Electrochemistry Communications 4(2002) 985–988 www.elsevier.com/locate/elecom
C.M.V.B. Almeida, B.F. Giannetti *
LaFTA – Laborat o rio de F ıısico-Qu ıımica Te o rica e Aplicada, Instituto de Ci ^ nciasExatas e Tecnologia da Universidade Paulista,
R. Dr. Bacelar 1212, Cep 04026-002, S a o Paulo, Brazil
Received 14October 2002; accepted 17 October 2002
Abstract
This work presents a new and simple electrode, which may be used to achieve the electrochemical response of ground solids or insoluble samples. Ore samples from Morro Velho Mine (Brazil) were employed to exemplify the use of such electrodes. The new electrode avoids the use of binders or other agents overcoming major deterioration problems.
Ó 2002 Published by Elsevier Science B.V.
Keywords: Carbon paste electrodes; Ground minerals; Electrochemical response
1. Introduction
The most popular electrode to study hardly soluble compounds is based on a carbon paste. These electrodes have been extensively used because of some advantages such as ease preparation and large potential window.
Surface renewal and modification are also simple. The use of carbon paste electrodes (CPEs) prepared mainly by direct mixing of electroactive compounds is well described in an excellent review [1]. However, these electrodes have has some inherent problems. For instance, the oxidation of the sample in the presence of the electrolytic binder means that CPEs lifetime is limited. In addition, the silicon oil binder causes deterioration and the binder viscosity has a significant effect on the electrode performance [2].
The methodology to immobilize mechanically solid particles on carbon surfaces (VMP – voltammetry of microparticles) was developed by Scholz et al. [3]. In this technique, the graphite electrode impregnated with paraffin is gently rubbed on the ground sample, which is transferred to the electrode surface. This electrode permits the direct analysis of solid ground samples. How-
*
Corresponding author. Fax: +55-11-55864073, or +55-11-55864170.
E-mail address: biafgian@unip.br
(B.F. Giannetti).
1388-2481/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S 1 3 8 8 - 2 48 1 ( 0 2 ) 0 0 5 1 1 - 8 ever, during electrode preparation part of the particles could be covered by the paraffin.
The aim of this note is to present a new and simple electrode, which may be used to achieve the electrochemical response of ground solids or insoluble samples.
With this purpose, ore samples from Morro Velho Mine
(Brazil) were used.
Some advantages of the new electrode can be listed:
(i) avoids the use of binders or other agents overcoming major deterioration problems, (ii) ease preparation, (iii) low cost, (iv) potential window, (v) ease modification,
(vi) excellent electrical conductivity, (vii) the use of solid particles of the same size guarantees the current values reproducibility, that is, similar surface areas are obtained.
2. Experimental
The carbon paste electrodes consist of 1.0 g graphite and 1.2 g paraffin wax (solidification point 68–74 ° C) containing 5 mg of ground mineral. The mineral is hand ground in an agate mortar and pestle. This material can be sieved to isolate a desired fraction and this procedure assures the reproducibility of the current values. A detailed explanation of the influence of the particle size on electrochemical response is reported in [4]. The mineral

986 C.M.V.B. Almeida, B.F. Giannetti / Electrochemistry Communications 4 (2002) 985–988
Fig. 1. Schematic representation of the electrode construction.
fractions used here to exemplify the electrode performance are less than 210 l m, in size.
For the construction of the carbon paste electrodes, a brass wire with diameter 3.0 mm was immersed in the mixture graphite/paraffin heated at 70 ° C, Fig. 1(1).
Then, 5.0 mg of ground mineral was placed in a Teflon cavity with diameter 4.5 mm, Fig. 1(2) and the extremity of the wire covered with the hot mixture was immediately pressed on the pyrite particles placed in the mould.
The resulting set is a pyrite disk with diameter nearly
4.0 mm, which covers the graphite surface almost completely, Fig. 1(3). The lateral parts of the electrode were then covered with Teflon tape to avoid contact with solution, Fig. 1(4).
The particles mechanically immobilized on the graphite/paraffin mixture are very stable. Besides the mechanical immobilization, the cooling process of the mixture in contact with the particles improves the solid stabilization. Thus, the electrode can be vigorously washed before its transfer to the electrochemical cell and can be also rotated to perform hydrodynamic experiments. Finally, the electrode-tip covered with mineral can be easily removed after every measurement and the brass wire can be used immediately to make a new electrode.
To compare the data with that obtained from the carbon paste electrode and with others found in literature [5,6], a compact crystal electrode was constructed.
A selected crystal of pyrite was cut to size and mounted in polyester resin with one surface exposed. The schematic representation of the electrode construction is shown in Fig. 2. The exposed surface was polished wet on 600-grit silicon carbide paper.
Potentiodynamic measurements of both electrodes were carried out in a standard electrochemical cell.
The reference electrode was an Ag/AgCl electrode placed in a Luggin–Haber capillary and the counter electrode was a platinized platinum wire of large area.
The electrolyte buffer, acetic acid/sodium acetate, pH
4.5 was prepared from Merck p.a. grade reagent and triply distilled water. Nitrogen was bubbled through
Fig. 2. Schematic representation of the crystal sample electrode.
the cell to deaerate the solution. Potential values quoted in this text are given on the standard hydrogen scale.
Fig. 3. SEM micrographs of pyrite electrode surfaces: (a) polished and
(b) ground electrode with particles < 210 l m.

metric area of 15 mm 2
C.M.V.B. Almeida, B.F. Giannetti / Electrochemistry Communications 4 (2002) 985–988
3. Results and discussion
SEM micrographs of the pyrite crystal sample (geo-
) and of the carbon paste electrode containing pyrite particles < 210 l m fixed as described above are shown in Fig. 3. It can be seen that the graphite/ paraffin mixture exposed to the solution is minimal.
The area of the ground electrodes can be roughly estimated. The geometric area of the polished electrode shown by the micrograph was estimated as about 0.95
mm 2 . In the same area, it is possible to place nearly 15 pyrite particles of 210 l m in size (Fig. 3). Considering the exposed particles as half cubes, the area in the micrograph of the carbon paste electrode can be calculated as ffi 1.9 mm 2 . Despite of the simplicity of the model employing geometric areas, it is clear that the area of the ground electrodes is larger than that of the polished ones, even without taking into account the roughness of each particle.
Voltammograms obtained from both pyrite electrodes with the sweep potential starting from the open circuit potential are shown in Fig. 4. The curve obtained from the ground electrode is similar to that obtained from the crystal sample, except that anodic current peak
A1 is present and the open circuit potential is more negative corresponding to the system Fe(II)/Fe(III).
Taking into account that the carbon paste electrode containing ground mineral cannot be polished, the surface of the pyrite particles may be partially covered by
987 iron oxides/hydroxides formed in air during grinding [4].
Thus, the presence of current peak A1 and the more negative potential could be expected. The other aspects of the voltammogram of ground pyrite are also identical to voltammograms found in literature [5,6].
Similar exploratory studies were carried out with other minerals. Fig. 5(a) shows the voltammogram obtained from pyrrhotite particles, which is similar to those
Fig. 5. Cyclic voltammograms of (a) the pyrrhotite crystal sample and
(b) the ground electrode, in acetic acid/acetate buffer, pH 4.5, sweep rate 20 mV s
1
, at 298 K.
E i
: open circuit potential, E a
¼ 0.95 V.
Fig. 4. Detail of the cyclic voltammograms of (a) the pyrite crystal sample and (b) the ground pyrite electrode, in acetic acid/acetate buffer, pH 4.5, sweep rate 20 mV s
1
, at 298 K.
E i
: open circuit potential, E a
¼ 0.95 V.
Fig. 6. Cyclic voltammograms of (a) the arsenopyrite crystal sample and (b) the ground electrode, in acetic acid/acetate buffer, pH 4.5, sweep rate 20 mV s
1
, at 298 K.
E i
: open circuit potential, E a
¼ 0.95 V.

988 obtained by Hamilton and Woods [6]. The curve obtained from ground arsenopyrite can be observed in Fig.
6(a). The cyclic voltammograms shown in Figs. 5(b) and
6(b) were obtained employing compact crystal electrodes.
Except for the presence of the peaks corresponding to the
Fe(II) oxidation (peak a2 in Fig. 5 and a1 in Fig. 6), the carbon paste electrode reproduces successfully the curves recorded employing the compact crystal electrodes.
4. Conclusion
It was shown that the carbon paste electrodes constructed as described above can reproduce data obtained from conventional constructed compact crystal electrodes. Besides this fact, this kind of electrode shows a number of advantages as low cost, easy construction, versatility for the use of ground minerals [7], mineral mixtures [8], metal deposition [9,10], that is, it offers the possibility to study the response of a several insoluble, but electroactive, substances.
Acknowledgements
C.M.V.B. Almeida, B.F. Giannetti / Electrochemistry Communications 4 (2002) 985–988
Financial support from Fundac
Pesquisa do Estado de S
~ a ao Paulo, FAPESP, and the valuable comments from Dr. Maria Elisa Martins are gratefully recognized.
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