, because of its volatility, corrosiveness and immiscibility with water. In the present work a simple and direct procedure is presented to determine simultaneously Pb(II) and Cu(II) in microemulsion of biodiesel, based on the differential pulse anodic stripping voltammetry (DPASV) at a mercury film electrode (MFE) prepared in situ. Additionally, the advantage of using MFEs are reported, especially regarding of very small quantity of mercury in the preparation of modified electrodes, providing mechanical stability and a larger surface/volume ratio [16,17].
L.C. Martiniano et al. / Fuel 103 (2013) 1164–1167
2. Materials and methods 2.1. Apparatus A three-electrode system of a working electrode, Ag/AgCl (KCl saturated) reference electrode and Pt counter electrodes was used throughout the study. The working electrode was a MFE prepared by in situ procedure on glassy carbon (GC) electrode surface. Voltammograms of DPASV were obtained by using a Potentiostat BAS voltammetric analyzer CV-50W model. Ultrapure water prepared from the Nanopure D4741 model (Ultrapure Water System), was used throughout the study. 2.2. Reagents and solutions Pb(II) and Cu(II) standard stock solutions were prepared from Titrisol standard, 1000 mg L1 (Merck). The other standard solutions were prepared by placing 31.8 lL of Cu(II) (1000 mg L1) and 103.7 lL of Pb(II) (1000 mg L1) into a 50.0 mL volumetric flask to obtain an aqueous solution whose final concentration of both ions was 1.00 105 mol L1. The microemulsion was prepared with the following reagents: 4.00 mL of nitric acid (PA and suprapure) 1.00 102 mol L1 aqueous solution with 1.50 mL of biodiesel, leaving under stirring for 5 min [14]. Then, 8.00 mL of propan-1-ol, under mild agitation for about 10 min, were added. This microemulsion had an apparent pH around 1.5. For all electrochemical experiments, 10.00 mL of this mixture was added into the electrochemical cell and deaerated with nitrogen for 20 min. For the optimization of the operational parameters of the equipment, we also used 10.00 mL of the same microemulsioned mixture but without biodiesel. 2.3. MFE preparation The GC electrode surface was, firstly, adequately polished (alumina 0.3 lm) and cleaned, washing it with purified water and putting it into an ultrasonic bath for 15 min. The electrochemical deposition of mercury film was performed by adding 60 lL of standard solution of Hg(II) 1000 mg L1 diluted to a final volume of 10.0 mL with the electrolyte solution, resulting in a concentration solution of Hg(II) ions 3.00 105 mol L1. After this step the solution was deaerated with nitrogen for 10 min to remove molecular oxygen and then a deposition potential of 900 mV was applied to the system for 10 min before analyte deposition. 2.4. Analysis of Pb(II) and Cu(II) in biodiesel The experiments were realized in same solution of MFE preparation. MFE was applied to metallic standard solution and samples
1165
of biodiesel in an electrochemical cell with 10.00 mL of microemulsion solution with apparent pH 1.5. The measurements of metallic ions in biodiesel were performed by differential pulse anodic stripping voltammetry (DPASV). In each series of analysis, a new MFE was prepared in order to get a better reproducibility.
3. Results and discussion For the experiments of the MFE response in presence of Pb(II) and Cu(II) the electrode was first immersed in an electrochemical cell containing 9.00 mL of blank and 60 lL of Hg(II). After mercury deposition, 1.00 mL of biodiesel microemulsion was added and a deposition potential of 700 mV for 5 min was applied, sweeping the potential until +100 mV. The influence of various parameters were considered as most important, such as, time of pre-concentration (tpre), deposition potential (Edep), potential scan rate (t), amplitude of application of potential pulses (Amp) and composition of the microemulsions for the simultaneous determination of Pb(II) and Cu(II), using an in situ MFE preparation. This study was carried out in the same electrolytic mixture used for the metal determination in biodiesel microemulsion. All studied parameters were observed to affect the electroanalytical performance of the DPASV measurements referent to the stripping peaks of Pb(II) (around 400 mV) and Cu(II) (around 100 mV). These optimization studies were performed with Pb(II) and Cu(II) ion concentration of 9.00 108 mol L1 in the supporting electrolyte mixture. According to the optimization of the parameters of DPASV, the values chosen for tpre, Edep, Amp and t were 5 min, 700 mV, 50 mV and 20 mV s1, respectively, because they meet the expectations in terms of resolution and sensitivity for the system and the experimental conditions in question. The same behavior obtained before for biodiesel microemulsion was observed for all parameters in this medium, in absence of biodiesel. 3.1. Stability of the electrode Subsequent to the optimization studies seeking for better experimental conditions, a feasibility trial on the stability, sensitivity and reproducibility of MFE, adequate for the simultaneous determination of metals in biodiesel microemulsions was carried out. The aim was to find out the ideal analytical conditions to determine Pb(II) and Cu(II) in this medium. In this study, the mercury film was found to be stable even after 12 cyclic voltammetric scans, because the electrode, after the formation of the mercury film, was used only for a same series of measurements. On the other hand, it was not removed from the medium where it has been prepared, being the sample added to this supporting electrolyte. Another new fresh surface was formed in a very simple way
Fig. 1. Voltammograms (A) and standard addition curves (B) for biodiesel microemulsion sample containing standard additions between 2.00 108 and 9.00 107 mol L1 of Pb(II) and Cu(II). Edep: 700 mV, Amplitude: 50 mV; t: 20 mV s1; tpre: 300 s.
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L.C. Martiniano et al. / Fuel 103 (2013) 1164–1167
Table 1 Figures of merit for metal determination in biodiesel microemulsion considering 95% of confidential level for n = 3. Evaluated parameters 00
E Linear concentration range LOD R Coefficient of variation for metal determination in biodiesel microemulsion
Pb(II)
Cu(II)
420 mV 2.00 108–1.00 107 mol L1 2.91 109 mol L1 0.993 19.4%
105 mV 2.00 108–1.00 107 mol L1 4.69 109 mol L1 0.996 18.0%
Table 2 Recovery tests for Pb(II) and Cu(II). Determination in biodiesel microemulsion samples. Added value of Pb(II) and Cu(II) (mol L1)
3.00 108 6.00 108 9.00 108 1.20 107 1.50 107
Recovered value (mol L1)
Recovered percentage (%)
Pb(II)
Cu(II)
Pb(II)
Cu(II)
2.85 108 5.55 108 8.92 108 1.17 107 1.41 107
3.14 108 6.22 108 9.26 108 1.31 107 1.65 107
95.0 92.5 99.1 97.5 94.0
104.7 103.7 102.9 109.2 110.0
(as presented in Section 2), where mercury film was electrodeposited and made ready for the second measurement series.
3.2. Determination of metals in biodiesel samples The voltammetric experiments and corresponding analytical curves for the simultaneous determinations of Pb(II) and Cu(II) were carried out in the electrolyte solution by DPASV with MFE prepared in situ. In these conditions, after scanning, we observed two peaks at potentials near 420 mV and 105 mV, attributed to Pb(II) and Cu(II), respectively. With the addition of increasing aliquots of standard solution of these metal ions, we observed clearly the linear increase of the peaks current intensity, however, there is also a shift of positions of these peaks, mainly of copper for the anodic region. The same peaks and behavior were observed when biodiesel microemulsion aliquot were added to the electrolyte solution, as showed in Fig. 1A. All measurements were made in triplicate, whose averages were plotted to get the analytical curves. On the other hand, it was observed that above this region of concentration there is a deviation from the linearity. The linear response obtained for Pb(II) and Cu(II) in both, electrolyte solution and biodiesel microemulsion, showed the same behavior. For the analytical curve in the electrolyte solution, two lines passing very close to the origin were obtained for the two ions, on the studied conditions, in the 2.00– 10.00 108 mol L1 concentration range (not showed). When the method was applied to the biodiesel samples, using the standard addition method (Fig. 1B), the linear regression for Cu(II) presented the equation: y = 1.84 106 + 79.19 x with a correlation coefficient (R) of 0.996. The equation for Pb(II) is y = 1.87 106 + 86.00 x, with a correlation coefficient of 0.993. Good linearity was obtained for all situations with R P 0.993. The standard additions method produced similar slopes for each analyte, indicating compensation of possible matrix effect. The limit of detection was defined as three times the standard deviation of ten measurements of the blank divided by the slope of the calibration curve and calculated for the biodiesel samples (3 SD/b), considering a 5 min of preconcentration time, and the values found were 2.91 109 mol L1 for Pb(II) and 4.69 109 mol L1 for Cu(II). The precision was evaluated through the repeatability, presenting values of variation coefficient of 19.4% and 18.0% for Pb(II) and Cu(II), respectively. Table 1 shows the results for the application to samples of biodiesel.
Recovery test was carried out with a microemulsion biodiesel sample fortified with metallic analytes and the values found were close to 100.0%, ranging from 92.5% to 99.1% for Pb(II) and 102.9 to 110.0% for Cu(II) (Table 2 ), with average values of (95.6 ± 2.7)% and (106.1 ± 3.3)% for Pb(II) and Cu(II), respectively. According to the results, the proposed method was successfully applied for the simultaneous determination of Pb(II) and Cu(II) in biodiesel microemulsion samples. 4. Conclusion The use of microemulsions has proved to be a very interesting way to overcome problems associated with the high viscosity of biodiesel and the required ionic strength to carried out electrochemical measurement on this organic medium. The results obtained by differential pulse anodic stripping voltammetry presented satisfactory values for accuracy and precision and the procedure was successfully employed in biodiesel samples. The simple, improved and optimized procedure proposed for the direct and simultaneous quantification of Cu(II) and Pb(II) at trace level in biodiesel, displays well-defined wave form peaks and showed good results in terms of analytical performance, indicating a very good alternative to determine these metals. Acknowledgements This study (Research Project EMEPETRO /No. 3548/05) was supported by Grants from PETROBRAS (Contrato 0050.0018742.06.04), FINEP (Conv. 01.05.0883.00), CNPq (Projeto Casadinho, Processo 620089/2008-3) and CAPES (Project 211/2007-PROCAD), and doctoral Fellowship for Lorena C. Martiniano from FAPEMA (Fundação de Amparo a Pesquisa do Estado do Maranhão). References [1] Conselho nacional de Política energética – CNPE, resolution N° 6 (09.16.2009). DOU Sect. 1; 2009. p. 16. [2] Grosser ZA, Davidowski LJ, Wee P. The analysis of biodiesel for inorganic contaminants, including sulfur, by ICP-OES. Shelton: PerkinElmer Inc.; 2009. [3] ASTM D 6751: standard specification for biodiesel fuel blend stock (B100) for middle distillate fuels. [4] EN 14214: automotive fuels – fatty acid methyl esters (FAME) for diesel engines – requirements and test methods. [5] Speight JG. Handbook of petroleum analysis. New York: John Wiley & Sons; 2001.
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