~~~
Table I. Measured Per Cent Reflectance Blue Green Red As freshly prepared 21.5 i 1 . 0 22.5 i 1 . 3 49.0 f 1 . 0 Unalkalized, after three 26.5 & 2 . 0 28.8 i 2 . 0 54.5 f 2 . 7 months Alkalized, after ten 21.5 i 1 . 8 21.3 i 1 . 4 47.3 f 2 . 3 months ~~~~
Other data are at hand to substantiate this conclusion. Other data show that in more concentrated solutions of the phosphorodithioate, the effect of alkalinization on stability is less marked. It seems plausible that a trace of nitrite in the distilled water used could be an important factor (3). The ratio of nitrite to phosphorodithioate would be higher in the more dilute solutions of the latter. When the solution is alkalized, nitrite is rendered less potent as an oxidant.
~~
Freshly prepared stains from the two solutions agreed within acceptable limits. The values in Table I represent two values for the unalkalized and two for the alkalized solution as the overall average of the four, expressed with the average deviation. The values for the unalkalized solution after three months and for the alkalized solution after ten months are each the average of four determinations. Though the data illustrate the degree of imprecision inherent in the analytical method, they demonstrate the advantage of adding Na2HP04 when preparing the solutions.
ACKNOWLEDGMENT
The writer is grateful to the American Can Company for the support of the earlier phases of this investigation, and for donating to Stetson University the reflectometer used in the later work. W. N. Jensen, whose contribution to the investigation is in part described above, is now associated with the Milwaukee Department of Health. RECEIVED for review May 17, 1968. Resubmitted May 20, 1969. Accepted June 13,1969.
Precision Assembly for Rotating Ring-Disk Electrodes and Related Techniques R. H. Sonner, B. Miller, and R. E. Visco Bell Telephone Laboratories, Znc. Murray Hill, N. J. THE INTRODUCTION of ring-disk electrochemistry was made in the USSR by Frumkin, Nekrasov, and Levich (1-3). Theory appropriate for this electrode configuration has been given by Levich (4) and significantly extended in a series of papers by Albery and Bruckenstein (5, 6). Descriptions of several specialized rotating assemblies are available (7-11). Inexpensive rotators are available from laboratory supply houses but do not provide for the necessary electrode contacts and mechanical stability. A complete electrode rotator with 9 fixed speeds has been recently offered commercially by Pine Instruments, Grove City, Pa. The rotating electrode system used in our laboratory was developed to satisfy a requirement for a precision multispeed unit. Two bench models have been in use for several years. A third unit has been installed in a controlled atmosphere dry box where it is used for studies in nonaqueous solvents. The modular control system enables the latter unit to be (1) A. N. Frumkin and L. N. Nekrasov, Dok. Akad. Nauk SSSR, 126, 115 (1959). (2) A. N. Frumkin, L. N. Nekrasov, V. Levich, and Ju. Ivanov, J. Electroanal. Chem., 1,84 (1959). (3) . , L. N. Nekrasov and N. P. Berezina, Dok. Akad. Nauk SSSR, 142, 855 (1962). (4) V. G. Levich, “Physicochemical Hydrodynamics,” PrenticeHall, Englewood Cliffs, N. J., 1962, Chapter VI. (5) W. J. Albery and S . Bruckenstein, Trans. Faraday Soc., 62, 1946 (1966). (6) Ibid., p 1920. (7) ~, M. P. Belvanchikov. Yu. V. Pleskov. and B. G. Pominov, Russ. J. PhG. Chem. English Transl., 34, 782 (1960). (8) H. E. Hintermann and E. Suter, Rev. Sei. Instrum. 36, 1610 (1965). (9) J. Wojtowicz and B. E. Conway, J. Electroanal. Chem., 13, 333 (1967). (10) D. C. Johnson, Ph.D. Thesis, U. of Minnesota, Minneapolis, Minn., 1967. (11) D. T. Napp, Ph.D. Thesis, U. of Minnesota, Minneapolis, Minn., 1967. 1498
ANALYTICAL CHEMISTRY
operated remotely. A line drawing of the mechanical system is shown in Figure 1 and a photo of the complete system with cell and electrode mounted is shown in Figure 2. The foundation for the unit is a rigid aluminum base and backplate assembly. Suitable openings for signal cables are provided in the base and rear plates. A cell positioning assembly (Velmex, Inc., Holcomb, N. Y . #A15120) is bolted to the backplate. The spindle bracket (K),Figure 1, is mounted on the backplate and provides support for the spindle and the adjustable brush assembly (I). The spindle (H. P. Smith Co., Cheshire, Conn.) is a precision, pre-loaded ball bearing assembly with dimensions of 2.0 inches body diameter and 6 inches overall length. It will accept nylon insulating collets (C) for electrodes of either 0.250 or 0.375 inch 0.d. Total run-out of the electrode tip at 4 inches is less than 0.001 inch. The spindle is driven from the side with a belt and pulley system at either one-half or twice the motor speed. Ball bearing idler pulleys ( H ) adjust belt tension. The servo motor drive system, including tachometergenerator and SCR amplifier is available from Electro Devices, Inc., Paterson, N. J. The system has a speed range of 50 to 5000 rpm with a constant torque capability of 6 inch-ounces. The speed of the motor (F)is adjusted with a 10-turn potentiometer on the motor control station. The regulation of the spindle speed is at least 0.5% and improves at higher speeds. The desired motor speed is maintained automatically by comparing the speed reference signal with the output of the tachometer generator (C) and feeding back the error signal to the SCR speed control amplifier. The exact speed of the spindle is determined with a photoelectric pickoff ( D ) (General Radio, West Concord, Mass. Model 1536-A or equivalent) together with a reflecting strip on the top collet nut. The period of one or ten revolutions is displayed on a digital frequency counter (General Radio Model 1151-AP or equivalent).
I
Figure 1. Drawing of rotation assembly A B C
D E
F G
H I J K
Three contact brush assembly Split ring-disk electrode Insulating collets Photoelectric pickoff and reflecting strip Spindle Servo motor Speed control tachometer Two range drive system, gears, belt, idlers Brush height adjustment Brush release Spindle mount
The brush assembly ( A ) allows as many as three independent circuits to be used in an electrode, as for example, in the split-ring disk system (12). This brush assembly is adjustable to allow the use of various electrode lengths. Silver-graphite brushes ( A ) (Graphite Metallizing Corp., Yonkers, N. Y.) are used to make electrical contact to the rotating electrode. The brushes are soldered on a phosphor bronze leaf spring assembly. Additional pressure is supplied by coil springs. The brush-spring assembly can be changed to accommodate either electrode diameter given above. With this system and an X-Y recorder, meaningful submicroampere signals can be recorded when conventional operational amplifier instrumentation is employed. A Plexiglas shield is bolted to each side of the base and rear plates and together with a folding front door provides pro(12) B. Miller and R. E. Visco, J. Electrochem. SOC.,115, 251 (1968).
Figure 2. Photo of complete assembly showing cell and electrode mounted tection for the operator when high rotational speeds are used. All purchased items are identified. The remainder of the unit was designed and fabricated in our shops. Several investigations employing this apparatus are complete and have been either published (13) or presented orally (13-16). A detailed description of the construction of the split-ring disk electrode is being prepared for publication (17). Additional details of the rotating electrode equipment are available from the authors upon request. ACKNOWLEDGMENT
The authors acknowledge the helpful suggestions made by Harold Smith (H. P. Smith Associates) and the drafting department of Bell Telephone Laboratories, particularly Henry Wlodarczyk, during the design and construction of this device. RECEIVED for review March 21, 1969. Accepted May 7, 1969. (13) B. Miller, Paper presented at the Chicago meeting of the Electrochemical Society, October 1967, Abstract 53. (14) B. Miller, Paper presented at the Chicago meeting of the Electrochemical Society, October 1967, Abstract 57. (15) R. E. Visco and R. H. Sonner, Paper presented at the Boston meeting of the Electrochemical Society, May 1968, Abstract 205. (16) B. Miller, Paper presented at the Montreal meeting of the Electrochemical Society, October 1968, Abstract 291. (17) B. Miller, J. Electrochem. Sue., in press.
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