378
Anal. Chem. 1985, 57,378-379
Technical Development, National Heart, Lung, and Blood Institute) for helpful discussions.
(6) Macfarlane, R. D. Anal. Chem. 1083, 55, 1247A-1264A and refer-
Registry No. 1 (RI-R6 = Me3, H3), 93454-66-1; 1 (R1-R6 = Me4,H2),93454-67-2; 1 (R1-R5 = Me, R6 = H), 603-47-4; 1 (R1-& = Me), 548-62-9; 2, 93454-65-0; methyl violet 2B, 8004-87-3.
Henry M. Fales* Lewis K. Pannell Edward A. Sokoloski
LITERATURE CITED (1) Conn, H. J. “Biological Stains”; Williams & Williams Co.: Baltimore, MD, 1961; pp 106, 146. (2) Hiranaka, P. K.: Kleinman, L. M.; Sokoloski, E. A,; Fales, H. M. Am. J. Hosp. Pharm. 1075, 32, 926-930. (3) Pannell, L.; Sokoloski, E. A.; Fales, H. M.; Tate, R . submltted for publication in Anal. Chem. (4) Scheifers, S. M.; Verma, S.; Cooks, R. G. Anal. Chem. 1083, 55,
2260-2266. (5) Ito, Y.; Conway, W. D. Anal. Chem. 1084, 56, 534A-554A.
ences therein.
Laboratory of Chemistry National Heart, Lung, and Blood Institute Bethesda, Maryland 20205
Peter Carmeci PC, Inc. 11805 Kim Place Potomac, Maryland 20854 RECEIVED for review September 5, 1984. Accepted October 23, 1984.
AIDS FOR ANALYTICAL CHEMISTS Considerations for Polishing Glassy Carbon to a Scratch-Free Finish Duane E. Weisshaar and Theodore Kuwana*
Department of Chemistry, T h e Ohio State University, Columbus, Ohio 43210 Currently a major research endeavor in this laboratory is the correlation of electrochemical behavior with the physical properties of glassy carbon electrode surfaces (1-6). A primary concern in these studies is the preparation of highly polished, reproducible surfaces. Until recently the usual procedure was to polish the electrode with successively smaller sizes of alumina on a polishing pad or cloth which produced scratch-free surfaces. It has been observed in this laboratory that electrodes polished on such pads tend to be less active electrochemically (at least for some reactions) than electrodes polished on a glass plate (6). However, polishing electrodes on a glass plate with a slurry of alumina and water often results in visible scratching of the electrode surface. A severe example of this scratching is depicted in Figure 1. This scratching is caused by large agglomerates of alumina that are inherent in the alumina preparation process (7,8). Recently, so-called, “deagglomerated” aluminas have become available. These aluminas are produced by an alternate process that minimizes the formation of agglomerates during production (8). We have observed that when Ni is electrodeposited on glassy carbon, the Ni tends to deposit along scratches (5). For our purposes it is imperative that the electrode surfaces be free of such scratches. A comparison of the polishing ability of deagglomerated aluminas from two different sources and the “regular” grade aluminas was undertaken to find a suitable source of alumina that would minimize this problem. The purpose of this paper is to pass the results of this comparison to others who may have similar polishing requirements. The polishing procedure described below is used routinely in this laboratory for the preparation of metal oxide dispersed electrodes (1-3) and polyvinylacetic acid coated electrodes (4-5). Research toward a fundamental understanding of the electrochemical behavior of these electrodes is continuing and will be published separately.
EXPERIMENTAL SECTION A ground glass plate (Barnes Analytical Division, Stamford,
CT) was polished on a padded lapping wheel (Ecomet 111, Buehler,
Table I. Alumina Samples TestedQ crystal
particle size, Mm
structure
1.0
C Y
0.3
a Y
0.05
source Ab
source Bb
R, S, D R, S,D R, S, D
D, s D D
R, regular; S, suspension; D, deagglomerated. Sources available on request.
Ldt., Lake Bluff, IL) for 15 min each with 600 grit aluminum oxide and 1.0 km alumina. The pad was thoroughly flushed with water between steps. This process did not produce a smooth surface, but the roughness was reduced drastically. The plate, as received, was quite rough and polishing on it without prior smoothing of the plate produced a glassy carbon surface with a high density of scratches. Three GC-10 Tokai glassy carbon electrodes (Tokai Carbon Co., Ltd., Tokyo Japan) of approximately 1 cm2 were used to evaluate the polishing ability of the various samples of LO-, 0.3-, and 0.05-pm alumina listed in Table I. Each electrode was polished in a slurry of alumina and double distilled water on the plate using a circular motion with relatively heavy hand pressure for approximately 30 s. A fresh aliquot of alumina was used to polish each electrode after which the polishing plate was rinsed with distilled water and wiped with a tissue. The alumina suspensions were diluted with double distilled water to give a slurry of about the same consistency as with the powders. The relatively heavy hand pressure was necessary to break up loose agglomerates that otherwise would scratch the surface. All photomicrographs were obtained with a Model PME-100-FF Olympus Tokyo Inverted Bench Model Metallographic Optic Microscope (LECO Corp., St. Joseph, MI). The alumina sample code that will be used here has the format: type-source. The types of alumina employed in this study were: regular (R), suspension (S), and deagglomerated (D). The two sources of alumina will be referred to as A and B. (The sources will be provided upon request.) Thus, S-B will refer to an alumina suspension from source B. The term “visible”,as used herein, refers to artifacts that are visible to the naked eye such as the scratches in Figure 1.
0003-2700/65/0357-0378$01.50/00 1984 American Chemlcal Society
ANALYTICAL CHEMISTRY. VOL. 57. NO. 1. JANUARY 1985 .-
w e 1. olassy carbon pollshed with 1.0-pm R-A alumina. Bar Is 15 pm.
Flpun 2. &say 15 em.
carbon
WMI 1.0-pm D-A alumlna. Bar is
RESULTS AND DISCUSSION
AU samples of the 1.0-pm aluminas except R-A could p r e duce visibly scratch-free surfaces (e.g., Figure 2 The faint black spots in Figure 2 are holes in the glassy carbon that result from the fabrication process and not from the polishing proteas). OcCasionaUy, a large agglomerate did not break up, in which cane some scratching resulted. Renewed polishing with a new aliquot of alumina usudy eliminated the scratches. It was almost impossible to get a visibly scratch-free surface with the R-A sample, hut usually there were only one or two scratches. The D-B, S-B, and S-A samples produced, by far, the best results. None of these gave any visible evidence of scratches on any of the tested electrodes. Sample D-B was uaed to produce a scratch-freesurface on each electrode before testing the effect of another alumina sample on that electrode. For the 0.3-pm samples, only the D-B and the R-A samples produced visibly match-free surfaces. The other samples did not cause severe scratching, hut it was impossible to obtain a scratch-free surface. It is interesting to note that the D-A sample produced more scratches than the R-A sample.
S70
All samples of 0.05-pm alumina except the D-A sample could produce visibly match-free Surface. The D-A sample produced many tiny matches (not visible) that a c t d y degraded a mirror finish produced by the 1.0-pm aluminas. Again the D-A sample produced more visible scratches than the R-A sample. At low magnification there was no apparent difference in the fhkproduced by the three sizes of aluminan. At higher magnification (ca. 1 X I@)the differences produced by the different sizes are readily apparent. The width of the microscopic scratches correlates with the nominal size of the alumina used to polish the surface. When this polishing procedure was carried out in the absence of alumina, i.e., polishing the electrode on a wet plate, the GC-10 surfaces remained scratch free. Thin implies that the polishing plate surface is not responsible for the matches that occur when alumina is used. Repeating this procedure on an optical tlat that had bean wed as a polishing plate for several years, however, resulted in matches in the electrode surfaces. The extent of scratching depended on which area of the plate wm used. This plate also seemed to have some shallow hills and valleys on the surface. Since the optical flat was smoother than the gnnmd plate, the important factor seem to be the htnesa of the polishing plate surface. It may be necessary to "resurface" a polishing plate periodically to maintain such a flat surface. Since there are undoubtedly more than two sources of alumina polishing compounds, and since thia investigation did not include enough samples from each source for compilation of statistics, it cannot be concluded that any one source of alumina is better than another. These results do indicate that it may be necessary to evaluate several grades of alumina from several production runs from a given supplier to obtain a batch that produces the desired results. The results also indicate, for polishing, one must consider not only the nominal size of the alumina particles hut ala0 the condition of the polishing plate surface. With the appropriate choice of the above criteria and a little patience, it is possible to obtain highly polished, reproducibly scratch-free electrode surfaces. ACKNOWLEDGMENT We gratefully acknowledge the gifts of deagglomerated alumina samples from source A and source B. Helpful discussions with Dale Karweik, Bart Jones, and Marc Porter are greatly appreciated. Registry No. Carbon, 7440-44-0: alumina, 1344.23-1. LITERATURE CITED (1) Lak. J.: KUVIIM. 1.J . Am. m.SOC. 1982. 101. 5.5144515. (2) Lak. J.; K w ~ M . 1.J. €bmmm/.cy*mr. 1983. 150. US-864. (3) Dong. S.; KUW~M. 1.J . E b m d m m . SOC. 19M. !SI. 813-819. (4) KBO. W.H.; KUW~M, 1. J . Am. Cnsm. See. 1984. 108. 47-76. (5) Webhaor. D. E.; Kwm. T. J . ELSAWMI. chan. 1984. 169. 395-399. (6) Hu. I.
