Article pubs.acs.org/EF
Voltammetric Determination of TBHQ Individually and Mixed with BHT in Petroleum Products Using a Gold Disc Electrode Markéta Tomásǩ ová,*,† Jaromíra Chýlková,† Tomás ̌ Navrátil,‡ and Renáta Šelešovskᆠ†
University of Pardubice, Faculty of Chemical Technology, Institute of Environmental and Chemical Engineering, Studentská 573, 532 10 Pardubice, Czech Republic ‡ J. Heyrovský Institute of Physical Chemistry of the AV CR, v.v.i, Dolejškova 3, 182 23 Prague 8, Czech Republic ABSTRACT: A new method is proposed for determination of TBHQ (tert-butylhydroquinone) individually and in a mixture with BHT (butylated hydroxytoluene) in mineral oil and biodiesel when using linear-sweep voltammetry in combination with a gold disc electrode. Both of these analytes gave rise to the respective anodic peak in the supporting electrolyte based on a mixture of dilute H2SO4 and isopropanol. The samples of mineral oil were extracted using ethanol, and the samples of biodiesel were analyzed without any special sample treatment or separation. The electroanalytical method developed has enabled the determination of antioxidants in model and real samples with satisfactory results and good prospects for practical analysis.
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INTRODUCTION Oils are commercial products and, as with any other product with a potential impact on the environment, detailed analysis is necessary. Users of petroleum products have understood that they deteriorate during use being degraded by evaporation, oxidation, and contamination.1 The major problem is oxidation of the oils. Resistance to oxidation is increased by addition of various synthetic antioxidants.2 Antioxidants are substances that play a very important role in many processes where free radicals are present.3,4 Antioxidants block free radicals, converting them into stable products via redox reactions.5 The most commonly used synthetic antioxidants are butylated hydroxytoluene (BHT), butylated hydroxyanisol (BHA), tert-butylhydroquinone (TBHQ), propyl gallate (PG), and pyrogallol (PA).6 In practice, a mixture of two or more antioxidants is found to be more effective than the use of a single compound. Because these substances are chemically similar, analysis becomes difficult at trace concentrations without prior separation.7 Several analytical methods have been reported, including spectrophotometry,8−11 liquid chromatography12−16 (mostly in combination with electrochemical detection17,18 but also with mass spectrometry)19,20 and gas chromatography with various detection methods21−23 for analysis of a wide variety of synthetic antioxidants in different types of samples. Generally, chromatographic methods are used for the analysis of these compounds in food,24−31 but even then, prior separation or sample clean up is sometimes required, resulting in fairly complicated procedures. Electrochemical methods have recently been used for the analysis of antioxidants. Many methods for determination of BHT and TBHQ, individually or simultaneously, have been reported. Ameye et al.32 present an extraction-based sample treatment for analysis of amine and phenolic antioxidants in motor and turbine oil. The oil sample was extracted in a solution of supporting electrolyte (sodium perchlorate dissolved in acetone or potassium hydroxide dissolved in © 2014 American Chemical Society
ethanol) in the presence of sand. Whereas soluble products, including antioxidants, were extracted into the solution and analyzed, the suspension adhered to the sand. Robledo et al.33 determined four antioxidants, namely, BHA, BHT, TBHQ, and PG, in olive oil. In this case, square-wave voltammetry (SWV) with a platinum ultramicroelectrode in the supporting electrolyte containing acetonitrile (AcN) and 0.1 mol L −1 (C4H9)4NPF6 was used. The analysis was done either directly in an oil matrix dissolved in a benzene−ethanol (1:2) mixture in the presence of 0.1 mol L−1 or 1 mol L−1 H2SO4 or the antioxidants were isolated with double extraction using AcN. The authors in paper34 developed a method for determination of TBHQ in biodiesel. In their paper, the addition of surfactant T-100 to the supporting electrolyte (8 mL of Britton-Robinson buffer at pH 8 per 2 mL of methanol) provided an adequate medium for TBHQ analysis in soybean biodiesel using SWV with a mercury-drop electrode. The use of T-100 was decesive for direct analysis of TBHQ only requiring prior dilution of biodiesel samples in methanol. In a next paper,35 the authors describe quantitative determination of phenolic antioxidants (BHA, BHT, and TBHQ) in mixtures using different voltammetric techniques, working electrodes, and supporting electrolytes. Glassy carbon (GC) and platinum (Pt) working electrodes were investigated. The supporting electrolytes investigated were Britton−Robinson buffer 0.1 mol L−1 (pH 2) and HCl 0.1 mol L−1 (pH 2), both with 2 g L−1 of methanol. The results in this paper show that, for food samples, the parameters investigated were satisfactory for some determination of antioxidant mixtures using square wave voltammetry (SWV) without chemometric approaches and without suffering overlapping problems. In the current paper, we describe the voltammetric determination of TBHQ both individually and simultaneously with BHT at a gold disc electrode. This method was Received: April 2, 2014 Revised: May 27, 2014 Published: June 13, 2014 4731
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successfully used for determination of BHT.36 The parameters for the determination (TBHQ and mixture of TBHQ and BHT) were optimized and the main attention was focused on the experimental testing of this method in analyses of mineral oil or spiked biodiesel samples and the simultaneous determination of TBHQ and BHT.
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Article
RESULTS AND DISCUSSION
Investigation of the Electrochemical Behavior of TBHQ. The papers in refs 36−38, which deal with voltammetric determination of particular antioxidants in petroleum products, showed that a supporting electrolyte containing isopropanol and H2SO4 can be used. As found in ref 36, isopropanol can be used as the solvent assuring higher solubility of the target component. First, the electrochemical behavior of TBHQ was investigated using cyclic voltammetry. It was found that this compound gives a peak of anodic oxidation in the range 640 mV − 1150 mV (see Figure 1). As can be seen in Figure 1, the process is irreversible.
EXPERIMENTAL SECTION
Chemicals and Reagents. All chemicals used were of analytical reagent grade and purchased from Penta or SigmaAldrich. Stock solution of TBHQ (4 g L−1) was prepared by dissolving the appropriate amount of TBHQ (Sigma-Aldrich, CAS: 121-00-6) in a 96% by volume solution of ethanol (EtOH), and the stock solution of BHT (4 g L−1) contained the appropriate amount of BHT (AppliChem, CAS: 128-37-0), made also in a 96% solution of ethanol. For voltammetric determinations of both antioxidants, that is, TBHQ and BHT, typical supporting electrolytes were based on dilute H2SO4 (Penta, CAS: 7664-93-9) mixed with isopropanol (i-PrOH, Penta, CAS: 603-117-00-0). Antioxidants were determined in a matrix of oil and biodiesel. Biodiesel was a mixture of standard diesel fuel and 30% fatty acid methyl esters (rapeseed oil methyl ester). The mineral oil samples were extracted with 96% EtOH. Anhydrous sodium sulfate, Na2SO4 (Sigma-Aldrich, CAS: 239313), was used to remove the rest of the oils from the extract. Apparatus and Accessories. Voltammetric analyses were performed using an EP 100VA electrochemical analyzer (HSC Servis; Bratislava, Slovak Republic) in a three-electrode cell which was composed of the gold disc (AuDE, OD 2 mm, HSC servis, Bratislava, Slovak Republic) as the working electrode, Ag|AgCl|3 mol L−1 KCl as the reference, and the Pt plate (3 × 5 mm) as the counter electrode. Procedure. All analyses were performed in i-PrOH containing 0.1 mol L−1 H2SO4. The volume analyzed amounted to 15 mL. Biodiesel samples were analyzed directly without any treatment. The antioxidants detected in the mineral oil had to be isolated by extraction. Antioxidants were extracted from 4 to 5 g of the mineral oil matrix by means of 20 mL of 96% EtOH using ultrasound for 10 min. After the suspension settled, the upper layer was separated and analyzed free of any oil residue. The limits of detection (LOD) and quantification (LOQ) parameters of the calibration curves (e.g., slope, intercept) and other statistical data were calculated using the statistical program Adstat.39 Samples. Spiked mineral oil and biodiesel samples were prepared by dissolving a known amount of antioxidants and using pure mineral oil or biodiesel. Model and real samples of mineral oil were prepared as follows; the latter had to be extracted with 96% EtOH (according to the procedure described in ref 37), when taking 4−5 g of synthetic or mineral oil, accompanied yet by thoroughly sonication (for 10 min). Anhydrous sodium sulfate, Na2SO4 (Sigma-Aldrich, CAS: 239313), was used to remove the residual of oils from the extracts because a small quantities of oil interfered all analyses. After achieving the separation of the suspension formed, the upper organic layer was taken for analysis and the oil residue entrapped with Na2SO4. The fraction separated was filtered and then analyzed immediately.
Figure 1. Cyclic voltammograms of TBHQ in supporting electrolyte (0.2 mL 48% H2SO4 + 14.8 mL i-PrOH). Experimental conditions: EIN = +0.5 V, EFIN = +1.3 V, scan rate 40 mV s−1, c (TBHQ): 45.8 μg mL−1.
The investigation of the electrochemical behavior of TBHQ (with a concentration 45.8 μg mL−1) during electrochemical oxidation was performed using a scan rate from 2.5 to 125 mV s−1 (see Figure 2). As can be seen in Figure 2, the peak values at
Figure 2. Cyclic voltammograms of TBHQ in supporting electrolyte (0.2 mL of 48% H2SO4 + 14.8 mL of i-PrOH) by different values of scan rate. Experimental conditions: EIN = +0.5 V; EFIN = +1.3 V; Curves 1−6, scan rate from 2.5 to 125 mV s−1, c (TBHQ), 45.8 μg mL−1.
higher rates shift to higher potentials. Using the dependence of peak height on the square root of the polarization rate a line was obtained with the form: y = 0.19x − 2.4 × 10−3. Because of the low value in this segment, we can say that the process at the electrode is driven only by diffusion. For subsequent analyses, a scan rate of 40 mV s−1 was chosen. The first method used for determination of TBHQ was square wave voltammetry (see Figure 3). 4732
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evaluation of individual voltammograms. The average (15.9 μg mL−1) for n = 7 was within the 95% interval of reliability and differs by +4.2% from the real value. The standard deviation was 0.43 μg mL−1. Determination of TBHQ in Mineral Oil and Biodiesel. It was found that this mix of oil makes it impossible to obtain direct voltammetric determination of antioxidant TBHQ using a working gold electrode in the suggested supporting electrolyte of isopropanol containing 0.1 mol L−1 H2SO4. It was necessary to carry out prior extraction of the sample using ethanol (96%) (this has been published in ref 38). Linear sweep voltammograms performed on the extract obtained after extraction steps when the mineral oil sample had been previously spiked with TBHQ (concentration of TBHQ in spiked sample: 5.41 g kg−1) are shown in Figure 5. The
Figure 3. Square wave voltammograms of TBHQ in supporting electrolyte (0.2 mL 48% H2SO4 + 14.8 mL i-PrOH). Experimental conditions: EIN = +0.5 V; EFIN = +1.0 V; scan rate, 40 mV s−1; Curve 1, supporting electrolyte; Curves 2−6, c (TBHQ) in range from 15.2 to 75.1 μg mL−1.
As depicted in Figure 3, the peak height is linearly proportional to the concentration and the following dependence was found: h = 8.4 × 10−3c + 5.2 × 10−2 (where h is in μA and c in μg mL−1), whereas the standard deviation for the slope of the calibration curve was 3.02 × 10−4 and the intercept is 1.77 × 10−2. The limit of quantification 0.258 μg mL−1 and limit of detection 0.143 μg mL−1 were evaluated. Similar sensitivity was achieved using the LSV method. Figure 4 presents the curves of anodic oxidation of TBHQ using LSV method. Figure 5. Anodic linear sweep voltammograms of TBHQ (isolated from the oil matrix) in the supporting electrolyte (0.2 mL of 48% H2SO4 + 14.8 mL of i-PrOH). Experimental conditions: EIN = +0.5 V; EFIN = +1.1 V; scan rate, 40 mV s−1; Curve 1, supporting electrolyte; Curve 2, TBHQ in an ethanol extract of mineral oil (c (TBHQ): 5.41 g kg−1); Curve 3, the first addition of a TBHQ standard solution (c = 30.9 μg mL−1); Curve 4, the second addition of the some standard.
standard addition method had provided fairly reproducible results. Nine replicated determinations (n = 9) gave an average value of 5.64 g kg−1 with standard deviation 0.15 g kg−1; 95% interval of reliability, with lower limit 5.53 g kg−1 and upper limit 5.76 g kg−1. In the case of determination of TBHQ in biodiesel samples, it is not necessary to use any special procedure for analysis because isopropanol ensures excellent solubility of the biodiesel mixture. To use the proposed LSV method, an appropriate volume of the biodiesel sample (see ref 36) was placed directly into an electrochemical cell for analysis. Biodiesel samples spiked with TBHQ to the level of 330 mg kg−1 were subjected to voltammetric analysis. The voltammetric curves of these samples were very similar to curves found in the mineral oil extract (see Figure 5). Five replicated determinations (n = 5) with average value 315 mg kg−1 and standard deviation 10.7 mg kg−1 were provided using the standard addition method (95% interval of reliability with lower limit 301.8 mg kg−1 and upper limit 328.3 mg kg−1). Simultaneous Determination of TBHQ and BHT in Mineral Oil. Because a mixture of antioxidants is generally used to prevent degradation of oils by oxidation, the simultaneous voltammetric determination of TBHQ and BHT was investigated. Figure 6 shows the linear sweep voltammetric curves obtained at a gold electrode in the suggested supporting electrolyte. The standard addition
Figure 4. Anodic linear sweep voltammograms of TBHQ in the supporting electrolyte (0.2 mL 48% H2SO4 + 14.8 mL i-PrOH). Experimental conditions: EIN = +0.5 V; EFIN = +1.0 V; scan rate, 40 mV s−1; Curve 1, supporting electrolyte; Curves 2−8, c (TBHQ) in range from 15.2 to 104.4 μg mL−1.
As depicted in Figure 4, the peak height is linearly proportional to the concentration, which was also the result obtained from statistical evaluation of the individual voltammograms. The following dependence was found: h = 3.2 × 10−2 + 7.3 × 10−2 (where h is in μA and c in μg mL−1), whereas the standard deviation for the slope of the calibration curve was 2.03 × 10−4 and the intercept is 1.36 × 10−2. According to the statistical evaluation,39 a linear model was confirmed, and from further analysis, the limit of quantification 0.210 μg mL−1 and limit of detection 0.142 μg mL−1 were evaluated. Thus, in this paper, the LSV method was chosen. The recovery of TBHQ determination was tested by repeated determination of a solution of 15.3 μg mL −1 TBHQ. The results obtained show that the recovery of the individual determinations does not exceed 110% (all results exhibit positive errors). The reason could be connected to 4733
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Figure 7. Anodic linear sweep voltammograms of TBHQ and BHT mixture (isolated from the oil matrix) in the supporting electrolyte (0.2 mL of 48% H2SO4 + 14.8 mL of i-PrOH). Experimental conditions: EIN = +0.65 V; EFIN = +1.5 V; scan rate 40 mV s−1; Curve 1, supporting electrolyte; Curve 2, TBHQ and BHT mixture in an ethanol extract of mineral oil (c (TBHQ), 2.52 g kg−1; c (BHT), 2.47 g kg−1); Curves 3, 4, (solid lines) the first and second addition of BHT standard solution (c = 16.7 μg mL−1); Curves 5, 6, (dashed lines) the first and second addition of TBHQ standard solution (c = 15.5 μg mL−1).
Figure 6. Anodic linear sweep voltammograms of TBHQ and BHT mixture in the supporting electrolyte (0.2 mL of 48% H2SO4 + 14.8 mL of i-PrOH). Experimental conditions: EIN = +0.5 V; EFIN = +1.3 V; scan rate, 40 mV s−1; Curve 1, supporting electrolyte; Curve 2, TBHQ and BHT mixture in support electrolyte (c (TBHQ), 15.3 μg mL−1; concentration of BHT, 16.5 μg mL−1); Curves 3, 4, (solid lines) the first and second addition of BHT standard solution (c = 16.7 μg mL−1); Curves 5, 6, (dashed lines) the first and second addition of TBHQ standard solution (c = 15.5 μg mL−1).
method (two levels of addition) was used. Well-defined peak currents were obtained for both antioxidants. The mixture of TBHQ and BHT was in a concentration ratio about 1:1. The maximum peak potential for TBHQ was registered at 0.755 V and the peak potential for BHT at 1.07 V. This good separation of 0.315 V of the peak potentials clearly allows simultaneous determination of the antioxidants. The accuracy of the mixture of TBHQ and BHT determination was tested by repeated determination of a solution of 15.3 and 16.5 μg mL−1, respectively. The results obtained show that the averages (15.4 and 16.5 μg mL−1, respectively) for n = 5 were within 95% interval of reliability and differ by +0.8% and +0.3%, respectively, from the real values. The standard deviations are 0.99 μg mL−1 for TBHQ and 1.07 μg mL−1 for BHT. The proposed method was used for the simultaneous determination of TBHQ and BHT in spiked mineral oil samples. In these cases, it was observed that the mix of the extracted solution shifted the maximum peak potential of these antioxidants and the background affected the shape of the voltammetric curves of the antioxidants under study: this problem is presented in Figure 7, where the signal for oxidation of the mixture overlaps with the background and it is thus impossible to evaluate it properly. The error of determination in this case was about 15% for TBHQ and 20% for BHT. Because the software of the EP 100 analyzer provides various mathematical operations, it is possible to overcome this problem using a mathematical approach, which is illustrated by Figure 8A for TBHQ and 8B for BHT. The voltammetric curves were differentiated by subtraction of the baseline (blank or supporting electrolyte) and then the curves were derived. As can be seen in Figure 8A and B, the peaks are clearly distinguishable. Such an approach was used for all analyses of spiked mineral oil samples, all of which were prepared by dissolving a known amount of antioxidant and using pure mineral oil. The results of the analyses for these samples are presented in Table 1. From the values in Table 1 it can be stated that the method of voltammetric determination of TBHQ and BHT using the proposed method of evaluation provides reliable results in complicated matrices of mineral oils. Because the amounts of
Figure 8. Anodic linear sweep voltammograms of (A) TBHQ and (B) BHT mixture in extract of mineral oil sample after mathematical approach. Curve 1,1′, TBHQ and BHT mixture in ethanol extract of mineral oil (c (TBHQ), 2.52 g kg−1; c (BHT), 2.47 g kg−1); Curves 2, 3, the first and second addition of TBHQ standard solution (c = 15.5 μg mL−1); Curves 2′, 3′; the first and second addition of BHT standard solution (c = 16.7 μg mL−1). All curves are obtained from the mathematical approach.
antioxidants in the samples were known, it was possible to compare the correctness of the results obtained. It can be observed from Table 1 that there are no significant differences between the values found and the declared values for the amounts of TBHQ and BHT in mineral oil. Biodiesel samples were the next to be studied. From previous studies, it is known that such samples can be determined directly without any other treatment (see the section entitled 4734
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background signal from the biodiesel matrix, and thus, such a determination is impossible to evaluate as in the case of the extract of mineral oil samples. In order to overcome this problem, we again used the mathematical approach, the results of which are presented in Figure 9B, where the voltammetric curves were differentiated by subtraction of the baseline of the supporting electrolyte and then the curves were derived. The results obtained are summarized in Table 2. The spiked
Table 1. Results of determinations of antioxidants in spiked mineral oil samplesa TBHQ
BHT
sample #
found [g kg−1]
error [%]
found [g kg−1]
error [%]
1 2 3 4 5
2.29 2.64 2.45 2.44 2.73
−8.9 +4.9 −2.5 −3.1 +8.6
2.70 2.37 2.60 2.71 2.62
+9.7 −4.0 +5.3 +9.8 +5.9
Table 2. Results of Determination of Antioxidants in Spiked Biodiesel Samplesa
Declared concentration of TBHQ: 2.52 g kg−1. Declared concentration of BHT: 2.47 g kg−1. a
TBHQ
“Determination of TBHQ in Mineral Oil and Biodiesel”). Antioxidant TBHQ mixed with BHT gave good results without any treatment. In the case of direct determination of BHT in biodiesel, it was observed that the biodiesel matrix affects the shape of the voltammetric curves for the antioxidant. This case is presented in Figure 9. Figure 9A presents the voltammetric curves of TBHQ and BHT mixtures in spiked biodiesel samples, where the standard addition method was used. Antioxidant TBHQ was not affected by the biodiesel matrix and hence was determined by the same procedure as is described above. Close attention was paid to the determination of BHT because the signal of BHT oxidation overlaps with the
−1
BHT
sample #
found [g kg ]
error [%]
found [g kg−1]
error [%]
1 2 3 4 5
2.25 2.18 2.11 2.18 2.34
0 −3.1 −6.3 −3.1 +4.0
2.32 2.57 2.40 2.20 2.61
−7.6 +2.5 −3.2 +3.2 +3.9
Declared concentration of TBHQ: 2.25 g kg−1. Declared concentration of BHT: 2.51 g kg−1. a
biodiesel samples were prepared by dissolving known amounts of TBHQ and BHT in the appropriate amount of biodiesel. From Table 2, it can be seen that there no significant difference between the values found and the declared values for the amounts of TBHQ and BHT in biodiesel. Using the Method Developed Here for Determination of TBHQ and BHT in Real Samples. In this work, only real biodiesel samples with a declared content of antioxidant TBHQ were available. The proposed method gives the results summarized in Table 3. From this table, it can be seen that Table 3. Results of Determination of TBHQ in Real Samples sample #
declared [ppm]
1 2 3 4
100 120 100 120
a
obtaineda [ppm]
recovery [%]
RSD [%]
± ± ± ±
94.4 101.9 94.1 102.2
−5.57 +1.85 −5.86 +2.21
94.4 122.3 94.1 122.6
2.48 6.34 0.89 6.20
Average of three measurements.
the content of TBHQ in real samples was in good agreement with the values declared by the manufacturer. Excellent recovery values were obtained for the analyte, associated with low values of %RSD, corroborating the accuracy and precision of the voltammetric method and suggesting that the proposed procedure is applicable to analysis of antioxidants in real samples. The suggested method is applicable in practical laboratories that deal with effective use of lubricants and fuels.
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CONCLUSIONS In this article, the method for determination of TBHQ, both individually and simultaneously with BHT, using linear-sweep voltammetry is proposed. The results for TBHQ allow us to conclude that the determination is not only possible but indeed low detection limits were obtained in determination of TBHQ (LOQ, 0.210 μg mL−1; LOD, 0.142 μg mL−1). For these purposes, a working gold disc electrode was used, exhibiting a satisfactorily reproducible response of anodic oxidation in the supporting electrolyte of choicea mixture of isopropanol and dilute H2SO4.
Figure 9. Anodic linear sweep voltammograms of TBHQ and BHT mixture (A) in biodiesel sample and (B) in detail after mathematical approach. Curve 0, supporting electrolyte; Curves 1,1′, TBHQ and BHT mixture in biodiesel (c (TBHQ), 2.25 g kg−1; c (BHT), 2.51 g kg−1); Curves 2, 3, (solid lines) the first and second addition of BHT standard solution (c = 66.7 μg mL−1), Curves 2′,3′, additions of BHT standard solution after the mathematical approach; Curves 4, 5, (dashed lines) the first and second addition of TBHQ standard solution (c = 61.9 μg mL−1). 4735
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The simultaneous voltammetric determination of antioxidants TBHQ and BHT in the supporting electrolyte was also investigated. The peak potential of TBHQ was registered at 0.755 V and the peak potential of BHT at 1.07 V. This good separation of 0.315 V of the peak potential clearly allows simultaneous determination of these two antioxidants. As expected, in the case of determination of spiked samples, the mixture of mineral oil extract or biodiesel affected the analyses. The signal for oxidation of the analyte overlaps with the background, and thus, it is impossible to evaluate it properly. This had to be overcome by additional mathematical approaches to the original voltammograms (recorded in the LSV mode), when the voltammetric curves were differentiated by subtraction of the baseline of the supporting electrolyte, and then the curves were derived. This procedure had to use for determination of a mixture TBHQ and BHT in mineral oil extract and for BHT in biodiesel samples. In conclusion, the method proposed can be characterized as relatively simple, quick, and capable of being carried out with satisfactory accuracy and precision. Also, contrary to other techniques, it does not require expensive instrumentation and the operational costs are minimal. On the basis of the results of analyses of spiked and real samples, it seems that the method developed can be recommended for practical analysis in the laboratories of oil refineries and similar industrial service stations.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support was provided by the SGFChT05/2014 project. REFERENCES
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