Determination of S(IV) Oxoanions at Poly[Ru(5-NO2-Phen)2Cl] Tetrapyridylporphyrin Glassy Carbon...

Determination of S(IV) Oxoanions at Poly[Ru(5-NO2-Phen)2Cl]Tetrapyridylporphyrin Glassy Carbon Modified Electrode

Paulina Dreyse,a Diego Quezada,a Jessica Honores,a Mar�a Jesffls Aguirre,b Leonora Mendoza,b Betty Matsuhiro,b

Diego Villagra,c Mauricio Isaacs*a

a Facultad de Qu�mica, Pontificia Universidad Cat�lica de Chile, VicuÇa Mackenna 4860, Macul, Santiago, Chileb Departamento de Qu�mica de los Materiales, Facultad de Qu�mica y Biolog�a, Universidad de Santiago de Chile, Avenida

Libertador Bernardo O�Higgins 3363, Casilla 40, Correo 33, Santiago, Chilec Departamento de Qu�mica, Facultad de Ciencias, Universidad de Tarapac�. Avenida General Vel�squez 1775, Casilla 7-D, Arica,

Chile*e-mail: [email protected]

Received: January 17, 2012;&Accepted: May 29, 2012

AbstractGlassy carbon electrodes modified with conducting polymers of Ni(II), Zn(II) and metal free tetraruthenated por-phyrin were evaluated for reduction and oxidation processes of S(IV) oxoanions in Na2SO3/water-ethanol at pH 1.0and 3.5, showing electrocatalytic activity. A Ni(II) film was able to reduce the S(IV) oxoanions selectively in pres-ence of high concentration of gallic acid. The Ni(II) film was also used as an amperometric sensor toward S(IV) ox-oanions reduction in white wine samples showing a detection and quantification limit of 1.40 mg L�1 and4.68 mgL�1, respectively. These results are promising for the electrochemical determination of S(IV) using conduct-ing polymers from these macrocycles.

Keywords: Sulfite, Tetraruthenated porphyrins, Conducting polymers, Electrocatalysis, Modified electrodes

DOI: 10.1002/elan.201200038

1 Introduction

Sulfite is a compound used as a preservative in an exten-sive variety of foodstuff because this anion prevents theenzymatic activity and inhibits the growth of microorgan-isms [1]. But this reagent, in high concentrations, mayproduce adverse reactions in asthmatic individuals, forthis reason since 1986, the Food and Drug Administrationhas required the labeling of products that contain morethan 10 mg L�1 of sulfite in foods or beverages, for exam-ple in wines [2, 3]. When sulfite is used in high concentra-tions in wine, its organoleptic properties can be altered;therefore its level must be carefully controlled [4].

At present, several analytical methods are used in thedetermination and quantification of S(IV) oxoanions inwines. Ripper�s method is one of them, which consists ina classic volumetric process where SO3

2� is titrated with I2

[5]. Another method, called Monier-Williams, is based inacid-base volumetric titrations [6, 7]. In spite of the exten-sive utilization of these methods, they underwent severaldrawbacks because the analyses are time consuming andthe results would eventually present precision and accura-cy problems [8]. In this context, the use of electrochemi-cal methods appears as a feasible alternative for the de-velopment of new techniques of quantification. Some ofthese methods consist in the detection of SO2 by poten-tiometric techniques using gas-diffusion electrodes inboth static and flow injection analysis [9–13].

Wine composition involves the presence of organiccompounds such as, anthocyanines, poly-saccharides,tanins, catequins, flavonoids phenolic acids among others,all of them have antioxidant properties, presenting lowoxidation potentials that eventually mask the oxidationwave of sulfite when this analyte is measured by directelectrochemical methods [14]. For this reason, it is desira-ble to find an electrocatalyst able to reduce sulfite be-cause this process takes place at negative potential wherethe antioxidant compounds do not show any interference.

On the other hand, the electrochemical procedures thatinvolve the use of modified electrodes with macrocycliccomplexes containing transition metals are good candi-dates for the detection of S(IV) oxoanions. Depending onthe electrocatalyst, these electrodes can promote chargetransfer between the coordinated analyte and the activecenter in the macrocycle (generally metal center), im-proving the catalytic activity and selectivity [15–19]. Ithas been demonstrated that in alkaline media, glassycarbon (GC) modified with metaloporphyrins displayscatalytic activity in the oxidation of S(IV) oxoanions [20].Ni(II) protoporphyrin, as sensor molecule, shows thata polymeric arrangement of this species generates a mono-layer distribution allowing sulfite detection in both staticand flow injection analysis [21,22].

Under this context, tetrapyridylporphyrins (TPyP) co-ordinated with four Ru(II) pyridyl complexes in the pe-

Electroanalysis 2012, 24, No. 8, 1709 – 1718 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1709

Full Paper

riphery of the macrocycle are particularly attractive be-cause they display unusual electrocatalytic [23], electroa-nalytic [24,25], photophysic [26] and photoelectrochemi-cal [27–29] properties. These porphyrins are called tetrar-uthenated porphyrin (TRP) and have been used inHSO3

� detection by reduction and oxidation reactions[30,31]. In particular, Co(II) meso-tetra(4-pyridyl)por-phyrin (CoTPyP), coordinated in the periphery with four[Ru(bpy)2Cl]+ (bpy: 2,2’-bipyridyl) complexes, were ad-sorbed onto GC electrode surfaces, using a Nafion film[30,31]. These modified electrodes show catalytic activityin water-ethanol solutions for oxidation and reduction ofHSO3

� species. [30,31].Another methodology used to obtain stable electro-

chemical sensors is the preparation of conductive films byelectropolymerization [32]. Some electropolymerizablefunctional groups in the structure of polypyridyl and mac-rocyclic complexes are: 5-NH2-phen (phen: phenanthro-line) [33], 5,6-NH2-phen [34] and 5,5’-NH2-bpy [35]. Re-cently, the role of nitro functional groups as an electropo-lymerization precursor in coordination compounds con-taining transition metals has been reported [36,37].

In this paper we propose a potential electrochemicalmethod for detection and quantification of S(IV) oxoanions,using modified GC electrodes with tetraruthenated porphyr-ins obtained by the coordination of four [Ru(5-NO2-phen)2Cl]+ moieties to Ni(II), Zn(II) and metal free TPyP.The catalytic activity of the polymeric films for oxidationand reduction of S(IV) oxoanions was evaluated at pH 1.0and 3.5, in wine model solution and white wine samples.

2 Experimental

2.1 Materials

All the reagents and solvents were of analytical grade orbetter and were used without further purification. The re-agents (suppliers) are: 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphyrin (H2TPyP), 1,10-phenanthroline mon-ohydrate, sodium hydroxide, silver hexafluorophosphateand nickel(II) acetate tetrahydrate, 3,4,5-trihydroxibenzo-ic acid (gallic acid) (Sigma-Aldrich); lithium chloride(Fisher); ruthenium(III) chloride trihydrate (PressureChemical Co.); zinc(II) acetate dihydrate (Merck);sodium perchlorate monohydrate (Fluka); tetraethylam-monium perchlorate (TEAClO4) (G. Frederick SmithChemical Company); sodium sulfite, tartaric acid(Riedel-deHa�n). The solvents (suppliers) are: 2,2,2-tri-fluoroethanol, perchloric acid (Sigma-Aldrich); absoluteethanol (Riedel-deHa�n); N,N-dimethylformamide(DMF) (J. T. Baker); nitric acid, sulfuric acid, chloroform,acetonitrile (ACN), methanol (MeOH), acetone, diethylether and glacial acetic acid (Merck).

2.2 Synthesis

5-NO2-phen ligand [38], Ru(5-NO2-phen)2Cl2·2H2O com-plex [39], Zn(II), Ni(II) and metal free TPyP were pre-

pared following the methods reported in the literature[36–37].

2.3 Measurements

All the electrochemical measurements were performedwith a CH Instruments Potentiostat 760C, a GC workingelectrode (r=1.5 mm, CH Instruments), a saturated Ag/AgCl reference electrode and a Pt wire counter electrode.All the potential values were measured using a Ag/AgClreference electrode.

2.3.1 Modified Working Electrode

Polymeric films were grown by cyclic voltammetry (CV)using samples of 1 mM of tetraruthenated porphyrins ofNi(II), Zn(II) and metal free (MTRP)/DMF with 0.1 MTEAClO4, during 50 potentiodynamic cycles between�1.20 and 1.50 V at scan rate of 0.5 V s�1. After polymeri-zation, the Poly-MTRP-GC modified electrode wasrinsed with DMF, ethanol and then with doubly distilledwater.

2.3.2 Electrocatalytic Process of S(IV) Oxoanion

The reduction and oxidation of S(IV) oxoanions wasstudied by CV with Poly-MTRP-GC modified electrodesin a 126 mg L�1 Na2SO3 in wine model solution (WM Sol:ethanol:H2O 12% v/v, 5 � 10�2 M tartaric acid, 0.1 MNaClO4, at pH 1.0 adjusted with perchloric acid and atpH 3.5 with 6 M NaOH, respectively) between 0.20 V and�0.80 V for the reduction process, and between 0 V and1.20 V for the oxidation process, at 100 mVs�1.

2.3.3 Study of the Potential Scan Rate

Experiments of the variation of the peak current asa function of the potential scan rate were carried out byCV with Poly-NiTRP-GC modified electrodes at126 mg L�1 Na2SO3 with WM Sol at pH 1.0 and 3.5, be-tween 0.20 V and �0.80 V (reduction) and between 0 Vand 1.20 V (oxidation), varying the scan rate from 5 to1200 mVs�1.

2.3.4 Interference Studies

The presence of polyphenols as interfering species wasevaluated by CV. The experiment was carried out at Poly-NiTRP-GC modified electrodes at 126 mg L�1 Na2SO3 inWM Sol at pH 1.0 and 3.5 and 1000 mg L�1 gallic acid be-tween �0.20 V and �0.80 V (reduction) and between 0 Vand 1.20 V (oxidation) at 100 mVs�1.

2.3.5 Amperometry Experiments in Wine Model Solution

Amperometry experiments were evaluated with bare GCand Poly-NiTRP-GC electrodes at �0.57 V with WM Sol/

1710 www.electroanalysis.wiley-vch.de � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 8, 1709 – 1718

Full Paper P. Dreyse et al.

http://www.electroanalysis.wiley-vch.de

1000 mg L�1 gallic acid at pH 1.0 adding aliquots of 50 mLof 27.8 mM Na2SO3 each 50 s.

2.3.6 Amperometry Experiments in Real Wine Samples

According to a modified protocol described in the litera-ture [40], amperometry experiments were evaluated withbare GC and Poly-NiTRP-GC electrodes at �0.54 V in0.1 M KCl samples of commercial Chilean white wine,adding aliquots of 50 mL of 27.8 mM Na2SO3 each 50 s.

3 Results and Discussion

The stability of the polymeric films in aqueous solution(0.1 M NaClO4) was studied in a previous publication

[37]. The results showed that these surfaces are advisablefor the electrocatalytic studies of different analytes underthose experimental conditions. In this work, the reprodu-cibility of the electrode preparation was corroborated bythe evaluation of the RSD value, measuring the charge ofRu(II)/Ru(III) redox couple (see below) in a 0.1 MNaClO4 solution after polymerization obtaining a RSDvalue of 1.49 %.

3.1 Electrocatalytic Process of S(IV) Oxoanions

The electrocatalytic behavior of Poly-MTRP-GC for thereduction and oxidation of S(IV) oxoanions at pH 1.0 and3.5 is shown in Figure 1. The reduction processes are pre-sented in Figure 1A and B, the oxidation processes in Fig-ure 1C and D.

Fig. 1. Voltammetric profiles of bare GC (dotted line), Poly-H2TRP-GC (dashed line), Poly- NiTRP-GC (solid line) and Poly-ZnTRP-GC (dash-dotted line) in 126 mgL�1 of Na2SO3/WM Sol at 100 mVs�1. Reduction process at pH 1.0 (A) and 3.5 (B). Oxida-tion process at pH 1.0 (C) and 3.5 (D). Insets: Voltammetric profiles of Poly-ZnTRP-GC in absence (dotted line) and presence (solidline) of 126 mg L�1 of Na2SO3 at 100 mV s�1.

Electroanalysis 2012, 24, No. 8, 1709 – 1718 � 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 1711

Determination of S(IV) Oxoanions


Figure 1A shows the electrocatalytic activity of Poly-MTRP-GC modified electrodes in the SO2 reduction(pH 1.0). An increase in the cathodic current of the vol-tammetric profiles of modified electrodes, compared tothe bare GC electrode is observed.

The onset in the voltammetric wave at ca. �0.37 V inthe case of modified electrodes indicates an electrocata-lytic behavior compared to bare GC that shows the onsetat �0.45 V. The reduction peaks of SO2 can be observedat �0.58 V, �0.54 V, �0.56 V and �0.52 V, to GC, H2TRP,NiTRP and ZnTRP, respectively.

Figure 1B displays profiles similar to those observed inthe Figure 1A, but the reduction peaks, in the case ofHSO3

� , appear around �0.70 V and the observed currentsare smaller compared to pH 1.0. This fact can be ex-plained in terms of the coordination properties of the tet-raruthenated porphyrins, which may present stronger co-ordination for SO2 than for HSO3

� . This behavior pro-motes the reduction of SO2 at more positive potentialthan the reduction process of HSO3

� . Similar reductionpotential of SO2, can be observed when comparable tet-raruthenated porphyrins have been used in this reaction[31].

Figure 1C compares cyclic voltammograms of the dif-ferent Poly-MTRP-GC modified electrodes, in presenceof Na2SO3, showing the electrocatalytic activity of allmodified electrodes in the HSO3

� oxidation process. Theoxidation peaks of HSO3

� can be observed at 0.91 V,0.88 V and 0.89 V, to H2TRP, NiTRP and ZnTRP, respec-tively.

According to the information of related studies, the ox-idation of S(IV) oxoanions strongly depends on the elec-trode material [41]. In the case of analogous Co(III) andNi(II) tetraruthenated porphyrins, it has been demon-strated that the catalysis of these anions is driven by theoxidation of Ru(II) complexes attached in the peripheryof the porphyrin ring [42]. It is possible to observe thisbehavior for pH 1.0 and 3.5 at all modified electrodesused in this work (see insets in Figures 1C and 1D)

Figure 1D also displays features similar to Figure 1Cwith oxidation peaks of HSO3

� vs. Ag/AgCl appearing at0.92 V, 0.91 V and 0.92 V, to H2TRP, NiTRP and ZnTRP,respectively.

In the results observed in Figure 1, the polymeric sur-face with Ni(II) macrocycle reaches the higher current,this result might be related with the amount of electroca-talyst on the electrode surface instead of real catalytic ac-tivity. Indeed, it has been demonstrated that Ni2+ con-taining macrocycles are able to grow as a much thickerfilm than the analogous M2+ first row transition metalseries, i.e., the polymerization kinetics are faster. Al-though the reason for this phenomenon is not clear yet, itis recognized that this property involve planarity of theresultant macromolecules, specific morphology and theirelectrocatalytic properties [32, 43–48]. This fact originatesmore ordered polymers, thus more active sites are avail-able on the electrode surface with a corresponding in-crease in charge. The surface coverages of the modified

electrodes were calculated in 0.1 M NaClO4 aqueous solu-tion, and the amounts of mol of catalyst obtained were:1.52� 10�10 molcm�2, 8.35� 10�10 molcm�2 and 4.33 �10�10 mol cm�2, to H2TRP, NiTRP and ZnTRP, respective-ly, then, these values are correlated with the behavior ob-served in the case of Poly-NiTRP-GC. On the other handcomplexation properties of Ni2+ as ion metal center, inthe porphyrin, cannot be neglected because Ni2+ can co-ordinate small molecules such as SO2 and HSO3

� , beingthe last one more favored. Consequently the activation ofS(IV) oxoanions should enhance the catalytic activity.

The electrocatalytic properties of Poly-NiTRP-GCallows to propose this macrocycle as the best candidate inorder to design an efficient sensor of S(IV) oxoanions, inthe oxidation and reduction reactions at pH values evalu-ated in this work. For this reason, Poly-NiTRP-GC wasutilized in the following studies.

3.2 Study of the Potential Scan Rate

The electrochemical systems, where catalytic reactionsare coupled to reversible or irreversible charge transfer,can be studied by correlations of kinetic and experimentalparameters, to understand the electrochemical mecha-nisms involved. The diagnostic criterion used is based inthe characterization of the variation of the ratio of anodicto cathodic peak current as a function of potential scanrate [49].

In this work, the studies of the mechanisms involved inthe reduction and oxidation of S(IV) oxoanions, at pH 1.0and 3.5 by the diagnostic criteria of Nicholson and Shain[49] are displayed. Experimentally, several cyclic voltam-mograms were recorded at different scan rate, obtainingthe peak current (Ip) in all the cases, and in this way itwas possible to plot Ipv

�1/2 vs. v. The behavior plotted wascompared with the current function plots defined byNicholsan and Shain [49].

Figure 2A displays the Ipv�1/2 vs. v plots obtained on

Poly-NiTRP-GC in presence of 126 mgL�1 of Na2SO3 forthe reduction processes at pH 1.0 and 3.5. In the case ofpH 1.0, it is not possible to observe an important varia-tion of the Ipv

�1/2 axis while the scan rate increases, there-fore, the tendency can be interpreted as a straight line,and this behavior can be used as a diagnostic criterion todemonstrate the absence (or unimportance) of some cou-pled chemical reaction. Consequently, this case only canbe interpreted as an irreversible charge transfer. Thisresult agrees with several reports, where it has been dem-onstrated that the reduction potential of SO2 in acidmedia is independent of the electrodic surface employed,and the reduced species can be the reactive radical anionSO2

�C, which undergo dimerization to form dithionite[50]. In the case of pH 3.5, the Ipv

�1/2 vs. v plot for HSO3�

reduction is related with a mechanism of irreversiblecharge transfer with catalytic reaction. According to pre-vious works [51], related with the HSO3

� reduction, usinga polymer of polypiridyl iron(II) complex as a catalyticspecies, the formation of equilibrium between reduced




polymer and HSO3� at negative potential is confirmed

[51]. When �0.5 V are applied, the electrocatalyst is re-duced, redirecting the electronic density from phenan-throline moiety towards S(IV) oxoanions. The formationof an adduct between HSO3

� and the aromatic moiety ofthe polymer can explain the behavior observed in thecase of Poly-NiTRP-GC, first an irreversible charge trans-fer that promotes the reduced macrocycle, and then, anintermediary complex or adduct that is electrocatalytical-ly reduced to dithionite or thiosulfate recovering the elec-trocatalyst [31].

Figure 2B shows the Ipv�1/2 vs. v plots obtained with

Poly-NiTRP-GC in presence of 126 mgL�1 of Na2SO3 foroxidation processes at pH 1.0 and 3.5. At pH 1.0, the be-havior observed agrees with a mechanism that involvesa charge transfer followed by a reversible chemical reac-tion. In this situation, three limiting cases are possible, de-pending of the magnitude of the kinetic rate constant.According to the description of Nicholson and Shain [49],the behavior observed fits with a second limiting casewhere the chemical reaction is very slow, so that essen-tially no chemical reaction takes place during the experi-ment. Therefore, in these situations, the equilibrium con-stants are bigger and the chemical reaction can be consid-ered irreversible. A similar description has been reportedin the study of the reaction between S(IV) oxoanions andtris(1,10-phenanthroline) iron(III), in aqueous solution[52]. In general, the metal ion mediated oxidation ofS(IV) oxoanions, promotes the formation of sulfate, dithi-onate or a mixture of them [53]. However, dithionate isthermodynamically unstable with respect to disproportio-nation; therefore, the most favorable product in this pro-cess is sulfate [53]. The last antecedents allow us to de-scribe a prototype of mechanism concordant with the de-signed by diagnostic criteria, where a reversible charge

transfer takes place first, corresponding to the redoxcouple of Ru(III)/Ru(II), and then, an irreversible chemi-cal reaction which produces the SO2 oxidation probablyto SO4

2�. At pH 3.5, the Ipv�1/2 vs. v plot for HSO3

� oxida-tion have relation with the same mechanism described toSO2 oxidation, where a first step is a reversible chargetransfer, attributed to the redox couple of Ru(III)/Ru(II),followed by a reversible chemical reaction, which is re-sponsible for promoting of HSO3

� oxidation.

3.3 Interference Study

The composition of the wine is very complex due to thepresence of various components from the grape and thefermentation process. Of the total wine composition,81% correspond to water, 12% is ethanol and a 7 % cor-respond to an ample range of compounds within which0.5 % is sulfite and 1.5 % are poly-phenols like gallic acid[8]. Therefore, polyphenols are highlighted due to theirhigh concentration and also by their powerful antioxidantproperties [54], and for this reason it is important to iden-tify how they can interfere in the determination processof S(IV) oxoanions.

Gallic acid is a type of phenolic acid with important an-tioxidant properties, provided for the presence of theeasily oxidizable ortho-diphenol group, which helps toprotect human cells against oxidative damage [55]. Thetotal amount of phenols in wines is determined in arbitra-ry units based in a phenolic standard compound which isgallic acid [56]. The phenolic composition in wines esti-mated in gallic acid equivalent lies between 215 and1215 mg L�1 [56], therefore, voltammetric experimentswere carried out in high concentration of gallic acid(1000 mg L�1), in order to represent the total contents ofpolyphenols present in wines and it was compared to the

Fig. 2. Current function plots at pH 1.0 and 3.5 of the reduction (A) and oxidation process (B).




voltammetric response of S(IV) oxoanions at differentpH values.

Figure 3 shows the results observed in the characteriza-tion by cyclic voltammetry for Poly-NiTRP-GC in a winemodel solution with Na2SO3 in absence and presence ofgallic acid, at negative and positive potential, at both pHvalues evaluated (1.0 and 3.5).

In Figure 3A it is possible to observe that the voltam-metric profile of the modified electrode in presence ofNa2SO3 (solid line) shows a reduction peak of SO2

around �0.55 V, which is similar to the observed one inFigure 1A. In the voltammetric profile of modified elec-trode only in presence of gallic acid (dotted line), redoxprocesses are not observed, therefore, in the voltammetricprofile of the Poly-NiTRP-GC with the mixture ofNa2SO3/gallic acid (dashed line), the redox process that

can be observed is ascribed only to SO2 reduction. Thesame behavior is observed in Figure 3B, but in this casethe predominant S(IV) oxoanion is HSO3

�. In the reduc-tion processes at both values of pH (1.0 and 3.5), it is pos-sible to observe that the current in the case of the mixtureis smaller than the current observed in the case when onlyNa2SO3 is present. This fact can be explained since a possi-ble formation of adduct between S(IV) oxoanions andgallic acid [57]. Comparatively, the current decreases areof 6.51% and 13.54% for pH 1.0 and 3.5, respectively.

Figure 3C shows the voltammetric profiles of the Poly-NiTRP-GC. In presence of Na2SO3, the behavior is theexpected (solid line), according to the observed in theFigure 1C but in the presence of gallic acid (dotted line)it is possible to observe a small oxidation process at0.65 V (ortho-diphenol oxidation) and other oxidation

Fig. 3. Voltammetric profiles of Poly-NiTRP-GC at 100 mVs�1 in WM Sol with: 126 mgL�1 Na2SO3 (solid line), 1000 mgL�1 gallicacid (dotted line) and 126 mgL�1 Na2SO3/1000 mg L�1 gallic acid (dashed line). Reduction process at pH 1.0 (A) and 3.5 (B). Oxida-tion process at pH 1.0 (C) and 3.5 (D). Insets: Voltammetric profiles of bare GC electrode.




process of great intensity at 0.86 V. This last process re-flects the electrocatalytic activity of the polymeric film inthe oxidation of one –OH group by Ru(III)/Ru(II) redoxcouple. In the case of the Na2SO3/gallic acid mixture(dashed line), the behavior is similar, this fact shows thatthe oxidation process of gallic acid mask oxidation pro-cess of SO2, and the unique difference is a slightly shift at0.94 V, which can be associated with adduct formation be-tween gallic acid-SO2.

The inset of Figure 3C shows the voltammetric profilesof the bare GC electrode. In presence of Na2SO3, no elec-trochemical response is observed (solid line), but in thepresence of gallic acid two redox processes take place(dashed line), one of them at 0.70 V is recognized asortho-diphenol group oxidation and the other at 1.02 Vcorresponds to oxidation of the phenol group adjacent tothe ortho-diphenol group [55]. The redox processes de-scribed to gallic acid also are observed in the voltammet-ric profile of Na2SO3/gallic acid mixture (dotted line),therefore, these processes only represent the oxidationsof phenolic compound.

Figure 3D shows a comparative condition, at pH 3.5,but in the presence of Na2SO3/gallic acid (dashed line),the predominance of the phenol group oxidation is notclear under HSO3

� oxidation, because the peak around0.90 V is smaller than in the case of the peak only withgallic acid. The results observed suggest that at pH 3.5the conditions are better for the formation of an adductbetween gallic acid and HSO3

� , as reported in the litera-ture [58].

On the basis of the results displayed in Figure 3 it ispossible to assume that the gallic acid, as a representative

compound of the phenolic composition in wine, acts likean interferent species in the oxidation process of S(IV)oxoanions (at pH 1.0 and 3.5) and does not interfere inthe reduction process of the same analytes. This factallows proposing a quantification method of sulfur com-pounds by a reduction process at pH 1.0.

3.4 Amperometric Determination of SO2 in Wine ModelSolution

According to the behavior described for Poly-NiTRP-GCin the reduction process of S(IV) oxoanions in the previoussection, amperometric studies were carried out at pH 1.The experiments took place in presence of 1000 mgL�1 ofgallic acid in order to represent the total content of poly-phenols in wines, aliquots of 27.8 mM Na2SO3 were addedeach 50 s, registering the amperometric response in a rangeof SO2 concentration between 0 to 150.72 mgL�1. The am-perometric experiments were accomplished in triplicate,using a new modified electrode in each experiment, deter-mining for each concentration of SO2 the correspondingstandard deviations. Figure 4 displays the results obtainedwhere a linear behavior is observed. Using the calibrationcurve, the detection and quantification limits were calculat-ed using the following equations:

LOD ¼ 3sy=x=b

LOQ ¼ 10sy=x=b

Where LOD is the detection limit, LOQ is the quan-tification limit, sy/x is the standard deviation calculated for

Fig. 4. Amperometric experiment for S(IV) oxoanions reduction at �0,57 V vs. Ag/AgCl in 1000 mgL�1 gallic acid/WM Sol pH 1.0with Poly-NiTRP-GC. Aliquots of 50 mL of 27.8 mM Na2SO3 each 50 s.




the linear regression according to Miller and Miller [59],and b is the slope of the calibration curve. The calculatedvalues are 3.32 mg L�1 and 11.06 mg L�1 for detectionlimit and quantification limit, respectively

The obtained relative standard deviation (RSD) was1.34% for three successive measurements renewing theelectrode surface, indicating a good reproducibility of themethod.

Worth of noting is the Inset in Figure 4 where it is pos-sible to observe that the noise signal increases with in-creasing concentration of the analyte. This fact suggestsa saturation of reduced species on the surface which ob-structs the active sites of the polymeric film i.e. an ineffi-cient mass transfer from the electrodic surface to the bulksolution [60]

3.5 Amperometric Determination of SO2 in White WineSamples

Using similar experimental conditions to the proposedmethod described above, a commercial Chilean whitewine was analyzed. The wine samples were prepared with0.1 M KCl, pH of the sample was not adjusted (pH 3.5),in order to avoid loss of analyte due to the acid medium.The amperometric experiments were carried out directlyin the wine sample without dilution, adding 50 mL ali-quots of Na2SO3 27.8 mM as standard each 50 s.

Figure 5 shows the calibration curve in terms of abso-lute value of the current. On the other hand, it was possi-ble to determine the initial SO2 concentration in the uti-lized sample as 32.7 mg L�1. The detection and quantifica-tion limits (using Equations 1 and 2) calculated were1.40 mgL�1 and 4.68 mg L�1, respectively. The RSD calcu-lated for the concentration was 6.24%, indicating thegood reproducibility of the proposed amperometricsensor.

The results obtained by the proposed electrochemicalmethod were validated through a Monier-Williams analy-sis, obtaining 31.1 mg L�1, using a correction factor al-ready published [58], which means that the percentage ofrecovery achieved by the electrochemical method pro-posed in this work is 95.2%

On the other hand, sulfite concentrations in wine havebeen measured with the same electrode by triplicate, theRSD value for the analytical signal was not satisfactory(ca. 20%). This means that the modified electrodes mightbe used as disposable devices because their preparation isreliable.

4 Conclusions

In this work, surfaces of GC electrodes, modified withNi(II), Zn(II) and metal free tetraruthenated porphyrins,are able to act as electrocatalysts in the reduction and ox-idation processes of S(IV) oxoanions at pH 1.0 and 3.5, inwine model solutions and white wine.

According to these results, Poly-NiTRP-GC was thepolymeric film chosen for the studies related with the re-action mechanisms and behavior as sensor in the reduc-tion and oxidation processes of S(IV) oxoanions.

Interpretation of Ipv�1/2 vs. v carried out with Poly-

NiTRP-GC, make it possible to propose that the reactionmechanism in the reduction process of S(IV) oxoanions,at pH 1.0, is only mediated by irreversible charge transfer,but at pH 3.5, the mechanism involved an irreversiblecharge transfer followed by a catalytic reaction. For theoxidation, at both pH values tested, the mechanism con-sist in a first electrochemical step, corresponding to redoxcouple of Ru(III)/Ru(II) and a second step where a rever-sible chemical reaction is involved, which can be consid-ered irreversible when this occurs very slowly, corre-sponding to oxidation of S(IV) oxoanion.

In the reduction process, gallic acid does not act as in-terferent species, and for this reason an amperometricmethod for determination and quantification of S(IV) ox-oanions was proposed in the reduction process at pH 1.0.The detection limit calculated is 3.32 mg L�1 and the limitof quantification is 11.06 mg L�1. The range of determina-tion utilized is between 17.41 and 150.72 mg L�1.

The determination of sulfite in a white wine sampleshowed a detection limit of 1.40 mg L�1, which is quite farfrom the typical sulfite concentrations detected in com-mercial wines. On the other hand, a good percentage ofrecuperation was achieved (95.2 %), which implies thatthe proposed method exhibits a high accuracy, an indis-pensable requirement for routine analysis.

This determination method is an excellent alternativecompared to the traditional method, allowing the sulfitequantification in a precise and reliable way, being a plausi-ble method for applications in the wine industries, even

Fig. 5. Calibration curve obtained for the determination of sul-fite in Chilean white wine/0.1 M of KCl with Poly-NiTRP-GCmodified electrode.




when the RSD values for the repeatability suggest thatthe electrode must be used in just one determination.Currently, efforts to improve electropolymerization con-ditions such as potential scan rate, monomer concentra-tion, number of cycles among others are in progress toobtain a more stable electrode in this matrix.

Acknowledgements

This work was supported by Programa Bicentenario enCiencia y Tecnolog�a, CONICYT-Banco Mundial, Proyec-to Anillo ACT-24, Project ICM-P10-003-F, CILIS, grantedby Fondo de Innovaci�n para la Competitividad, del Min-isterio de Econom�a, Fomento y Turismo, Chile. FONDE-CYT Program 1100203 and 1120071 Projects. P. D. andQD acknowledge CONICYT Doctoral and AT-24080067Fellowships.

References

[1] M. Zhao, D. B. Hibbert, J. J. Gooding, Anal. Chim. Acta2006, 556, 195.

[2] H. Schwartz, J. Allergy Clin. Inmunol. 1983, 71, 487.[3] H. Vally, A. Carr, J. El-Saleh, P. Thompson, J. Allergy Clin.

Immunol. 1999, 103, 41.[4] B. Zoecklein, K. Fugelsang, B. Gump, F. Nury, An�lisis y

producci�n de vino, 1st ed., Acribia S. A., Zaragoza,EspaÇa 2001, ch. 9.

[5] M. Ripper, J. Prakt. Chem. 1892, 46, 428.[6] L. F. Burroughs, A. H. Sparks, Analyst 1964, 89, 55.[7] L. F. Burroughs, A. H. Sparks, J. Sci. Food Agric. 1964, 15,

176.[8] I. Hornsey, The Chemistry and Biology of Winemaking,

RSC Publishing, Cambridge, UK 2007.[9] G. Shi, M. Luo, J. Xue, L. Jin, J. Y. Jin, Talanta 2001, 54,

773.[10] P. Abu-Nader, S. Sagrado-Vives, H. Mottola, J. Electroanal.

Chem. 1990, 284, 323.[11] F. Falcone, K. C. Maxwell, J. Agric. Food Chem. 1992, 40,

1355.[12] A. K. Abass, J. P. Hart, D. Cowell, Sens. Actuators B, Chem.

2000, 62,148.[13] P. Linares, M. D. Luque de Castro, M. Valcarcel, Anal.

Chim. Acta 1989, 225, 443.[14] P. A. Kilmartin, H. Zou, A. L. Waterhouse, J. Agric. Food

Chem. 2001, 49, 1957.[15] S. M. Chen, S. W. Chiu, Electrochim. Acta 2000, 45, 4399.[16] S. M. Chen, J. Mol. Catal. A Chem. 1996, 112, 277.[17] J. Zhang, W. J. Pietro, A. V. P. Lever, J. Electroanal. Chem.

1996, 403, 93.[18] S. Daunert, S. Wallace, A. Florido, L. G. Bachas, Anal.

Chem. 1991, 63, 1676.[19] Y. H. Tse, P. Janda, A. B. P. Lever, Anal. Chem. 1994, 66,

384.[20] M. Lucero, G. Ram�rez, A. Riquelme, I. Azocar, M. Isaacs,

F. Armijo, J. E. Fçrster, E. Trollund, M. J. Aguirre, D. Lexa,J. Mol. Catal. 2004, 221, 71.

[21] R. Carballo, V. Campo Dall�Orto, A. Lo Balbo, I. Rezzano,Sens. Actuators B, Chem. 2003, 88, 155.

[22] J. H. Zagal, M. J. Aguirre, M. Isaacs, J. Pavez, Electroqu�mi-ca y Electrocat�lisis. Nuevos Materiales: Aspectos Funda-

mentales y Aplicaciones, Vol. Ia (Ed: N. Alonso-Vante), Ed-itorial e-libro.net, Buenos Aires 2003, ch. 6.

[23] C. Shi, F. C. Anson, Inorg. Chem. 1992, 31, 5078.[24] M. S. M. Quintino, K. Araki, H. E. Toma, L. Angnes, Talan-

ta 2006, 68, 1281.[25] M. S. M. Quintino, H. Winnischofer, K. Araki, H. E. Toma,

L. Angnes, Analyst 2005, 130, 221.[26] A. Prodi, M. T. Indelli, C. J. Kleverlaan, E. Alessio, F. Scan-

dola, Coord. Chem. Rev. 2002, 229, 51.[27] M. R. Wasielewski, Chem. Rev. 1992, 92, 435.[28] K. Araki, C. A. Silva, H. E. Toma, L. H. Catalani, M. H. G.

Medeiros, P. Di Mascio, J. Inorg. Biochem. 2000, 78, 269.[29] H. Winnischofer, A. L. Barboza Formiga, M. Nakamura,

H. E. Toma, K. Araki, A. F. Nogueira, Photochem. Photo-biol. Sci. 2005, 4, 359.

[30] K. Calfum�n, M. J. Aguirre, D. Villagra, C. YaÇez, C. Ar-valo, B. Matsuhiro, M. Isaacs, J. Solid State Electrochem.2010, 14, 1065.

[31] K. Calfum�n, M. Garc�a, M. J. Aguirre, B. Matsuhiro, L.Mendoza, M. Isaacs, Electroanalysis 2010, 22, 338.

[32] M. Isaacs, F. Armijo, G. Ram�rez, E. Trollund, S. R. Biaggio,J. Costamagna, M. J. Aguirre, J. Mol. Catal. A, Chem. 2005,229, 249.

[33] M. Vol�pin, G. Novodarova, E. Kolosova, N. Guzhova, A.Kononenko, Y. Lejkin, Inorg. Chim. Acta 1981, 5, 21.

[34] S. Bodige, F. MacDonnell, Tetrahedron Lett. 1997, 38, 8159.[35] C. Janiak, S. Deblon, H-P. Wu, M. Kolm, P. Klfers, H. Pio-

trowski, P. Mayer, Eur. J. Inorg. Chem. 1999, 1507.[36] P. Dreyse, M. Isaacs, P. Iturriaga, D. Villagra, M. J. Aguirre,

C. P. Kubiak, S. Glover, J. Goeltz, J. Electroanal. Chem.2010, 648, 98.

[37] P. Dreyse, M. Isaacs, C. C�ceres, K. Calfuman, A. Aliaga, D.Villagra, M. J. Aguirre, Electrochim. Acta 2011, 56, 5230.

[38] E. Amouyal, J. C. Chambron, A. Homsi, J. P. Sauvage, J.Chem. Soc. Dalton Trans. 1990, 6, 1841.

[39] B. P. Sullivan, D. J. Salmon, T. J. Meyer, Inorg. Chem. 1978,17, 3334.

[40] A. Isaac, C. Livingstone, A. J. Wain, R. G. Compton, J.Davis, Tr. Anal. Chem. 2006, 25, 589.

[41] N. Rea, B. Loock, D. Lexa, Inorg. Chim. Acta 2001, 312, 53.[42] I. Mayer, H. E. Toma, K. Araki, J. Electroanal. Chem. 2006,

590, 111.[43] H. Li, T. F. Guarr, J. Chem. Soc. Commun. 1989, 832.[44] Y. H. Tse, P. Janda, H. Lam, J. Zhang, W. J. Pietro, A. B. P.

Lever, J. Porphyrins Phthalocyanines 1997, 1, 3.[45] P. Ardiles, E. Trollund, M. Isaacs, F. Armijo, J. C. Canales,

M. J. Aguirre, M. J. Canales, J. Mol. Catal. A, Chem. 2001,165, 169.

[46] J. Wang, Anal. Lett. 1996, 29, 1575.[47] F. Xu, H. Li, Q. Peng, T. F. Guarr, Synth. Met. 1993, 55,

1668.[48] K. L. Brown, H. A. Mottola, Langmuir 1998, 14, 3441.[49] R. S. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706.[50] L. E. Barrose-Antle, D. S. Silvester, L. Aldous, C. Hardacre,

R. G. Compton, J. Phys. Chem. C 2008, 112, 3398.[51] C. Garc�a, C. Dom�nguez, A. Aliaga, B. Matsuhiro, L. Men-

doza, M. J. Aguirre, M. Isaacs, Electroanalysis 2011, 23,1401.

[52] D. W. Carlyle, J. Am. Soc. Chem. 1972, 94, 4525.[53] D. M. Yost, H. Russell, Systematic Inorganic Chemistry,

Prentice-Hall, New York 1944.[54] B. C. Prasongsidh, G. R. Skurray, Food Chem. 1998, 62, 355.[55] P. A. Kilmartin, H. Zou, A. L. Waterhouse, J. Agric. Food.

Chem. 2001, 49, 1957.[56] E. Bordeu S., J. Scarpa B-B. An�lisis Qu�mico del vino, 2nd

ed., Ediciones Universidad Cat�lica de Chile, Chile 2000.




[57] E. M. Francia-Aricha, M. T. Guerra, J. C. Rivas-Gonzalo, C.Santos-Buelga, J. Agric. Food. Chem. 1997, 45, 2262.

[58] C. Montes, J. H. Vlez, G. Ram�rez, M. Isaacs, R. Arce,M. J. Aguirre, Sci. World J. 2012, Art. No. 168148.

[59] J. N. Miller, J. C. Miller, Estadistica y Quimiometria paraQuimica Analitica, 4th ed., Pearson Education, New York,2002, ch. 5.

[60] Y. Fu, P. Li, L. Bu, T. Wang, Q. Xie, J. Chen, S. Yao, Anal.Chem. 2011, 83, 6511.




Determination of S(IV) Oxoanions at Poly[Ru(5-NO2-Phen)2Cl] Tetrapyridylporphyrin Glassy Carbon...

Documents

Transcript of Determination of S(IV) Oxoanions at Poly[Ru(5-NO2-Phen)2Cl] Tetrapyridylporphyrin Glassy Carbon...