When G9 is “1” the motor rotates clockwise, and when it is “O”, the motor rotates counterclockwise. The control of relay drivers RD1, RD2, and RD3 can be transferred to a Manual Drive Control (Figure 10) using a 2-pole, 2-position switch. Thus by means of a push-button (PB) the monochromator can be started or stopped manually. Two single-pole, 2-position switches allow scanning at full
speed (Fast) or medium speed (Slow), and up or down. RECEIVED for review June 28, 1972. Accepted October 2, 1972. Work supported in part by NSF Grant G P 18910. Presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March, 1972.
Demountable Ring-Disk Electrode George W. Harrington,l H. A. Laitinen, and Vlado Trendafilov2 Departmenl oj’Chemistry, School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Ill. 61801 THERING DISK ELECTRODE has, within recent years, become a valuable tool for the study of a wide variety of electrochemical processes ( I ) . The use of different electrode materials, however, requires the fabrication of an entirely new electrode for each material, and the use of coated electrodes requiring heat treatment, etc., is impossible with existing designs. Another difficulty involves the centering of the electrode. In order to ensure that laminar flow occurs at the surface, the electrode must rotate symmetrically about its center axis. Such rotation is difficult to achieve if a long shaft is required. Demountable electrodes have been previously described (2-4). One of these ( 2 ) suffered from leakage about the electrodes, and the second ( 3 ) appears structurally unsound and subject also to leakage problems. The design used for this electrode does not ensure that the liquid-tight seal, if it forms, occurs at the face of the electrode. Neither design permits accurate centering. The kind of demountable electrode described in ( 4 ) will require fabrication of a new gap each time that the electrode is reassembled. The electrode described below was designed specifically to permit the use of different materials for the disk and to eliminate problems associated with centering the electrode. EXPERIMENTAL
The rotating assembly and motor system used were those described by Miller et a/. ( 5 ) . With only minor modifications, the potentiostatic circuitry was that described by Miller (6). The solutions used to test the electrode were prepared from reagent grade materials. The CuC& was used without further purification. The KCl was recrystallized three times from triply distilled water. Triply distilled water was also used in the preparation of all solutions. RESULTS AND DISCUSSION Platinum Disk-Platinum Ring Electrode. A cross sectional
diagram of the partially assembled electrode is shown in Figure 1. The diameter of the disk, at the surface, was Department of Chemistry, Temple University, Philadelphia, Pa. 19122 2 Present address, University of Skopje, Skopje, Yugoslavia. (1) R. N. Adams, “Electrochemistry at Solid Electrodes,” Marcel Dekker. New York. N.Y.. 1969, pp 92-110. (2) G . V. Zhutaeva and N. A. Sliumilova, Elekrrokhinziyu, 2, 606 (1966). (3) A. N. Doronin, ibid., 4, 1193 (1968). (4) R . D. Cowling and H. E. Hintermann, J . Electrochem. SOC., 118,1912(1971). (5) R. H. Sonner, B. Miller, and R. E. Visco, ANAL.CHEM.,41, 1498 (1969). (6) B. Miller. J . Elrciroclieni. Soc., 116, 1117 (1969).
0625 4
Figure 1. Demountable ring disk electrode-cross sectional view A . Disk electrode B. Ring electrode C. Plexiglas mount
D. Stainless steel plunger Stainless steel ring Set screws (6-32) Holes for connecting bolts (6-32)
E. F. G.
0.155 inch. The inner diameter of the ring was 0.177 inch and the outer diameter was 0.220 inch. Only two major dimensions are given in the figure since the other dimensions are not critical and can be varied to suit individual needs. Basically, the electrode consisted of a fixed ring imbedded in a cylindrical Plexiglas (Rohm & Haas) mount that had a hole in the center ending in a taper. The disk electrode was machined as a tapered cylinder to fit this taper. The angles of the taper of the disk and that of the hole differed by thirty minutes to ensure that the liquid tight seal was made at the surface and
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not somewhere back along the taper, resulting in leaks that would yield thin-layer effects. The tapered disk was held in place by a hollow stainless steel plunger (D)and a spring. The spring exerted pressure on the disk when the flanges were bolted together. The gap between the ring and the disk was established by the Plexiglas at the end of the taper. Connection to the ring was made by the wire shown which passed through a notch in the end of the plunger and then up through its center. This wire terminated in a small jack, at the flange end of the Plexiglas mount. This jack mated with a plug attached to an insulated wire that continued up through the main shaft. Connection to the external circuitry was made at the top of the shaft by spring-loaded graphite brushes as in reference (5). The main shaft and flange was made of stainless steel. This flange contained three symmetrically placed set screws ( F ) that were used to center the electrode. With the disk, plunger, and spring in place, the jacks were connected and the steel flange was slid onto the Plexiglas flange. The set screws were then adjusted until the electrode assembly was centered as determined by a machinist's centering gauge. Once centered, the flange were secured by three bolts inserted in the symmetrically placed holes (G), on the flanges. This centering operation required only a few minutes. The bolt holes in the Plexiglas flange were oversize to permit movement of the electrode during the centering operation. The center hub on the Plexiglas flange was surrounded by a press-fitted stainless steel ring ( E ) to prevent distortion of the Plexiglas by the set screws. When fully assembled, the electrode face was polished on a flat surface using various grades of alumina polishing material. This ensured, in the case of platinum, a usable electrode surface and also a uniformly flat surface along the ring and disk electrodes. The machining of the Plexiglas part could be done in a variety of ways. In this case, the cylinder and flange were cut from a solid block of Plexiglas and then the end machined back to leave a solid taper. The center hole was partially drilled, and the ring electrode and wire were set in place with epoxy adhesive. The entire end was then reformed with the epoxy. When the adhesive was set, the end was cut and the final taper machined. This procedure was followed so that the ring could add mechanical strength to the end during final cutting. Two such electrodes have been constructed and tested with solutions of Cu(I1) in 0.5M KC1. The results for these solutions agreed exactly with those of Napp et al. (7). The reaction Cu(I1)-Cu(1) was used to determine the experimental collection efficiencies. The collection efficiency, N , is defined as the ratio of ring current to disk current. In each case, the experimental value agreed with the theoretical value of 0.345 within less than 2 z (8). The experimental value of N was also found to be independent of rotation speed up to the limit of the assembly which was 2500 rpm. Each electrode had the same N value since ring and disks of the same dimensions were used in each. The electrodes were disassembled and reassembled several times with no changes in the experimental collection efficiency observed. Extensive polishing, as described above for the initial assembly, was not necessary after each disassembly-assembly operation. Tin Oxide-Coated Glass Disk-Platinum Ring Electrode.
A glass button with tapered sides was fashioned from a No. 8 (7) D. T. Napp, D. C. Johnson, and S. Bruckenstein, ANAL. CHEM., 39,481 (1967). (8) W. J. Albery and S. Bruckenstein, Trans. Faraday Soc., 62, 1920(1966).
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Pyrex (Corning Glass) flask stopper by polishing the sides with sandpaper and polishing alumina, and cutting to approximate size. Next, it was coated on all sides with a tin oxide coating, by heating it to 550 "C on a hot plate and spraying for short intervals with a solution containing 3M Sn(1V) chloride, 1.5M HCl, and 0.525M SbC13. After each spraying, the temperature was allowed to rise again to 550 "C, and the spraying repeated until a coating thickness of about 0.7 pm had been reached, as judged from a greenish blue interference color. The coated disk was mounted in the electrode assembly, and polished successively with sandpaper, 0.3 and 0.05 micrometer polishing alumina until the ring and disk were coplanar. Next, the electrode was disassembled, the tin oxide coating was removed by means of a solution containing chromium (111) chloride, HC1, and zinc amalgam, using nitrogen stirring. The button was then cleaned and coated once again with tin oxide as before. The initial coating was used only to achieve the final dimensions of the tapered button. Now, in order to minimize the electrical resistance due to the tin oxide that is not to be exposed to the solution, all surfaces to within about 1 mm of the disk surface were reduced to metallic tin by electrolysis in 0.5M H2S04, 0.5M Na2S0, at -0.8V zis. SCE. This treatment proved more successful than attempts to coat the glass with silver paint or silvering solution. The button was then remounted, and polished 1-2 minutes using only polishing alumina to produce the final electrode. Occasionally, electrodes fabricated in this manner showed leakage due to minor geometrical imperfections. Leakage was effectively prevented by immersing the button in melted paraffin wax and remounting. The wax was then removed from the disk area with filter paper wetted with acetone and then polished in the same way. To test the performance of such an electrode, of disk diameter 0.288 inch, inner ring diameter 0.295 inch, and outer ring diameter 0.360 inch, the collection efficiency was measured with the Cu(1I )solution used previously. The experimental value of N agreed within 2 % at various rotational speeds from 500 rpm to 2500 rpm with the theoretical value of 0.368 calculated from Equation 6.1 of reference (8)--i.e., at 500 rpm N = 0.368, and at 2500 rpm N = 0.363 To test the electrode for charge transfer behavior, the value of KSlhfor Fe(I1) in 1M H2SO4 was determined by using the method of Levich (9). The value determined in this way was 1.3 x 10-6cm sec-'as compared with 6.3 x sec-' (IO). An attempt was made to fabricate an electrode in which both disk and ring were of tin oxide-coated glass. For this purpose, a glass rod and a glass tube were used for the disk and ring, coating both as before. The electrode gave a collection efficiency which depended on rotational speed, varying from 0.320 to 0.281 at 1000 rpm to 2500 rpm. The effect was probably due to the fact that the rod and tube were not quite round. Fabrication with accurately dimensioned stock would presumably yield better results, but the matter was not pursued further. ACKNOWLEDGMENT
The authors would like to express their appreciation to Elmer L. Lash of the Machine Shop, School of Chemical Sciences, for his skill and many useful suggestions. RECEIVED for review May 10, 1972. Accepted September 27, 1972. The work done in this project was supported by National Science Foundation Grant NSF G P 26017. (9) Z. Galus and R. N. Adams, J . Phys. Chem., 67,866 (1963). (10) David L. Zellmer, Ph.D. Thesis, University of Illinois. 1969.
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