Graphite-Ethylene=Propylene=Diene Terpolymer CompositeElectrodes. A New Electrode Material for ElectrochemicalDetectionLuis Alonso, ConcepcioÂn Parrado, MarõÂa Pedrero, Lourdes AguÈõÂ, and Jose M. PingarroÂn*

Departamento de QuõÂmica AnalõÂtica, Facultad de Ciencias QuõÂmicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain

Received: September 30, 1998

Final version: November 18, 1998

Abstract

The preparation and electroanalytical performance of a new composite electrode material, fabricated by mixing graphite and theethylene=propylene=diene (EPD) terpolymer is reported. The voltammetric and ¯ow-injection with amperometric detection responses at thiscomposite electrode of several substances of different solubilities in water are discussed and compared with those obtained at graphite-Te¯on composite and glassy carbon electrodes. An EPD content around 2% ensures good compactness of the material and an adequateconductivity. Under ¯owing conditions, these electrodes show better signal-to-background ratios than those obtained with the otherelectrode materials tested. Furthermore, the new electrodes show very good resistance to fouling with no need of electrode surface pre-treatment. The presence of a high content of organic solvent in the carrier solution produces a sharp decrease in the amperometric currentthus limiting the advantageous use of the graphite-EPD composite electrodes to predominantly aqueous media.

Keywords: Graphite, Ethylene=propylene=diene terpolymer, Composite electrodes, Flow-injection with amperometric detection

1. Introduction

The development of new electrode materials capable ofimproving the functional characteristics of conventional electro-des such as renewability, manageability, versatility, sensitivityand low price has attracted great attention during the past [1],mainly in connection with the wider and wider use of electro-chemical detectors in ¯owing systems.

In this context, carbon-based materials have been extensivelystudied, and, besides the well-known carbon paste electrodes [2],materials such as reticulated vitreous carbon [3, 4], and compo-sites fabricated by mixing graphite with Te¯on [5, 6], Kel-F [7],PVC [8, 9], zeolites [10], sol-gel matrices [11] or paraseal wax[12] have been employed with analytical purposes.

In particular, and besides other important advantages, it hasbeen claimed that composite electrodes exhibit current densityenhancement when they are used for electrochemical detection in¯owing streams [1]. Recently, we have critically compared par-af®n carbon paste and graphite-Te¯on composite electrodesconcerning the voltammetric and ¯ow-injection amperometricbehavior of several antioxidants of different hydrophobicity [13].

This article reports on the preparation and electroanalyticalperformance of a new composite electrode material, constructedby mixing graphite and the ethylene=propylene=diene (EPD)terpolymer (poly(ethylene-co-propylene-co-5-methylene-2-nor-bornene)), a rubbery material with a 70 wt. % ethylene and a 4 wt% 5-methylene-2-norbornene content, used in the construction ofsecondary lithium ion batteries [14] offering a good bending andimpact resistance with improved surface properties. No ante-cedents were found in the literature regarding the use of thispolymer as binding agent for the fabrication of composite vol-tammetric electrodes. Various substances, differing in theirsolubility in water, were chosen as analytical probes. Thus, thevoltammetric and ¯ow-injection with amperometric detectionresponses of potassium ferrocyanide, ascorbic acid, penta-chlorophenol and the herbicide thiram (tetramethyl-thiur-amdisul®de) were examined at the graphite-EPD terpolymer

electrode and compared with those obtained at graphite-Te¯oncomposite and glassy carbon electrodes.

2. Experimental

2.1. Apparatus and Electrodes

The measurements were performed on a Eco Chemie AutolabPSTAT 10 potentiostat using the software packages GPES 3.1and 3.4.

A Metrohm 6.0805.010 glassy carbon electrode as well asgraphite (Ultra F purity, Carbon of America)-PTFE (polytetra-¯uorethylene)(Aldrich) and graphite-EPD (ethylene=propylene=diene terpolymer, Aldrich) composite electrodes were used asworking electrodes. A Ag=AgCl BAS MF2063 reference elec-trode, and a platinum wire counter electrode were also used inbatch experiments, while a Ag=AgCl=KCl 3 mol Lÿ1 Metrohm6.0727.000 reference electrode, and a Au 6.0333.010 counterelectrode were used for ¯ow injection measurements.

The ¯ow-injection arrangement consisted of a Gilson Mini-puls-3 peristaltic pump, and a Rheodyne Model 5020 injectionvalve with variable injection volumes. Detection was accom-plished by using a Metrohm EA-1096 wall-jet cell, and potentialswere controlled by means of the Eco Chemie Autolab mentionedabove.

A P-Selecta ultrasonic bath and a Carver pellet press were alsoused.

2.2. Composite Electrodes Preparation

Graphite-PTFE pellets were prepared following the procedurepreviously described [15]. Then, several 3.5 mm diameter diskportions of the pellet were bored, and each disk was press-®ttedinto a 3.5 mm i.d. Te¯on holder. Electrical contact was madethrough a stainless steel screw.

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Graphite-EPD pellets were prepared by thoroughly hand-mixing the appropriate amounts of graphite and a 1255 mg Lÿ1

solution of the terpolymer in cyclohexane (Panreac), dependingon the ®nal EPD percentage required, until cyclohexane wascompletely evaporated. Next, the mixture was pressed into pelletswith a 5.0 mm diameter Carver pellet press at 2000 Kg cmÿ2 for10 min. Afterwards, each pellet was press-®tted into a 3.5 mm i.d.te¯on holder where electrical contact, as occurred with graphite-PTFE pellets, was made through a stainless steel screw.

2.3. Reagents and Solutions

Pentachlorophenol, and thiram (Aldrich) stock solutions(1.0610ÿ3 mol Lÿ1) in methanol (Panreac), and potassium fer-rocyanide (Sigma), and ascorbic acid (Merck) stock solutions(1.0610ÿ3 mol Lÿ1) in water, were prepared weekly by weigh-ing. More dilute standards were obtained by suitable dilutionwith 0.1 mol Lÿ1 HClO4 (ascorbic acid), 0.1 mol Lÿ1 HCl(potassium ferrocyanide), and 0.1 mol Lÿ1 HPO4

2ÿ=H2PO4ÿ

(pentachlorophenol and thiram). All chemicals were of analyticalreagent grade, and the water used was obtained from a MilliporeMilli-Q puri®cation system. Acetonitrile (Panreac) was also used.

2.4. Procedures

The graphite-PTFE and graphite-EPD composite electrodeswere polished daily, ®rst for 5 s on a 150 grit SiC paper, and thenon weighing paper until a shiny surface was obtained. Thisprocedure was the same as that previously used for graphite-PTFE electrodes [5, 6]. No chemical or electrochemical regen-eration of the electrodes was then necessary during the wholeworking day.

The glassy carbon electrode was polished before each mea-surement with alumina powder (particle size lower than 0.3mm)for 1 min; then it was sonicated in water and ®nally dried with aclean tissue.

All measurements were carried out under ambient conditions.For the voltammetric studies linear sweep voltammetric scanswere initiated from 0.00 V towards more positive values.Amperometric measurements in ¯ow injection experiments wereperformed by applying a potential of �1:0 V to the electrode.Calibration plots for all the compounds under study wereobtained by injecting a 150mL aliquot of the compound stocksolution into the carrier at a ¯ow rate of 3.0 mL minÿ1.

3. Results and Discussion

As mentioned above, four different substances were selectedto test the behavior of graphite-EPD electrodes. From higherto lower solubility in water, these substances were potassiumferrocyanide, ascorbic acid, pentachlorophenol and thiram. Tak-ing into account the electroanalytical data from the literature [5,6], as well as some preliminary assays, different working media,in which these compounds have been shown to exhibit goodelectrochemical responses, were selected. Thus, aqueous solu-tions of potassium ferrocyanide and ascorbic acid, using0.1 mol Lÿ1 HCl and 0.1 mol Lÿ1 HClO4 as supporting electro-lytes, respectively, a 2 : 98 methanol: 0.1 mol Lÿ1 phosphatebuffer solution of pH 7.0 for pentachlorophenol, and a 10 : 90methanol : 0.1 mol Lÿ1 phosphate buffer solution of pH 7.4 forthiram were employed.

Table 1 summarizes the data (Ep and the peak current tobackground current ratio) obtained by cyclic voltammetry at thethree types of electrodes used. Two different percentages of EPDand Te¯on were tested in the case of composite electrodes. Ascan be observed, the oxidation response of each substanceappeared at similar potential values at the different electrodes.However, the ip=ib ratio was always considerably higher at gra-phite-EPD composite electrodes than at the other electrodematerials. This is specially true for the more hydrophilic analytechecked and, in general, this trend becomes more evident as thehydrophobicity of the compound decreased. As an example,Figure 1 shows cyclic voltammograms recorded for Fe(CN)4ÿ

6

and ascorbic acid at a graphite-2% EPD composite electrode.

3.1. Voltammetric Behavior Under Hydrodynamic

Conditions

The graphite-to-insulator (EPD, Te¯on) ratio is an importantcharacteristic of the electrode material regarding properties suchas mechanical resistance, electrical conductivity, compatibilitywith organic solvents and swelling phenomena in ¯owing sys-tems. In this case, composite pellets with EPD contents lowerthan 1% showed poor compactness which makes the electrodeconstruction dif®cult. The same drawback had been previouslyobserved for Te¯on composite electrodes with insulator percen-tages lower than 40% [6]. Furthermore, EPD contents above 10%(or 90% in the case of Te¯on) gave rise to a loss of conductivitypreventing the use of the pellets as electrode material. Therefore,EPD contents of 1, 2 and 4%, as well as Te¯on percentages of 40,60 and 80% were used for the fabrication of the compositeelectrodes, which were tested under hydrodynamic conditions forthe above-mentioned four substances.

Electroanalysis 1999, 11, No. 3

Table 1. Electroanalytical data obtained by cyclic voltammetry at different graphite-EPD and graphite-Te¯on composite electrodes. See text for theworking medium used for each compound. GC: glassy carbon electrode.

ip : ib=Ep, [V]

Electrode Fe(CN)4ÿ6 Ascorbic

acidPenthachlorophenol Thiram

Graphite-2% EPD 138=0.426 11.7=0.643 5.3=0.681 4.1=0.738Graphite-4% EPD 144=0.426 8.2=0.662 2.6=0.691 1.9=0.719Graphite-40% Te¯on 6.6=0.445 4.4=0.660 1.4=0.691 1.5=0.738Graphite-60% Te¯on 2.9=0.558 1.5=0.596 2.1=0.747 1.5=0.747GC 1.6=0.426 1.7=0.710 1.3=0.719 1.5=0.738

162 L. Alonso et al.

As an example, Figure 2 shows hydrodynamic voltammo-grams for 1.0610ÿ4 mol Lÿ1 ascorbic acid and thiram obtainedat the graphite-EPD composite electrodes with a ¯ow rate of3.0 mL minÿ1. Obviously, the carrier solution consisted in eachcase of the same working medium used for each particularcompound. As can be observed, well-de®ned current plateauswere obtained in all cases, showing no signi®cant differences inshape among them. Table 2 summarizes values for the half-wavepotential (E1=2) and the limiting current to background currentratio (il=ib) obtained from this type of voltammograms at thedifferent electrode materials tested. As expected, both, the lim-iting and the background currents decreased for the compositeelectrodes as the insulator percentage increased, and, therefore,the il=ib ratio constitutes a more appropriate parameter to checkthe performance of the different electrode materials.

In general, it can be said that the half-wave potentials for themore hydrophilic analytes (Fe(CN)4ÿ

6 and ascorbic acid)appeared at less positive potentials at graphite-EPD electrodesthan at graphite-Te¯on electrodes. Furthermore, the E1=2 valuesat the EPD composite electrodes are very similar to thoseobtained at a conventional glassy carbon electrode. However, likein cyclic coltammetry, the il=ib ratio for these compounds isnotably better at graphite-EPD composite electrodes, speciallyfor insulator percentages of 1±2%. Regarding pentachlorophenoland thiram, the more hydrophobic compounds tested, their E1=2

values are similar and even lower at graphite-Te¯on electrodesthan at graphite-EPD electrodes and GC (0.71� 0.01 V and0.91� 0.02 V, respectively), indicating a more hydrophobic nat-ure of the Te¯on composite electrodes. The il=ib ratio for pen-tachlorophenol is very similar at the different electrodes, whereasit is considerably higher at graphite-EPD electrodes in the case ofthiram.

One of the most important practical features in the behavior ofcomposite electrodes under ¯owing conditions is their stability,

that is, their resistance to fouling or their ability to yield repro-ducible responses with no need of surface regeneration pre-treatments. When consecutive hydrodynamic voltammogramswere registered with a conventional GC electrode, regeneration ofthe electrode surface by polishing was necessary to obtainreproducible voltammograms, specially for the more hydro-phobic analytes. However, no polishing was needed betweenconsecutive scans both with graphite-EPD and graphite-Te¯onelectrodes, with RSD values for E1=2 and il=ib ranging between 1and 5% (n � 5). These results indicate good reproducibility ofthe composite electrode responses with no need of pretreatmentof the electrode surface, which is specially advantageous under¯owing working conditions.

3.2. Flow-Injection with Amperometric Detection.

Analytical Characteristics

The obtention of hydrodynamic voltammograms with well-de®ned current plateaus at the composite electrodes allowed thedevelopment of ¯ow-injection with amperometric detectionanalytical methods for the four compounds tested. In order to beable to compare results, the same constant applied potential was

Electroanalysis 1999, 11, No. 3

Fig. 1. Cyclic voltammograms for Fe(CN)4ÿ6 (a) and ascorbic acid (b) at a

graphite ± 2% EPD composite electrode. Dotted lines show backgroundvoltammograms (0.1 mo Lÿ1 HCl and 0.1 mol Lÿ1 HClO4, respectively),v� 50 mVsÿ1.

Fig. 2. Hydrodynamic voltammograms obtained at graphite ± 1% (1), 2%(2) or 4% (3) EPD composite electrodes for 1.0610ÿ4 mol Lÿ1 ascorbicacid (a) and thiram (b). Dotted voltammograms correspond to thebackground voltammograms (0.1 mol Lÿ1 HClO4 and 10 : 90 methanol:0.1 mol Lÿ1 phosphate buffer solution of pH 7.4 for ascorbic acid andthiram, respectively); ¯ow rate, 3.0 mL minÿ1.

Graphite-Ethylene=Propylene=Diene Terpolymer Composite Electrodes 163

selected for all electrode materials and analytes. So, this potentialshould be high enough to allow the oxidation of these com-pounds and to be placed in the limiting current zone for each ofthem. A potential value of �1:00 V accomplished these condi-tions and, therefore, it was chosen to carry out the amperometricdetection under ¯ow-injection conditions.

Figure 3 shows calibration plots for Fe(CN)4ÿ6 (a) and penta-

chlorophenol (b) obtained at different electrode materials. As canbe observed, a considerable increase in the slope of the calibra-tion plots was produced when the graphite-EPD compositeelectrode was used as working electrode, which implies a highersensitivity assuming that the active area approaches the geometric

area of the electrode. The analytical characteristics of the corre-sponding calibration plots at the graphite-EPD composite elec-trode are summarized in Table 3. Relative standard deviationvalues were calculated from ten different solutions at a con-centration level of 1.0610ÿ6 mol Lÿ1 of each analyte. Moreover,the RSD values obtained for the ip measurements from 50repetitive injections of each analyte at the same concentrationlevel are also shown in Table 3, these indicate good resistance tofouling of the graphite-EPD composite electrode under ¯owingconditions. Limits of determination and detection were calculatedaccording to the 10 s [16] and 3 sb=m criteria [17], respectively,where m is the slope of the calibration plot and sb is the standarddeviation (n � 10) of the signals from 1.0610ÿ6 mol Lÿ1 ana-lyte. As can be deduced, similar ranges of linearity and detectionlimits were obtained for all the compounds tested independentlyof their hydrophobicity. These analytical characteristics areslightly better than those achieved with a conventional GCelectrode provided that a regeneration of the electrode surfacewas accomplished after each set of measurements when this latterelectrode material was used.

These results show fairly well that the developed graphite-EPDcomposite electrodes can be advantageously used as workingelectrodes for the amperometric detection of different types ofanalytes (hydrophilic or hydrophobic) under ¯owing conditions.

Finally, the changes in the amperometric responses of thecomposite electrodes with the decrease in the polarity of thecarrier solution, due to the presence of increasing percentages oforganic solvents such as methanol or acetonitrile, were alsochecked. As an example, Figure 4 shows the dependence of ip forascorbic acid and pentachlorophenol on the amount of theseorganic solvents in the carrier solution. As can be observed, thepeak current decreased when the percentage of organic solvent inthe carrier solution increased, which agrees with the behaviorpreviously reported for graphite-Te¯on composite electrodes[15]. This trend was much more pronounced in the case of thepresence of acetonitrile. Actually, methanol contents higher than30% for ascorbic acid or 40% for pentachlorophenol, or acet-onitrile contents higher than 20% for both compounds gave riseto such a big decrease in the ip=ib ratio that no useful amperometricresponses at the graphite-EPD composite electrode could beobtained. However, low but still useful signals could be measuredat a glassy carbon electrode even for methanol or acetonitrilecontents of 50±60%. This behavior can be extended, in general,also for Fe (CN)4ÿ

6 and thiram. In conclusion, the developedgraphite-EPD composite electrode exhibited an advantageousbehavior with respect to conventional glassy carbon electrodeswhen the carrier solution (or mobile phase) was constituted bymethanol:water mixtures with a low organic solvent content.

Fig. 3. Calibration plots for Fe(CN)4ÿ6 and pentachlorophenol obtained at

a graphite-2% EPD (m), a graphite - 60 % Te¯on (j), and glassy carbon(d) electrode by ¯ow-injection with amperometric detection; ¯ow rate:3.0 mL minÿ1, Vi� 150 mL; Eapp� � 1.00 V.

Table 2. E1=2 and limiting-to-background current ratio (il=ib) obtained by hydrodynamic voltammetry at graphite-EPD and graphite-Te¯on compositeelectrodes. Flow rate: 3.0 mL minÿ1; concentration for each compound: 1.0610ÿ4 mol Lÿ1.

E1=2 [V]=il : ib

Electrode Fe(CN)4ÿ6 Ascorbic acid Pentachlorophenol Thiram

Graphite-1% EPD 0.43� 0.02=20.47� 0.07 0.66� 0.01=9.2� 0.7 0.69� 0.02=1.6� 0.2 0.802� 0.006=10.1� 0.8Graphite-2% EPD 0.41� 0.01=13.17� 0.05 0.73� 0.7=8.8� 0.2 0.71� 0.01=3.0� 0.1 0.802� 0.006=3.81� 0.04Graphite-4% EPD 0.40� 0.01=9.51� 0.09 0.73� 0.2=5.4� 0.2 0.68� 0.02=1.80� 0.05 0.738� 0.006=4.61� 0.03Graphite-40% Te¯on 0.41� 0.02=3.86� 0.05 0.88� 0.2=5.2� 0.2 0.79� 0.01=1.82� 0.09 0.78� 0.01=1.10� 0.09Graphite-60% Te¯on 0.42� 0.02=2.92� 0.02 0.88� 0.1=5.6� 0.1 0.71� 0.01=2.18� 0.06 0.79� 0.01=1.9� 0.1Graphite-80% Te¯on 0.41� 0.01=2.763� 0.009 0.98� 0.02=4.43� 0.09 0.68� 0.02=2.37� 0.02 0.64� 0.02=2.43� 0.04

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164 L. Alonso et al.

4. Conclusions

All the above results demonstrate that the composite materialconstituted by a mixture of graphite and the ethylene=propylene=diene terpolymer can be used as voltammetric elec-trode material as well as indicator electrode under ¯owing con-ditions. EPD contents around 2% ensure a good compactness ofthe material, which facilitates the electrode fabrication, and alsoan adequate conductivity. These electrodes exhibit very goodsignal-to-background ratios for analytes of different hydro-phobicity, although this advantage is specially remarkable as thesolubility in water of the target compound is higher. Furthermore,the composite EPD electrodes yielded reproducible results withno need of pretreatment of the electrode surface, which is spe-

cially advantageous when comparing with a conventional elec-trode material such as glassy carbon. However, the presence of ahigh content of organic solvent in the working solution gives riseto a dramatic decrease in the amperometric response under¯owing conditions, which limits the advantageous use of theseelectrodes to predominantly aqueous media.

5. Acknowledgements

The ®nancial support of the SubdireccioÂn General de Forma-cioÂn y PromocioÂn del Conocimiento, Project PD96-0640 isgratefuly acknowledged.

Table 3. Analytical characteristics of the calibration plots obtained by ¯ow-injection with amperometric detection at graphite-2% EPD compositeelectrodes. Carrier solution: 0.1 mol Lÿ1 HCl (Fe(CN)4ÿ

6 ), 0.1 mol Lÿ1 HClO4 (ascorbic acid), 2 : 98 methanol: 0.1 mol Lÿ1 phosphate buffer pH 7.0(pentachlorophenol), and 10 : 90 methanol: 0.1 mol Lÿ1 phosphate buffer of pH 7.4 (thiram); ¯ow rate: 3.0 mL minÿ1, Vi� 150 mL; Eapp� � 1.00 V.

Fe(CN)4ÿ6 Ascorbic acid Pentachlorophenol Thiram

Linear range [mM] 0.7±10 0.5±10 1±10 0.5±8r 0.998 0.999 0.998 0.994Slope [mA L m molÿ1] 0.037� 0.001 0.033� 0.001 0.033� 0.001 0.10� 0.04Intercept [mA] 7 0.018� 0.006 7 0.005� 0.002 7 0.008� 0.007 0.0� 0.2Limit of determination [mM] 1.6 1.9 1.5 1.0Limit of detection [mM] 0.5 0.5 0.5 0.3RSD [%] [a]=[b] 8.3=5.6 8.6=5.9 7.5=7.4 4.3=6.9

[a] calculated from 10 different 1.0610ÿ6 mol Lÿ1 solutions of each analyte; [b] calculated from 50 repetitiveinjections of each analyte at a 1.0610ÿ6 mol Lÿ1 concentration level.

Fig. 4. Dependence of peak current for ascorbic acid (a), and pentachlorophenol (b), on the acetonitrile and methanol content in the carrier solution,obtained at graphite-2% EPD (m) and glassy carbon (d) electrodes by ¯ow-injection with amperometric detection. Other conditions as in Figure 3.

Electroanalysis 1999, 11, No. 3

Graphite-Ethylene=Propylene=Diene Terpolymer Composite Electrodes 165

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