- Email: [email protected]
Cuprite-modified electrode for the detection of iodide species
Sensors and Actuators B 59 Ž1999. 113–117 www.elsevier.nlrlocatersensorb
Cuprite-modified electrode for the detection of iodide species G. Lefevre, J. Bessiere, A. Walcarius
)
Laboratoire de Chimie Physique pour l’EnÕironnement, Unite´ Mixte de Recherche UMR 7564, CNRS-UniÕersite´ H. Poincare´ Nancy I, 405, rue de VandoeuÕre, F-54600 Villers-les-Nancy, France
Abstract A cuprite-modified carbon paste electrode was evaluated as an electrochemical sensor for iodide species in aqueous medium. The overall analysis involved a two-step procedure: an open-circuit accumulation step followed by voltammetric quantification. In the preconcentration step, iodide was accumulated on cuprite ŽCu 2 O. according to a surface precipitation mechanism leading to the formation of CuI. This solid was then detected either in the cathodic or in the anodic mode, the first process allowing multiple successive analyses without requiring any regeneration of the electrode surface, while the anodic scan resulted in the surface oxidation of Cu 2 O requiring the renewal of the electrode surface before any further accumulation experiment. The influence of various experimental parameters on the sensor response was investigated Ži.e., pH, preconcentration time, detection mode, iodide concentration, Cu 2 O content into the paste.. Reproducible results were obtained after optimization: a linear calibration was obtained in the 1 = 10y6 M to 2 = 10y5 M concentration range, with a detection limit of 5 = 10y7 M. The effect of chloride interference was also discussed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Cuprite; Iodide; Carbon paste; Preconcentration; Voltammetric detection
1. Introduction Iodine is an essential trace element playing an important role in mental development, growth and basic metabolisms w1x. In the same time, iodine species belongs to the toxic elements, especially because one of its radioactive isotopes, 129 I, is characterized by a long half-life Ž1.7 = 10 7 years., as well as weakly adsorbed on minerals w2x, making it able to reach the biosphere before decaying to insignificant levels. Therefore, in the context of radioactive waste management, it is important to have sensors at one’s disposal to detect any leaching of iodine species from containers stored in geological sites. Several methods have been worked out for the determination of total iodine, iodide and iodate, but most of them were expensive and relatively complicated Žsuch as gas or ion chromatography, inductively coupled plasma atomic emission spectroscopy or mass spectrometry, neutron activation analysis., and there is a lack of methods for the specific determination of iodide. Some electrochemical procedures were proposed recently. Besides the well-known cathodic stripping voltammetry using mercury electrodes
)
Corresponding author. [email protected]
Fax:
q 33-3-8327-5444;
E-mail:
w3x, they were most often based on anodic or adsorptive stripping voltammetry, applying either potentiostatic accumulation w4,5x or preconcentration at open-circuit through the formation of ion pairs at chemically modified electrodes w6,7x or using ion exchange resins w8x. Electrodesupported bilayer lipid membrane containing buckminsterfullerene was also reported w9x. The aim of the present study was to evaluate a carbon paste electrode modified with cuprite ŽCu 2 O. for the voltammetric determination of iodide, by exploiting the ability of Cu 2 O to accumulate Iy via the formation of iodide-contained surface precipitates w10,11x.
2. Experimental Iodide solutions were prepared from analytical grade KI ŽAldrich. and high-purity water Ž18 M V cmy1 . from a millipore milliQ water purification system. Cuprous oxide, Cu 2 O, was reagent grade ŽFluka. and treated in slightly acidic medium Ž30 g Cu 2 O in 250 ml HClO4 10y4 M during 12 h. before use for turning the mineral surfaces from hydrophobic to hydrophilic in order to assure proper hydration of Cu 2 O particles. HClO4 was chosen for its inertness with respect to any redox alteration of Cu 2 O and
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 0 6 - 3
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G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117
because the ClO4y ion was not involved in any adsorption process on the cuprite surface, as checked by X-ray photoelectron spectroscopy. Other chemicals were of analytical grade. Cuprite-modified carbon paste electrode ŽCu 2 O-MCPE. was prepared by mixing weighed amounts of Cu 2 O, high purity graphite ŽAlfa., and mineral oil ŽAldrich. thoroughly until obtaining an uniformly wetted paste. Portions of the resulting composite material were then packed into the end of a home-made PTFE cylindrical tube equipped with a screwing stainless steel piston. The surface was smoothed on a piece of weighing paper. Voltammetry was conducted with a Model 283 potentiostat galvanostat monitored by the M270 electrochemical research software ŽEG & G Princeton Applied Research.. Measurements were performed using a conventional single-compartment cell assembled with Cu 2 O-MCPE as working electrode, a saturated calomel reference electrode, and a Pt wire auxiliary electrode. All voltammograms were obtained with unstirred solutions, maintained under inert ŽN2 . atmosphere. Cyclic voltammetric experiments were usually performed in iodide solution containing 10y1 M Na 2 SO4 as the supporting electrolyte, after selected periods of time owing to the accumulation of Iy to take place at Cu 2 O particles. For analytical determination of the concentration of Iy in aqueous samples the following procedure has to be followed: Ž1. preconcentration was achieved from stirred solutions containing the target analyte at a given concentration and selected pH; Ž2. the electrode was then removed from the accumulation cell, rinsed with water, and transferred to the separate voltammetric cell containing only a supporting electrolyte Ž10y1 M Na 2 SO4 . where voltammetric monitoring was performed in the square wave mode. When cathodic detection was applied, the same electrode surface can be reused without any treatment, while the analysis in the anodic direction required a renewal of the surface by mechanical smoothing before each measurement, as usual for sensors based on carbon paste w12x, due to the alteration of the Cu 2 O surface by the formation of a CuO overlayer.
tion up to q1.0 V, and especially in the potential range q0.40 to q0.60 V where CuI oxidation is expected to occur w11x. Moreover, there was no cathodic peak in the potential range y0.5 to y0.8 V which is known to correspond to the reduction of iodide-contained surface precipitates on Cu 2 O-MCPE w11x. To demonstrate that the Cu 2 O-MCPE was sensitive to Iy species after their accumulation at open-circuit, its response was compared to that of a plain, freshly polished unmodified carbon paste electrode. Fig. 1a show the responses recorded at the plain electrode by scanning potentials towards the negative ŽA. or the positive ŽB. direction. As expected, no significant change in the electrode response was observed in the cathodic scan, because Iy cannot be reduced, while the anodic scan revealed the classical response of Iy at carbon paste revealing the reversible transformation of iodide into molecular iodine w4x. The behaviour of Cu 2 O-MCPE was studied in both the anodic and cathodic modes after selected periods of time allowed to the electrode to contact the solution Žaccumulation times.. Here, as shown in Fig. 1b, each increase in the time afforded to the electrode to contact the analyte solution gave rise to significant increase of the voltammetric peaks, either in the cathodic or anodic modes. The comparison with the unmodified electrode demonstrates clearly the uptake capacity of Cu 2 O with respect to iodide, with the concomitant ability of Cu 2 O-MCPE to detect the accumulated Iy species. In the presence of protons ŽpH - 7., the accumulation of Iy on Cu 2 O is known to occur via the formation of a CuI surface precipitate according to Eq. Ž1. w11x:
m 2CuI
Cu 2 OŽs. q 2Hqq 2Iy
Žs. q H 2 O
Ž 1.
where subscript «s» refers to the solid phase. The voltammetric peaks observed in Fig. 1b are therefore related to
3. Results and discussion The electrochemical response of carbon composite electrodes modified with copper oxides was previously characterised in alkaline medium in the goal to exploit the catalytic properties of these materials with respect to the amperometric detection of various organics w13–16x. In 0.1 M NaOH, a small broad peak at about q0.15 V was sometimes observed in the first cyclic voltammetric scan, and attributed to the formation of CuO, while disappeared in subsequent scans or by decreasing the NaOH concentration w13x. This is consistent with our observation that no significant electrochemical activity of Cu 2 O-MCPE was observed in 10y1 M Na 2 SO4 ŽpH 6. in the anodic direc-
Fig. 1. Cyclic voltammograms of 5.0=10y5 M KI recorded in 0.1 M Na 2 SO4 , in ŽA. the cathodic and ŽB. the anodic modes, using Ža. an unmodified carbon paste electrode and Žb. a 10%-Cu 2 O-modified carbon paste electrode ŽCu 2 O-MCPE.. Curves obtained with Cu 2 O-MCPE were recorded Ž1. 2 min, Ž2. 5 min, Ž3. 10 min and Ž4. 25 min after immersion of the electrode into the solution. Sweep rate: 50 mV sy1 .
G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117 Table 1 Iodide-uptake of Cu 2 O as a function of pH Ž1.5 g Cu 2 O in 30 ml 10y4 M KI. pH
Uptake Žmmol gy1 . a
6.5 6.8 7.2 7.7 8.0 8.5 9.0
1.78 Ž89. 1.52 Ž76. 1.22 Ž61. 0.88 Ž44. 0.74 Ž37. 0.60 Ž30. 0.42 Ž21.
a
Figures in parentheses are the uptake as a percentage of the maximum.
the redox processes of CuI. In the cathodic mode, CuI is reduced into metallic copper ŽEq. Ž2.. at y0.50 V, the peak potential being shifted to the negative direction for larger amount of CuI due to the longer time required for its complete reduction. In the anodic mode, however, both «CuŽqI. » and «I ŽyI. » elements are oxidised between q0.4 V and q0.6 V Žfor the same reason as above., leading to the formation of molecular iodine and copperŽII. oxide ŽEq. Ž3..; this potential range is consistent with values corresponding to the oxidation of Iy ŽFig. 1BŽa.. and CuŽI. w13x. The presence of free iodide was only observed when small currents were recorded for CuI oxidation, while occluded in the main peak at high accumulation degrees.
m Cu
CuI q 1ey y
Ž0 .
q
q Iy
CuI y 2e y 2H q H 2 O
m 1r2 I q CuO 2
115
Reduction of CuI was slightly influenced by the composition of the detection medium. The effect of pH was investigated and higher peak currents were observed in the 3–5 range so that pH 4 was selected as the most appropriate. At the same time, the preconcentration efficiency was found to be dramatically affected by pH, the presence of protons tending to favour the leaching of soluble copperŽI., as expected from the Pourbaix Diagram w17x, which would result in the formation of larger quantities of CuI as decreasing pH ŽTable 1.. This is shown in Fig. 2 where largest currents were clearly obtained for the lowest pH, down to pH 3 for the accumulation from a 1 = 10y3 M Iy solution and down to pH 4 for 2 = 10y5 M Iy. The same results were obtained from buffered and not buffered solutions, because the very low Cu 2 O-to-solution ratio and the continuous stirring of the solution maintain a constant
Ž 2. Ž 3.
As a consequence, the analysis performed in the anodic mode induced the formation of a CuO deposit on Cu 2 O which prevented any further accumulation of Iy species because they cannot be accumulated on cupric oxide, requiring therefore a mechanical surface renewing step between each preconcentration experiment. Nevertheless, successive measurements were allowed when working in the cathodic mode Ž7% standard deviation Ž n s 5. for the detection of 0.05 mM Iy at pH 4.0., because no alteration of the Cu 2 O surface other than that induced by Iy uptake was happening. It should be only ensured that the electrogenerated metallic copper deposit was entirely re-oxidised on scan reversal. Both anodic and cathodic peak currents were found to be directly proportional to the sweep rate Žin the range 5–200 mV sy1 . indicating a thin layer behaviour which again sustains the formation of a solid phase upon Iy accumulation on cuprite. For reasons mentioned above, detection in the cathodic mode was applied throughout. Despite this mode allowed in situ analysis Ži.e., both preconcentration and detection in the same medium., it was chosen to use two different cells for the accumulation and for the detection in order to investigate separately the factors affecting these two steps and to define the optimum conditions for the analytical procedure.
Fig. 2. Influence of pH on the detection of iodide accumulated at the Cu 2 O-MCPE. Preconcentration experiments were performed ŽA. during 1 min in a solution containing 1=10y3 M KI, and ŽB. during 5 min in the presence of 2=10y5 M KI.
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G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117
proton concentration in the diffusion layer, in spite of their consumption in the adsorption process according to Eq. Ž1.. This is therefore not surprising that the optimum pH for the accumulation of iodide was found to correspond to a proton concentration very close to that of the analyte ŽIy. : pH 3 for wIyx s 1 = 10y3 M and pH 4–4.5 for wIyx s 2 = 10y5 M. By decreasing pH further, however, a slight decrease in the uptake efficiency was observed because of the production of larger amounts of CuŽqI. species: some of them are subjected to dismutation before hitting an iodide anion or even can be lost in solution by diffusing rapidly far away from the electrode surface, limiting somewhat the deposition of CuI on this surface. Another avenue for exploiting the attractive uptake behaviour of Cu 2 O-MCPE is its use in square wave voltammetry ŽSWV. which led to significant improvement in sensitivity for CuI detection. Typically, a six-fold increase in the sensitivity was achieved by passing from linear sweep voltammetry to SWV, for the analysis of Iy in the 5 = 10y6 M to 5 = 10y5 M concentration range after 15 min accumulation. This detection mode was chosen for further experiments. The detection limit of Cu 2 O-MCPE was strongly affected by the accumulation time of Iy at Cu 2 O particles located at the electrode surface. To evaluate this effect, the accumulation time of the DPV for various copper concentrations Žfrom 5 = 10y6 M to 1 = 10y4 M. was varied between 2 and 120 min. Plots of peak currents Žor charges. against accumulation time exhibited a saturation curve where saturation currents were attained as faster as higher was the concentration of Iy. For example, at 8 = 10y5 M, the peak area corresponding to saturation was measured to be about 1.4 mC and the calculated ‘half peak time’ was approximately 25 min. At lower concentration and for short accumulation times, the relationship between accumulation time and voltammetric signals was almost linear ŽFig. 3.. The total amount of Iy accumulated on the electrode was calculated from the peak area. After normal-
isation with respect to Iy concentration, the plot of the CuI amount against accumulation time resulted in a straight line, indicating that iodide is accumulated at a constant rate. The amount of Cu 2 O in the paste was also found to affect peak currents. Peaks were rapidly growing up to 5 mass% Cu 2 O, then slowly levelling off between 10% and 20%, and finally decreasing at higher contents. Such behaviour can be explained by considering the uptake mechanism involving the production of CuŽI. and the subsequent formation of CuI. At very low Cu 2 O contents, the accumulation of Iy is limited by the amount of available CuŽI. species and therefore by the amount of Cu 2 O located at the electrode surface. When this amount becomes sufficient to accumulate all the Iy species located in the diffusion layer, a constant response of the electrode is observed. Finally, too high Cu 2 O contents result in an increase in the resistance of the electrode Žconcomitantly with a decrease in its active surface. which leads to lower peak currents. Keeping in mind all the above results, a calibration plot of peak currents against iodide concentration can be obtained. For a 15-min accumulation period in iodide solutions maintained at pH 3.6, using a 10%-Cu 2 O-MCPE, and detection by SWV at pH 4.0, linear evolution of peak currents was observed in the 1 = 10y6 M to 2 = 10y5 M concentration range. A detection limit of 5 = 10y7 M was achieved. In relation to the fact that chloride is probably the most significant interferent for iodide determination in underground waters, the effect of this species on the detection of iodide at Cu 2 O-MCPE was evaluated. For this purpose, various iodide concentrations Ž1 = 10y5 M, 5 = 10y5 M, 1.6 = 10y4 M and 5 = 10y4 M. were spiked with increasing amounts of NaCl to get chloride concentrations ranging from 1 = 10y4 M to 0.5 M. It was observed that chloride did not appear to have any influence on iodide detection at concentrations up to 100 times that of iodide. In addition, the sensor was found to be selective for iodide over bromide species, since the detection of 5 = 10y5 M Iy was not affected by the presence of 10y3 M Bry.
4. Conclusion
Fig. 3. Dependence of the Cu 2 O-MCPE response on the preconcentration time for the detection of Ža. 0.5=10y5 M and Žb. 2.0=10y5 M iodide at pH 4. Square wave voltammograms were obtained at a frequency of 20 Hz, a pulse amplitude of 25 mV and a step potential of 2 mV.
The Cu 2 O-modified carbon paste electrode is suitable for the voltammetric detection of iodide species after their accumulation at open-circuit. This was achieved by exploiting the ability of Cu 2 O to take up Iy and the electrochemical activity of the resulting surface precipitate, CuI, in both the cathodic and anodic modes. These two modes gave similar sensitivity, but the detection in the cathodic direction was preferred because allowing in situ analysis. The sensor exhibited the highest response when preconcentration and detection were performed at pH values between 3 and 4. The voltammetric response was linear in the
G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117
1 = 10y6 M to 2 = 10y5 M concentration range for a 15 min preconcentration time at open circuit. Significant selectivity was observed over chloride and bromide.
Acknowledgements We are grateful to the «Agence Nationale pour la Gestion des Dechets Radioactifs» ŽANDRA. for financial ´ support.
References w1x N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1984. w2x M. Sazarashi, Y. Ikeda, Adsorption of Iy ions on minerals for 129I waste management, J. Nucl. Sci. Technol. 31 Ž1994. 620–622. w3x R.C. Propst, Cathodic pulse stripping analysis of iodine at the parts-per-billion level, Anal. Chem. 49 Ž1977. 1199–1205. w4x I. Svancara, J. Konvalina, K. Schachl, K. Kalcher, K. Vytras, Stripping voltammetric determination of iodide with synergistic accumulation at a carbon paste electrode, Electroanalysis 10 Ž1998. 435–441. w5x J. Konvalina, I. Svancara, K. Vytras, K. Kalcher, Study of conditions for voltammetric determinations of iodine at carbon paste electrodes, Sci. Pap. Univ. Pardubice, Ser. A 3 Ž1997. 153–162. w6x L. Guanghan, Y. Min, Z. Qifang, W. Ailan, J. Zexiang, Anodic stripping voltammetry for the determination of trace iodide, Electroanalysis 7 Ž1995. 591–593. w7x M.-K. Kwak, D.-S. Park, M.-S. Won, Y.-B. Shim, Anodic differential pulse voltammetric analysis of iodide with a cinchonine-modified carbon paste electrode, Electroanalysis 8 Ž1996. 680–684. w8x K. Kalcher, Voltammetrische bestimmung von iodid mit einer mit anionenaustauschern modifizierten kohlepasteelektrode, Fresenius Z. Anal. Chem. 321 Ž1985. 666–670. w9x L.-G. Wang, X. Wang, A.L. Ottova, H.T. Tien, Iodide sensitive sensor based on a supported bilayer lipid membrane containing a cluster form of carbon Žfullerene C60., Electroanalysis 8 Ž1996. 1020–1022.
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w10x Z. Haq, G.M. Bancroft, W.S. Fyfe, G. Bird, V.J. Lopata, Sorption of iodide on copper, Environ. Sci. Technol. 14 Ž1980. 1106–1110. w11x G. Lefevre, A. Walcarius, J. Bessiere, ` Voltammetric investigation of iodide sorption on cuprite dispersed into a carbon paste electrode, Electrochim. Acta 44 Ž1999. 1817–1826. w12x K. Kalcher, J.-M. Kauffmann, J. Wang, I. Svancara, K. Vytras, C. Neuhold, Z. Yang, Sensors based on carbon paste in electrochemical analysis: a review with particular emphasis on the period 1990–1993, Electroanalysis 7 Ž1995. 5–22. w13x Y. Xie, C.O. Huber, Electrocatalysis and amperometric detection using an electrode made of copper oxide and carbon paste, Anal. Chem. 63 Ž1991. 1714–1719. w14x Q. Chen, J. Wang, G. Rayson, B. Tian, Y. Lin, Sensor array for carbohydrates and amino acids based on electrocatalytic modified electrodes, Anal. Chem. 65 Ž1993. 251–254. w15x X. Huang, J.J. Pot, W.Th. Kok, Electrochemical characteristics of conductive carbon cement as matrix for chemically modified electrodes, Anal. Chim. Acta 300 Ž1995. 5–14. w16x T.R.I. Cataldi, D. Centonze, Development of a carbon composite electrode made from polyethylene and graphite powder modified with copperŽI. oxide, Anal. Chim. Acta 326 Ž1996. 107–115. w17x M. Pourbaix, Atlas d’equilibres electrochimiques, GV Paris, 1963. ´ ´ G. LefeÕre is currently a PhD student in Chemistry at the «Universite´ ` Henri Poincare», ´ Nancy I, France. His research subject focuses mainly on the sorption properties of minerals towards iodine species and on the development of new sensors for Iy. J. Bessiere ` received his PhD in Analytical Chemistry from the University Pierre and Marie Curie, Paris, in 1969. He joined the University Henri Poincare, ´ Nancy, in 1973 where he is presently professor of Analytical Chemistry. His main interests are in electroanalytical chemistry applied to the liquidrsolid interfaces, environmental chemistry and sensors. A. Walcarius received his PhD in chemistry from the «Facultes ´ Universitaires Notre-Dame de la Paix» at Namur, Belgium, in 1994. His postgraduate research at the New Mexico State University, Las Cruces, USA, involved the bioelectroanalytical applications of zeolite-modified electrodes. In 1995, he joined the Laboratory of Physical Chemistry for the Environment, where he is currently employed as a research scientist by the National Centre for Scientific Research ŽCNRS.. His present research interests include the development of new electrochemical methodologies for in situ investigations of mass transfer reactions at the solidrliquid interfaces and their applications as amperometric sensors.
Cuprite-modified electrode for the detection of iodide species G. Lefevre, J. Bessiere, A. Walcarius
)
Laboratoire de Chimie Physique pour l’EnÕironnement, Unite´ Mixte de Recherche UMR 7564, CNRS-UniÕersite´ H. Poincare´ Nancy I, 405, rue de VandoeuÕre, F-54600 Villers-les-Nancy, France
Abstract A cuprite-modified carbon paste electrode was evaluated as an electrochemical sensor for iodide species in aqueous medium. The overall analysis involved a two-step procedure: an open-circuit accumulation step followed by voltammetric quantification. In the preconcentration step, iodide was accumulated on cuprite ŽCu 2 O. according to a surface precipitation mechanism leading to the formation of CuI. This solid was then detected either in the cathodic or in the anodic mode, the first process allowing multiple successive analyses without requiring any regeneration of the electrode surface, while the anodic scan resulted in the surface oxidation of Cu 2 O requiring the renewal of the electrode surface before any further accumulation experiment. The influence of various experimental parameters on the sensor response was investigated Ži.e., pH, preconcentration time, detection mode, iodide concentration, Cu 2 O content into the paste.. Reproducible results were obtained after optimization: a linear calibration was obtained in the 1 = 10y6 M to 2 = 10y5 M concentration range, with a detection limit of 5 = 10y7 M. The effect of chloride interference was also discussed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Cuprite; Iodide; Carbon paste; Preconcentration; Voltammetric detection
1. Introduction Iodine is an essential trace element playing an important role in mental development, growth and basic metabolisms w1x. In the same time, iodine species belongs to the toxic elements, especially because one of its radioactive isotopes, 129 I, is characterized by a long half-life Ž1.7 = 10 7 years., as well as weakly adsorbed on minerals w2x, making it able to reach the biosphere before decaying to insignificant levels. Therefore, in the context of radioactive waste management, it is important to have sensors at one’s disposal to detect any leaching of iodine species from containers stored in geological sites. Several methods have been worked out for the determination of total iodine, iodide and iodate, but most of them were expensive and relatively complicated Žsuch as gas or ion chromatography, inductively coupled plasma atomic emission spectroscopy or mass spectrometry, neutron activation analysis., and there is a lack of methods for the specific determination of iodide. Some electrochemical procedures were proposed recently. Besides the well-known cathodic stripping voltammetry using mercury electrodes
)
Corresponding author. [email protected]
Fax:
q 33-3-8327-5444;
E-mail:
w3x, they were most often based on anodic or adsorptive stripping voltammetry, applying either potentiostatic accumulation w4,5x or preconcentration at open-circuit through the formation of ion pairs at chemically modified electrodes w6,7x or using ion exchange resins w8x. Electrodesupported bilayer lipid membrane containing buckminsterfullerene was also reported w9x. The aim of the present study was to evaluate a carbon paste electrode modified with cuprite ŽCu 2 O. for the voltammetric determination of iodide, by exploiting the ability of Cu 2 O to accumulate Iy via the formation of iodide-contained surface precipitates w10,11x.
2. Experimental Iodide solutions were prepared from analytical grade KI ŽAldrich. and high-purity water Ž18 M V cmy1 . from a millipore milliQ water purification system. Cuprous oxide, Cu 2 O, was reagent grade ŽFluka. and treated in slightly acidic medium Ž30 g Cu 2 O in 250 ml HClO4 10y4 M during 12 h. before use for turning the mineral surfaces from hydrophobic to hydrophilic in order to assure proper hydration of Cu 2 O particles. HClO4 was chosen for its inertness with respect to any redox alteration of Cu 2 O and
0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 2 0 6 - 3
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G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117
because the ClO4y ion was not involved in any adsorption process on the cuprite surface, as checked by X-ray photoelectron spectroscopy. Other chemicals were of analytical grade. Cuprite-modified carbon paste electrode ŽCu 2 O-MCPE. was prepared by mixing weighed amounts of Cu 2 O, high purity graphite ŽAlfa., and mineral oil ŽAldrich. thoroughly until obtaining an uniformly wetted paste. Portions of the resulting composite material were then packed into the end of a home-made PTFE cylindrical tube equipped with a screwing stainless steel piston. The surface was smoothed on a piece of weighing paper. Voltammetry was conducted with a Model 283 potentiostat galvanostat monitored by the M270 electrochemical research software ŽEG & G Princeton Applied Research.. Measurements were performed using a conventional single-compartment cell assembled with Cu 2 O-MCPE as working electrode, a saturated calomel reference electrode, and a Pt wire auxiliary electrode. All voltammograms were obtained with unstirred solutions, maintained under inert ŽN2 . atmosphere. Cyclic voltammetric experiments were usually performed in iodide solution containing 10y1 M Na 2 SO4 as the supporting electrolyte, after selected periods of time owing to the accumulation of Iy to take place at Cu 2 O particles. For analytical determination of the concentration of Iy in aqueous samples the following procedure has to be followed: Ž1. preconcentration was achieved from stirred solutions containing the target analyte at a given concentration and selected pH; Ž2. the electrode was then removed from the accumulation cell, rinsed with water, and transferred to the separate voltammetric cell containing only a supporting electrolyte Ž10y1 M Na 2 SO4 . where voltammetric monitoring was performed in the square wave mode. When cathodic detection was applied, the same electrode surface can be reused without any treatment, while the analysis in the anodic direction required a renewal of the surface by mechanical smoothing before each measurement, as usual for sensors based on carbon paste w12x, due to the alteration of the Cu 2 O surface by the formation of a CuO overlayer.
tion up to q1.0 V, and especially in the potential range q0.40 to q0.60 V where CuI oxidation is expected to occur w11x. Moreover, there was no cathodic peak in the potential range y0.5 to y0.8 V which is known to correspond to the reduction of iodide-contained surface precipitates on Cu 2 O-MCPE w11x. To demonstrate that the Cu 2 O-MCPE was sensitive to Iy species after their accumulation at open-circuit, its response was compared to that of a plain, freshly polished unmodified carbon paste electrode. Fig. 1a show the responses recorded at the plain electrode by scanning potentials towards the negative ŽA. or the positive ŽB. direction. As expected, no significant change in the electrode response was observed in the cathodic scan, because Iy cannot be reduced, while the anodic scan revealed the classical response of Iy at carbon paste revealing the reversible transformation of iodide into molecular iodine w4x. The behaviour of Cu 2 O-MCPE was studied in both the anodic and cathodic modes after selected periods of time allowed to the electrode to contact the solution Žaccumulation times.. Here, as shown in Fig. 1b, each increase in the time afforded to the electrode to contact the analyte solution gave rise to significant increase of the voltammetric peaks, either in the cathodic or anodic modes. The comparison with the unmodified electrode demonstrates clearly the uptake capacity of Cu 2 O with respect to iodide, with the concomitant ability of Cu 2 O-MCPE to detect the accumulated Iy species. In the presence of protons ŽpH - 7., the accumulation of Iy on Cu 2 O is known to occur via the formation of a CuI surface precipitate according to Eq. Ž1. w11x:
m 2CuI
Cu 2 OŽs. q 2Hqq 2Iy
Žs. q H 2 O
Ž 1.
where subscript «s» refers to the solid phase. The voltammetric peaks observed in Fig. 1b are therefore related to
3. Results and discussion The electrochemical response of carbon composite electrodes modified with copper oxides was previously characterised in alkaline medium in the goal to exploit the catalytic properties of these materials with respect to the amperometric detection of various organics w13–16x. In 0.1 M NaOH, a small broad peak at about q0.15 V was sometimes observed in the first cyclic voltammetric scan, and attributed to the formation of CuO, while disappeared in subsequent scans or by decreasing the NaOH concentration w13x. This is consistent with our observation that no significant electrochemical activity of Cu 2 O-MCPE was observed in 10y1 M Na 2 SO4 ŽpH 6. in the anodic direc-
Fig. 1. Cyclic voltammograms of 5.0=10y5 M KI recorded in 0.1 M Na 2 SO4 , in ŽA. the cathodic and ŽB. the anodic modes, using Ža. an unmodified carbon paste electrode and Žb. a 10%-Cu 2 O-modified carbon paste electrode ŽCu 2 O-MCPE.. Curves obtained with Cu 2 O-MCPE were recorded Ž1. 2 min, Ž2. 5 min, Ž3. 10 min and Ž4. 25 min after immersion of the electrode into the solution. Sweep rate: 50 mV sy1 .
G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117 Table 1 Iodide-uptake of Cu 2 O as a function of pH Ž1.5 g Cu 2 O in 30 ml 10y4 M KI. pH
Uptake Žmmol gy1 . a
6.5 6.8 7.2 7.7 8.0 8.5 9.0
1.78 Ž89. 1.52 Ž76. 1.22 Ž61. 0.88 Ž44. 0.74 Ž37. 0.60 Ž30. 0.42 Ž21.
a
Figures in parentheses are the uptake as a percentage of the maximum.
the redox processes of CuI. In the cathodic mode, CuI is reduced into metallic copper ŽEq. Ž2.. at y0.50 V, the peak potential being shifted to the negative direction for larger amount of CuI due to the longer time required for its complete reduction. In the anodic mode, however, both «CuŽqI. » and «I ŽyI. » elements are oxidised between q0.4 V and q0.6 V Žfor the same reason as above., leading to the formation of molecular iodine and copperŽII. oxide ŽEq. Ž3..; this potential range is consistent with values corresponding to the oxidation of Iy ŽFig. 1BŽa.. and CuŽI. w13x. The presence of free iodide was only observed when small currents were recorded for CuI oxidation, while occluded in the main peak at high accumulation degrees.
m Cu
CuI q 1ey y
Ž0 .
q
q Iy
CuI y 2e y 2H q H 2 O
m 1r2 I q CuO 2
115
Reduction of CuI was slightly influenced by the composition of the detection medium. The effect of pH was investigated and higher peak currents were observed in the 3–5 range so that pH 4 was selected as the most appropriate. At the same time, the preconcentration efficiency was found to be dramatically affected by pH, the presence of protons tending to favour the leaching of soluble copperŽI., as expected from the Pourbaix Diagram w17x, which would result in the formation of larger quantities of CuI as decreasing pH ŽTable 1.. This is shown in Fig. 2 where largest currents were clearly obtained for the lowest pH, down to pH 3 for the accumulation from a 1 = 10y3 M Iy solution and down to pH 4 for 2 = 10y5 M Iy. The same results were obtained from buffered and not buffered solutions, because the very low Cu 2 O-to-solution ratio and the continuous stirring of the solution maintain a constant
Ž 2. Ž 3.
As a consequence, the analysis performed in the anodic mode induced the formation of a CuO deposit on Cu 2 O which prevented any further accumulation of Iy species because they cannot be accumulated on cupric oxide, requiring therefore a mechanical surface renewing step between each preconcentration experiment. Nevertheless, successive measurements were allowed when working in the cathodic mode Ž7% standard deviation Ž n s 5. for the detection of 0.05 mM Iy at pH 4.0., because no alteration of the Cu 2 O surface other than that induced by Iy uptake was happening. It should be only ensured that the electrogenerated metallic copper deposit was entirely re-oxidised on scan reversal. Both anodic and cathodic peak currents were found to be directly proportional to the sweep rate Žin the range 5–200 mV sy1 . indicating a thin layer behaviour which again sustains the formation of a solid phase upon Iy accumulation on cuprite. For reasons mentioned above, detection in the cathodic mode was applied throughout. Despite this mode allowed in situ analysis Ži.e., both preconcentration and detection in the same medium., it was chosen to use two different cells for the accumulation and for the detection in order to investigate separately the factors affecting these two steps and to define the optimum conditions for the analytical procedure.
Fig. 2. Influence of pH on the detection of iodide accumulated at the Cu 2 O-MCPE. Preconcentration experiments were performed ŽA. during 1 min in a solution containing 1=10y3 M KI, and ŽB. during 5 min in the presence of 2=10y5 M KI.
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proton concentration in the diffusion layer, in spite of their consumption in the adsorption process according to Eq. Ž1.. This is therefore not surprising that the optimum pH for the accumulation of iodide was found to correspond to a proton concentration very close to that of the analyte ŽIy. : pH 3 for wIyx s 1 = 10y3 M and pH 4–4.5 for wIyx s 2 = 10y5 M. By decreasing pH further, however, a slight decrease in the uptake efficiency was observed because of the production of larger amounts of CuŽqI. species: some of them are subjected to dismutation before hitting an iodide anion or even can be lost in solution by diffusing rapidly far away from the electrode surface, limiting somewhat the deposition of CuI on this surface. Another avenue for exploiting the attractive uptake behaviour of Cu 2 O-MCPE is its use in square wave voltammetry ŽSWV. which led to significant improvement in sensitivity for CuI detection. Typically, a six-fold increase in the sensitivity was achieved by passing from linear sweep voltammetry to SWV, for the analysis of Iy in the 5 = 10y6 M to 5 = 10y5 M concentration range after 15 min accumulation. This detection mode was chosen for further experiments. The detection limit of Cu 2 O-MCPE was strongly affected by the accumulation time of Iy at Cu 2 O particles located at the electrode surface. To evaluate this effect, the accumulation time of the DPV for various copper concentrations Žfrom 5 = 10y6 M to 1 = 10y4 M. was varied between 2 and 120 min. Plots of peak currents Žor charges. against accumulation time exhibited a saturation curve where saturation currents were attained as faster as higher was the concentration of Iy. For example, at 8 = 10y5 M, the peak area corresponding to saturation was measured to be about 1.4 mC and the calculated ‘half peak time’ was approximately 25 min. At lower concentration and for short accumulation times, the relationship between accumulation time and voltammetric signals was almost linear ŽFig. 3.. The total amount of Iy accumulated on the electrode was calculated from the peak area. After normal-
isation with respect to Iy concentration, the plot of the CuI amount against accumulation time resulted in a straight line, indicating that iodide is accumulated at a constant rate. The amount of Cu 2 O in the paste was also found to affect peak currents. Peaks were rapidly growing up to 5 mass% Cu 2 O, then slowly levelling off between 10% and 20%, and finally decreasing at higher contents. Such behaviour can be explained by considering the uptake mechanism involving the production of CuŽI. and the subsequent formation of CuI. At very low Cu 2 O contents, the accumulation of Iy is limited by the amount of available CuŽI. species and therefore by the amount of Cu 2 O located at the electrode surface. When this amount becomes sufficient to accumulate all the Iy species located in the diffusion layer, a constant response of the electrode is observed. Finally, too high Cu 2 O contents result in an increase in the resistance of the electrode Žconcomitantly with a decrease in its active surface. which leads to lower peak currents. Keeping in mind all the above results, a calibration plot of peak currents against iodide concentration can be obtained. For a 15-min accumulation period in iodide solutions maintained at pH 3.6, using a 10%-Cu 2 O-MCPE, and detection by SWV at pH 4.0, linear evolution of peak currents was observed in the 1 = 10y6 M to 2 = 10y5 M concentration range. A detection limit of 5 = 10y7 M was achieved. In relation to the fact that chloride is probably the most significant interferent for iodide determination in underground waters, the effect of this species on the detection of iodide at Cu 2 O-MCPE was evaluated. For this purpose, various iodide concentrations Ž1 = 10y5 M, 5 = 10y5 M, 1.6 = 10y4 M and 5 = 10y4 M. were spiked with increasing amounts of NaCl to get chloride concentrations ranging from 1 = 10y4 M to 0.5 M. It was observed that chloride did not appear to have any influence on iodide detection at concentrations up to 100 times that of iodide. In addition, the sensor was found to be selective for iodide over bromide species, since the detection of 5 = 10y5 M Iy was not affected by the presence of 10y3 M Bry.
4. Conclusion
Fig. 3. Dependence of the Cu 2 O-MCPE response on the preconcentration time for the detection of Ža. 0.5=10y5 M and Žb. 2.0=10y5 M iodide at pH 4. Square wave voltammograms were obtained at a frequency of 20 Hz, a pulse amplitude of 25 mV and a step potential of 2 mV.
The Cu 2 O-modified carbon paste electrode is suitable for the voltammetric detection of iodide species after their accumulation at open-circuit. This was achieved by exploiting the ability of Cu 2 O to take up Iy and the electrochemical activity of the resulting surface precipitate, CuI, in both the cathodic and anodic modes. These two modes gave similar sensitivity, but the detection in the cathodic direction was preferred because allowing in situ analysis. The sensor exhibited the highest response when preconcentration and detection were performed at pH values between 3 and 4. The voltammetric response was linear in the
G. LefeÕre et al.r Sensors and Actuators B 59 (1999) 113–117
1 = 10y6 M to 2 = 10y5 M concentration range for a 15 min preconcentration time at open circuit. Significant selectivity was observed over chloride and bromide.
Acknowledgements We are grateful to the «Agence Nationale pour la Gestion des Dechets Radioactifs» ŽANDRA. for financial ´ support.
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w10x Z. Haq, G.M. Bancroft, W.S. Fyfe, G. Bird, V.J. Lopata, Sorption of iodide on copper, Environ. Sci. Technol. 14 Ž1980. 1106–1110. w11x G. Lefevre, A. Walcarius, J. Bessiere, ` Voltammetric investigation of iodide sorption on cuprite dispersed into a carbon paste electrode, Electrochim. Acta 44 Ž1999. 1817–1826. w12x K. Kalcher, J.-M. Kauffmann, J. Wang, I. Svancara, K. Vytras, C. Neuhold, Z. Yang, Sensors based on carbon paste in electrochemical analysis: a review with particular emphasis on the period 1990–1993, Electroanalysis 7 Ž1995. 5–22. w13x Y. Xie, C.O. Huber, Electrocatalysis and amperometric detection using an electrode made of copper oxide and carbon paste, Anal. Chem. 63 Ž1991. 1714–1719. w14x Q. Chen, J. Wang, G. Rayson, B. Tian, Y. Lin, Sensor array for carbohydrates and amino acids based on electrocatalytic modified electrodes, Anal. Chem. 65 Ž1993. 251–254. w15x X. Huang, J.J. Pot, W.Th. Kok, Electrochemical characteristics of conductive carbon cement as matrix for chemically modified electrodes, Anal. Chim. Acta 300 Ž1995. 5–14. w16x T.R.I. Cataldi, D. Centonze, Development of a carbon composite electrode made from polyethylene and graphite powder modified with copperŽI. oxide, Anal. Chim. Acta 326 Ž1996. 107–115. w17x M. Pourbaix, Atlas d’equilibres electrochimiques, GV Paris, 1963. ´ ´ G. LefeÕre is currently a PhD student in Chemistry at the «Universite´ ` Henri Poincare», ´ Nancy I, France. His research subject focuses mainly on the sorption properties of minerals towards iodine species and on the development of new sensors for Iy. J. Bessiere ` received his PhD in Analytical Chemistry from the University Pierre and Marie Curie, Paris, in 1969. He joined the University Henri Poincare, ´ Nancy, in 1973 where he is presently professor of Analytical Chemistry. His main interests are in electroanalytical chemistry applied to the liquidrsolid interfaces, environmental chemistry and sensors. A. Walcarius received his PhD in chemistry from the «Facultes ´ Universitaires Notre-Dame de la Paix» at Namur, Belgium, in 1994. His postgraduate research at the New Mexico State University, Las Cruces, USA, involved the bioelectroanalytical applications of zeolite-modified electrodes. In 1995, he joined the Laboratory of Physical Chemistry for the Environment, where he is currently employed as a research scientist by the National Centre for Scientific Research ŽCNRS.. His present research interests include the development of new electrochemical methodologies for in situ investigations of mass transfer reactions at the solidrliquid interfaces and their applications as amperometric sensors.