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Determination of Sterically HinderedPhenols and α-Tocopherol by CyclicVoltammetryGuzel Ziyatdinova a , Aliya Khuzina a & Herman Budnikov aa Analytical Chemistry Department , A.M. Butlerov Institute ofChemistry, Kazan (Volga Region) Federal University, Kremlyevskaya ,Kazan , Russian FederationAccepted author version posted online: 13 Apr 2012.Publishedonline: 02 Aug 2012.

To cite this article: Guzel Ziyatdinova , Aliya Khuzina & Herman Budnikov (2012) Determinationof Sterically Hindered Phenols and α-Tocopherol by Cyclic Voltammetry, Analytical Letters, 45:12,1670-1685, DOI: 10.1080/00032719.2012.677788

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Electrochemistry

DETERMINATION OF STERICALLY HINDEREDPHENOLS AND a-TOCOPHEROL BY CYCLICVOLTAMMETRY

Guzel Ziyatdinova, Aliya Khuzina, and Herman BudnikovAnalytical Chemistry Department, A.M. Butlerov Institute of Chemistry,Kazan (Volga Region) Federal University, Kremlyevskaya, Kazan,Russian Federation

Sterically hindered phenols (2,6-di-tert-butyl-4-methylphenol (BHT) and its derivatives)

are irreversibly oxidized at þ0.96–1.30V on glassy carbon electrode (GCE) in 0.1M

LiCIO4 in acetonitrile in accordance to cyclic voltammetry data. a-Tocopherol gives oxi-

dation step at þ0.4V on voltammograms under the same conditions. Oxidation process

leads to formation of corresponding quinoid derivatives. Calibration graphs linearity is

1.5–2 order for all compounds under investigation. Limits of detection are in the range

9.6–44.3lM. The approach has been applied for determination of BHT and a-tocopherol

in vegetable and lubricating oils as well as pharmaceuticals and cosmetics using preliminary

single extraction of analytes with acetonitrile for 15min at oil/extractant ratio of 1:2.5.

Keywords: a-Tocopherol; BHT; Cyclic voltammetry; Food analysis; Lubricating oils; Pharmaceutical

analysis; Sterically hindered phenolic antioxidants

INTRODUCTION

Antioxidants play a very important role in many biological processes in whichfree radicals are present (Mariani et al. 2005). The primary antioxidants to whichlow-molecular weight compounds belong act by blocking free radicals and convert-ing them into stable products via redox reactions (Madhavi, Deshpande, andSalunkhe 1995). Phenolic compounds such as a-tocopherol, butylated hydroxyto-luene (BHT), and other sterically hindered phenols represent the major family ofboth natural and synthetic primary antioxidants (Denisov and Khudyakov 1987;Foti 2007). The mechanism generally used to explain their effectiveness is basedon the abstraction of the softly bound H atom by peroxyl radicals (Denisov andKhudyakov 1987; Scott 1993; Schnabel 1992). Phenolic antioxidants are commonlyused alone or together in commercial mixtures as additives in oils, foodstuff, and

Received 7 January 2012; accepted 4 February 2012.

Address correspondence to Guzel Ziyatdinova, Department of Analytical Chemistry, A.M.

Butlerov Institute of Chemistry, Kazan (Volga Region) Federal University, Kremlyevskaya, 18, Kazan,

420008, Russia. E-mail: Ziyatdinovag@mail.ru

Analytical Letters, 45: 1670–1685, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0003-2719 print=1532-236X online

DOI: 10.1080/00032719.2012.677788

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cosmetics, in order to prevent oxidative rancidity (Hudson 1990; Bruggemann et al.2004). On the other side, phenol-based antioxidants can exhibit dose-dependent toxiceffects (Clayson et al. 1986; Yu, Mandlekar, and Kong 2000) including carcinogen-esis (Hirose et al. 1998). Therefore, analytical control of these compounds in differ-ent samples is of considerable importance and requires the development of precisemethods for their determination.

Various analytical methods have been successfully used for determination ofsterically hindered phenols and a-tocopherol. Methods include different types ofchromatography (Ding and Zou 2012; Barros et al. 2008; Rodila et al. 2010; andPerrin and Meyer 2002) such as micellar electrokinetic capillary chromatography(Delgado-Zamarreno et al. 2007) and optical methods (Capitan-Vallvey, Valencia,and Nicolas 2004; Espinosa-Mansilla et al. 1996; Asgher et al. 2010).

Phenolic fragment of compounds under investigation is easily electrochemicallyoxidized on platinum (Raymundo et al. 2007; Litescu, Radu, and Diaconu 2001;Budnikov, Ziyatdinova, andGil’metdinova 2004), gold (Abdullin, Turova, Parshakovaet al. 2002; Chylkova et al. 2010), carbon-based (Abdullin 2002; Raymundo et al. 2007;Ziyatdinova and Budnikov 2011), and modified (Tormin et al. 2011; Freitas andFatibello-Filho 2010; Prabakar and Narayanan 2010) electrodes. A wide range ofvoltammetric techniques including differential pulse (Michalkiewicz, Mechanik, andMalyszko 2004; Beaulieu and Hadziyev 2006; Diaz et al. 2004) and square-wave(Medeiros, Rocha-Filho, and Fatibello-Filho 2010; Raymundo et al. 2007) modes havebeen applied for the determination of sterically hindered phenols and a-tocopherol infoodstuff, lubricating oils, and cosmetics. However, the detection limit in some casesis not sufficient for traces of compounds to be determined, and these methods are foundto be less sensitive and irreproducible due to the electrode surface fouling.

Recently, chemometric approaches, in particular principal componentregression and partial least square regression methods, have been introduced forthe determination of mixtures of synthetic antioxidants with three or more compo-nents (Diaz et al. 1998; and Richards, Bessant, and Saini 2002). Even being limited inuse, these approaches can be helpful in some cases.

The aim of present work is the investigation of electrochemical behavior ofphenolic antioxidants and development of voltammetric method for determinationof BHT and a-tocopherol in real samples.

EXPERIMENTAL

Reagents

The BHT 99% purity was purchased from Aldrich, Germany; 2,20-Thiobis(6-tert-butyl-4-methylphenol) 98% purity was purchased from Carbon ScientificCo, LTD, UK; 2,6-Di-isobornyl-4-methylphenol and BHT derivatives were grantedby professor E. Korotkova (Tomsk Polytechnic University) and professor E.Ofitserov (Mendeleyev University of Chemical Technology of Russia), respectively.Stock solutions (10mM) of sterically hindered phenols were prepared by dissolving aprecisely weighed amount of the substance in ethanol. More dilute solutions (testsolutions) were prepared daily before measurements by exact dilution of the stocksolutions.

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a-Tocopherol acetate oil solution of pharmaceutical grade purity (Hoffman-LaRoche, France) containing 97.9% of substance was used for the measurements. Itsstock solution was prepared by alkaline saponification of sample (0.15 g) in ethanolby refluxing with a threefold excess of KOH alcoholic solution for 30min in a waterbath. The obtained solution of a-tocopherol was transferred to volumetric flask,diluted to the mark with ethanol, and used for further investigations after coulo-metric standardization by titration with electrogenerated bromine (Abdullin,Turova, Ziyatdinova et al. 2002). The average degree of saponification was equalto 99%. The final concentration of stock solution was 12mM. Additional dilutedsolutions (test solutions) were prepared daily before measurements by exact dilutionof the stock solution.

Acetonitrile of HPLC grade (Panreac, Spain) and ethanol rectificate were usedfor the measurements. All other chemicals were analytical reagent grade purity andused as received. The experiments were carried out at laboratory temperature (20�C).All solutions were kept in glass vessels in the dark at laboratory temperature except-ing a-tocopherol and phenols stock solutions stored at þ4�C.

Apparatus

Voltammetric measurements were performed using voltammetric analyzer‘‘Ecotest-VA’’ (Econix-Expert Ltd., Russia). The electrochemical cell (V¼ 25mL)consisted of working GCE (3.14mm2 geometric surface area), silver-silver chloridesaturated KCl reference electrode, and counter electrode (platinum wire). TheGCE surface was carefully polished with alumina (0.05 mm) on polishing cloth. Then,it was rinsed with acetone and double distilled water before use.

Coulometric measurements were carried out using P-5827M potentiostat (ZIP,Belarus) with four-electrode two-compartment electrochemical cell. A bare platinumfoil with 1 cm2 surface area was used as the working electrode, and a platinum wireas the auxiliary electrode. A pair of polarized platinum electrodes was used for detec-tion of the titration end-point. Surface of platinum electrodes was cleaned by HNO3

and then rinsed thoroughly with double distilled water.

Procedures

Cyclic voltammetry. The 0.1M LiCIO4 in acetonitrile was chosen assupporting electrolyte. Cyclic voltammograms were recorded for the solution of15.0mL supporting electrolyte and aliquot portion of analyte test solution atpotential scan rate of 100mV s�1 and potential range from 0 to 1.5 V.

Coulometric titration. Electrochemical generation of bromine were carriedout at a current density 5mA=cm2 from 0.2M (C2H5)4NBr in 0.1M HClO4 in acet-onitrile. Hexacyanoferrate(III) ions were generated from 0.1M K4[Fe(CN)6] in0.5M NaOH. The titration end-point was measured biamperometrically(DE¼ 300mV). Coulometric titration was carried out in a 50.0mL cell. The support-ing electrolyte (20.0mL) and an aliquot portion of a-tocopherol stock solution (0.2–0.8mL) were inserted into the cell. After the electrodes immersion, the generating cir-cuit and timer were switched on simultaneously. Alteration of indicative current in

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time was marked. The mass of analyte was calculated using Faraday formulae(Monk 2001).

Sample Preparation

The objects of analysis are commercially available vegetable and lubricatingoils, pharmaceuticals and cosmetics from different producers. Codes A-Y corre-spond to different commercial brands of samples.

For the determination of a-tocopherol and BHT, preliminary liquid extractionby acetonitrile was used. An amount of 3mL of oil or 3 g of butter or cream wasmixed with 7.5mL of acetonitrile in a separating funnel, shaken for 15min, storeduntil the phase separation (5–10min), and then the acetonitrile layer was collectedfor further measurements. The aliquot portion of the extract (0.3–1.0mL) wasinserted in the electrochemical cell with 15.0mL of supporting electrolyte, and thevoltammograms were recorded with a scan rate 100mV s�1 in the range of 0–1.5V.

Statistical Analysis

All measurements were performed 5 times. Mathematical and statistical evalu-ation (for number of measurements n¼ 5 and for significance level a¼ 0.05) was per-formed using the software OriginPro 8.0 (OriginLab, USA). All data are expressedas the X�DX with X as average value and DX as confidence interval (Otto 1999).

RESULTS AND DISCUSSION

Cyclic Voltammetry of Phenolic Antioxidants

Voltammetric behavior of a-tocopherol and sterically hindered phenols onGCE in 0.1M LiClO4 in acetonitrile was investigated. The structures of compoundsunder investigation are shown in Fig. 1.

All substances are oxidized under these conditions showing one oxidation step(Fig. 2). It has the peak form in the case of BHT and 2,6-di-isobornyl-4-methylphe-nol (Fig. 2A and B). The oxidation of sterically hindered phenols was irreversible,which was confirmed by the absence of cathodic steps on the corresponding cyclicvoltammograms at a given range of polarization (Fig. 2A–C). The peak ofa-tocopherol oxidation is observed on voltammograms (Fig. 2D). The cathodic stepappears on cyclic voltammograms at the higher concentrations of a-tocopherol thatcould be only considered as a hint of reversibility.

All sterically hindered phenols and a-tocopherol were oxidized at potentialsshown in Table 1. The difference in oxidation potentials was caused by electronicand steric effects of substituents in the structure of corresponding phenol that agreedwell with the calculated gas-phase bond dissociation enthalpy and ionization poten-tial for phenolic antioxidants in dependence of their chemical structure (Wright,Johnson, and DiLabio 2001). The lowest oxidation potential (�1.0V) was observedfor amino derivatives of BHT.

The effect of potential scan rate on the voltammetric characteristics of phenolicantioxidants was evaluated. Figure 3 shows plots for BHT. Oxidation currents of

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compounds under investigation increased at higher scan rates and maximum currentwas observed at 100mV s�1. Electrochemical oxidation of phenols was controlled bythe diffusion of electroactive species as the slowest step in the electrochemical pro-cess. The diffusion nature of the voltammetric peaks was confirmed on the basisof linear dependence (Nicholson and Shain 1964) of the peak current on the squareroot of scan rate v1=2 with R¼ 0.9985–0.9995. The number of electrons participatingin oxidation is 2.0� 0.2 for 2,20-thiobis(6-tert-butyl-4-methylphenol), 2,6-di-tert-butyl-4-a,b-diacetylethylphenol, 2,6-di-isobornyl-4-methylphenol, and a-tocopherol;and 1.00� 0.05 for amino derivatives of BHT. The number of electrons is calculatedby comparison of oxidation currents of compounds under investigation with stan-dard compound (BHT) that undergoes two-electronic oxidation (Medeiros et al.2010; Ross, Barclay, and Vinqvist 2003). The hydroxyl group of phenol participatesin oxidation with formation of corresponding quinoid derivatives in accordance toScheme 1.

Figure 1. Phenolic antioxidants under investigation.

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In the case of 2,20-thiobis(6-tert-butyl-4-methylphenol), just one phenolic ringunderwent oxidation in accordance to Scheme 2, which corresponds well with oxi-dation of other bisphenols (Amorati et al. 2003). It was caused by the intramolecularhydrogen bond between the hydroxyl group and sulfur atom; whereas, the secondhydroxyl group, giving a weaker H-bond (presumably to the p-electrons of the otherbenzene ring), was less embedded in the bulk of the molecule and therefore more eas-ily accessible to oxidation. Thus, the hydrogen atom of the second hydroxyl group

Table 1. Oxidation potentials of phenolic antioxidants in 0.1M LiClO4 in acetonitrile

Compound under investigation Emax, V

BHT þ1.22

2,20-Thiobis(6-tert-butyl-4-methylphenol) þ1.13

2,6-Di-isobornyl-4-methylphenol þ1.11

2,6-Di-tert-butyl-4-b,b-diacetylethylphenol þ1.30

2,6-Di-tert-butyl-4-dimethylaminomethylphenol þ1.05

2,6-Di-tert-butyl-4-(N,N-bis-b-oxyethyl-amino)methylphenol þ0.96

a-Tocopherol þ0.40

Figure 2. Cyclic voltammograms of 290mM (curve 2), 480mM (3), 650mM (4), and 910mM (5) BHT (A);

2,6-di-isobornyl-4-methylphenol (B); 2,20-thiobis(6-tert-butyl-4-methylphenol) (C); and 158mM (curve 2),

314mM (3), 762mM (4), 1050mM (5), and 2090mM (6) a-tocopherol (D) on GCE in 0.1M LiClO4 in

acetonitrile (curve 1). Potential scan rate 100mV s�1.

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can be abstracted more easily because it is less sterically hindered and characterizedby lower bond strength (Amorati et al. 2006).

a-Tocopherol is easily oxidized to p-tocopherylquinone with participation oftwo electrons (Dryhurst 1985). The process involves several steps. a-Tocopherol isinitially oxidized by one-electron removal to form a radical cation which rapidlyloses a proton to form the neutral radical and then, immediately, further oxidizedby one-electron to form the diamagnetic cation (Marcus and Hawley 1970;Svanholm, Bechgaard, and Parker 1974; Williams and Webster 2004; Webster2007). In the presence of water containing acetonitrile, the reaction of cation withwater produces hemiketal which is rapidly converted into quinone (Tan et al.2011) (Scheme 3).

Scheme 1. Oxidation of sterically hindered monophenols.

Scheme 2. Oxidation of 2,20-thiobis(6-tert-butyl-4-methylphenol).

Figure 3. A) Cyclic voltammetry on GCE in 0.1M LiClO4 in acetonitrile in the absence (curve 1) and in

the presence of 480mM BHT at the following scan rates: 10mV s�1 (curve 2), 25mV s�1 (3), 50mV s�1 (4),

81mV s�1 (5), and 100mV s�1 (6). B) Relationship between BHT oxidation current and t1=2.

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There are linear relationships between oxidation current and analyte concen-tration for compounds under investigation. Analytical characteristics of phenolicantioxidant oxidation are presented in Table 2. The limits of detection (LOD) andquantification (LOQ) were calculated using statistical treatment (3SDa=b) and(10SDa=b), respectively, where SDa is the standard deviation of the average arithme-tic of 10 voltammograms of the blank obtained in the same potential oxidation ofanalyte and b is the slope of the calibration curve. The LOD and LOQ values indi-cated satisfactory sensitivity of the proposed method.

Quantitative determination of phenolic antioxidants in model solutions wascarried out. The accuracy of results obtained was evaluated by the added-foundmethod (Table 3).

Analytical Application

Based on the results obtained, a voltammetric method for determination ofa-tocopherol and BHT in real samples was developed. Vegetable and lubricating oils,

Table 2. Analytical characteristics of voltammetric determination of phenolic antioxidants

Regression equation

y¼ a þbx

Analyte

LOD,

mMLOQ,

mMAnalytical

range, mM a (mA)

b � 10�3

(mA L mol�1) R

BHT 27.6 92.0 99� 1450 0.86� 0.08 17.7� 0.1 0.9999

2,20-Thiobis(6-tert-butyl-4-methylphenol)

44.3 148 200� 1600 �1.5� 0.2 21.5� 0.2 0.9997

2,6-Di-isobornyl-4-methylphenol 14.5 48.2 50� 1600 �0.5� 0.2 16.3� 0.2 0.9993

2,6-Di-tert-butyl-4-b,b-diacetylethylphenol

37.5 125 200� 1600 0. 8� 0.3 18.2� 0.3 0.9990

2,6-Di-tert-butyl-4-di-

methylaminomethyl-phenol

13.9 46.4 50� 1300 1.0� 0.1 5.3� 0.1 0.9982

2,6-Di-tert-butyl-4-(N,N-bis-b-oxyethyl-amino)methylphenol

9.6 32.0 50� 1300 0.63� 0.04 5.89� 0.05 0.9998

a-Tocopherol 18.2 60.5 64� 2090 0.61� 0.06 7.6� 0.1 0.9994

Scheme 3. Electrooxidation of a-tocopherol.

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butter, pharmaceuticals, and cosmetics have been chosen as objects of analysis. Allof them have complex matrices. Therefore, preliminary separation of components isrequired prior to analyte determination.

Due to the lipophilic nature of BHT and a-tocopherol, isolation of these com-pounds from oil samples should be accomplished by extraction with an organic sol-vent. Therefore, simple liquid-liquid extraction by acetonitrile was proposed becauseit has been used for the extraction of BHT in other matrixes such as biscuits andchewing gum (Gonzalez et al. 1998; Gonzalez, Gallego, and Valcarcel 1999; Witter2005). For this purpose, the model oil free of phenolic antioxidants was obtainedby pretreatment of oil with isopropanol (Podolina et al. 2008) and further separationof oil layer.

This oil was spiked with a known amount of BHT or a-tocopherol and furtherextraction by acetonitrile was applied. The effectiveness of extraction was controlledby coulometric titration with electrogenerated hexacyanoferrate(III) ions. Quantitat-ive recovery was obtained by single extraction for 15min at oil=extractant ratio of1:2.5 (Fig. 4). The average recovery values for BHT and a-tocopherol under theseconditions were 98.4 and 99.3%, respectively.

In order to check the compounds providing the analytical signal from theextracts of real samples, the supplementary test was performed by addition of aBHT standard solution. The increase in oxidation currents proportional to theamount of BHT added was observed at the same potential. The results for spikedsamples were in good agreement with calculated data.

Voltammetric determination of BHT and a-tocopherol in vegetable oils wasperformed (Table 4). As one can see, a-tocopherol content in vegetable oils was

Table 3. Voltammetric determination of phenolic antioxidants in test solutions

Analyte Added, mg Found, mg RSD, %

BHT 0.22 0.23� 0.01 2.8

1.54 1.542� 0.003 0.1

3.74 3.76� 0.04 0.7

2,20-Thiobis(6-tert-butyl-4-methylphenol) 0.72 0.722� 0.008 0.7

3.59 3.577� 0.007 0.2

6.1 6.07� 0.03 0.5

2,6-Di-isobornyl-4-methylphenol 0.19 0.191� 0.007 2.2

3.81 3. 8� 0.1 1.6

7.23 7.25� 0.07 0.6

2,6-Di-tert-butyl-4-b,b-diacetylethylphenol 0.64 0.637� 0.003 0.2

3.18 3.1� 0.1 1.3

6.04 6.09� 0.06 0.6

2,6-Di-tert-butyl-4-di-methylaminomethyl-phenol 0.26 0.26� 0.02 4.7

1.84 1.85� 0.02 0.7

3.95 3.94� 0.02 0.6

2,6-Di-tert-butyl-4-(N,N-bis-b-oxyethyl-amino)methylphenol 0.32 0.32� 0.02 4.6

2.26 2.26� 0.02 0.9

4.85 4.84� 0.03 0.4

a-Tocopherol 0.28 0.28� 0.02 0.4

4.82 4.8� 0.2 0.9

6.89 6.8� 0.2 0.9

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low which was caused by the technological process of oil production. a-Tocopherolloss can reach 50% depending on the processing method used. The biggest amountsare lost during deodorization (Eitenmiller and Lee 2004). The exception is brand‘‘A’’ characterized by high a-tocopherol concentration, which is explained by theextra addition of Vitamin E as a nutrient to refined oil that was marked on the labelby the producer.

The BHT is an edible antioxidant that may be added as a fortifying agent inoils and butters for protection of both vitamin A and oil from oxidative damage.This approach was confirmed by results of the tests of this study. Linseed oils andbutter under investigations showed the presence of BHT (Table 4).

Figure 4. Recovery of BHT and a-tocopherol from model oil by acetonitrile.

Table 4. Voltammetric determination of BHT and a-tocopherol in vegetable oils and butter (n¼ 5,

P¼ 0.95)

Oil type

Commercial

brand

Found

a-tocopherol, g L�1 RSD, %

Found

BHT, gL�1 RSD, %

Sunflower oil refined A 0.23� 0.01 0.8 – –

B 0.048� 0.005 0.3 – –

C 0.011� 0.003 0.2 – –

D 0.08� 0.02 1.0 – –

Sunflower oil E 0.129� 0.009 0.4 – –

Olive oil F 0.049� 0.002 0.2 – –

G 0.044� 0.006 0.3 – –

Linseed oil H 0.089� 0.006 0.4 0.023� 0.002 0.1

I <LOD 0.015� 0.001 0.1

Butter J <LOD 0.061� 0.006 0.2

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At the same time, BHT is widely used as antioxidant additive in lubricatingoils. The results of BHT voltammetric determination are presented in Table 5. Thereare oxidation steps at 0.8V on voltammograms (Fig. 5) of several oils that indicatethe presence of other antioxidant additives. This step is probably connected with zincdialkyldithiophosphates that is presend in engine oils and able to oxidize at theaforementioned potential.

Oxidation reactions play an important role in the decomposition of pharma-ceutical products and are considered to be a prime cause of product instability(Conners, Amidon, and Kennon 1986). The BHT has shown the advantage as anauxiliary component preventing oxidation of active substance in pharmaceuticals.Therefore, BHT content evaluation in pharmaceuticals is of interest. Oil solutionsof retinol esters and ointment ‘‘Mycosoral’’ have been investigated as dosage forms.

Oxidation steps at 0.92 and 1.22V were observed on voltammograms ofextracts from retinol acetate and palmitate oil solutions (Fig. 6). The first step cor-responded to oxidation of retinoic fragment, the second one corresponded to BHT

Figure 5. Cyclic voltammograms of 0.3mL engine oil extract in the absence (curve 2) and in the presence

of 129mM of BHT (curve 3) in 0.1M LiClO4 in acetonitrile (curve 1). Potential scan rate 100mV s�1.

Table 5. Voltammetric determination of BHT in lubricating oils (n¼ 5, P¼ 0.95)

Lubricating Oil Commercial brand Found BHT, gL�1 RSD, %

Engine oil K 11.7� 0.1 0.66

L 0.97� 0.03 1.60

M – –

N 1.40� 0.03 1.10

Gear oil O 0.9� 0.1 6.40

Power steering oil P 0.098� 0.003 0.20

Roughing Pump Oil Q – –

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oxidation. The difference in oxidation potentials was 0.3V which enabled the selec-tive determination of BHT without recourse of additional separation of components.

Dosage forms under investigation contain significant amounts of BHT(Table 6) that can be varied for the same type of pharmaceutical from different pro-ducers. The BHT concentration decreases during the storage and reaches the 30%loss in comparison with initial value after expiration of storage time which corre-sponds well with literature data (Finkel’shtein et al. 1977).

The use of BHT as a preservative in cosmetic formulations helps to protectagainst the deterioration of cosmetic products. It is usually added in concentrationsfrom 0.0002 to 0.5% (Lanigan and Yamarik 2002). As one can see from Table 7, cos-metic creams are characterized by a high level of BHT although there is no infor-mation on the label regarding the presence of BHT. Anti-aging cream ‘‘R’’showed 1.37% of BHT which exceeds the allowable rate. Dermatological cosmeticsare free of BHT which was confirmed by investigation results.

Figure 6. Cyclic voltammograms of 1.0mL retinol palmitate extract in the absence (curve 2) and in the

presence of 95 mM of BHT (curve 3) n 0.1M LiClO4 in acetonitrile (curve 1). Potential scan rate

100mV s�1.

Table 6. Voltammetric determination of BHT in pharmaceutical dosage forms (n¼ 5, P¼ 0.95)

Dosage form BHT concentration,mg g�1 RSD, %

Retinol palmitate oil solution (best before 2011) 0.240� 0.002a 0.9

Retinol palmitate oil solution (best before 2012) 0.350� 0.003a 1.3

Retinol acetate oil solution 2.09� 0.08b 3.7

Mycosoral 1.03� 0.04 3.4

aProducer 1.bProducer 2.

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CONCLUSIONS

The results of the present work indicate the possible use of cyclic voltammetryin the determination of sterically hindered phenolic antioxidants and a-tocopherol.The schemes of their electrochemical oxidation are proposed. The approach foranalysis of BHT and a-tocopherol in oil samples and creams with a rapid method-ology including liquid extraction with acetonitrile and voltammetric detection wasdeveloped. The proposed procedure for the control of phenolic antioxidants in realsamples is simple and reliable, which promotes its use in pharmaceutical and foodindustry laboratories as well as quality control centers.

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Table 7. Voltammetric determination of BHT in cosmetics (n¼ 5, P¼ 0.95)

Cream Commercial brand BHT content,mg g�1 RSD, %

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