Simultaneous voltammetric determination of TBHQ and PG in

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Simultaneous voltammetric determination of TBHQ and PG in biodiesel-ethanol at a Pt ultramicroelectrode Andrea Anilda Hoffmann da Rocha, Marcella Casagrande, Lívia de Souza Schaumlöffel, Yara Patrícia da Silva, and Clarisse Maria Sartori Piatnicki Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Simultaneous voltammetric determination of TBHQ and PG in biodiesel-ethanol at a Pt ultramicroelectrode Andrea Anilda Hoffmann da Rochaa, Marcella Casagrandea, Lívia de Souza Schaumlöffela, Yara Patrícia da Silvaa, Clarisse Maria Sartori Piatnickia*

a

Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento

Gonçalves, 9500, CP 15003, 91501-970, Porto Alegre, RS, Brazil. E-mail: [email protected]

* Corresponding author: Email: [email protected]

ACS Paragon Plus Environment

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Abstract

A direct non-aqueous method for simultaneous determination of propyl gallate (PG) and tert-butylhydroquinone (TBHQ) in biodiesel is described. Differential pulse voltammetry (DPV) using a Pt disk ultramicroelectrode (ume) in biodiesel:ethanol 1:1, v/v, containing tetrahexylammonium perchlorate 0.02 mol L-1 showed two oxidation peaks in the range from 1.20×10-3 mol L-1 to 8.9×10-3 mol L-1 for TBHQ and from 8.6×10-4 mol L-1 to 4.9×10-3 mol L-1 for PG. Simultaneous quantification and additionrecovery experiments were carried out in antioxidants-free biodiesel samples spiked with variable concentrations of TBHQ and PG. Because of significant overlapping in oxidation signals, the amount of TBHQ and PG in B100 was calculated through calibration graphs after voltammograms deconvolution using Microcal Origin® software (version 8.0). The method provides a direct, low cost, non-aqueous and small volume procedure for simultaneous quantification of TBHQ and PG in biodiesel with, respectively, 116% and 104% recovery values.

Keywords: Biodiesel, Tert-butylhydroquinone, Propyl gallate, Simultaneous quantification, Non-aqueous.


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Abbreviations TBHQ, tert-butyl hydroquinone; PG, propyl gallate; BHA, butyl-hydroxyl-anisole; BHT, butyl-hidroxyl-toluene; B100, biodiesel; ume, ultramicroelectrode; N(Hex)4ClO4, tetrahexilammonium perclorate; GCE, glassy carbon electrode; BDD, boron-doped diamond electrode; BR, Britton–Robinson; LSV, linear sweep voltammetry; DPV, differential pulse voltammetry; SWV, square wave voltammetry; CV, cyclic voltammetry; CTAB, cetyltrimethylammonium bromide.


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1. Introduction Biofuels are promising alternatives to petroleum-based fuels 1,2. However, biodiesel chemical and physical properties are affected by oxidation during storage giving rise to products such as alcohols, aldehydes, carboxylic acids and epoxides, which cause damage to combustion engines 3. Synthetic phenolic antioxidants such as tert-butyl-hydroquinone (TBHQ), propyl gallate (PG), butyl-hydroxyl-anisole (BHA) and butyl-hidroxyl-toluene (BHT) are usually added to biodiesel to prevent chemical degradation 4. In order to improve biodiesel stability, industry often employs mixtures of two or more antioxidants, which should be regularly monitored 5–7. Accelerated oxidation methods (Rancimat induction period, IP, or oil stability index and pressurized differential scanning calorimetry) are generally used for this purpose 8,9. However, since these measurements require several hours, rapid and direct quantification of antioxidants in biodiesel is important to monitor and adjust their concentration still in the production line, to ensure biofuel quality 10. Several analytical methods such as gas chromatography 11,12, high performance liquid chromatography 13 and electroanalysis 14,15 have been developed for synthetic antioxidants determination, mostly in edible oils and foods. However, generally at least one extraction step is required. Electrochemical methods are promising alternatives to traditional approaches due to its relatively low operating cost, possibility of miniaturization, simpler operation, high sensitivity and fast response 16. Regarding biodiesel, few studies on antioxidants quantification 6–8,17,18 have been reported. Caramit et al. investigated successfully direct simultaneous determination of TBHQ+BHA 8 and TBHQ+BHA+PG 6 in biodiesel using linear voltammetry at a carbon-nanotube-modified screen-printed electrode in Britton-Robinson (BR) buffer


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containing cationic cetyltrimethylammonium bromide (CTAB). Biodiesel samples were diluted in ethanol and added to aqueous BR electrolyte without 6 and with 8 10% methanol. Moreover, amperometric techniques in simultaneous determination of TBHQ and BHA in biodiesel using batch injection analysis were developed by Tormin et al. 17 and da Silva et al. 18 in hydroethanolic solution containing 0.1 mol L-1 HClO4 at borondoped diamond (BDD) and GCE. Further, simultaneous determination of TBHQ and BHT has been also reported by Tomásková et al. 7 in diesel fuel containing 30% biodiesel. The mixture was quantitatively analysed by LSV in isopropanol-0.1 mol L-1 H2SO4 supporting electrolyte at a gold disc microelectrode (2 mm diameter). Electrochemical studies on simultaneous determination of binary and ternary mixtures of TBHQ, BHA, BHT and PG in biodiesel and in a diesel-biodiesel mixture using different electrodes and working electrolytes are summarized in Table 1. Table 1 Due to molecular structures similarity, phenolic antioxidants mixtures are difficult to analyze because usually voltammograms overlap partially. In this concern much progress has been made in applying chemometrical techniques to electroanalytical determinations 19. Jitmanee et al. 20 applied deconvolution to potentiometric acid–base titration and anodic stripping voltammetry of copper. In turn, Caramit et al. 6 successfully employed signals derivatization to solve analytical problems in voltammetry using Microcal Origin® software, which was also applied in the present work. The aim of this study is to develop a non-aqueous, direct, rapid, environmentally friendly, low cost and simpler procedure than those reported in the literature for simultaneous determination of TBHQ and PG in biodiesel. No extraction step, hydro-


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organic medium or electrode pre-treatment is required. Besides, as much as the authors know, direct simultaneous quantification of TBHQ and PG in biodiesel using DPV at a Pt ume and non-aqueous electrolyte has not been reported yet.

2. Experimental

2.1. Instrumentation A µAUTOLAB TYPE III potentiostat/galvanostat (Holland) system was used to perform voltammetric measurements. All experiments were carried out in a threeelectrode configuration glass cell. The software packages used were the General Purpose Electrochemical System (GPES) for the acquisition of the data and Microcal Origin® software (version 8.0) for data treatment. Quasi-reference (PQRE) 21, working, and auxiliary electrodes were a Pt wire, a 5 µm radius Pt disk ume from EG&G PAR (Princeton Applied Research, Wellesley, MA, USA), and a Pt ribbon, respectively.

2.2. Reagents and solutions Ethanol (99%), tetrahexylammonium perchlorate (99%), TBHQ (97%) and PG (98%) were purchased from Acros Organics (USA) and used without further purification. Separate stock solutions of TBHQ and PG, 0.12 mol L-1 and 0.01 mol L-1, respectively, were prepared in biodiesel:ethanol 1:1 (v/v).


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2.3. Procedures 2.3.1. Procedure for sample preparation and analysis Fresh antioxidants-free soybean biodiesel samples were provided by a local producer and diluted in ethanol in the 1:1 (v/v) ratio. All experimental samples contained 0.02 mol L-1 N(Hex)4ClO4 as background electrolyte. Adequate amounts of stock solutions were added to the electrochemical cell containing 10.00 mL of biodiesel:ethanol sample to obtain calibration samples in concentrations ranging from 1.20×10-3 to 8.90×10-3 mol L-1 for TBHQ and from 8.60×10-4 to 4.90×10-3 mol L-1 for PG. For both mutual interference experiments TBHQ concentration was set at 4.3×10-3 mol L-1 varying that of PG, as well as setting PG concentration at 3.4×10-3 mol L-1 and varying that of TBHQ. Since there is no certificated reference material for biodiesel, validation of the proposed method was performed through a recovery test by adding known amounts of TBHQ and PG, simultaneously, to biodiesel-ethanol solutions spiked with 1.2×10-3 mol L-1 TBHQ and 9.4×0-4 mol L-1 PG. Ten DPV replicates were carried out to quantify both antioxidants using the standard addition method. Apparent percent recovery was calculated from the ratio of the observed value obtained via deconvolution of the standard addition voltammograms divided by the reference value, that is, the value for spike. For all experiments differential pulse voltammograms were carried out between 0.000 and 1.200 V at 5 mV s-1 scan rate, 50 mV pulse amplitude and 50 ms pulse width. Unless otherwise indicated, in order to improve reproducibility, the working electrode was cleaned prior to each set of experiments by sonication in concentrated H2SO4 for 5 minutes and then washed with Milli-Q® water (R ≥ 18.2 MΩ cm). All


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electrochemical measurements were carried out at room temperature in the presence of dissolved oxygen.

2.3.2. Deconvolution procedure Due to the observed oxidation peaks overlapping when both antioxidants were present in the mixture, a mathematical peak deconvolution procedure via Microcal Origin® software (version 8.0) was necessary for satisfactory separation and simultaneous determination. For the deconvolution process, the baselines of all voltammograms were adjusted, and subsequently, the “Gaussian-analysis” tool was used to select the number of peaks, peak width at a half height and peak potential 19. When carried out as described above the deconvolution procedure allowed satisfactory separation of the antioxidants target peaks and their subsequent quantification.

3. Results and discussion

3.1 TBHQ oxidation at a Pt ume in ethanol Increased conductivity, higher N(Hex)4ClO4 solubility, lower viscosity and stable background currents in voltammetric measurements were obtained using ethanol as diluent for biodiesel. By adding increasing amounts of biodiesel to ethanol it was observed that the ethanol oxidation limiting current remains constant starting from the ethanol:B100 1:1, (v/v) ratio with a significant decrease in medium viscosity. Electrochemical parameters of a redox couple such as formal potential, halfwave potential and/or peak potential can be modified by the solvent nature. Ethanol electrochemical oxidation has been extensively studied in the last decades at platinum electrodes in aqueous medium, mostly by voltammetric methods 22.


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Preliminary DPV experiments on TBHQ oxidation carried out in ethanol 0.2 mol L-1 in N(Hex)4ClO4 showed a small wave around 0.7 V (see Figure 1, curve a), most probably due to ethanol oxidation and, coincidently, very close to TBHQ oxidation potential. Voltammograms in Figure 1 (curves b to f) evidence a linear relation between TBHQ concentration and peak current. Figure 1 A study by S. Andreescu et al. 23 on phenol oxidation at a Pt electrode in aqueous 2×10-3 mol L-1 phenol and 50% v/v ethanol concluded that phenol is not appreciably deposited through the Pt oxide layer on the electrode surface.

3.2 PG and TBHQ calibration graphs Generally, oxidation of phenolic compounds produces unstable phenoxy radicals that can be further oxidized to quinones or react to form dimmers, which readily polymerize into polyaromatic compounds 24,25. Moreover, phenoxy radicals are able to interact with each other or with another phenol monomer, giving rise to a strongly adherent insulating film that passivates the working electrode surface 25,26. However, no passivation of the Pt electrode has been observed in the present experiments. This behavior is assigned to adsorption of biodiesel esters polar molecules on Pt through the carbonyl moiety, thus hindering dimmers deposition and higher molecular associations on the electrode surface 27–29. Investigating PG oxidation in microemulsions Szymula, and Narkiewicz-Milchalek 30 observed that addition of surfactants to the system influences both the oxidation potential and the peak current. Anionic and nonionic surfactant films formed at the electrode/solution interface hinder electron transfer decreasing the transport rate of PG molecules to the electrode.


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Figure 2 shows PG oxidation voltammograms after blank subtraction. The peak potential shifts from 0.7 to 0.8 V with PG increasing concentration, a behavior that is assigned to the irreversible character of phenolic compounds oxidation processes 31,32. At the same time, probably there is adsorption of biodiesel on the electrode surface so that PG oxidation occurs at a more positive potential. Figure 2 Insert in Figure 2 shows the relationship between peak intensity and PG concentration in the range from 8.6×10-4 to 4.9×10-3 mol L-1. Detection (LOD) and quantification (LOQ) limits, were calculated by using 3Sb/b and 10Sb/b, respectively, where Sb is the standard deviation of ten blank sample replicates and b is the analytical curve slope. Values for LOD and LOQ are 1.70×10-4 mol L-1 and 5.7×10-4 mol L-1, respectively; 1.40×10-8 A L mol-1 calibration sensitivity and 0.993 correlation coefficient, r. Small deviation from linearity for higher PG concentrations as observed in Figure 2 has been already reported 33. Figure 3 shows differential pulse voltammograms for individual quantification of TBHQ, after background subtraction. At the oxidation potential, around 0.6 V, there is a linear relationship between peak current intensity and TBHQ concentration in the range from 1.20×10-3 to 8.9×10-3 mol L-1. Figure 3 In this case, values for LOD, LOQ, calibration sensitivity and correlation coefficient, r, are, 2.12×10-4, 7.0×10-4 mol L-1, 1.47×10-8 A L mol-1 and 0.997, respectively. By comparing voltammograms in Figures 3 and 4 it is seen that the potential shift with increasing concentration is more significant for PG than for TBHQ. This behavior indicates varying degrees of irreversibility as well as the influence of


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biodiesel adsorption on the electrode surface hampering electron transfer as discussed above 31,32.

3.3 Mutual interference In order to assess the extent of both compounds mutual interference, voltammograms of samples containing either 3.4×10-3 mol L-1 PG and different concentrations of TBHQ, or 4.3×10-3 mol L-1 TBHQ and variable PG concentrations were carried out. After blank subtraction, two oxidation peaks were observed around 0.6 V for TBHQ and 0.9 V for PG in (A) and (B) as shown in Figure 4. Figure 4 In the presence of TBHQ, a change in slope of PG calibration curve for higher concentrations (insert in Figure 4B) suggests a lower PG concentration in the diffusion layer as evidenced by a lesser current value compared to the expected one. Since the corresponding radical cation formed upon oxidation is a relatively strong acid a proton will be spontaneously released 34 and the resulting TBHQ phenoxyl radical may be further oxidized to phenoxonium. Then, both phenoxyl and phenoxonium radicals from TBHQ, which represents electrophiles, may partially attack PG, thus explaining the slight interference between TBHQ and PG.

3.4 Simultaneous quantification Simultaneous quantification of TBHQ and PG (Figure 5A) in biodiesel required a deconvolution treatment (Figure 5B) due to a significant overlapping in oxidation currents. Since both antioxidants were affected by the background signal from the


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biodiesel matrix, voltammetric curves were differentiated by baseline subtraction and then derived. Figure 5 In experiments on mutual interference deviations from linearity at higher concentrations are more significant for PG than for TBHQ (Figure 4), whereas on simultaneous determination (Figure 5) deviations for TBHQ prevail. This behavior seems to indicate that, apart from chemical reactions as discussed above, a kinetic component could control both antioxidants concentration in the diffusion layer so that they would not be completely independent of each other. Results so far are summarized in Table 2. Table 2 Data in Table 2 for TBHQ and PG simultaneous determination show that the calibration curve presents excellent sensitivity and limits of detection of the same order of magnitude for both antioxidants.

3.5. Recovery test Addition-recovery experiments were carried out in an antioxidant-free biodiesel sample spiked with variable concentrations of TBHQ and PG. Figure 6A shows differential pulse voltammograms obtained with the standard addition method. After background subtraction and voltammograms differentiation, peaks for TBHQ and PG become distinguishable as shown in Figure 6B. Figure 6 Recoveries of 116% for TBHQ and 104% for PG were achieved with the proposed method, indicating satisfactory precision and accuracy. Therefore, it could be


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employed for TBHQ and PG direct determination in biodiesel without sample pretreatment.

4. Conclusions Differential pulse voltammetry at a Pt ultramicroelectrode in biodiesel-ethanol medium showed to be a successful alternative approach allowing simultaneous determination of TBHQ and PG without prior extraction. The novel non-aqueous method afforded fast response, excellent sensitivity, selectivity, low cost and satisfactory repeatability in simultaneous determination of the target antioxidants, requiring a single, simple and rapid dilution step. Therefore, it can be successfully employed to improve antioxidants electroanalysis in routine quality control of biodiesel samples and recommended for practical analysis in laboratories of biodiesel plants.

5. Acknowledgments

The authors are grateful to: CECOM-IQ-UFRGS; FINEP and CNPq for financial support, respectively, through the Bioarmaz2 Project and Processes 478998/2012-0 and 405011/2013-0. To CNPq for post-doctoral and CAPES for postdoctoral and PhD grants.


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LIST OF FIGURES

Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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FIGURE CAPTIONS

Figure 1. DPV at a 5 µm radius Pt disk ume in ethanol 0.2 mol L-1 N(Hex)4ClO4: (a) in the absence of TBHQ; (b-f) in the presence of TBHQ concentration: 6.0×10-4; 1.2, 1.8, 2.4 and 3.0×10-3 mol L-1. Potential range 0.000 V to 1.200 V, respectively, 10 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt.

Figure 2. Differential pulse voltammetry at a 5 µm radius Pt disk ume in B100:ethanol, 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4: (a-f) PG concentration from 8.6×10-4 mol L-1 to 4.9×10-3 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, 50 ms pulse width, potential scan rate 5 mV s-1, counter and quasi reference electrodes in Pt. Insert: PG linear calibration.

Figure 3. Differential pulse voltammetry at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4: (a-h) TBHQ concentration from 1.20×10-3 mol L-1 to 8.9×10-3 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt. Insert: TBHQ linear calibration.

Figure 4. DPV at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4. (A) (a-h) TBHQ concentration from 1.20×10-3 to 8.9×10-3 mol L-1 in the presence of 3.4×10-3 mol L-1 PG. Insert: TBHQ linear calibration curve. (B) (a-h) PG concentration from 9.3×10-4 to 7.0×10-3 mol L-1 in the presence of TBHQ 4.3×10-3 mol


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L-1. Insert: PG linear calibration curve. Potential range 0.000 V to 1.200 V, 50 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt.

Figure 5. A) DPV at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4. Successive additions of TBHQ from 1.20×10-3 to 8.9×10-3 mol L-1 and of PG from 9.3×10-4 to 7.0×10-3 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt. B) Deconvolution process. Insert: TBHQ and PG linear calibration curves.

Figure 6: A) DPV at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4. a) Biodiesel sample spiked with 1.2×10-3 mol L-1 TBHQ and 9.4×10-4 mol L-1 PG and (b-h) successive additions of both antioxidants: TBHQ from 2.3×10-3; 3.4; 4.4; 5.5; 6.4; 7.4 and 8.3 mol L-1; PG from 1.8; 2.7; 3.5; 4.3; 5.0; 5.8 and 6.5 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, 50 ms pulse width, potential scan rate 5 mV s-1, counter and quasi reference electrodes in Pt. B) Deconvolution process. Insert: TBHQ and PG standard addition curves.


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TABLES

Table 1

Table 1: Simultaneous determination of phenolic antioxidants in biodiesel:

electroanalytical approaches. Analytes

Electrode/Technique

Electrolyte/Sample preparation

Reference

TBHQ+PG+BHA

MWCNT-SPE/

BR buffer pH 4.0, 2.00×10-4 mol L-1 CTAB/

6

LSV

Dilution in ethanol

SPE–MWCNT/

0.04 mol L-1 BR buffer pH 2.0,

LSV

2.0% methanol, 5.0×10-4 mol L-1 CTAB /

TBHQ+BHA

8

biodiesel:ethanol, v/v TBHQ+BHA

*TBHQ+BHT

TBHQ+BHA

GCE/

Hidroethanolic v/v 0.1 mol L-1 HClO4 /

Amp

biodiesel:ethanol 1:40, v/v

Gold disk electrode/

0.1 mol L-1 H2SO4 isopropanol /

LSV

Direct analysis

BDD or GCE/

0.1 mol L-1 aqueous HClO4-5% ethanol/

Amp

Dilution in ethanol

17

7

18

LSV: linear sweep voltammetry; Amp: Amperometry; GCE: glassy carbon electrode; BR: Britton– Robinson buffer; MWCNT-SPE: multi-walled-carbon-nanotube-modified screen-printed electrodes; BDD: boron doped diamond. *30% biodiesel in diesel.


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Table 2

Table 2. Figures of merit for TBHQ and PG determination in B100:ethanol, 1:1, v/v. LWR

Intercepta

(mol L-1)

A

TBHQ

1.2×10-3 – 8.9×10-3

5.2×10-11

PG

8.6×10-4 – 5.7×10-3

Analyte

r

LD

Sensitivity

(mol L-1)

(A mol L-1)

0.997

2.12×10-4

1.47×10-8

1.71×10-13

0.993

1.70×10-4

1.40×10-8

3.89×10-11

0.998

3.64×10-4

1.31×10-8

1.01×10-11

0.982

9.24×10-5

1.98×10-8

(A) separate

(B) PG (3.4 x 10-3 mol L-1) varying TBHQ TBHQ

1.2×10-3 – 8.9×10-3

(C) TBHQ (4.30 x 10-3 mol L-1) varying PG PG

9.3×10-4 – 7.0×10-3

(D) TBHQ and PG simultaneous TBHQ

1.2×10-3 – 8.9×10-3

8.6×10-12

0.994

1.90×10-4

1.23×10-8

PG

9.3×10-4 – 7.0×10-3

3.6×10-14

0.998

1.18×10-4

1.77×10-8

LWR: linear working range; r: correlation coefficient; LD: limit of detection a

A = ampere


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Figure 1. DPV at a 5 µm radius Pt disk ume in ethanol 0.2 mol L-1 N(Hex)4ClO4: (a) in the absence of TBHQ; (b-f) in the presence of TBHQ concentration: 6.0×10-4; 1.2, 1.8, 2.4 and 3.0×10-3 mol L-1. Potential range 0.000 V to 1.200 V, respectively, 10 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt. 297x210mm (300 x 300 DPI)


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Figure 2. Differential pulse voltammetry at a 5 µm radius Pt disk ume in B100:ethanol, 1:1, v/v, 0.02 mol L1 in N(Hex)4ClO4: (a-f) PG concentration from 8.6×10-4 mol L-1 to 4.9×10-3 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, 50 ms pulse width, potential scan rate 5 mV s-1, counter and quasi reference electrodes in Pt. Insert: PG linear calibration. 297x210mm (300 x 300 DPI)


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Figure 3. Differential pulse voltammetry at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L1 in N(Hex)4ClO4: (a-h) TBHQ concentration from 1.20×10-3 mol L-1 to 8.9×10-3 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt. Insert: TBHQ linear calibration. 297x210mm (300 x 300 DPI)


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Figure 4. DPV at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4. (A) (ah) TBHQ concentration from 1.20×10-3 to 8.9×10-3 mol L-1 in the presence of 3.4×10-3 mol L-1 PG. Insert: TBHQ linear calibration curve. (B) (a-h) PG concentration from 9.3×10-4 to 7.0×10-3 mol L-1 in the presence of TBHQ 4.3×10-3 mol L-1. Insert: PG linear calibration curve. Potential range 0.000 V to 1.200 V, 50 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt. 160x60mm (300 x 300 DPI)


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Figure 5. A) DPV at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4. Successive additions of TBHQ from 1.20×10-3 to 8.9×10-3 mol L-1 and of PG from 9.3×10-4 to 7.0×10-3 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, potential scan rate 5 mV s-1, 50 ms pulse width, counter and quasi reference electrodes in Pt. B) Deconvolution process. Insert: TBHQ and PG linear calibration curves. 160x60mm (300 x 300 DPI)


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Figure 6: A) DPV at a 5 µm radius Pt disk ume in B100:ethanol 1:1, v/v, 0.02 mol L-1 in N(Hex)4ClO4. a) Biodiesel sample spiked with 1.2×10-3 mol L-1 TBHQ and 9.4×10-4 mol L-1 PG and (b-h) successive additions of both antioxidants: TBHQ from 2.3×10-3; 3.4; 4.4; 5.5; 6.4; 7.4 and 8.3 mol L-1; PG from 1.8; 2.7; 3.5; 4.3; 5.0; 5.8 and 6.5 mol L-1. Potential range 0.000 V to 1.200 V, 50 mV amplitude, 50 ms pulse width, potential scan rate 5 mV s-1, counter and quasi reference electrodes in Pt. B) Deconvolution process. Insert: TBHQ and PG standard addition curves. 160x60mm (300 x 300 DPI)


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Simultaneous voltammetric determination of TBHQ and PG in

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