cathodic electrochemical determination of herbicides in acid media using a bismuth film electrode

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Cathodic Electrochemical Determination of Herbicides in AcidMedia Using a Bismuth Film Electrode

Monica Moreno,a* Esperanza Bermejo,a Manuel Chicharro,a Antonio Zapardiel,b Alberto Sanchez Arribasa

a Departamento de Qu�mica Anal�tica y Analisis Instrumental, Universidad Autonoma de Madrid, Calle Francisco Tomas yValiente 7, 28049 Madrid, Spain

*e-mail: [email protected] Departamento de Ciencias Anal�ticas, Universidad Nacional de Educacion a Distancia (UNED), Paseo Senda del Rey 9,

28040 Madrid, Spain

Received: July 19, 2008Accepted: September 19, 2008

AbstractA revised procedure for the preparation of bismuth film electrodes (BiFEs), adapted to improve its performance inacid media, is presented and applied for the analysis of herbicides under these conditions. Common carbon-basedsubstrates have been studied for the preparation of ex situ BiFE as well as the influence of relevant parameters uponthe bismuth film formation (plating solution, deposition potential and deposition time). A plating solution consistingof 0.60 mM Bi(NO3)3 in 0.10 M H2SO4 and a careful selection of the plating potential (equal to the cathodic peakpotential in this solution) applied during 360 s yields to a suitable BiFE in terms of stability and electroanalyticalperformance. The applicability of the BiFE was investigated for the analysis of the herbicides metamitron, metribuzin,maleic hydrazide and atrazine in nondeaerated solutions by square-wave voltammetry (SWV). In all cases theenhanced electrochemical performance of the BiFE over bare glassy carbon electrode (GCE) is evident. Herbicidessignals at BiFE have improved sensitivity and a positive potential shift is observed when compared to bare GCE. Theherbicide metribuzin (4-amino-6-tert-4,5-dihydro-3-methyl-thio-1,2,4-triazin-5-one) was determinated in nondeaer-ated solutions by square-wave voltammetry (SWV) using BiFE. Under optimal conditions, the dynamic linear rangeof concentrations is comprised between 10 and 200 mM and the detection limit is 6 mM.

Keywords: Bismuth film electrode, Electrochemical detection, Square-wave voltammetry, Triazine herbicides,Metribuzin

DOI: 10.1002/elan.200804405

1. Introduction

The usage of chemical pesticides is know to have asignificant positive impact on the expansion of agriculturaland food industries. As a consequence of the massive usageof these compounds, their occurrence in foodstuffs anddrinking water has increased progressively in the past yearswith subsequent new potential risks for the consumers�health. For this reason, new analytical strategies aredemanded for the fast and simple monitoring of thesecompounds.

Electroanalytical techniques, particularly the voltammet-ric ones, could meet these requirements since they arerelatively simple to apply, fast and reasonably cheap,providing good sensitivity and selectivity to electroactiveanalytes [1]. This favorable performance is closely related tothe properties of the working electrode material employedfor analyte detection.

Cathodic electrochemical detection of inorganic as well asorganic compounds at different electrodes has been studiedexhaustively, essentially at mercury electrodes [2, 3]. Thesemercury electrodes exhibit a wide negative potentialwindow and good electrochemical behavior (low back-

ground, high hydrogen overpotential). However the toxicityof mercury might cause banning of the use of theseelectrodes in laboratories in a close future, following theGreen Analytical Chemistry trends.

Since its introduction in stripping analysis [4], the bismuthfilm electrode (BiFE) has been accepted in many electro-analytical laboratories worldwide. Bismuth is an environ-mental friendly element, with very low toxicity and advanta-geous electrochemical properties, such as high hydrogenoverpotential, limited oxygen interference or the ability toform fused or low-temperature alloys with heavy metals [5, 6],very similar to those attained when using mercury electrodes.

BiFEs have found a wide range of applications in anodicstripping voltammetry [7 – 9], or potentiometric strippinganalysis of trace metals [10, 11] as well as in cathodicadsorptive stripping voltammetry [12 – 14] for the determi-nation of some metals. On the other hand, its application forthe cathodic electrochemical determination of organiccompounds has been scarcely studied [15 – 17]. For example,the detection of some azo dyes [15], insecticides [16] and theherbicide metamitron [17] has been described, where thesuitability of BiFE for the replacement of mercury electro-des was obvious.

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� 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2009, 21, No. 3-5, 415 – 421

Most of the herbicides are organic compounds and many ofthem have electrochemical properties. Among them, triazinicherbicides could undergo electrochemical reduction, studiedmainly at mercury electrodes in acidic medium [18, 19]. Thiswork describes a revised procedure for the preparation ofbismuth film electrode (BiFE), adapted to improve itsperformance in acid media, with the aim to apply it for theanalysis of herbicides under these conditions, concept notreported to date. Optimization experiments were carried outto find the best experimental conditions for bismuth filmdeposition which provide suitable BiFE for the voltammetricanalysis of herbicides (atrazine, metamitron, maleic hydra-zide and metribuzin) in nondeaereated acidic solutions.

2. Experimental

2.1. Reagents

Metamitron, metribuzin, atrazine, maleic hydracide andmethanol grade Pestanal were purchased from Riedel-de-Haen (Madrid, Spain). The herbicide stock solutions wereprepared by dissolving an appropriate amount of thecompound either in methanol or in water containing 50%methanol. Bismuth stock solutions were prepared bydissolving bismuth nitrate (Carlo Erba, Italy) in HNO3

0.5%. All stock solutions were kept away from the lightand stored under refrigeration. Diluted solutions wereprepared daily from these stock solutions. All the otherchemicals were analytical-reagent grade and they were usedwithout any further purification. Ultrapure water (1>18 MW cm) from a Millipore-MilliQ system was used forpreparing all solutions.

2.2. Apparatus

All the electrochemical measurements were carried outusing a BAS 100B electrochemical analyzer (BAS, WestLafayette, USA) in a conventional three electrodes config-uration. Bismuth film as well as carbon paste and glassycarbon (BAS, model MF-2012) electrodes (3 mm diameter)were used as working electrodes. A platinum wire and a Ag/AgCl, 3 M NaCl (BAS, model RE-5B) were used as counterand reference electrode respectively. All potentials werereferred to this reference system. Data storage and con-version to �txt� files were performed with BAS W 2.3software (BAS, West Lafayette, USA) through a PentiumIV PC computer connected to the electrochemical analyzer.

2.3. Procedures

2.3.1. Electrode Preparation

Carbon paste electrodes (CPE) were prepared by mixing inan agate mortar the desired amount of graphite powder(Grade #38, Fisher Scientific, Spain) with mineral oil

(Aldrich) in 8 :2, 7 : 3 and 6 :4 ratios for 30 minutes. Aportion of the resulting paste was packed firmly into thecavity (3.0 mm diameter, 3.0 mm depth) of a Teflon tube.The electric contact was established via a stainless steelscrew. Glassy carbon electrode (GCE) was polished with 0.3and 0.05 mm alumina slurries (Buehler, Spain) on BAS softlapping pads.

2.3.2. Plating Conditions

Bismuth films were prepared by applying a constantpotential (typically equal to bismuth cathodic peak, ca.�530 mV) during a fixed time (typically 360 s) in airsaturated 0.10 M H2SO4 solutions containing the appropri-ate bismuth concentration, placed in a 5 mL electrochemicalcell. During the bismuth deposition, the solution was stirredwith a magnetic bar at ca. 200 rpm. Once the bismuth filmwas plated, the electrode was gently rinsed with 0.10 MH2SO4 solution and placed in the fresh measurementsolution.

2.3.3. Electrochemical Measurements

Electrochemical measurements of herbicides were carriedout in nondeaerated solutions containing the desired con-centration of these herbicides in 0.10 M H2SO4. Before eachvoltammetric measurement, the solution was gently stirredfor 5 seconds. All measurements were done at roomtemperature (20 – 25 8C).

3. Results and Discussion

3.1. BiFE Preparation

There are many organic herbicides with reductive moietiesonly electroactive in acidic medium, so the performance ofBiFE was studied in this medium whose exploitation hasbeen dealt in rarely. Optimization experiments were carriedout to find the best experimental conditions (substrateelectrode, composition of the plating solution, platingpotential and plating time) for bismuth film formationwhich provides suitable BiFE for the voltammetric analysisin acidic medium, using the signal of the triazinic herbicidemetamitron as probe. Metamitron undergoes electrochem-ical reduction processes in acidic, neutral and slightly basicpH described in mercury electrodes [20, 21], with twoconsecutive steps involving two electrons each one. On theother hand, the voltammetric detection of this herbicide at aBiFE in 0.10 M acetate buffer solution of pH 4.5 has beenreported earlier [17].

Several carbon-based substrate electrodes (GCE andCPE) were investigated for studying bismuth deposition.Firstly each substrate was ex situ plated in air saturatedacetate buffer solution of pH 4.5 containing 0.50 mM Bi(III)following the procedure described by Sanchez Arribas et al.[17]; afterwards, the voltammograms for 200 mM metami-

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tron in air saturated acetate buffer solution of pH 4.5 wererecorded between �400 and �1100 mV in SWV mode.

As could be seen in Figure 1, the background signal incarbon paste and glassy carbon electrodes is highly influencedby the oxygen reduction and the metamitron signal is severelyinterfered; instead, BiFE background signal is flat and a well-defined metamitron signal is observed at more positivepotential in all cases. CPE showed in general higher back-ground currents and hydrogen evolution was positivelyshifted (more pronounced when increasing the carbon-to-oilratio) when compared to GCE under these conditions. Theseelectrodes present a porous structure where oxygen remainsoccluded, even after solution purging, which greatly contrib-utes to the observed background differences and difficultiesthe film formation, resulting in modified electrodes withworse voltammetric properties and low stability. On theopposite, glassy carbon closed structure has lower perme-ability to gases, favoring lower background currents and theformation of BiFE of enhanced electroanalytical perfor-mance. Therefore, GCE was chosen as substrate electrode forbismuth film plating for subsequent experiments.

The performance of the ex situ-plated bismuth film ontoGCE was studied in acidic media and Figure 2 shows thevoltammograms obtained for 200 mM metamitron in airsaturated 0.10 M H2SO4 solution at BiFE and bare GCE.

The BiFE shows lower background under these condi-tions and no adverse influence of the oxygen reduction wasnoticed, in the opposite to bare GCE; moreover, BiFEshows a reasonable overpotential to hydrogen evolution,allowing a sufficiently wide negative potential window atthis pH. The metamitron signals under these conditionsshowed an increase in the peak current (about 25%) and apotential shift to more positive values when compared to

those obtained in acetate buffer, indicating that protons areinvolved in the electrochemical process, in accordance withliterature reports [17, 20]. It is clear that an appropriateperformance of BiFE in such acidic conditions would lead toattain better analytical results. In this sense a revision of theprocedure for preparing bismuth films adapted to theserequirements was carried out and it is described in thefollowing lines.

An important parameter influencing the quality ofbismuth coating is the plating solution composition. Theeasily accessible sources of Bi(III) ions are simple salts likeBi(NO3)3 5H2O or BiCl3; however, the noncomplexedBi(III) ions hydrolyze very easy, forming insoluble com-pounds even in weak acidic media. Because of the risk of

Fig. 1. Voltammograms obtained in the presence and in the absence of metamitron at bismuth film electrodes plated onto differentsubstrates (solid lines). Dashed lines correspond to voltammograms at bare electrode. Measuring conditions: 0.10 M acetate buffer ofpH 4.5; air saturated solution; 200 mM metamitron. SWV parameters: step, 5 mV; amplitude, 25 mV; frequency, 20 Hz. Bismuth platingconditions: 0.10 M acetate buffer of pH 4.5 containing 0.50 mM Bi(III) at �600 mV during 240 s.

Fig. 2. Voltammograms of 200 mM metamitron in 0.10 M H2SO4

at bare electrode (a) and at BiFE plated onto glassy carbonelectrode (b). Dashed lines correspond to background signal inthe absence of metamitron. SWV parameters and bismuth platingconditions as in Figure 1.

417Cathodic Electrochemical Determination of Herbicides

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hydrolysis, different acidic media including 0.10 M acetatebuffer of pH 4.5 and 0.10 M H2SO4, HNO3 and HClsolutions were tested for 0.50 mM Bi(III) and the resultsobtained are shown in Table 1.

Metamitron signals were very similar in all cases but filmsplated in H2SO4 showed more stability for consecutiveanalysis than the rest and, for this reason, these conditionswere chosen for the following studies.

For the study of the plating potential, first, the linearvoltammetric curve of the plating solution containing0.50 mM Bi(III) in 0.10 M H2SO4 was recorded at GCE(Fig. 3A) and then the plating potential was studied forvalues more positive and negative than peak potential.

As could be seen in the figure 3B, when the platingpotential was set at a value equal to the peak potential, theBiFE obtained under these conditions showed the bestanalytical performance, similarly to Toth et al. [22] findings

for bismuth plating solution containing HCl and KBr ascomplexant. More positive potentials led to poor-coatedelectrode surfaces while the films obtained at more negativepotentials looked thicker and less stable, probably due tofaster film nucleation and the competitive hydrogen evolu-tion.

The concentration of Bi(III) in the plating solutioncontaining 0.10 M H2SO4 was examined in the 0.10 to1.20 mM range. It was found that the metamitron peakincreased and the peak potential shifted to more positivepotential when the concentration of Bi(III) in the platingsolution increased, as shown in Figure 3C, but the back-ground current increased as well. 0.60 mM was chosen asBi(III) plating concentration for further studies, where agood compromise between high sensitivity for metamitronand low background current was obtained. Finally, theplating time was studied, showing an increase in the peakcurrent and a decrease in the metamitron peak potential upto 100 s and remaining constant for higher plating time(Fig. 3D). On the other hand, the film stability improvedwhen the plating time increased up to 360 s while longertimes led to unstable coverages. So 360 s plating time waschosen for further experiments since higher times could leadto thicker bismuth films, usually less conductive an unstableas above explained.

Table 1. Effect of the plating buffer solution on the response of200 mM metamitron.

Plating buffer solution Ep (mV) (n¼ 3) Ip (mA) (n¼ 3)

Acetate 0.10 M pH 4.5 �418� 3.5 �19� 1.1H2SO4 0.10 M �411� 6.3 �23� 0.9HNO3 0.10 M �415� 5.0 �21� 1.9HCl 0.10 M �403� 2.9 �20� 1.5

Fig. 3. Effect of plating conditions on the voltammetric signal of 200 mM metamitron in 0.10 M H2SO4. Linear sweep voltammogram ofplating solution containing 0.50 mM Bi(III) in 0.10 M H2SO4 (A). Effect of plating potential (B), bismuth concentration (C) and platingtime (D). Squares and circles correspond to peak current and peak potential respectively. LSV operational parameters: scan rate, 50 mVs�1. SWV parameters as in Figure 1. General plating conditions: 0.10 M H2SO4; stirring rate, 200 rpm. Other conditions: B) 0.50 mMBi(III) during 200 s; C) Bismuth cathodic peak potential during 240 s; D) 0.60 mM Bi(III) at bismuth cathodic peak potential.




3.2. Voltammetric Application of BiFE

The applicability of the BIFE prepared by the optimizedplating procedure was investigated for the determination ofthe herbicides metamitron, metribuzin, maleic hydrazideand atrazine. All the studied compounds undergo electro-chemical reduction processes described in mercury electro-des [18 – 21]. The voltammetric response for air saturatedsolutions of metamitron, metribuzin, maleic hydrazide andatrazine in 200 mM concentration each, was compared withthat of the GCE under identical conditions (Fig. 4).

As could be seen in Figure 4, in all cases the enhancedelectrochemical performance of the BiFE over GCE isevident. The metamitron and metribuzin signals obtainedwith the BiFE are sharper and 50 and 125 mV-positivelyshifted, respectively, when compared to those obtainedusing the GCE, indicating that the electron transfer rate isfavored in BiFE; moreover, metribuzin presents a secondwave around�735 mV which is not observed in GCE. Thissecond wave was also observed at a dropping-mercuryelectrode [19] and it is attributed to the reduction of theC�SCH3 bond. Atrazine and maleic hydrazide did notpresent reduction signal at bare GCE while both herbicidespresent a clearly defined cathodic peak at �750 and�760 mV respectively at BiFE, allowing their detectionand determination using this electrode. These facts con-firmed the favorable electrochemical performance of thebismuth electrode under these conditions, always in thepresence of dissolved oxygen, stated in the observeddecrease in the overpotential needed to promote the studiedelectrochemical processes. The reduction potentials report-ed for these compounds at dropping mercury electrodes inapproximate pH conditions were similar while there are no

available equivalent studies at mercury film electrode,confirming in this way the suitability of BiFE as substituteof mercury-based electrodes.

Using the optimum plating conditions, we examined somecharacteristics of the metribuzin electrochemical reductionprocess at the BiFE and GCE. The effect of scan rate wasstudied by linear sweep voltammetry in the 10 to 441 mV/srange.

The voltammograms obtained at the BiFE (Fig. 5)showed two cathodic waves which shifted to more negativepotential with the scan rate while the peak currents have alinear relationship with the square root of the scan rate(Fig. 5 inset). This behavior indicates that the metribuzinreduction is a diffusion-controlled irreversible electrochem-ical process under these conditions. This fact would beadvantageous for the development of new methodologiesfor the determination of the metribuzin in flow systems(flow injection analysis (FIA), high-performance liquidchromatography (HPLC) and capillary electrophoresis(CE)) by cathodic amperometry using BiFE without oxygenremoval.

For an irreversible electrochemical reaction, the relation-ship between the peak potential (Ep) and the scan rate (v) inlinear sweep voltammetry is expressed by [23]

Ep ¼RT

2 a n Fln nþK (1)

where a is the transfer coefficient, n is the number ofelectrons involved in the reaction and K is a constant.According to Equation 1, the plot of Ep vs. ln v is linear andan could be calculated from its slope. Values of an of 1.57�0.06 and 1.3� 0.1 were obtained for BiFE (first wave) andGCE respectively for metribuzin reduction in 0.10 M H2

SO4, indicating that the Bi layer enhances the electrontransfer rate observed at plain GCE and is even faster when

Fig. 4. Voltammograms obtained in the presence and in theabsence of 200 mM metamitron (A), atrazine (B), metribuzin (C)and maleic hydrazide (D) in BiFE (solid lines) and GCE (dashedlines). SWV parameters: initial potential: �300 mV; step 5 mV;amplitude 25 mV; frequency 20 Hz. Bismuth plating conditions:0.10 M H2SO4; plating potential: bismuth cathodic peak; bismuthconcentration: 0.60 mM; plating time: 360 s; stirring rate 200 rpm.

Fig. 5. Effect of scan rate on the linear sweep voltammetry signalof 200 mM metribuzin in 0.10 M H2SO4 at BiFE. Relationshipbetween scan rate and wave current (inset). Scan rates: a) 10, b)25, c) 49, d) 81, e) 121, f) 169, g) 225, h) 289, i) 361, j) 441 mV s�1.Bismuth plating conditions as in Figure 4.




compared to Skopalova et al. work at dropping mercuryelectrode (an¼ 1.24 at pH 2.5) [19].

Calibration curves for the determination of metribuzin atthe BiFE and GCE were carried out by SWV under optimalcondition. The SWV for different concentrations of theherbicide were illustrated in Figure 6. Well-defined andsharp peaks with relatively low background were obtainedusing the bismuth electrode in contrast to the broader andlower GCE signals, severely interfered by dissolved oxygen.The corresponding calibration plot for the BiFE shows goodlinearity over the 10 mM to 200 mM range, and extends up to200 mM and levels off for higher metribuzin concentrations(not shown). In contrast, 10 mM metribuzin signals can notbe measured at the GCE. The sensitivity of the BiFE ishigher than the obtained using GCE, with better correlation(results summarized in Table 2). The favorable performanceof the BiFE is supported by the calculated LOD value of6 mM, about four times lower than that found for glassycarbon electrode (22 mM).

The repeatability in the signal was checked using 200 mMmetribuzin solutions in 0.10 M H2SO4 for different platedBiFE (n¼ 8) and different freshly polished GCE surfaces(n¼ 8), showing highly remarkable RSD results at BiFE incomparison with the poor precision obtained with GCE dueto the oxygen interference (Table 2). Furthermore, it shouldbe pointed out the high reproducibility (RSD¼ 3.7%) and

stability of the metribuzin signals obtained after 30 consec-utive SWV measurements using an unique bismuth film,demonstrating the high stability and suitability of theseelectrodes for the cathodic voltammetric detection ofelectroactive substances.

4. Conclusions

The prospects of electroanalytical application of bismuthfilm electrodes in air saturated acid solutions have beenillustrated for the cathodic voltammetric detection ofherbicides, where a judicious choice of the preparationconditions of bismuth films has demonstrated to play a keyrole in the their final electroanalytical performance. BiFEprepared using the proposed conditions have shown en-hanced electrochemical properties than GCE for detectingherbicides in acidic medium in the presence of oxygen, withrelatively low and stable background. The analytical applic-ability of these BiFE has been demonstrated for thecathodic voltammetric detection of the herbicide metribu-zin, showing higher sensitivity, improved reproducibilityand stability and lower detection potential than at GCE,allowing in this way lower detection limit. These resultsconstitute a necessary preliminary approach to furtherdeveloping of advanced methodologies for the determina-tion of herbicides in batch and flow systems (FIA, HPLCand CE) by cathodic amperometry without oxygen removal.

5. Acknowledgements

The authors wish to thank the Ministerio de Ciencia yTecnolog�a de Espana, Fondo de Desarrollo Regional de laUnion Europea, Comunidad Autonoma de Madrid andUniversidad Autonoma de Madrid for the financial supportof these projects (CTQ2004-06334-C02-01-02, CCG06-UAM/SEM-0124, CCG07-UAM/SEM-1752).

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Fig. 6. Effect of metribuzin concentration upon the SWV signalat BiFE and GCE in 0.10 M H2SO4. Metribuzin concentration: a)0, b) 9.0, c) 18, d) 36, e) 90, f) 125, g) 200 mM. Bismuth platingconditions as in figure 4. SWV parameters as in Figure 4.

Table 2. Calibration values of metribuzin at BiFE and GCE.

Sensitivity(mA mM�1)

Regression LOD [a](mM)

RSD [b](%)

BiFE (wave 1) 101 0.997 6 5.9GCE 68 0.98 22 12.5

[a] Calculated using the S/N¼ 3 criterion, being S the sensitivity and N thestandard deviation of 12 background signal.[b] Calculated for 8 different surfaces.




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cathodic electrochemical determination of herbicides in acid media using a bismuth film electrode

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