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New Approaches to the Characterization of Carbon Paste Electrodes Using the Ohmic Resistance Effect and Qualitative Carbon Paste Indexes ˇ vancara,† Kurt Kalcher,‡ Martin Bartosˇ,† Karel Vytrˇas,† and Jirˇı´ Ludvı´k*,§ Toma´sˇ Mikysek,† Ivan S Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, CZ-532 10 Pardubice, Czech Republic, Institute of Chemistry-Analytical Chemistry, Karl-Franzens University Graz, A-8020 Graz, Austria, and J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ-182 23 Prague, Czech Republic In this article, some new approaches to characterize the carbon paste mixtures and the respective carbon paste electrodes (CPEs) are presented, discussed, and critically evaluated. Particular attention has been paid to the changes of the ohmic resistance, relative to the dependence on composition of the CPE, the materials used, the time, and the position of storage. Four types of carbon pastes were examined, and for the interpretation of experimental data, a new simple model of “close-packing of spheres” has been applied. This model resembles the percolation theory for solid matter. In our case, however, it is possible to explain not only the “bent” or “broken” shape of the dependence of the electrode resistance upon the binder:carbon ratio and the corresponding electrochemical current response, but also differences caused by various material used and three various effects observed during the electrode aging. Furthermore, the report presents the significance of practical utilization of the recently introduced carbon paste index (denoted as χCPE), which is a qualitative hitherto unused factor based on the evaluation of cyclic voltammograms for standard redox systems (e.g., [Fe(CN)6]3-/4-) and specifying the electrochemical properties of a CPE. Some problems connected with homogeneity and stability of carbon pastes, their handling, storage, or eventual aging effects are also discussed. Carbon paste, which is a mixture of carbon powder and a suitable liquid binder, represents one of the most frequent laboratory-made electrode materials and apparently is the most flexible substrate for chemical and biological modifications.1-3 (In * To whom correspondence should be addressed. Tel.: +420-266 053 217. Fax: +420-286 582 307. E-mail address:
[email protected]. † Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice. ‡ Institute of Chemistry-Analytical Chemistry, Karl-Franzens University Graz. § J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic. (1) Kalcher, K.; Kauffmann, J. M.; Wang, J.; Sˇvancara, Vytrˇas, K.; Neuhold, C.; Yang, Z. Electroanalysis 1995, 7, 5–22. (2) Gorton, L. Electroanalysis 1995, 7, 23–45. (3) Kalcher, K. Sˇvancara, I. Metelka, R. Vytrˇas, K. Walcarius, A. In The Encyclopedia of Sensors, Vol. 4; Grimes, C. A., Dickey, E. C., Pishko, M. V., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2006. 10.1021/ac9004937 CCC: $40.75 2009 American Chemical Society Published on Web 07/08/2009
2008, the field of electrochemistry with carbon paste electrodes (CPEs) celebrated a 50-year anniversary4 since the discovery of carbon paste by Adams.5) Moreover, the modification of carbon pastes represents a unique approach, when the electrode is modified simultaneously on the surface and in the bulk. Carbon paste (similar to mercury) represents a very valuable possibility to renew the electrode surface easily and reproducibly. Among numerous specifics of carbon pastes, the rather individual behavior of the corresponding mixturessdiffering from paste to paste and from laboratory to laboratorysis often emphasized. On one hand, one can appreciate the advantage to control the desired properties of a particular mixture; on the other hand, it can be a certain limitation, with regard to the choice of a CPE for routine analysis, where commercially available electrodes with unambiguously defined behavior are usually preferred.6,7 However, the heterogeneous character of CPE, the specific influence of the liquid binder, the variability of the used carbon material, and the various proportional compositions require a broad and reliable spectrum of characterization methods. The first efforts of this type were made by the inventor’s research team,8,9 and it is interesting to note that the most conclusive results have been reported as late as in their final contribution to the field.10 Herein, extensive studies by Farsang,11 Monien et al.,12 Lindquist,13 or Soederhjelm14 can be mentioned; all of them being elaborated as more or less systematic comparisons of carbon pastes prepared from different graphite powders, as well as binders (pasting liquids). In the studies on preconcentration of neutral molecules onto the carbon paste bulk,15-17 one can trace the pioneering steps to (4) Sˇvancara, I.; Vytrˇas, K.; Kalcher, K.; Walcarius, A.; Wang, J. Electroanalysis 2009, 21, 7–28. (5) Adams, R. N. Anal. Chem. 1958, 30, 1576. (6) Sˇvancara, I.; Vytrˇas, K.; Zima, J.; Barek, J. Cr. Rev. Anal. Chem. 2001, 31, 311–345. (7) Ostapczuk, P. Anal. Chim. Acta 1993, 273, 35–40. (8) Olson, C.; Adams, R. N. Anal. Chim. Acta 1960, 22, 582–589. (9) Olson, C.; Adams, R. N. Anal. Chim. Acta 1963, 29, 358. (10) Rice, M. E.; Galus, Z.; Adams, R. N. J. Electroanal. Chem. 1983, 143, 89– 102. (11) FarsangGy., Acta Chim. Acad. Sci. Hung. 1965, 45, 163–175. (12) Monien, H.; Specker, H.; Zinke, K. Fresenius Z. Anal. Chem. 1967, 225, 342. (13) Lindquist, J. J. Electroanal. Chem. 1974, 52, 37–46. (14) Soderhjelm, P. J. Electroanal. Chem. 1976, 71, 109–115. (15) Chambers, C. A. H.; Lee, J. K. J. Electroanal. Chem. 1967, 14, 309–314.
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propose unified numeric parameters that would typify CPEs, according to their extraction capabilities defined as the so-called separation factors.15 Such a characterization via explicit and easily to-calculate constants had once been recommended by Adams himself;18 nevertheless, any continuator was not able to offer the proper and straightforward solution. The topic of characterization measurements with CPEs and related sensors has also been reviewed,19 together with numerous practical hints with regard to how to select the proper carbon paste components and how to test the freshly made carbon paste mixtures. For testing of the electrode properties, some authors had also used commonly recommended reversible redox systems (e.g., FeIII/FeII; AgI/Ag0; and I2/I-) or organic compounds (quinone/ hydroquinone, Q/H2Q; ferrocenium/ferrocene, Fc+/Fc; ascorbic acid/dehydroascorbic acid, or phenol) whose electrochemical behavior, reaction kinetics, or some specifics (e.g., adsorption properties) are well-known from measurements with standardized electrodes such as glassy carbon or platinum disk.20 Although the concrete indexes (or quotients, respectively) have already been proposed (see the discussion in ref 3), their practical usefulness could not yet be assessed, because of a lack of relevant experimental data. One specific feature of commonly used carbon paste mixtures is a very low ohmic resistance, usually being on the order of ohms or tens of ohms only,1,3,21,22 which increases with growing proportion of the binder.10 Up until now, this feature has not yet been in CPEs satisfactorily described nor explained, despite of several existing hypotheses (e.g., percolation theory or tunneling effect known for semiconductors).6 The presented paper is focused to physicochemical investigation of CPEs, namely, to the changes of their resistivity39 (using resistance measurements) and analysis of the electrochemical response. Because the electrode characterization itself should involve an explanation of the actual behavior and a way of certain prediction of electrode properties in dependence of the component ratio, as well as the preparation procedure and aging, a simple model based on close-packing of spherical particles was postulated, experimentally proven, and discussed. This model resembles the percolation theory for rigid lattices23 but seems to be more practical and qualitatively applicable also to the viscous “moving” systems. For the electrochemical characterization of CPEs, the carbon paste index (which is denoted by χCPE and is based on an evaluation of cyclic-voltammetric data of the reversible ferricyanide/ferrocyanide redox system) was utilized and verified. The mentioned characterization methods were also used for investigation of the CPE stability and aging in dependence of the carbon paste composition. (16) Wang, J.; Luo, D.-B. J. Electroanal. Chem. 1984, 179, 251–261. (17) Wang, J.; Deshmukh, B. K.; Bonakdar, M. J. Electroanal. Chem. 1985, 194, 339–353. (18) Adams, R. N. Rev. Polarog. (Kyoto) 1963, 11, 71–78. (19) Sˇvancara, I.; Schachl, K. Chem. Listy 1999, 93, 490–499. (20) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969. (21) Davis, D. G.; Everhart, M. E. Anal. Chem. 1964, 36, 38–40. (22) Beilby, A. L.; Mather, B. R. Anal. Chem. 1965, 37, 766–768. (23) Grimmet, G. Percolation, Second Edition; Springer: Berlin, 1999.
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EXPERIMENTAL SECTION Chemicals and Reagents. Potassium hexacyanoferrate(III), K3[Fe(CN)6] (p.a. grade), and potassium chloride, KCl (Suprapur), were used as received (both from Merck). Throughout the experimental work, all solutions were prepared from doubly deionized water obtained by passing through a laboratory purification system (Milli-Q, Millipore). Apparatus. A modular electrochemical system AUTOLAB (model “PGSTAT 30”; Ecochemie, Utrecht, The Netherlands), equipped with PGSTAT-12 and ECD modules (from the same manufacturer), was used in combination with GPES 4.9 software (from the same manufacturer). This assembly was connected to an external electrode stand that incorporates the three-electrode cell with the working electrode (CPE, see below), Ag | AgCl | 3 M KCl reference, and platinum plate (ca. 0.5 cm2) counter electrodes. Powder X-ray diffraction (XRD) data were obtained with a D8Advance diffractometer (Bruker AXS, Germany) with BraggBrentano θ-θ geometry (40 kV, 40 mA) using Cu KR radiation with a secondary graphite monochromator. The diffraction angles were measured at room temperature from 5 to 65° (2θ) in 0.02° steps with a counting time of 5 s per step. Carbon Paste Electrodes. Carbon paste mixtures were prepared by intimately hand-mixing graphite powder with a liquid binder in a laboratory mortar. The carbon moiety was represented by either (i) graphite powder (“CR-5” type, with a microcrystalline structure and with a particle diameter of 5-10 µm (cf. Figure 2a); this is a product for gear lubrication supplied by Maziva Ty´n, Czech Republic) or (ii) glassy carbon powder (“Sigradur-G”, formed by spherical particles with a distribution of 5-20 µm (cf. Figure 2b), HTW Meitingen, Germany). The pasting liquid was either (i) paraffin oil (PO) for spectroscopy (Uvasol grade, Merck) or (ii) silicone oil (SO) with high viscosity (“Lukooil MV 8000” of the methyl-/vinyl-polysiloxane type; Lucˇebnı´ za´vody Kolı´n, Czech Republic). After homogenization, the individual carbon pastes were packed into a set of piston-driven electrode holders with identical configuration and geometry,24,25 ensuring the same diameter for the active carbon paste surface (L ) 2 mm). In this work, four types of carbon paste mixtures were studied and characterized by evaluating the actual experimental data for (i) a standard mixture made of graphite and paraffin oil (C/PO), (ii) a mixture containing silicone oil instead of paraffin oil (“C/SO”), (iii) a mixture of glassy carbon powder (with spherical particles) and paraffin oil (GC/PO), and (iv) a mixture of glassy carbon and silicone oil (GC/SO). The percentage of a binder in the mixtures is given usually as a weight percentage (w/w) for easier preparation. However, for the consideration in the frame of the model used for the close packing of spheres, the amount of a binder must be recalculated to a volume percentage (v/v). The actual resistances of the individual pastes were measured and the electrochemical experiments were performed immediately after the surface renewal by smoothing with a wet filter paper. In addition, the pure carbon powders (hereafter referenced as having 0% binder) were also used in some tests. (24) Sˇvancara, I.; Vytrˇas, K.; Metelka, R. Czech Pat. Appl. PV 3939, 2002. (25) Sˇvancara, I.; Metelka, R.; Vytrˇas, K. In Sensing in Electroanalysis; Vytrˇas, K., Kalcher, K., Eds.; University of Pardubice: Pardubice, Czech Republic, 2005; pp 7-18.
Procedures. Ohmic Resistance Measurements. In all cases, the CPE body was placed vertically and the freshly smoothed electrode surface touched to a conductive plate at the desk. The second contact was then linked to a multimeter (Model Voltcraft; Conrad Electronics, Germany) that was connected to a metallic piston (composed of highly conductive steel having alone negligible electric resistance). Cyclic Voltammetry (CV). If not stated otherwise, CV measurements were performed in a solution of 0.1 M KCl that contained 0.005 M K3Fe(CN)6. The typical experiment was started at a potential of +1.0 V vs Ag/AgCl, with a vertex potential of -0.7 V and a scan rate of 50 mV/s, when usually five cycles had been recorded to reach the steady-state conditions, and the representative (average) peak heights were computed and evaluated by software. Typically, each series of CV curves was measured at the freshly cleaned carbon paste surface, and dissolved oxygen in the sample solutions was removed by bubbling with argon gas (purity: 99.99%, Linde Technoplyn, Czech Republic). RESULTS AND DISCUSSION Ohmic Resistance of Carbon Pastes. The electric resistance of CPEs (RCP), despite the presence of insulating binder, is surprisingly low for “drier” mixtures that are composed of paraffin or silicone oils, reaching an order of only a few ohms.3,4 (The graphites exhibit a negligible resistivity of ca. 10-50 µΩ m,26 whereas both oily binders that have been used are typical insulators with an extremely high resistivity (∼5 × 1013 Ω m).27) With a higher proportion of pasting liquid in the mixture, the value of RCP increases; nevertheless, the resultant resistances of commonly used CPEs are still fairly low and can be measured in an interval of 30-100 Ω,19 depending on the quality of both main carbon paste constituents and on the increasing content of liquid binder. Similar conclusions were then made in further studies (e.g., see refs 9, 13, 28-30), also paying attention to the specific resistance of CPEs. Slightly higher resistances can be ascertained for carbon paste mixtures that are composed of graphite with larger and rougher particles21,22 or using some special carbon pastes that are made of liquid organic esters (e.g., dioctyl phthalate or tricresyl phosphate) whose resistance may exceed even 200 Ω.31,32 It is generally known that a high modifier content in the carbon paste mixture causes an undesirable increase in the ohmic resistance.33 This effect is illustrated on Figure 1 in ref 10, where a high proportion of the binder added causes a strong decrease in the electrode rate constant (corresponding to the increase in R). This dependence was interpreted as a monotonic paraboloid (26) Powell, R. L.; Childs, G. E. In American Institute of Physics Handbook, 3rd Edition; Gary, D. E., Ed.; McGraw-Hill: New York, 1972; pp 4-160. (27) Mark, J. In Polymer Data Handbook; Oxford University Press: New York, 1999; p 425. (28) Mesaric´, S.; Dahmen, E. M. F. Anal. Chim. Acta 1973, 64, 431–438. (29) Albahadily, F. N.; Mottola, H. A. Anal. Chem. 1987, 59, 958–962. (30) Hvı´zdalova´, M. Studies with Carbon Paste Electrodes in Voltametric Measurements (in Czech). Diploma Thesis, University of Pardubice, Pardubice, Czech Republic, 1994. (31) Sˇvancara, I.; Vytrˇas, K. Anal. Chim. Acta 1993, 273, 195–204. (32) Sˇvancara, I.; Ogorevc, B.; Novicˇ, M.; Vytrˇas, K. Anal. Bioanal. Chem. 2002, 372, 795–800. (33) Marcoux, L. S.; Prater, K. B.; Prater, B. G.; Adams, R. N. Anal. Chem. 1965, 37, 1446–1447.
Table 1. Carbon Paste Components Used in This Study and Their Corresponding Densities component glassy carbon powder graphite powder silicone oil paraffin oil
specification Sigradur-G CR-5 Lukoil Nujol
density [g/cm3] 1.45 2.15 0.97 0.87
or an exponential curve empirically showing the usable composition of the CPE. However, our actual set of more than 100 experimental data, measured for all four types of carbon pastes and for each particular mixture differing in the carbon-to-pasting liquid ratio, has shown that the resistance of the mixture changes as the amount of binder increases in a specific way: The dependences are not smooth and exhibit break-points at ∼20%-25% (w/w) of pasting liquid (depending on the type of carbon powder and binder). If we take into account the density of the carbon material and that of the pasting liquid (see Table 1), the break in the dependence curve occurs between 30%-45% (v/v) of the paraffin or silicone oil (see Figures 1a-d). Hence, the graph shows always two linear segments: below this threshold, the resistance is low and almost constant. As the amount of binder increases above this ratio, the resistance of the mixture increases abruptly. This observation has indicated that there would be a general principle behind this phenomenon having the same origin for all four CP mixtures that have been examined. For the fundamental explanation of this shape, we propose to use the simple model of “close-packing of spherical particles” known from crystallography: in the case of the close cubic packing (space group Fm3m), as well as in the close hexagonal packing (space group P63/mmc), the space that is filled with spheres represents 74% of the total volume, in the case of the spacecentered cubic packing (space group Im3m), it is 68% and in the rare simple cubic packing (spheres are placed only in the corners of a cube, space group Pm3m), it is 52.4%.34,35 When formally replacing the graphite particles in carbon pastes by such solid spheres, the application of this model gives the threshold volume proportion for the carbon content (g65% of the total volume of the carbon paste), when the electrically conducting graphite particles are in maximal permanent multiple contact and, apparently, almost full conductivity of the CPE material is thus attained. In this case, the nonconducting binder represents only a filler of the free space between the graphite particles and has practically no effect on the overall conductivity. Therefore, up to a content of ca. 35% (v/v) PO/SO, the observed resistance of the electrode is almost constant and close to zero. In contrast, in the case of higher proportions of the pasting oily liquid, the carbon particles are “floating” in oil and, therefore, permanent contact among the particles is not fully ensured. In addition, because the incidence of random touches of the graphite units in these oily masses decreases with the increasing fraction of pasting liquid, a rapid increase of the electrode resistance is (34) Sedmidubsky´, D. Introduction to the Principles of Structural Chemistry Crystalline Inorganic Substances (in Czech);VSˇCHT Praha: Prague, Czech Republic, 2008. (35) Hahn, T., Ed. International Tables for Crystallography, Vol. A; Reidel Publishing: Dordrecht, The Netherlands, and Boston, 1983.
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Figure 1. Dependence of the resistivity on carbon paste composition for four different carbon paste mixtures: (a) C/PO type (composed of graphite powder and paraffin oil); (b) C/SO type (composed of graphite powder and silicone oil); (c) GC/PO type (with glassy carbon powder and paraffin oil), and (d) GC/SO type (with glassy carbon powder and silicone oil). In all four carbon paste mixtures, the content of the binder varied over a range of 0%-60% (v/v).
the result for all carbon paste mixtures with >35% (v/v) PO/SO. This model was confirmed experimentally by two different approaches (cf. Figures 1 and 4 (presented later in this work)), when its interpretation was best-fitted to the results obtained for the GC/PO (i.e., a carbon paste mixture comprised of glassy carbon powder, Sigradur-G, with regularly spherical particles.36 (Its microstructure is shown in Figure 2, together with the C/SO carbon paste, which has a completely different particle shape.) This model resembles the principles of percolation theory for solid matter.23 Recently, a series of papers appeared where percolation theory was used in the case of solid polymer/graphite conducting composites with rigid lattices (e.g., refs 37 and 38) but not in the case of rather viscous CPEs. Working with CPEs, we therefore expected the existence of the point of discontinuity, a “threshold” value of the proportion of components in the CPEs; (36) Sˇvancara, I.; Hvı´zdalova´, M.; Vytrˇas, K.; Kalcher, K.; Novotny´, R. Electroanalysis 1996, 8, 61–65. (37) Zou, J.-F.; Yu, Z.-Z.; Pan, Y.-X.; Fang, X.-P.; Ou, Y.-Ch. J. Polym. Sci. B 2002, 40, 954–963. (38) Heaney, M. B.; Schmitz, G. P. U.S. Patent 2006/0046036 A1, March 2, 2006. (39) One of the important physical quantities related to resistance measurements and characterizing the CPE is the resistivity (specific resistance, denoted as R′CP) parameter, which enables a better comparison of resistance data. R′CP ) [RCP(EXP) × ACPE]/lCP [Ω/m], where RCP(EXP) [Ω] is the experimentally obtained electrical (ohmic) resistance of a CPE, ACPE [m2] is the crosssectional area of the actual CPE surface, and lCP [m] represents the length of a column of carbon paste packed in the electrode holder. Conductivity is then just a reciprocal (inverse) value of the resistivity (expressed in terms of S/m).
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however, in the literature about CPEs, such a study is missing. In addition to this, our simple model seems to be suitable, namely, for rather viscous material without a rigid lattice. Besides the observations described, two additional effects were also noticed: (i) for each proportion chosen, the mixtures of the graphite powder with the paraffin oil were more compact (more solid, viscous) than those containing silicone oil; and (ii) the mixtures with glassy carbon (Sigradur-G) were distinctly more fluid than those composed of graphite at identical carbon-to-pasting liquid ratio. The proper interpretation of both effects (the latter has already been reported, but not explained),30,36 can be made when considering the differences in morphology and the properties of both carbon materials and specific molecular structure of both oils: Whereas glassy carbon is formed by smooth spheres (see Figure 2b), the particles of CR-5 graphite powder are porous, with undefined shape (Figure 2a), having a hexagonal multilayer structure (P63/mm) and, as X-ray spectra reveal, it is a mixture of carbon and two graphite poly types: graphite 2H and graphite 3R, which are in minor quantity. Simultaneously, the molecules of paraffins (i.e., mixtures of hydrocarbons with the C20-C50 chains) are fiber-shaped and much smaller than voluminous structures of alkylated/arylated polysiloxane in the used silicone oil. Based on approximate calculations of the atomic distances, it can be deduced that graphite particles are able to accommodate
Figure 3. Typical cyclic voltammograms for the [Fe(CN)6]3-/ [Fe(CN)6]4- redox couple obtained with two different silicone oil-based CPEs. Experimental conditions: CV; carbon pastes of the C/SO type; 0.1 M KCl + 0.005 M K3Fe(CN)6. Potential range, from +1.0 V vs Ag/AgCl, to -0.7 V and backward; scan rate, 50 mV/s.
Figure 2. Scanning electron microscopy (SEM) images showing the surface microstructure of two different carbon pastes: (a) C/PO and (b) GC/PO.
a certain portion of paraffin oil in their pores and/or in the space between the graphitic layers (interlayer distance is 0.335 nm), which causes a more solid texture than that for the silicone oilbased mixtures, where a penetration of large polysiloxane molecules is less probable. Regarding the glassy carbon powder, Sigradur-G, its surface is specially treated already during synthesis (details are given in ref 36), thanks to which its absorption capabilities with respect to gases and liquids are to be minimized. The X-ray spectra of Sigradur-G confirm the amorphous character of its glassy carbon particles. This undoubtedly deteriorates the adherence of Sigradur-G with both PO and SO, when the resultant mixtures remain fluid and less compact even for relatively low contents of pasting liquid. The effect of soaking with paraffin oil by CR-5 graphite can be also seen in Figure 1a, where the break-point upon the plot is somewhat shifted toward a higher proportion of the binder compared to the curves for C/SO, GC/PO and GC/SO pastes (see Figures 1b, 1c, and 1d). Cyclic Voltammetry of the [Fe(CN)6]3-/[Fe(CN)6]4- System, Qualitative Parameter - χ. The conductivity of the carbon paste electrode material has a crucial impact on the electroanalytical signals at CPEs. This effect can also be expressed using typical electrochemical data. A simple and quick experimental test is based on the evaluation of the anodic/cathodic peak difference in cyclic voltammetry of a one-electron reversible system. There is a number of available suitable redox systems (Tl+/Tl, Fc+/ Fc, Ru-complexes, etc.) when the choice is dependent on the
Figure 4. Dependence of the parameter χ on the content of binder in a carbon paste mixture of the C/PO type.
solvent, electrode material. and the operational potential range. In our case, we selected the [Fe(CN)6]3-/[Fe(CN)6]4- system. In the case of totally reversible one-electron redox reaction, the cathodic/anodic peak difference is theoretically 59 mV. With increasing resistance of the CPE, a stepwise loss of reversibility should occur, accompanied by the increase of the peak potential difference. For an experimental description of degree of reversibility, the parameter χ could be used,6
χ)
∆Ep([Fe(CN)6]3-/4-) ∆Ep,theor
where ∆Ep([Fe(CN)6]3-/4-) is the experimentally measured difference (in millivolts) between the peak potentials of the cathodic and anodic processes; ∆Ep,theor is the theoretical difference of 59 mV. One example of a typical CV experiment is then depicted in Figure 3, which makes comparison for two different mixtures of the graphite/silicone oil (C/SO)-based carbon paste. The dependence of the parameter χ on the amount of binder in the case of graphite/paraffin oil (C/PO) mixtures is shown in Figure 4. Again, the graph has a typical shape: up to 40% (v/v) of the binder, the parameter χ is practically constant and corresponds to the anodic/cathodic peak difference of 90-110 mV. A breakAnalytical Chemistry, Vol. 81, No. 15, August 1, 2009
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point evident on the dependence at higher proportion of the binding liquid indicates that the rate constant of the selected redox process is suddenly slowing, the reversibility is decreasing, and, as a consequence, the peak difference increases to ca. 300 mV at 60% (v/v) of the binder. This shape, as well as the position of the break on the curve, fits perfectly to the resistance measurements shown in Figure 1a. The analogous series of CV data was recorded for all four types of studied CPE mixtures. By comparing the respective results, it was found that, in all cases, the reversibility of the [Fe(CN)6]3-/ [Fe(CN)6]4- couple decreases as the content of pasting liquid increases over ca. 40% (v/v). This trend is more pronounced (and the degree of irreversibility is higher) for the GC/PO, compared to both C/PO and C/SO mixtures, which reflects the aforementioned less-compact consistence and generally higher resistance of the GC/PO paste. Stability of Carbon Paste Mixtures. Carbon paste, as a typical heterogeneous material,1-3 also must be considered, with respect to possible changes in consistency and functional properties of otherwise uniform mixtures. Naturally, aging effects are dependent on the type of carbon particles, as well as on the nature of the binder. Regarding traditional carbon pastes of optimal composition based on paraffin or silicone oils, such effects are usually minor and the behavior of the respective CPEs is stable for months9,20 or, in extreme cases, years.4 In the case of less-common or even inappropriate composition, however, pertinent changes in the homogeneity of electrode material occur and undesirable aging can be observed. When the excess of the binding liquid is too high, the electrodes exhibit a “bleeding effect”, which is a spontaneous loss of the binder.1,3 In CPEs with >40% (w/w) binder, the resistance of the electrode increases gradually within several days or weeks. The explanation of such increasing resistance is based on the previously mentioned model of “packed spheres”. From experimental data and their interpretation, one can deduce that, within the range of ca. 25% and 35% (w/w) of the pasting liquid (i.e., just behind the breakpoint on the resistivity-to-composition dependence), the carbon particles are not already tightly packed and are losing permanent contact. However, because of a high viscosity of the binder, the sedimentation effect does not cause changes in homogeneity, and the paste remains “compact” and, thus, applicable for longer time. When the amount of the binder exceeds 40% (w/w), the carbon particles are “floating” in the liquid medium and the separation of solid phase and liquid phase occurs due to the gravity, accompanied by a substantial increase of resistivity. The experimental proof for this explanation is the difference in aging between horizontally and vertically stored electrodes. In vertically stored electrodes, the sedimentation of the graphite particles (which have higher density than the binder, cf. Table 1) causes dehomogenization, where the upper part of the carbon paste filling is more oily and, therefore, is less conductive. In consequence of such fractionation in a column of carbon paste with a length of ∼2 cm, the overall conductivity of CPE (vertically measured) after four days of storage is getting low and, consequently, the CV responses (∆Ep and χ) are less favorable. In horizontally stored electrodes, the same effect leads to the 6332
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Table 2. Comparison of Characterization Parameters of Two CPEs (Mixture of CR-5 and PO) Stored for 4 Days in Horizontal and Vertical Positions, Respectively After 4 Days of Storage Carbon Paste Electrode
Horizontally
Vertically
% v/v
% w/w
∆Ep
Fresh χ
∆Ep
χ
∆Ep
χ
53.8 60.0
32.0 37.7
183 358
3.1 6.1
142 222
2.4 3.8
191 256
3.2 4.3
sedimentation along the CPE piston; the conductivity of the entire electrode filling is thus retained and the electrochemical responses are better (see Table 2). This is in accordance with our previously mentioned observation:4 a silicone fluid-based carbon paste with an optimal composition (25% (w/w)) that has been tightly packed in the Teflon electrode body and horizontally stored in a cooler place was fully usable, even after 4 years. There are also surprisingly positive changes: freshly prepared carbon paste mixtures (namely, those containing graphite powder) cannot be used immediately and require a certain time for “selfhomogenization”, when their physicochemical and electrochemical properties (resistance, background currents, the parameter χ, etc.) are improved and stabilized during next 12-24 h.19 Table 2 illustrates both discussed effects: the difference between horizontally and vertically stored electrodes, as well as the better electrochemical properties after four days. All of the above processes of agingsthe positive “selfhomogenization”, as well as the undesirable “bleeding” or sedimentationsare most probably caused by the surface tension effects, i.e., by wetting of the carbon particles with pasting liquid. The powder particles generally have a large specific area covered by oxygen functional groups and adsorbed air.1,3 The homogenization procedure during the preparation of CPEs yields only partially wetted particles that are slowly undergoing an additional wetting process until equilibrium is attained. This is not an instantaneous process, and its duration is dependent on the materials used, among others. Therefore, the complete wetting either can cause bleeding of the electrode (microdroplets of oil on the partially wetted surface slowly merge and tend to leak) or, on the other hand, the complete wetting is accompanied by partial penetration of the binder into the surface structure of the solid particles (see above), resulting in better homogenization. Some other unwanted changes in the behavior of carbon pastes can occur when using prefilled CPEs that have been stored for longer time, dipped in a beaker with distilled water.19 In these cases, however, refreshment of the electrode is often being accomplished by removing the water-saturated portion of the paste.32 Certain complications with homogeneity of carbon paste mixtures may appear for special mixtures from more volatile binders, because of rapid desiccation of the corresponding carbon paste.31 The last evidence of the aging process is observed in the case of CPEs in frequent use: after several weeks (months) in a vertical position, disintegration of the electrode material occurs, accompanied by changes in conductivity. Such an effect is most probably caused by a slow variation of composition; when every renewal procedure removes slightly more binder than carbon from the electrode surface (a very slow leakage), or the partial
permeation of oil into the graphite structure can contribute to the drying effect. As a result, a slow but noticeable “drying” of the electrode occurs and the electrode material finally is shattered. CONCLUSIONS In conclusion, because of the increasing interest in the development of various specific sensors in electroanalytical chemistry, many chemists are deciding to work with modified carbon paste electrodes (CPEs). Many users prepare CPEs intuitively and empirically, mainly according to the observed final plasticity, hoping that pastes with the same plasticity have the same electrochemical properties. Also, some new users of CPEs believe that the same proportion of the carbon/binder material ensures the same electrochemical properties, even if different materials are used. However, neither of the aforementioned expectations are not fully correct. On the other hand, measuring the conductivity of each CPE is time-consuming. Therefore, our contribution could save time, mainly for those who are starting to work with CPEs, and help in the fundamental decisions about the choice of materials, proportions, and ways of characterization. In this article, particular attention has been paid to the characterization of CPEs based on changes of the ohmic resistance in dependence on the composition of CPEs, on the type of carbon paste constituents used, and on the time and position of CPE storage. Based on extensive conductivity measurements and electrochemical experiments, a characteristic “breakpoint” on the graph of resistivity versus CPE composition has been found, which can be further exploited diagnostically, as an indication of the optimal carbon paste composition for the given components. For the interpretation of all experimental data, a simple model of “close-packing of spheres” has been proposed. This model resembles the percolation theory for solid matter. In our case,
however, it is possible to explain not only the “bent” or “broken” shape of the dependence of the electrode resistance on the binder: carbon ratio and the corresponding electrochemical current response, but also differences caused by the various materials used and effects observed during the electrode aging. The applicability of the presented model has been confirmed experimentally using cyclic voltammetry. For a simple specification of electrochemical properties of the studied CPEs, besides the measured difference between anodic and cathodic peak potential, the usefulness of the carbon paste index (χCPE) has been shown. The overall shape of the χ-vs-c(binder) plot, as well as the breakpoint itself, corresponds fully to the results of resistance measurements. Finally, the “close-packing of spheres” model was also used for investigation and interpretation of the stability of carbon pastes, including some associations with their manipulation and storage. The presented model, accompanying experiments, and observed experiences could be thus generally useful for everyday practical use: for the preparation, handling, and storage of various types of CPEs; for interpretation of their behavior; for optimization of their electroanalytical properties; and for further development of CPEs or related sensors. ACKNOWLEDGMENT Financial support from the Ministry of Education, Youth, and Sports of the Czech Republic (No. MSM0021627502 and Project No. LC 510) is gratefully acknowledged. The authors thank Assoc. Prof. Ludvı´k Benesˇ for performing the accompanying measurements of X-ray spectra. Received for review March 6, 2009. Accepted June 17, 2009. AC9004937
Analytical Chemistry, Vol. 81, No. 15, August 1, 2009
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