Voltammetric Studies for Detection and Degradation Assessment of some Synthetic Food Dyestuffs

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1 Voltammetric Studies for Detection and Degradation Assessment of some Synthetic Food Dyestuffs I. Tartrazine - E 102 ELENA DIACU, ELEONORA-MIHAELA UNGUREANU 1 *, CORNELIA PETRONELA ENE, ALEXANDRU ANTON IVANOV University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Polizu Str., , Bucharest, Romania Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used for electrochemical characterization of Tartrazine (TAR), one of the most used synthetic food dyestuffs in view of its detection and degradation assessment from soft drinks. The voltammetric behaviour was investigated at different ph values ( ) on glassy carbon electrode. The influence of concentration, ph and scan rate on the currents and peak potentials was established. Good correlations of the electrochemical parameters of the two methods were found. Both methods are simple, very sensitive and inexpensive and can be applied to determine TAR in commercial soft drinks, containing very small concentrations of it. The possibility to assess the degradation of TAR using the voltammetric data is also discussed. Keywords: Tartrazine, cyclic voltammetry, differential pulse voltammetry, food dyestuff It is well known that food quality is closely associated with colour and flavour, these last qualities making the food more attractive for the consumer. In addition, food dyestuffs are considered as source of stability during the food processing and storage. However, not all coloured substances are suitable for the coloration of foodstuffs, being already demonstrated that some of them, especially the synthetic ones, may be dangerous from the human health point of view [1, 3]. For this reason, the synthetic food dyes are regulated, being the subject to restricted use status [4, 5], and some of them (azogeranine- E128 Red 2G and those similar to Sudan class) fall under the provisions of the zero-tolerance [6]. Tartrazine (TAR) and Sunset Yellow (SUN) are synthetic food dyestuffs, being known by consumers as additives E 102 or FD&C Yellow Number 5, and E 110 or FD&C Yellow 6, respectively. These two food colours were selected for this study because of their wide use in various soft drinks and due to their possible human health issues, such as allergy, diarrhea, vomiting, migraines, thyroid tumours, chromosomal damage and hyperactive behaviour in children [7]. In European Union these food dyestuffs are permitted under certain conditions and they are limited in use. The maximum accepted level by international and national legislation is of 100μg/mL for TAR and 50μg/mL for SUN, when they are individually considered, and no more than 100μg/mL where they are used together [4]. Therefore, the content of these dyestuffs must be followed by applying reliable methods for their precise identification and quantification in food and drinks. There are reported numerous papers on the analysis of synthetic food dyestuffs, especially by using chromatographic methods [8-17] and also by voltammetric methods [18-25]. However, only few works [26, 27] addressed the degradation of the synthetic dyes TAR and SUN and the preservation of properties for which they are used as food additives by voltammetric methods. TAR (trisodium 5-hydroxy 1-(4-sulfonatophenyl)-4-(4- sulfonatophenyl azo)-h-pyrazole-3 carboxylate) and SUN (disodium 6-hydroxy-5-(4-sulfonatophenyl azo)-2- naphthalene-sulfonate) are electroactive organic compounds mainly due to the presence of an azo - group in their molecule. The first part of our study deals with electrochemical characterisation of TAR that is shown in figure 1. Fig. 1. Chemical structure of TAR Experimental part The voltammetric experiments were conducted using glassy carbon electrode in buffered aqueous solutions of TAR at different ph values, concentrations and scan rates, according to the usual procedures [28], in order to establish its electrochemical behaviour and evaluate the optimal analytical parameters. Reagents All reagents were of analytical purity and the solutions were prepared using bidistilled water. TAR was purchased from SIGMA-Aldrich. Buffer solutions (citrate for ph = 3.0 and 4.0, acetate for ph = 5.0) were prepared according to standard procedures. Bidistilled water was obtained from an Elgastat water purification system (5 MΩ cm). Instrumentation and Procedure Electrochemical experiments were carried out using a PAR283 potentiostat in conventional three-electrode cell. A glassy carbon electrode disk (3-mm diameter from CH Instruments) working electrode, counter electrode, a platinum wire and a Ag/AgCl/aqueous saturated KCl solution were reference electrode, respectively.. All * em_ungureanu2000@yahoo.com REV. CHIM. (Bucharest) 62 No

2 potentials were referred to the reference potential (0.197V at 25 o C). Denver Instrument Model 220 ph-conductivity meter was used to measure the ph of solutions. The electrochemical characterization was performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Before each experiment the glassy carbon disk was polished with 0.2μm diamond paste, then rinsed in absolute ethanol and distilled water. CV experiments were performed with different scan rates (0.1 1V/s) for investigation of the scan rate influence. DPV experiments were recorded with pulse of 25mV for 50ms, step height of 2mV, step time 100ms, and scan rate of 20mV/s. Solutions were degassed with a stream of argon for 20 min before each measurement and kept under argon atmosphere during the entire experiment, conducted at 25 o C. Results and discussions The CV and DPV experiments have been conducted in aqueous buffer solutions as supporting electrolyte in milimolar concentrations of TAR (1-10mM), at different ph values, selected within the ph range of the real soft drinks samples, in the range 3-7. In order to establish the reversible character of the involved electrochemical processes and TAR degradation, the CV curves have been recorded at different scan rates and potential ranges. In the following section there are presented the results concerning the behaviour of TAR at ph 3, where all the electrochemical processes are best highlighted. Electrochemical characterization of TAR at ph 3 The electrochemical behaviour of TAR has been investigated in milimolar solutions of TAR in aqueous buffer solutions 0.1M as supporting electrolyte. DPV and CV studies have been performed at different concentrations of TAR, and at different scan rates and potential ranges in CV. Figure 2A shows the DPV curves for TAR at different concentrations. One anodic peak (1a) and two cathodic peaks (1c, 2c) can be observed. The peak potentials vary linearly with the logarithm of the TAR concentration (upper inset in fig. 2) and the peak currents vary linearly with the TAR concentration (lower inset in fig. 2). The corresponding equations of these linear dependences are given in table 1. We mention that the linear dependences of the peaks currents could be used in analytical determination of TAR by DPV method. The single anodic peak identified in the DPV curves could be attributed to the oxidation of TAR to its stable radical cation. After ionization the positive charge is mainly delocalized on the whole molecule. The oxidation potential of peak 1a is in the range characteristic for azo compounds. The first cathodic peak 1c identified in the DPV curves could be attributed to the reduction of TAR to its stable radical anion. After ionization, the negative charge is also delocalized on the whole molecule. The reduction potential values are also in the range characteristic for these azo compounds. It is very likely that both oxidation and reduction of TAR involve electrochemical and chemical steps. In case of TAR, at this ph value (ph=3), a second multielectron transfer can be seen as peak 2c at very negative potentials. The behaviour with two cathodic peaks is specific for this acid ph at which there is a high concentration of protons that favors further reduction of TAR molecule. The same processes are identified for TAR in the CV curves (fig. 3) with three peaks situated at potential values in agreement with those obtained by DPV. The processes are electrochemically irreversible, showing the electrochemical degradation of TAR during the potential scanning. The peak potentials vary linearly with the logarithm of the TAR concentration (upper inset in fig. 3) and the peak currents vary linearly with TAR concentration (lower inset in fig. 3 show). The equations of these dependences are also given in table 2. Obviously these linear dependences of the peaks currents could be also used in analytical determination of TAR by CV method. Figure 4 shows the CV curves on different scan rates in the domains of the first anodic and cathodic processes. Both anodic and cathodic scans are plotted on the same graph. The electrochemical reversibility of the first anodic (1a) and cathodic (1c) processes has been carefully evaluated from the CV curves obtained at different scan rates. As it can be seen these processes are all irreversible. Linear dependences of the peak potentials on the logarithm of the scan rate have been experimentally obtained (upper inset in fig. 4). Linear dependences of the peak currents (1a, 1c) on the square root of the scan rate have been obtained (lower inset in fig. 4) demonstrating a diffusion Fig. 2. DPV curves for TAR at different concentrations (mm) on glassy carbon electrode (3mm in diameter) at ph 3; upper inset - Linear dependences of the anodic peak potential (E1a) and cathodic peaks potentials (E1c, E2c) on TAR concentration logarithm (log c); lower inset - Linear dependences of the anodic (i1a) and cathodic peaks currents (i1c, i2c) on TAR concentration (c) REV. CHIM. (Bucharest) 62 No

3 Fig. 3. CV curves (v = 0.1V/s) for TAR at different concentrations (mm) on glassy carbon electrode (3mm in diameter) at ph3; upper inset - Linear dependences of the anodic peak potential (E1a) and cathodic peak potentials (E1c, E2c) on TAR concentration logarithm (log c); lower inset - Linear dependences of the anodic (i1a) and cathodic peaks currents (i1c, i2c) on TAR concentration (c) Fig. 4. CV curves in the first anodic and cathodic scans at different scan rates (V/s) on glassy carbon electrode (3mm in diameter) in a TAR solution (2 mm); upper inset - Linear dependences of the anodic peak (E1a) potential and first cathodic peak (E1c) potential on the logarithm of the scan rate (log v); lower inset - Linear dependences of the anodic (i1a) and first cathodic (i1c) peak currents on the square root of the scan rate (sqrt v) controlled processes. The values of ip/v 1/2 c ratio of around 200 and 300 μa(v/s) -1/2 (mm) -1 have been calculated for peaks 1a and 1c, respectively. In the range of investigated scan rates, and TAR concentrations the processes are irreversible. The diffusion coefficient of ~10-5 cm 2 s -1 for TAR species has been calculated using Randless-Sevcik equation for by supposing a single electron transfer. However, the lower inset in figure 4 shows that the slope of peak current dependence on the scan rate is higher (1.5 times) for 1c than for 1a. The ph influence The influence of ph on the peak potentials of the anodic and first cathodic processes has been studied from the DPV and CV curves obtained in TAR solutions in different buffer solutions. These dependences are all linear. The DPV curves show better correlation coefficients than CV curves, tables 1 4 show. Figure 5 shows the influence of ph on the peak potentials for the anodic and first cathodic processes from DPV curves for TAR. It can be seen from table 1 that, in DPV, experiments, the E1a peak dependence on ph has slope about three times lower than E1c peak (around 31mV and 84mV, respectively). The corresponding values from CV are about 10mV higher than the DPV values (around 39mV and 96mV, respectively), as normally expected. These data express the difference between the numbers of electrons involved in the corresponding electrode processes, which are not simple electrochemical reactions, but rather complex ECE processes, consisted in successive electron transfer (E) and chemical (C) steps. It can be seen from table 2 that, in DPV, i1a peak has a slope of its dependence on ph quite similar to that of i1c peak (around 8.4μA and 9.2μA, respectively). However, the corresponding values from CV are very different, about 27.6μA and 41.9μA, respectively, both higher than the DPV values, as normally expected, at each ph studied. These data confirm also the difference between the numbers of electrons involved in these complex electrode processes. From analytical point of view, the CV method offer better possibilities than DPV method for the detection of TAR. However, in the case of dye mixtures the DPV method could be a preferred choice due to the its higher selectivity. From table 3 it can be seen that, in DPV, E1a peak potential has a negative slope of its dependence in respect REV. CHIM. (Bucharest) 62 No

4 Table 1 POTENTIAL E (in V) FOR THE ANODIC PEAK 1a AND THE FIRST CATHODIC PEAK 1c ON TAR CONCENTRATION LOGARITHM (R 2 IS THE CORRELATION COEFFICIENT) Table 2 CURRENTS (in A) FOR THE ANODIC PEAK i1a AND THE FIRST CATHODIC PEAK i1c ON TAR CONCENTRATION C (in mmole L -1 ) (R 2 IS THE CORRELATION COEFFICIENT) Fig. 5. Dependences on ph for different concentrations of TAR from DPV curves: A -the anodic (E1a) and first cathodic (E1c) peak potentials; B - the anodic (i1a) and first cathodic (i1c) peak currents Table 3 POTENTIAL E (in V) FOR THE ANODIC PEAK E1a AND THE FIRST CATHODIC PEAK E1c ON ph AT DIFFERENT TAR CONCENTRATION (c) FROM DPV AND CV EXPERIMENTS. (R 2 IS THE CORRELATION COEFFICIENT) Table 4 CURRENTS (in A) FOR THE ANODIC PEAK i1a AND THE FIRST CATHODIC PEAK i1c ON ph AT DIFFERENT TAR CONCENTRATION (c) FROM DPV AND CV EXPERIMENTS. (R 2 IS THE CORRELATION COEFFICIENT) to ph, a value of about -33mV, showing that the electrochemical oxidation rate occurs slower at acidic ph, and therefore, the TAR degradation is slowed down in acidic media. The slope value of E1a from DPV experiments is about three times smaller than that for E1c peak. The corresponding values from CV have about the same slopes as DPV (around 33mV and 97mV, respectively). Also, the data analysis expresses the difference between the numbers of electrons involved in these electrode processes, which are not simple electrochemical reactions, but complex ECE processes. Analyzing the data from table 3 it can be observed that for E1a and E1c, the two voltammetric methods present similar dependences on the solution ph. Consequently, to determine TAR content in a drink sample, REV. CHIM. (Bucharest) 62 No

5 it is recommended to perform the measurements at potential E1a, because it is more stable than E1c potential in respect with the ph variation. Table 4 displays the results concerning ph influence on peak currents for the anodic peak (1a) and first cathodic peak (1c). It can be seen that there are some variations with ph of solution. For instance, in DPV, the slope is of around 0.82μA and 3.8μA, for i1a and i1c, respectively. The corresponding values from CV are of about 3.3μA and 5.4μA, respectively, being a little bit higher than the DPV values. This leads to the conclusion that i1a value from DPV can be used as analytical signal for TAR analysis. Conclusions The voltammetric study of TAR has evidenced its electrochemical behaviour in both anodic and cathodic potential domains. The TAR characterization by CV and DPV methods allowed the assessment of its electrochemical properties and the evaluation of parameters which can be further exploited in the analysis of the dyestuffs present in soft drinks by one of this simple, sensitive and inexpensive method. The experimental data show that both methods are suitable for TAR detection, although but DPV method is more sensitive. This electrochemical study shows that TAR degradation comes during the potential scanning, all electrochemical processes being irreversible. Also, the electrochemical oxidation occurs slower at acidic ph, and therefore, the TAR degradation is slowed down in acidic media. Our next study plans to establish the analytical performance parameters for TAR determination in order to be applied in monitoring of commercial soft drinks using. References 1.GHORPADE, V.M., DESHPANDE, S.S., SALUNKHE, D.K., Food colours. In Food Additive Toxicology (MAGA J.A., TU A.T. Eds), p. 179, Marcel Dekker, New York. 2.GOLKA, K., KOPPS, S., MYSLAK, Z. W., Toxicology Letters. 2004, 151, no. 1, p ALI, M.A., BASHIER S.A., Food Additives & Contaminants. 2006, 23, p EUROPEAN COMMISSION DIRECTIVE European Parliament Council Directive 94/36/EC. Official Journal of the European Communities. L 237/13. 5.EUROPEAN COMMISSION DIRECTIVE 2006/33/EC, European Parliament, Official Journal of the European Communities. L 82/10. 6.Commission Regulation (EC) 2007/884/EC, European Parliament, Official Journal of theeuropean Communities, L 195/8. 7.MCCANN, D., BARRETT, A., COOPER, A., CRUMPLER, D., DALEN, L., GRIMSHAW, K., KITCHIN, E., LOK, K., PORTEOUS, L., PRINCE, E., SONUGA-BARKE, E., WARNER, JO., STEVENSON, J., Lancet, 370, 2007, p ZHANG, J., GAO, N., ZHANG, Y., Analytical Letters. 40, 2007, no. 16, p DIACU, E., ENE, C. P., Rev Chim. (Bucharest). 60, no. 8, 2009, p ERTAS, E., OZER, H., ALASALVAR, C., Food Chemistry. 105, 2007, p AITINOZ, S.,TOPTAN, S., J. of Food Composition and Analysis , 6, p AL-DEGS, Y. S., Food Chemistry. 117, 2009, 3, p GARCÍA-FALCÓN, M. S., SIMAL-GÁNDARA, J., Food Control. 16, 2005, no. 3, p YOSHIOKA, N., ICHIHASHI K., ALGHAMDI, A.H., Food Analytical Methods, 74, 2008, p DIACU, E., ENE, C. P., Rev. Chim. (Bucharest). 61, 2010, no. 12, p MINIOTI K, S., SAKELLARIOU, C. F., THOMAIDIS, N. S., Anal. Chem. Acta. 583, 2007, p TATEO, F., BODONI, M., GALLON, F., Czech J. Food Sci. 28, 2010, no. 5, p BERZAS NEVADO, J. J., RODRIGUEZ FLORES, J., VILLASENOR LLERENA, M. J., Fresenius J. Anal. Chem. 1997, 357, no. 7, p DUBENSKA, L., LEVYTSKA, H., POPERECHNA, N., Talanta. 54, 2001, p LOPEZ-DE-ALBA, P. L., LOPEZ-MARTINEZ, L., DE-LEON- RODRIGUEZ, L. M., Electroanalysis. 14, 2002, 3, p ALGHAMDI, A. H., J. of AOAC International, 88, 2005, 5, p GULABOSKI R., PEREIRA, C. M., Electroanalytical Techniques and Instrumentation in Food Analysis. in Handbook of Food Analysis Instrumentses, chap. 17,.Ed. Semih Otl, Bosa Roca, WANGZ, L.H., HUANG, S.J., Russ. J. Electrochem., 46, 2010, 12, p WROLSTAD, R.E., SMITH, D.E., Color Analysis, in Food Analysis, Fourth Edition. Food Science Text Series., p. 573, Ed Nielsen S., New York, SONG, Y. Z., XU, J. M., LV, J. S., ZHONG, H., YE, Y., XIE, J. M., Indian. J. Chem., 49A, 2010, p THAKUR, B. R., ARYA, S. S., Food, 37, 1993, p TURTURA, G.C., MINGUZZI, A., Zentralbl. Mikrobiol. 147, 1992, p UNGUREANU, E.-M., PILAN, L., MEGHEA, A., LE FLOCH, F., SIMONATO, J.-P., BIDAN, G., Rev. Chim. (Bucharest), , no. 4, p. 400 Manuscript received: REV. CHIM. (Bucharest) 62 No