A.?B.?Lever

Bard

Baumgartner

Bechtold

Brown

Camacho

Denney

Gasana

Govaert

Kilroy

Leznoff

Ozkaya

Ozoemena

Serjeant

Shaikh

Wael

Westbroek

Zagal

English

De Wael, Karolien

Westbroek, Philippe

Temmerman, Eduard

Ghent University Academic Bibliography

Full PaperElectrocatalytic Oxidation of Dithionite at aCobalt(II)tetrasulfonated Phthalocyanine and5,10,15,20-Tetrakis-(4-sulfonatophenyl)porphyrin Cobalt(II)Modified Gold Electrode in Alkaline SolutionKarolien De Wael,a Philippe Westbroek,*b Eduard Temmermanaa Ghent University, Department of Analytical Chemistry, Krijgslaan 281 S12, B-9000 Gent, Belgiumb Ghent University, Department of Textiles, Technologiepark 907, B-9052 Gent, Belgium*e-mail: philippe.westbroek@Ugent.beReceived: January 7, 2004Final version: May 17, 2004AbstractIn this paper it is shown that the charge transfer kinetics of the oxidation of sodium dithionite at a gold electrode inalkaline solution can be increased by modifying this electrode with cobalt(II)tetrasulfonated phthalocyanine, sodiumsalt (CoTSPc) or 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin cobalt(II), tetrasodium salt (CoTSPor). Electro-deposition of these catalysts is discussed as well as the oxidation of sodium dithionite at these immobilized catalysts.Despite an observed acceleration in the charge transfer kinetics, sodium dithionite is still oxidized irreversibly.However, the improvement in kinetics has beneficial consequences in the field of electroanalysis of sodium dithionite.Comparison of the oxidation at bare gold and modified gold electrodes showed that with improved charge transferkinetics at the modified electrodes a higher sensitivity and selectivity and a much lower detection limit is obtained.Keywords: Gold, Electrocatalysis, Sodium Dithionite, Oxidation1. IntroductionSodium dithionite finds several applications as a reducingagent, such as in the reduction of textile dyes for dyeingprocesses. This reduction is crucial for the process becausethe oxidized form of most of these dyes (indigo, indan-threnes, ...) is virtually insoluble in aqueous solutions.However, sodium dithionite is relatively unstable and reactswith oxygen, which may cause a fast decline of its concen-tration during processing [1 – 2]. This can have a seriousinfluence on the actual reduced dye concentration and onthe quality and reproducibility of the dyed textile product.In the past, Westbroek et al. devoted research to thedevelopment of sensors to measure concentrations ofchemical components important in the textile industry,such as hydrogen peroxide in bleaching [3 – 6], sodiumdithionite [7 – 10] and sulfite [8, 10] in dyeing as well as thedyes themselves such as indigo [10] and indanthrenes [7, 9].They focussed mainly on voltammetric and amperometricdetection of these components using rotating disc [3 – 9],ultramicro [5] and wall-jet electrodes positioned in flowthrough cells [10]. To support the insight in the analyticaldetermination of the concentration of the above mentionedchemicals they also investigated the mechanism [11 – 13],kinetics [11 – 14] and reagent transport properties [8, 1 – 14]of the oxidation/reduction reactions used for their analyticaldetection.The reason to start the abovementioned investigations onsodium dithionite and associated dye sensor developmentwas the fact that drawbacks of previously developedmethods prevented their widespread use in practice. Severalmethods were described in literature to determine theconcentration of sodium dithionite, sulfite [15 – 20] and/orindigo [20 – 22]. The sodium dithionite concentration can bedetermined by volumetric titration with jodium [15 – 17] orwith K3[Fe(CN)6] [20]. The endpoint detection of thesetitrations can be done visually [17 – 19] or potentiometri-cally [18 – 19]. Besides a fotometric and spectrofotometricreflectance method [21 – 22], indigo can be determined bypotentiometric titration [20]. The difference in standardpotential of indigo and sodium dithionite is about 150 mV,which allows determination of both concentrations in thesame titration [20].In this paper the authors will show that the detection ofsodium dithionite on bare gold electrodes can be improvedby electrocatalysis using a cobalt(II)tetrasulfonatedphtha-locyanine, sodium salt (CoTSPc) (Fig. 1a) or a 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin cobalt(II), tetraso-dium salt (Fig. 1b) (CoTSPor) as catalyst.The choice for these catalysts was based on their welldescribed behavior in literature (but not in combinationwith gold), the knowledge in electrodeposition of thesecompounds on gold surfaces and their good water solubility,making it possible to perform electrodeposition in aqueous(read environmental friendly) solutions.263Electroanalysis 2005, 17, No. 3  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.2004030922. ExperimentalA saturated calomel reference electrode (SCE) with twocompartments from Radiometer and a carbon counterelectrode were used. The working electrodes were goldelectrodes from BAS (diameter 1.6 mm) and were pre-treated by mechanical and electrochemical polishing. Be-fore its first use the electrode surface was scoured briefly onSiC-emery paper 1200 grit to obtain a fresh surface. Tosmoothen this relatively rough surface it is further subjectedto sequential polishing on polishing cloth covered withalumina (Buehler) powder of 1, 0.3 and 0.05 mmparticle sizefor respectively 5, 10 and 20 minutes. To remove anyadherent Al2O3 particles the electrode surface was rinsedthoroughly with doubly deionized water and cleaned in anultrasonic bath (Branson 3210) for 2 minutes. Finally theelectrode was pretreated electrochemically by scanning it ina buffer solution, pH 12, between 1.2 and 0.6 V (vs. SCE)until 5 subsequent scans were identical.A PGSTAT20 potentiostat fromECOChemie controlledby GPES 4.9 software package running on a Pentium IIcomputer (Eknadata) was used to record the voltammetriccurves. Measurement of the pH of the solution was donewith aOrionBenchtoppH-meterModel 420A.TheCoTSPcsodium salt was purchased from Midcentury (Posen, Il,USA). The buffer solution of pH 12 (Na2HPO4/NaOH) andsodium dithionite were purchased from Riedel-de Hae¨n.CoTSPor tetrasodium salt was obtained from PorphyrinFig. 1. Molecular structure of cobalt(II)tetrasulfonatedphthalocyanine, sodium salt (a) and 5,10,15,20-tetrakis-(4-sulfonatophenyl)por-phyrin cobalt(II), tetrasodium salt (b).264 Karolien De Wael et al.Electroanalysis 2005, 17, No. 3  2005 WILEY-VCH Verlag GmbH&Co. KGaA, WeinheimSystems (Lbeck, Germany). Before each experiment, purenitrogen was bubbled through the cell solution for 20 mi-nutes.3. Results and Discussion3.1. Electrodeposition of the CatalystsPrior to each deposition experiment a tenfold scanning ofthe gold electrode was performed between 1.2 and 0.6 V(vs. SCE) in a pH 12 buffer solution. The deposition itselfconsists of scanning the electrode for about 100 scans in thesamepotentialwindow in apH 12buffer solution containing8 103 mol L1 of CoTSPc or CoTSPor. Figure 2 shows thefinal scans of the deposition of CoTSPc (curve 1) andCoTSPor (curve 2).It can clearly be seen that for both catalysts, peaks due tometal ion oxidation/reduction occur at potentials around0.3and0.2 V(vs. SCE)andaround0.9 and1.1 V(vs. SCE) forringactivity.Additionally forCoTSPcalsoanoxidationpeak isdetected around 0.4 V (vs. SCE) (curve 1). The intensity ofthese peaks increased gradually with scanning of the electrodein the buffer containingCoTSPc orCoTSPuntil a stable curvewas obtained, curves 1 and 2 in Figure 2.The reversibly behaving redox system around 0.2 V (vs.SCE) can be attributed to Co(III)/Co(II) [23 – 26] andCo(II)/Co(I) [26 – 28] corresponds to the quasi reversiblesystem around 0.3 V (vs. SCE). Reduction peaks around0.9 and 1.1 V (vs. SCE) correspond to ring reduction ofCo(II)TSPc at Co(II)TSPc modified gold surfaces as con-firmed by literature results [26].A hypothesis for the electrodeposition of Co(II)TSPc atgold electrode surfaces has been formulated [26] based onelectrochemical observations by correlating the evolution intime of the different peaks observed as a function of scannumber and Co(II)TSPc concentration in solution. Peaksdue to Co(III)/Co(II) and Co(II)/Co(I) redox systems wereindentified and ring oxidation/reduction peaks were ob-served. In addition, some of these peaks showed electro-catalytic activity for respectively reduction or oxidation ofCo(II)TSPc at the Co(II)TSPcmodified gold electrode. Themechanism of deposition was found to be a multistepprocess where running through the entire anodic andcathodic potential window is an important condition, initialdeposition is controlled by kinetics and reorganization ofthe initially formed layer occurs as a function of time. Onlywhen the procedure given above final voltammetric curvesare obtained as shown in Figure 2. These curves are suitablefor further work according to the electrocatalytic oxidationof sodium dithionite.3.2. Electrocatalysis with Modified Gold ElectrodesTowards Sodium DithioniteCommon possible steps in an overall electrochemicalreaction are electron transfer, chemical reaction, transportand ad(de)sorption steps, the slowest one being ratedetermining.It is known that depositing a catalyst at the surface of anelectrode can increase the reaction rate, in general byaccerelerating the charge transfer or chemical reactionkinetics. This means that if improvement of the overallelectrochemical reaction rate is the goal, such a kinetic stepshould be rate determining at the unmodified electrode.This also means that for redox systems behaving reversiblyat the unmodified electrode, electrocatalysis is not usefulbecause for reversible systems transport controls the overallrate of the reaction at all potentials.Fig. 2. Current-potential curves recorded at CoTSPc (curve 1) and CoTSpor (curve 2) modified gold electrodes in pH 12 buffersolution containing 8 103 mol L1 CoTSPc (curve 1) or CoTSpor (curve 2).265Electrocatalytic Oxidation of DithioniteElectroanalysis 2005, 17, No. 3  2005 WILEY-VCH Verlag GmbH&Co. KGaA, WeinheimDespite the fact that for the oxidation of sodiumdithionite at unmodified gold electrodes well-definedvoltammetric waves are obtained [12], the reaction stillbehaves irreversibly [12 – 14]. Therefore it is in principlepossible to improve the oxidation rate of this reaction byusing an appropriate catalyst immobilized at the goldelectrode surface.In Figure 3 cyclic voltammetric curves are shown of theoxidation of sodium dithionite at a gold electrode (curves 1,4), at aCoTSPcmodified gold electrode (curves 2, 5) and at aCoTSPor modified gold electrode (curves 3, 6) for twodifferent sodium dithionite concentrations. Several obser-vations indicate electrocatalytic behavior. First the voltam-metric waves are shifted towards less positive potentials.This means that less overpotential (or generally activationenergy) should be applied to the system because of thepresence ofCoTSPc that provides an alternative, less energydemanding, oxidation reaction path for sodium dithionite.Secondly, the slope of the inclining part of the wave is higherat the modified electrodes, revealing faster charge transferkinetics. Thirdly, the shape of the voltammetric wave isbetter defined (more peak shaped wave) and the peakcurrents are higher compared to the peak currents obtainedat an unmodified electrode. Finally, experiments withFig. 3. Voltammetric curves of sodium dithionite at a bare gold electrode (curves 1 and 4), at a CoTSPc modified gold electrode (curves2 and 5) and at a CoTSPor modified gold electrode (curves 3 and 6), in pH 12 buffer solution containing 9.85 104 (curves 1, 2 and 3)and 1.46 103 mol L1 (curves 4, 5 and 6) of sodium dithionite. T¼ 298.0 K; v¼ 50 mV s1.Fig. 4. Oxidation of sodium dithionite at a CoTSPc modified gold electrode in a pH 12 buffer solution containing 1) 9.85 104,2) 1.46 103, 3) 1.93 103, 4) 2.39 103 and 5) 2.84 103 mol L1 of sodium dithionite. T¼ 298.0 K; v¼ 50 mV s1.266 Karolien De Wael et al.Electroanalysis 2005, 17, No. 3  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheimvarying concentrations of sodium dithionite at gold electro-des and CoTSPc modified gold electrodes (Fig. 4) showedthat the peak current shift towards more positive potentialsis much more expressed at unmodified gold electrodes thanis the case at the modified ones. This again indicates that forthe modified electrodes the influence of the charge transferkinetics in the overall oxidation rate is smaller. Important tonote is that the oxidation of sodium dithionite at themodified gold electrodes is still behaving irreversibly, socharge transfer kinetics still play a role in the overall rate,especially at small overpotentials.Another remark is the relatively mild electrocatalyticeffect resulting in a shift of the wave of 20 mV and anincrease of the peak currents by approximately 17%. Thiscan be explained by the fact that the dithionite oxidation atbare gold occurs with relatively fast kinetics (well-defined,transport controlled peak), despite the irreversible behaviorof this reaction. Therefore, it may be clear that it is notpossible to improve much on the kinetics. This partlyexplains the relatively mild electrocatalytic effect ofCoTSPc. Another reason is that the oxidation of dithioniteis performed at Co(III)TSPc, a species that first needs to beformed at the electrode surface. FromFigure 2 it can be seenthat its formation is complete around 0.1 V (vs. Ag jAgCl),the same potential where the oxidation of dithionite atCoTSPc starts to occur.Electrocatalysing the oxidation of sodium dithionite hasadvantageous consequences for its analytical application,especially in sensor development to monitor the sodiumdithionite concentration during processing. Calibrationplots obtained at gold electrodes and CoTSPc modifiedgold electrodes (Fig. 4) are shown in Figure 5. The peakcurrents of these calibration plots are the net peak currents,obtained by subtracting the background current recorded atE¼ 0 V (vs. SCE) from the experimental peak current.Notethat this background current is dependent on the dithioniteconcentration (see Fig. 4) for bothmodified andunmodifiedelectrodes.However, repetition of the concentration studiesshowed that this current is very reproducible for themodified electrodes,which is not the case for theunmodifiedones. This explains why the error margins (Fig. 5) forunmodified electrodes were relatively high, especially forsmall concentrations.The large difference in error margin observed at the baregold and modified gold electrode also has its consequenceson the detection limit. Taking into account the criterion thatthe detection limit corresponds to twice the standarddeviation results in a detection limit of about 2 104 molL1 for the unmodified gold electrode, which was also foundin literature [10], and 8 106 mol L1 for the modifiedelectrode. Asmentioned above, this is mainly due to amuchmore stable and reproducible background current.FromFigure 5 it can also be seen that the sensitivity of thedetection of sodium dithionite has improved on themodified gold electrode because a higher slope is obtainedfor the calibration plot.Finally it can be observed from the curves in Figure 3 thatthe second oxidation peak of dithionite (due to oxidation ofsulfite to sulfate [7, 8, 10, 12, 13]) does not shift to lesspositive potentials at modified gold electrodes. In otherwords this reaction is not electrocatalyzed. Thismeans that abetter peak separation will be obtained at the modifiedelectrode or a higher selectivity for dithionite will beobtained.4. ConclusionsIt is clearly shown in this paper that CoTSPc or CoTSPormodified gold electrodes electrocatalyze the charge transferkinetics of the oxidation of sodium dithionite. This isbeneficial for the analytical detection of sodium dithionitebecause it improves sensitivity, selectivity and detectionlimit. This is important for industrial applications because amore precise and accurate monitoring of sodium dithioniteconcentration during processing will lead to amore efficientFig. 5. Calibration plot between peak current of the sodium dithionite oxidation reaction and its concentration at a bare gold electrode(plot 1) and at a CoTSPc (plot 2) and CoTSPor (curve 3) modified gold electrode. T¼ 298.0 K.267Electrocatalytic Oxidation of DithioniteElectroanalysis 2005, 17, No. 3  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheimuse of the chemicals. This makes it possible that lesschemical will be consumed and that waste water from theprocess is less heavily loaded with hazardous compounds.5. References[1] F. Camacho, P. Paez, C. Jimenez, M. Fernandez, Chem. Eng.Sci. 1997, 52, 1387.[2] A. A. Shaikh, S. M. Zaidi, J. Chem. Technol. Biotechnol.1993, 56, 139.[3] P. Westbroek, E. Temmerman, P. Kiekens, Anal. Commun.1998, 35, 21.[4] P. Westbroek, E. Temmerman, P. Kiekens, Anal. Chim. Acta1999, 385, 423.[5] P. Westbroek, B. Van Haute, E. Temmerman, Fres. J. Anal.Chem. 1996, 354, 405.[6] P. Westbroek, E. Temmerman, P. Kiekens, F. Govaert, J.Autom. Chem. 1998, 20, 185.[7] F. Govaert, E. Temmerman, P. Westbroek, Anal. Commun.1998, 35, 153.[8] E. Gasana, P. Westbroek, E. Temmerman, H. P. Thun, Anal.Commun. 1999, 36, 387.[9] F. Govaert, E. Temmerman, P. Kiekens, Anal. Chim. Acta1999, 385, 307.[10] E. Gasana, P. Westbroek, E. Temmerman, H. P. Thun, P.Kiekens, Anal. Chim. Acta 2003, 486, 73.[11] P. Westbroek, E. Temmerman, J. Electroanal. Chem. 2000,482, 40.[12] P. Westbroek, J. De Strycker, K. Van Uytfanghe, E. Temmer-man, J. Electroanal. Chem. 2001, 516, 83.[13] E. Gasana, P. Westbroek, K. De Wael, E. Temmerman, K. DeClerck, P. Kiekens, J. Electroanal. Chem. 2003, 553C, 35.[14] E. Gasana, P. Westbroek, E. Temmerman, H. P. Thun, F.Twagiramungu, Electrochem. Commun. 2000, 2, 727.[15] L. Brown, L. Szekeres, Talanta 1979, 26, 414.[16] W. P. Kilroy, Talanta 1979, 27, 343.[17] W. P. Kilroy, Talanta 1983, 30, 419.[18] E. P. Serjeant, Potentiometry and Potentiometric Titrations,Wiley, Chichester 1984.[19] A. J. Bard, R. Parsons, J. Jordan, Standard Potentials inAqueous Solution, Marcel Dekker, New York 1985.[20] U. Baumgartner, Mell. Textilber. 1987, 68, 189.[21] R. C. Denney, R. Sinclair, Visible and Ultraviolet Spectro-scopy, Wiley, Chichester 1987.[22] T. Bechtold, E. Butscher, O. Bobleter, Textil Praxis Int. 1992,6, 44.[23] K. Ozoemena, P. Westbroek, T. Nyokong, J. Porphyr.Phthalocyanine 2002, 6, 98.[24] A. B. Lever, A. Milaeva, G. Speier, in Phthalocyanines:Properties and Applications (Eds: C. C. Leznoff,A. B. Lever), VCH, New York 1989.[25] A. R. Ozkaya, E. Hamuryudan, Z. A. Bayir, O. Bekaroglu, J.Porph. Phthal. 2000, 4, 689.[26] K. De Wael, P. Westbroek, E. Temmerman, J. Electroanal.Chem., in press.[27] C. C. Leznoff and A. B.P. Lever, Phthalocyanines: Propertiesand Applications, Vol. 3, VCH, Weinheim 1993.[28] K. Ozoemena, P. Westbroek, T. Nyokong, Electrochem.Commun. 2001, 3, 529.[29] J. Zagal, R. K. Sen and E. Yeager, J. Electroanal. Chem. 1977,83, 207.268 Karolien De Wael et al.Electroanalysis 2005, 17, No. 3  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

Electrocatalytic oxidation of dithionite at a cobalt(II)tetrasulfonated phthalocyanine and 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin cobalt(II) modified gold electrode in alkaline solution

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Electrocatalytic Oxidation of Dithionite at a Cobalt(II)tetrasulfonated Phthalocyanine and 5,10,15,20-Tetrakis-(4-sulfonatophenyl)porphyrin Cobalt(II) Modified Gold Electrode in Alkaline Solution

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Electrocatalytic oxidation of dithionite at a cobalt(II)tetrasulfonated phthalocyanine and 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin cobalt(II) modified gold electrode in alkaline solution

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