ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
1015
Construction of a Rotating Ring-Disc Electrode from Irregular Electrode Materials Peter G. Rowley” Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Janet G. Osteryoung Department of Microbiology, Colorado State University, Fort Collins, Colorado 80523
Investigation of modified electrodes consisting of electroactive reactants attached to electrode surfaces has received close attention in the past five years (1-9). Substrates used in modified electrode experiments have included platinum ( I ) , tin oxide (6, 8, 9 ) , indium oxide (6), spectroscopic graphite ( 2 ) ,glassy carbon (j), pyrolytic graphite ( 4 , 7) and highly oriented pyrolytic graphite ( 3 ) . Murray a n d co-workers ( 2 , 5 ) have shown how x-ray photoelectron spectroscopy (ESCA) may be utilized for extensive surface characterization; however, for in-situ characterization and mechanism elucidation, Anson has recently described ( 4 , 7) how cyclic and, in particular, differential pulse voltammetry can be used t o advantage. T h e power of the rotating ring-disc electrode (RRDE) technique for unravelling electrode mechanisms has recently been drawn into focus by Bruckenstein and Miller (IO). T h e capability of a RRDE’s ring electrode to detect, virtually simultaneously, species generated a t t h e disc electrode, provides a n elegant a n d powerful procedure for elucidating complex chemical and electrochemical reaction mechanisms. However, because of t h e physical nature of some of these electrode substrates, in particular highly oriented pyrolytic graphite, a problem arises in how t o construct a ring-disc electrode using a disc substrate which is almost impossible to machine into a disc form. This is particularly the case when t h e end-plane rather than basal-plane surfaces of both pyrolytic and highly-oriented pyrolytic graphite are required to be mounted. We have been successful in fabricating an inexpensive rotating ring-disc electrode incorporating irregular end-plane sections of pyrolytic and highly-oriented pyrolytic graphite.
EXPERIMENTAL Electrochemical Instrumentation. Ring and disc potentials were independently controlled using a dual electrode potentiostat manufactured by SPEL, Specific Electronic Ltd., and the variable electrode potential was produced by a basic potential sweep generator (constructed in this laboratory). A Pine Instrument Company rotator was used to rotate the ring-disc system at specific rotation rates. Current-voltage traces for both ring and disc were recorded simultaneously on a Hewlett-Packard H P 7046A X-Y-Y recorder. Reagents. The chemicals used were Merck “Suprapur” grade; no further purification was carried out. Solutions were prepared with double deionized-double distilled water. Electrochemical Cell. The cell was constructed from an acid-washed 150-mL Berzelius tall form beaker, fitted with a Teflon top machined to allow the insertion of the ring-disc electrode, two degassing tubes, a Pt spiral counter electrode contained in a glass tube with a “pinhole” solution connection, and finally a PAR salt bridge assembly including a saturated calomel reference electrode. Electrode Construction. Initial investigations were carried out using disc-only electrodes. These were constructed by mounting the electrode samples onto the end of a hollow glass tube using heat-shrinkable polyolefin tubing (FIT 300, Alpha Wire Corp.) which has the property that the inside surface melts while the rigid outer surface shrinks on application of hot air. This polyolefin tubing has a shrinking temperature of approximately 115 “C. This allows the irregular disc substrate samples to be mounted easily by contraction around the irregular structure, ensuring a leak-tight seal. 0003-2700/78/0350-1015$01.OO/O
The rotating ring-disc electrodes were fabricated according to the following outline. The irregular disc electrode substrate (pyrolytic and highly oriented pyrolytic graphite; Union Carbide) was mounted onto a glass shaft as described below and depicted in Figure 1. The glass tubing onto which the disc substrate was mounted was a piece of Ace 5027 adapter tube which terminates (at opposite end) in a glass (female) screw-thread. This thread was fitted with a machined Teflon thread press-fitted onto the end of the stainless steel rotator shank. Electrical connection to the disc was made by pressure contacts onto the back (inside) of the disc, and a t the opposite end, onto the narrow portion of the stainless steel rotator shank over which the Teflon screw thread was pressed. A removable thin (2 mm) brass rod with a spring spot-welded on either end served this purpose, as shown in Figure 2. The diameter of the glass shaft and the disc substrate were chosen so that either one or two layers of heat shrinkable polyolefin tubing resulted in a snug fit for the piece of glassy carbon (G.C.-20, Tokai Electrode Manufacturing Company) pipe (10-mm i.d.) which was placed over it and served as the ring electrode. To ensure a leak-tight seal, the gap between the polyolefin tubing and the G.C.-20 tubing was filled with epoxy cement by coating the polyolefin as the glassy carbon ring was placed onto it. Excess epoxy was removed with acetone before it dried. With adequate clamping, the glassy carbon ring can be set so as to be relatively concentric to the center of the glass shaft. Contact between the ring and the rotator (which has two carbon brushes for electrical contact onto the revolving shaft) is made by first press-fitting a brass ring onto the upper portion of the machined Teflon connection between the glass shaft and stainless steel rotator shank. The lower carbon brush makes an electrical pressure contact onto this ring. Then a piece of copper wire is soldered (low temperature solder) onto the bottom of the brass ring; the length of it follows closely the contour of the glass shaft, down until it overlaps onto the outside of the glassy carbon ring. To secure the wire to the ring, and at the same time mask the side regions of the glassy carbon as well as the exposed copper contact wire, a larger diameter piece of heat-shrinkable polyolefin tubing (FIT 300-13 mm) was placed over the ring to cover the entire length of the electrode, up to the screw thread. The final stage in electrode preparation involved carefully filling surface gaps between the ring and disc with epoxy. The surfaces of both ring and disc were then polished using a series of fine papers, finishing with a fine camel polishing cloth to remove particulates. It should be recognized that this polishing process of the edge-plane has a tendency to bend over some of the protruding basal plane, thus exposing it on the mounted edge orientation. Although correct alignment of the electrode should be ensured by relatively careful machining of the stainless steel shank and the Teflon (male) screw thread, any eccentricity can be easily eliminated before mounting the disc electrode. Simply secure the joined steel shank and glass shaft in the rotator, and apply a mild flame to the top of the glass shaft (just below the glass screw thread) while the electrode is revolving. This enables the electrode tip to run true despite eccentricities due to electrode construction. The shapes of substrates mounted in this manner varied from quasi-circular samples of pyrolytic graphite to almost square samples of highly oriented pyrolytic graphite. The typical area of these electrodes was 0.5 cm2 with an average disc to ring gap of approximately 1 mm. The assembled electrode demonstrates a thermal mechanical-stability up to about 100 O C . The polyolefin tubing exhibits both chemical and mechanical stability in the following list of 1978 American Chemical Society
1016
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Rotator Shank
5ol
40
Teflon Thread
101
L Hlghly Oriented Pyrolytic Graphite
0
0
1000
2000
3000 4000
Rotation Rate (rpm)
Figure 3. Collection efficiency of the ring with respect to the disc at various rotation rates Substrate
Figure 1. Exploded diagram of a ring-disc electrode without outer tubing sheath
Teflon
. (Threaded)
- DISCElectrical
Connection
electrode. However, a working curve can be constructed (Figure 3), enabling quantitative use of the ring-disc electrode system for mechanistic studies. Once constructed, the ring and disc substrate materials used in these electrodes can be easily removed for further experiments, such as surface analysis (Auger, ESCA), by simply using a razor blade and a hot-air gun. The use of such a system can result in definitive information about electrochemical mechanisms of modified electrode systems, as long as the limitations introduced by the use of this electrode system are realized. First, a working curve for ring collection efficiency must be used in the absence of the ideal behavior usually associated with precision made, thin-gap, ring-disc electrodes. Second, the use of a wider annulus between the ring and disc increases the average transit time (13) for the transport of a species from the disc to the ring. These are minor disadvantages weighed against the power and convenience of the ring-disc technique.
ACKNOWLEDGMENT We thank A. W. Moore, Union Carbide Corporation, Parma Technical Center, Parma, Ohio, for helpful discussions and the supply of graphite samples. ?14rnmk
Figure 2. Completed ring-disc electrode
solvents. In each case, no mechanical deformation nor dissolution of the tubing was observed after immersion for 48 h. The solvents were acetonitrile, ethanol, acetone, dichloromethane, ethyl acetate, chloroform, carbon tetrachloride, and benzene.
RESULTS AND DISCUSSION Two rotating ring-disc electrodes were constructed, one using a piece of pyrolytic graphite and the other with a piece of highly oriented pyrolytic graphite. Both were mounted to expose the edge-plane to the solution. The collection efficiency (11) of the ring with respect to the disc electrode was determined using a M Cu(II)/Cu(I)/Cu system in 0.5 M KCl (12). The curves obtained a t various rotation speeds are shown in Figure 3. As perhaps would be expected, these electrodes do not behave ideally in t h a t collection efficiency is not independent of rate of rotation, due to some eccentricity as well as the irregular nature of the disc
LITERATURE CITED R. F. Lane and A. T. Hubbard, J. Phys. Chem., 77, 1401 (1973); ibu.. p 1444. C. M. Elliott and R. W. Murray, Anal. Chem., 48, 1247 (1976). L. L. Miller et al., J. Am. Chem. Soc., 98, 8271 (1976). A. P. Brown, C. Koval. and F. C. Anson, J . E/ectroanal. Chem., 72. 379 (1976). J. C. Lennox and R. W. Murray, J. Nectroanal. Chem., 78, 395 (1977). T. Kuwana et al., Anal. Chem., 48, 741 (1976). A. P. Brown and F. C. Anson, Anal. Chem., 49, 1598 (1977). P. P. Moses, L. Weir, and R. W. Murray, Anal. Chem., 47, 1882 (1975). D. G. Davis and R. W. Murray, Anal. Chem., 49, 194 (1977). S.Bruckenstein and B. Miller, Acc. Chem. Res., 10, 54 (1977). W. J. Albery and S. Bruckenstein, Trans. Faraday SOC.,82, 1920 (1966). D. T. Napp, D. C. Johnson, and S. Bruckenstein, Anal. Chem., 39, 481 (1967). S. Bfuckenstein and G. A. Feldman, J. Electmnal. Chem., 9,395 (1967).
RECEIVED for review December 28,1977. Accepted February 13, 1978. This work was funded in part by the National Science Foundation under Grant Number CHE 75-00332 and by the Environmental Protection Agency under Grant Number R 805-183.
