lower bearing is cut into the bottom support, and the rotating member (which is suitably machined to hold the diffraction grating) is located by the centered pin. The hole for the second bearing is cut into the top of the rotating member, and the axis of rotation is then set by the centering pin, i.e., by the location of the hole in the top support. Although not sought, it is evident that angular misalignment between the centering and centered pins will not affect the smoothness of rotation of the rotating member, a situation that would cause severe problems with conventional bearings. The two bearings are aligned by pushing the centering pin down into the top nest of stationary balls by turning the loading screw. This is done until the rotating member spins freely with no shake, wobble, or play. A very light film of fine oil on the balls is an aid. In practice, a sine drive arm may be attached to the rotating member or fastened around the centered pin. We have fabricated one holder for a 102 X 256 mm plane grating using a worm wheel and gear for the drive. Here, the center hole of the worm wheel was used in place of the centered pin and held the rotating ball. The grating holder was mounted directly on top of the worm wheel. To test the alignment and smoothness of the bearings, the zero-order reflection from a helium-neon laser was observed visually as the grating was rotated. The grating was firmly mounted on a stable 5.5-m optical bed, and the laser beam multi-passed along the bed seven times to provide an optical level arm in excess of 38 m. The grating could be rotated in steps of less than 0.002O over angles up to 70° with no cogging, jerking, or vertical wobble observed in the spot image of the laser beam. The manner in which the above observations apply when the grating is used spectroscopically is indicated in Figure 3. Here, the grating was rotated with a stepper motor driving the worm gear at 0.000069° per step, with approximately 7% relative repeatability in step sizes. The grating was operating in the second order in a Czerny-Turner spectrometer of 5.0-m focal length arranged such that the distance from the camera mirror to the monochromator exit slit was approximately 5 m. This is the optical lever arm, over which cogging or jerking imperfections in the grating bearing are amplified when the grating is rotated. The equivalent mechanical lever arm in the bearing is the distance from any contact point between the balls and the central axis of rotation, which in this example is approximately 3 mm. Thus, the magnification of bearing imperfections is 1667.
In Figure 3, the entrance and exit slits of the monochromator were set at 0.050 mm, meaning that approximately 52 steps of the stepper motor are required to scan the geometrical image of the monochromator entrance slit across the exit slit. It is expected that during this time, the photoelectric signal observed in scanning up the leading edge of a line (e.g., Hg 5461) would show a regular rise, Le., show no discontinuities, unless the bearing and associated gearing involved in the grating rotation did not rotate, or detented in the reverse direction, etc., because of mechanical imperfection. Two such scans are indicated a t the bottom of Figure 3. The first scan shows one irregularity (A), while the second (repeat) scan shows two (B and C). If we interpret the presence of the second irregularity in the second scan as due to imperfections in the bearing, and note that it consists of three steps, then the irregularity in equivalent linear motion a t the ball contact points inside the bearing would be approximately 2 X mm. While this can be considered only an estimate, we understand that such minute irregularities are possible if the contact points in the bearing occur only through a film of oil. The data in Figure 3 clearly verify the practical utility of this simple bearing.
ACKNOWLEDGMENT The assistance of Russel Riley, Robert Schmelzer, and Robert Lang of the Chemistry Department InstrumentMachine Shops is acknowledged and appreciated. LITERATURE CITED (1) R . M. Badger, L. R. Zumwalt, andP. A. Giguere, Rev. Sci. Instrum.,19,861
(1948). (2)J. P. Walters, Anal. Chem.. 39,770 (1967). (3)E . P.Turner, U S . Patent 2,351,890, June 20, 1944. (4) B. G. Carlson, US. Patent 2,352,469, June 27, 1944. (5)G. L. Jones, U S .Patent 2,990,221, June 27, 1961. (6) R. F. Moore, US. Patent 3,463,564, August 26, 1969.
RECEIVEDfor review December 19,1975. Accepted February 13,1976. Portions of this work were conducted as part of the requirements for the Master of Science degree in chemistry a t Wisconsin (BDH). The financial support of the National Science Foundation under Grant GP 13902-X is acknowledged.
High-Output Potentiostat for Electrosynthesis Studies Rodney L. Hand' and Robert F. Nelson** Department of Chemistry, University of Idaho, Moscow, Idaho 83843 and Department of Chemistry, University of Georgia, Athens, Ga. 3060 1
Recent articles have demonstrated the ability of electrochemistry, in conjunction with a thorough knowledge of the kinetics and mechanisms of chemical reactions, to exert a significant degree of control over product distributions through variation of such parameters as applied potential, current density, concentration of electroactive species, and electrolysis time, thus illuminating exciting long-range possibilities in the field of electrosynthesis. To achieve meaningful results, a potentiostat is required which is capable of performing electrolyses on at least gram quantities of ma-
'
Present address, Allied Chemical Corp., 550 2nd St., Idaho Falls, Idaho 83401. Present address, Department of Chemistry, University of Georgia, Athens, Ga. 30601.
terial in a short period of time. Unfortunately, most currently available commercial instruments are not well suited to this task and/or are quite costly. These commercial instruments generally fall into two broad categories which can be classified as high voltage/low current or low voltage/high current power sources. The former group of instruments, which is noted for low-output and fast-response characteristics, is designed primarily for kinetic studies employing electrochemical relaxation techniques. These instruments are poorly suited for electrosynthesis studies because of the prohibitively long times required for electrolyses of reasonable amounts of material. Those instruments capable of high output currents usually have low applied potential limits and are thus designed primarily for work in media with very low resistances; they ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
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lack the higher voltage limits necessary to pass large currents in high-resistance solutions such as the commonly used nonaqueous solvents. Although the major design requirements for potentiostats have been present for quite some time in the literature (1-3), and many fine instrument designs are available, the majority of these, like the commercial models, have either inadequate voltage or current limits for electrosynthesis studies. An exception to this is the use of a power supply/amplifier as a booster on an operational amplifier-based potentiostat ( 4 ) . This combination provides a capability of f 5 0 V at f l A, which is more than adequate for coulometric determinations of n-values and very low-level preparative work, but, as noted by the authors, problems involving amplifier saturation are frequently encountered under routine electrosynthesis conditions. The power booster design recently published by Reilley and co-workers ( 5 )is another qualified exception, as is the synthesis-scale potentiostat of Bewick and Brown (6). The former instrument, while being very inexpensive to construct, has a rather low current output of 100 mA and, as with the power supply/amplifier booster mentioned above, must be used in conjunction with an operational amplifierbased potentiostat which must be available, purchased, or independently built. The latter potentiostat has ratings of f 4 0 V a t f 4 A, which would be somewhat acceptable for all but very large scale applications. It is, however, complex and expensive to construct. Because of the foregoing instrument deficiencies and our on-going interest in electrosynthesis studies, it was decided to build a potentiostat with more flexible specifications. Such a unit, presented herein, has been in operation in our laboratory for over 2 years and appears to be well suited to routine studies as well as those on an extraordinarily large scale. In addition to its wide applicability, with the incorporation of normally available commercial power supplies, it is a very inexpensive instrument. General Considerations. The potentiostat has output limits of f 6 5 V a t f 2 5 A at the final stage. The voltage limit was largely dictated by the ready availability of power transistors with an adequate breakdown voltage. This voltage limit can no doubt be increased, if desired, by using a larger array of lower power, higher voltage transistors but it was felt that, with care in cell design, 65 volts is adequate for most applications, with the possible exception of very low dielectric nonaqueous solvents. The current capability was primarily dictated by the power supplies on hand, and in this case corresponds to an electrolysis rate of nearly one equivalent per hour a t full output. Obviously, the unit can be used with lower current power supplies, but the maximum rating would only be increased by use of a different array of power transistors as described above. In constructing potentiostats, a design requirement of considerable import is the response time; however, we did not treat it as a primary consideration for two major reasons. First, the potentiostat will be operating into cells with large electrodes and therefore large capacitances and long cell time constants. In addition, the large power supplies required usually have relatively slow recovery times associated with the rather large voltage and current excursions taking place. Since either or both of these conditions may commonly be limiting factors, a fast-rise potentiostat would be no better suited to synthesis work than a much slower one. The minimum response time required for the potentiostat, then, is simply that adequate to “beat” these parameters. In preparative studies, the only time that this may be a drawback is in pulse electrolyses where the pulsing rate would be comparable to the cell time constant and/or power supply recovery times. Considering the difficulties involved, pulse electrolyses on a preparative scale are of questionable utility, so this is not considered to be a major shortcoming of the in1264
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strument. This is not to say that pulse electrolyses cannot be performed with it, but rather that the maximum pulse rate is somewhat limited. Circuitry. I t should be noted a t this point that because of the large number of excellent circuit designs available in the literature, it would be presumptuous to claim originality. The design settled upon is a simple operational amplifier potentiostat with a large transistorized power booster, shown in Figure 1. It is somewhat unique in its flexibility and power ratings, as well as the nominal cost exclusive of power supplies. The control amplifier is a general purpose Burr-Brown 1506C, though any good general purpose operational amplifier would suffice. The output of the control amplifier is applied to the base of QI or Q2. Transistors Q1 and Q3 (oxidation side) or Q2 and Q4 (reduction side) form a voltage amplifier with a gain of approximately 6.5; the output of this stage is then applied to the base of Q5 or Q6. Transistors Q5 and Q 7 or and Qs are simple emitter follower current amplifiers. Q 5 and Q7 and Q6 and QS are attached to heat sinks that enable each combined stage to dissipate approximately 190 watts. At this stage, the instrument is used in conjunction with two Hewlett-Packard HP 6443B 120 V/2.5 A power supplies (set at 70 V) with 2.5 A at f 6 5 V available a t the emitters of Q 7 and Qs. When i t is used at this stage, the instrument is capable of operating into cells with a resistance as low as 8 ohms. At cell resistances less than 8 ohms, the output voltage of the power supplies must be proportionately reduced. These capabilities have proved to be ideal for routine coulometric and small scale (0.5-2.0 g) electrosynthesis studies. For electrolyses of larger amounts of material, an additional stage of current gain, $9 and &lo, is available to provide a final current capability of 25 A. This final stage consists of 8 parallel transistors on each side mounted on air-blown heat sinks giving a total power dissipation capability of 1200 watts for either side. With the final stage connected, the potentiostat can operate into cells with resistances as low as 1 ohm, and again for lower resistances the power supply voltage must be reduced. At this stage, the potentiostat has been used with two Kepco 36 V/30 A power supplies. Due to the fact that only two such power supplies were available, it was not possible to test and operate the unit at the last stage at f 7 0 V. However, each side was tested individually at 70 V and the unit was tested and operated a t f 3 6 V. Performance Characteristics. The potentiostat’s response into resistive loads a t switching potentials of +1volt to -1 volt a t the reference electrode a t currents from fl yA to f 1 5 A is between 20 and 200 gs. While this is admittedly not exceptionally fast, it should be more than adequate for the bulk of routine electrosynthesis work. Construction Notes. For fabrication of our three laboratory instruments, we have used an 11-in. X 17-in. X 3-in. chassis fitted with a standard 8-in. x 19-in. relay rack face plate. The front panel and chassis layouts can be varied in any manner desired; the layouts that we employ are available upon request. Many of the jacks shown in the front panel are not necessary for routine operation, but are a great aid if any trouble shooting becomes necessary. Care must be taken to ensure that the heat sinks used for the power transistors are insulated from the chassis. The final stages (Q9 and Qlo) are mounted on 6-in. X 6-in. X 36-in. heat sinks separate from the main instrument chassis. It is advisable to enclose these heat sinks with a wood or plastic cover to provide better air circulation and to prevent an accidental short between the heat sinks with a device such as a watch or screwdriver. The power supplies are also mounted separate from the main instrument chassis, but generally in the same relay rack. They are then wired in series as you would wire simple batteries. Unit Operation. One point that must be strongly emphasized a t this juncture is that this unit has essentially no pro-
t AUXILIARY ELECTRODE LEAD TO
REF
ELECTRODE JACK
ON-OFF
-
GND
ELECTRODE LEAD
--
WORKING ELECTRODE LEAD (CKT GND)
CELL
0.5A
Figure 1. Potentiostat
schematic
(a) R2 is a high-precision IO-turn potentiometer.(b)All resistors are 1% tolerance. (c) R7 and RBare bias adjustments for Q1 and Q? to eliminate crossover time between channels; they are set at -4K for operation at 30 V or less and at -2K for >30-V operation. (d)R1 can be variable as a coarse voltage adjust
tective circuits. Errors in operation such as shorting the working electrode to the auxiliary or operating a t too high a voltage into a low resistance load can, and in most cases will, damage the output transistors. Lack of protective circuits requires that particular care be exercised in operating the potentiostat, but it need not be a serious shortcoming, as evidenced by the fact than over a period of two years and many hundreds of electrolyses and coulometry runs we have experienced only one instrument failure. The sole source of protection is provided by the voltage and current adjustments on the power supplies. It is recommended that all electrolysis runs be started a t a low voltage, roughly 20 V, and then increase the voltage up to 70 V if it is required. A complete set of operating instructions is available upon request. T o operate the potentiostat as a coulometer a calibrated resistance is inserted in series with the auxiliary electrode and the voltage drop across the resistor is applied to a voltageto-frequency converter (floating input) with associated counter, an integrating digital voltmeter, or a stripchart recorder. If the instrument is being used a t the 2.5-A stage, a resistance of 0.5 to 2.5 ohms (with the appropriate power rating) is used. At the 25-A stage, a 0.1 ohm (at least 60 watts) resistor is employed. We routinely leave these resistors in the auxiliary electrode circuits, as they provide some measure of short-circuit protection and have only a minor effect on the output voltage. In actual day-to-day operation, we have found the 2.5-A output stage to be ideal for routine electrosynthesis studies in which one wishes to electrolyze 1-2 gram quantities in a matter of minutes. Although we have tested it extensively, we have not employed the 25-A stage for electrosynthesis work
and any potential user should consider cell design very carefully here, since heat dissipation and gas buildup could be severe problems a t these current levels.
APPENDIX The following is a listing of the major components, most of which are available from standard electronics supply houses. The only exception is the 2N 3146 transistors, which could be obtained only from either Germanium Power Devices Corp., P.O. Box 65, Andover, Mass. 01810, or from Semiconductor Technology, Inc., 124-14 22nd Ave., College Point, N.Y. 13356. 1) Amplifiers: AI, Az, As-Burr Brown 1506/C. 2) Heat sinks: (2)-Thermalloy 6560B (5-in. X 5-in. X 6-in.); (2)Thermailoy 6660 (6-in. X 6-in. X 36-in.). 3) Transistors: (1) 2N 3499, (1) 2N 3501, (1)2N 3635, (1)2N 3637, (1) 2N 5052, (1)2N 5344, (9) 2N 3146, (9) 2N 3773. 4) Power supplies: (1) Burr Brown Model 501 f 15 V Power Supply; (2) Hewlett-Packard H P 6443B 120 V/2.5 A Power Supplies; (2) Kepco 36M30 Power Supplies.
ACKNOWLEDGMENT Special thanks are due to R. N. Adams and D. E. Smith for their support and encouragement. LITERATURE CITED (1) G. L. Booman and W. B. Holbrook, Anal. Chem., 35, 1793 (1963). (2) A. Bewick and M. Fleischmann, Electrochim. Acta, 8, 89 (1963). (3) J. A. Von Fraunhofer and C. H. Banks, "The Potentiostat and Its Applications", Butterworth and Co. Ltd., London, 1972. (4) S.C. Creason and R. F. Nelson, J. Chem. Educ., 48, 775 (1971). (5) W. S. Woodward, T. H. Ridgway and C. N. Reilley, Anal. Chem., 45, 435 (1973). (6) A. Bewick and 0.R. Brown, J. Elecfroanal. Chem., 15, 129 (1967).
RECEIVEDfor review August 28, 1975. Accepted March 5 , 1976. Financial support of this work through NSF Grant No. GP-30606 is gratefully acknowledged.
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