Anal. Chem. 1984, 56, 118-119
118
4 7
- 1.0
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-0.5
0.0
v SCE Flgure 4. Cyclic voltammograms of background currents (standard Pine rotating disk silver working electrode, area 0.0442om2, vs. SCE): (1) hand-polished electrode, pH 9.4 borate buffer; (2)mechanically polished electrode, pH 9.2 borate buffer; (3) handpolished electrode, 0.2 M NaOH; (4)mechanically polished electrode, 0.1 M NaOH. YS
with 0.3 and 0.05 wm alumina for a t least 15 min. In both cases, background currents were decreased substantially by the use of automated polishing methods. In polishing relatively soft metals like gold and silver, one must be especially careful to keep all materials scrupulously clean. Also, one must pay particular attention to match the hardness of the electrode material with the hardness of the abrasive. In cases where the electrode material is harder than the insulating material, the insulating material can be worn away preferentially. As a result the electrode protrudes out of the insulating material. This leads to varying degrees of nonlinear diffusion when the electrode is being used. To minimize these problems, we switched to a nylon polishing cloth. The incorporation of the outrigger described previously also helped to minimize this problem. For both soft materials and electrodes for which the insulator and active area differ in hardness, the modified Minimet makes it possible to polish for very long times using very gentle
conditions. During long unattended periods of polishing, the heat generated by friction tended to evaporate the lubricant. To prevent this the polishing bowls were modified to hold dry ice. A small opening was made in the lip and the underside of the collar of the bowl was closed off by using a snugly fitting piece of plastic tubing. Crushed dry ice placed in the opening just under the glass plate kept the area cold. If electrodes are small or light, it may be difficult to get them to rotate. In such cases, the electrode should be secured off center in the sleeve. In addition, a clamp holder or hose clamp attached to the top of the electrode can provide a moment of inertia large enough for rotation with no compromise in the quality of the polished surface. Narrow-barreled electrodes posed one other problem: that of proper alignment of the surface with the polishing cloth. Care must be taken that they are exactly perpendicular to one another. Construction of a collar/roller bearing assembly employing a smaller diameter roller bearing would eliminate this problem.
ACKNOWLEDGMENT The authors wish to thank Gary Sagerman for constructing the modified load arm, Jerry Ptak for providing a technical drawing of the same, and Mizuho Iwamoto for supplying the cyclic voltammograms. LITERATURE CITED (1) Vydra, F.; Stullk, K.; Julakova, E. "Electrochemical Stripping Analysis"; Wiley: New York, 1976. (2) Bard, A. J., Ed. "Electroanalytical Chemistry": Marcel Dekker: New York, 1973: Vol. 6. (3) Ewing, G. W. "Instrumental Methods of Chemlcal Anaiysls", 3rd ed., McGraw-HIII: New York, 1960. (4) Bard, A. J; Faulkner, L. R. "Electrochemical Methods"; Why: New York, 1980. (5) Delahay, P. "Double Layer & Electrode Klnetics"; Wiley-Interscience: New York, 1985.
RECEIVED for review June 27,1983. Accepted October 3,1983. This work was supported by the National Science Foundation under Grant No. CHE 7917543 and CHE 8305748.
Rotating Ring-Disk Electrode with Wide Temperature Range David K. Roe* and Mario Aparicio-Razo
Department of Chemistry, Portland State University, Portland, Oregon 97207 With few exceptions, rotating ring-disk electrodes have been constructed from organic polymers and metals, a combination that has a ratio of thermal expansion coefficiences between 5 and 10. Deformation of the polymer usually occurs a t 10 to 20 OC above room temperature resulting in solution penetration along the sides of the metal electrode. This problem restricts all electrochemical experiments to a narrow temperature range when this type of electrode is used. Phillips et al. (I) were successful in construction of a glassy carbon electrode assembly using borosilicate glass to insulate the ring and the disk. In this case, the coefficients of thermal expansion are sufficiently close that the electrode was usable in fused salts up to 450 "C. By properly selecting the type of glass, a few other electrode materials could be used. Two recent electrode designs ( 2 , 3 )appear to avoid extreme sensitivity to temperature of the seal between insulator and electrodes by employing heat shrinkable polyolefin tubing. Operation at 100 OC was said to be possible with the one design (2) and may also apply to the other (3) due to similarity of sealing technique. A rotating disk electrode made by thermal
compression ( 4 ) of a platimum disk in Kel-F has been used in aqueous solutions up to 60 OC without leakage problems (5). In principle, the assembly technique could be extended to include a ring electrode. A quite different approach to the design of a ring-disk electrode assembly has been used in our laboratory to permit use over a wide temperature range. The body of the electrode assembly is fabricated from a machinable ceramic and coated by thermal evaporation or sputtering to form the conductive surface that becomes the ring and the disk. Electrical contact to the two electrodes is provided by a simple and reliable means: a platinum rivet.
EXPERIMENTAL SECTION Details of the major features of the ceramic ring-disk electrode are given in Figure la. The electrode body is a piece of ceramic machined from Macor (Corning Glass Works) with an internal thread that mates with a shaft of stainless steel tubing. A length of Macor rod of nominal 0.5 in. diameter was turned to 12 mm and cut into 20 mm lengths. Each piece was drilled and tapped with a 1/4 X 28 thread to a depth of about 15 mm. The top 5 mm
0003-2700/84/0356-0118$01.50/00 1983 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
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rotator and dual potentiostat were designed and constructed in this laboratory.
a
b
Figure 1. (a) Partial cross section of electrode and shaft. (b) Enlarged view of lower end of electrode body. Drawings are not to scale.
of the hole was machined to a diameter of 6.5 mm to provide accurate centering with the shaft. A second 2 mm hole was drilled in the wall directly above the intended ring location, again to a depth of 15 mm. Both this hole and the center one were continued to the lower end at a diameter of 0.5 mm and the exit hole was slightly flared. Platinum wire of the same diameter was stretched slightly to provide a tight fit and then one end was melted to form a small ball. After threading the wire through one of the holes, it was cut off at a distance of 0.5 mm from the ceramic surface and the end peened flat to finish the rivet. During this step, the ball-end of the platinum wire was supported on a steel rod. After installation of the second rivet, the end of the ceramic was machined in the form shown in Figure lb. Ring and disk diameters were defined in this step by cutting into the surface to a depth of about 0.1 mm, leaving a ridge with a base width of about the same dimension. This surface was then polished to a mirror finish with 0.5-pm alumina. Metalization of the lower end of the ceramic was by thermal evaporation under vacuum or by sputtering; both methods have been used successfully. Good adherence depends upon applying first an intermediate layer of chromium or titanium, about 1to 5 nm thick, followed by the desired metal, in this case gold, to a thickness of 10-50 nm. A number of ceramic pieces can be metalized at the same time. When properly applied, the metal layer resisted the pull of adhesive tape. Next, the gold was removed from the outside and intermediate ridges by a very thin cut on a lathe. The depth of this cut determines the space between disk and ring and it has been made as small as 0.025 mm. Electrodeposition was used to build up the thickness of the ring and disk areas. For gold, a 0.1 F solution of AuC13 in 1F HCl was found to produce pure, dense deposits at 50 "C and a Several steps of plating followed current density of 1 mA by polishing produced a surface that was flush with the edges of the ridges that defined the areas of the ring and disk, as is illustrated in Figure l b. Electrical contact to the platinum rivets was provided by springs, as shown in Figure la. The upper end of the spring from the ring contacted a stainless steel disk centered on a ceramic hub for insulation and a wire from the disk was connected to a brass slip ring on the shaft. This arrangement allows easy removal of the electrode body yet reliable electrical contact. Collection efficiencies were measured with Fe(CN)63-in 0.1 F H2S04over the range of rotation rates from 500 to 5000 rpm. The
RESULTS AND DISCUSSION Calculated collection efficiency for an electrode assembly with a disk radius of 0.381 cm, inner ring radius 0.407 cm, and outer ring radius of 0.597 is 0.488 according to the method of Albery and Hitchman (6). Measured efficiency a t room temperature with Fe(CN):- was 0.493 and it remained constant within 1% over the entire rotation rate range. This electrode design has also been used extensively over the past 2 years in a study of sulfur and selenium electrochemistry using dimethyl sulfoxide as solvent. In these experiments, temperatures up to 125 "C were used with no apparent failure due to thermal changes. Details of these measurements will soon be published. The sucess of this electrode design rests upon the nearly perfect match of thermal expansion coefficients of platinum ((9.7 X IOT7)/OC)and Macor ((9.4 X lO-')/OC), providing a leak-free electrical contact to the electrode surface. Gold and most other metals have different expansion coefficients but in thin layers on the ceramic the dimensional changes can be accommodated to some degree. We have not yet prepared electrodes of metals other than gold and so cannot comment on their properties. It is obvious that the critical step is evaporation or sputtering of the intermediate metal layer; lack of adhesion is always due to poor surface preparation or improper conditions during this operation. Through a series of empirical tests, conditions for good adhesion were finally obtained. This seems to be the usual approach in thin film preparation by these techniques. In comparison with other designs, this electrode assembly appears to be simpler and more easily made. One shaft assembly can be used with a variety of electrode bodies and their exchange is very simple. A number of metals can be electroplated in satisfactory purity, so there are no serious limitations in choice of materials. It is also possible to deposit carbon on inert substrates in the pyrolytic form, although we have not as yet explored this technique. Finally, it should be noted that very narrow disk-to-ring gaps are possible and, with modification of the ring contact, multiple, thin rings can also be made. ACKNOWLEDGMENT Many of the details of this electrode design as well as the construction were the result of the ingenious efforts of the late Thomas B. Hutchins, 111. R e g i s t r y No. Gold, 7440-57-5. LITERATURE CITED (1) Phillips, J.; Gale, R. J.; Wler, R. G.; Osteryoung, R. A. Anal. Chem.
1978, 4 8 , 1268. Rowley, P. G.; Osteryoung, J. G. And. Chem. 1978, 50, 1015. Geiger, T.; Anson, F. C. Anal. Chem. 1980, 52, 2448. Tench, D.; Odgen, C. J . Nectrochem. SOC. 1978, 125, 194. Haak, R. P.; Ogden, C. Rockwell International Science Center, Thousand Oaks, CA, personal communication, 1981. (6) Albery, W. J.; Hltchman, M. L. "Ring-Disc Electrodes"; Clarendon Press: Oxford, 197 1. (2) (3) (4) (5)
RECEIVED for review July 29, 1983. Accepted September 16, 1983.
