In the Laboratory The Microscale Laboratory
Modified Carbon Electrodes for Microscale Electrochemistry Enrico Mocellin* AMC2 – Australasian Microscale Chemistry Centre, School of Biological and Chemical Sciences, Deakin University, Geelong, Victoria 3217, Australia Teresa Goscinska School of Biological and Chemical Sciences, Deakin University, Geelong, Victoria 3217, Australia
The development of modified carbon electrodes offers an inexpensive method for electrochemical investigations of the small quantities of compounds encountered in microscale chemistry (1). The technique involves the transfer of the analyte to the working surface of the modified graphite electrode, either by abrasion from a solid target compound or by evaporative deposition from organic solutions. The electrode, whose surface is coated with a thin film of the analyte, is placed in a conventional aqueous electrochemical cell and redox reactions are observed in the usual manner. Pyrolytic carbon offers access to a wide anodic potential range, low electrical resistance, low residual current, and a reproducible structure of the electrode surface and is available in morphological diversity as fibers, vitreous (glassy), and graphite pastes, making it suitable for use in the microscale laboratory (2). Modification of carbon electrodes with waxes and oils provides a lipophilic surface suitable for electrochemistry of water insoluble organic, organometallic compounds (3) and synthetic products. The paraffin-impregnated graphite electrode (PIGE) (4) and the carbon paste electrode (CPE) (5) are able to retain on their working surfaces a wide range of lipophilic organic and organometallic compounds. After insertion of the electrode immobilized with analyte into an aqueous electrochemical cell (as seen in Fig. 1a), the potential is scanned and the current is monitored in the usual manner using such conventional techniques as cyclic voltammetry (CV) or differential pulse voltammetry (DPV). With a little practice it is possible to achieve good reproducibility of the background current and almost constant electrode area. The proposed methodology is both simpler and cheaper. Either conventional or pseudo reference electrode (e.g., Pt wire) can be used in the cell and the relative potential difference can be established by the use of a standard reference substance such as ferrocene, as shown in Figure 1b, to determine the actual potential (see box). Traditionally, KCl rather than NaClO4 has been used with the Ag/AgCl reference electrode; but this precipitates KClO4 at the reference electrode frits and jacketed interfaces in contact with NaClO4. To obtain comparable values in organic solvents, the observed potential differences can be algebraically adjusted to the E1/2 value of ferrocene, + 0.44 V for the SCE in CH2Cl2. Reference/Pseudo-reference Electrode
This relatively new abrasive stripping voltammetry (ABSV) method makes use of the fact that extremely small amounts of samples are sufficient to give easily measurable current. The use of the PIGE and CPE method has proved to be very effective. The theory of electron transfer at solid surfaces by solid compounds with low or inadequate innate electron conductivity is not fully understood, and the influence of the organic binder on the electrochemistry and faradaic reactions has not been fully elucidated. This lack of in-depth theory, however, has not discouraged researchers from utilizing these techniques, and many other variations, in the study of mineralogy, superconductors, and the intercalation of ions into host lattices. An excellent review article covering all the above points can be obtained in reference 4. Experimental Procedure
Preparation of Electrodes PIGE. The 5 × 50-mm graphite rods (VEB Electrokole, Lichtenberg) are placed in a suitable flask containing molten paraffin (60 °C ) and a vacuum is applied for 2 or 3 hours to the whole assembly until air bubbles leaving the graphite rods counter electrode
working electrode
(a)
aqueous
electrolyte
reference electrode
Figure 1. (a) Schematic diagram of aqueous electrochemical cell. Note that the WE is just touching the electrolyte solution surface. (b) CV of ferrocene at PIGE electrode using 0.5 M NaClO4 as aqueous electrolyte. The electrochemical configuration was completed utilizing 0.5 M aqueous NaClO4. Scan rate was 50 mV/s.
(b)
E1/2 of Working Electrode (V)
Ag/AgCl in 0.5 M aq NaClO4
0.25
Pt
0.33 *Corresponding author.
JChemEd.chem.wisc.edu • Vol. 75 No. 6 June 1998 • Journal of Chemical Education
771
In the Laboratory
(a)
(b)
(a)
copper
hot water (60 C)
wire
teflon tubing
Current
20 µa µA 20 molten paraffin
working surface
carbon paste
Figure 2. Schematic diagrams of (a) the PIGE preparation and (b) the assembled CPE.
Apparatus Electrochemical tests can be performed on conventional CV equipment (potentiostat, an X-Y recorder, and a threeelectrode cell using platinum wire as the counter electrode and an Ag/AgCl reference housed in a fritted glass jacket containing 0.5 M NaClO4 solution.). The PIGEs or the CPEs were used as the working electrodes. For all experiments the 0.5 M aqueous solution of NaClO4 was used as the electrolyte. Each experiment was performed in quiescent electrolyte solutions under a nitrogen blanket. Procedure The analytes were immobilized on the electrode in two ways: by applying a 2-µL drop of the analyte (dissolved in ethanol), from a micropipet, on the working surface of the PIGE; or by abrasion from a filter paper on the working surface of the CPE. In the latter case the sample was dissolved in Nujol to form a paste, which was then spread on filter paper. The variations of the potential differences obtained at different electrodes are due to the unconventional jacketed reference electrodes used. These can be converted to standard potentials by algebraic adjustment to the E1/2 value of ferrocene. Conclusions The proposed approach of integrating electrochemical instrumental techniques is useful to characterize the product of synthesis, especially the small quantities obtained in microscale chemistry. The concept not only allows inexpensive electrochemical examination of very small quantities of compounds, but also gives students the opportunity for further investigative and developmental work in quantitative and qualitative aspects 772
-0.50 -0.60 V vs Ag/AgC
-0.70
(b) 25 ma µA 25
Current
are no longer visible (Fig. 2a). After the vacuum is slowly released, the graphite rods are taken from the bath and the excess paraffin is removed with tissue paper. After the paraffin has solidified at room temperature, the working surface of the electrode is cleaned by wiping it and conditioned by scribing lines on a sheet of filter paper. CPE. In an agate pestle and mortar, 2 mL of Nujol is added dropwise to 3 g of high-purity graphite powder and ground until a uniform paste is achieved. The carbon paste is then packed into Teflon tubing with a copper wire to make an electrical contact (Fig. 2b). The working area of the CPE can be renewed before each use by cutting off a thin slice with a scalpel and polishing the surface on filter paper.
-0.40
-0.30
-0.10
-0.30
-0.50
-0.70
-0.90
-1.10
-1.30
V vs Ag/AgCl Figure 3. Cyclic voltammograms of (a) 1,4-naphthoquinone abraded from a Nujol paste, on filter paper at CPE; (b) (η3-C3H5)Fe(CO)3I (3) at PIGE, transferred as a 2-µL drop from ethanolic solution onto electrode surface. In both cases the electrolyte used was 0.5 M aqueous NaClO4 solution.
of specific electrodes and sensors. Use of aqueous electrochemical cells overcomes the stringent electrochemical requirements for purity of solvents and supporting electrolytes, thus keeping costs to a minimum. Acknowledgment E. M. gratefully acknowledges a grant in aid from the Clive and Vera Ramaciotti Foundations, which complemented the purchase of equipment. Literature Cited 1. Mocellin, E.; Ravera, M.; Russell, R. A.; Hynson, T. J. Chem. Educ. 1996, 73, A99. 2. Gilmartin,. A. T.; Hart J. P. Analyst 1995, 120, 1029. 3. Mocellin, E.; Russell, R. A.; Ravera, V. M. J. Chem. Educ. 1998, 75, 773–775. 4. Scholz, F.; Meyer, B. Chem. Soc. Rev. 1994, 23, 341. 5. Kuwana, T.; French, W. G. Anal. Chem. 1964, 36, 241.
Journal of Chemical Education • Vol. 75 No. 6 June 1998 • JChemEd.chem.wisc.edu
