enough to detect the arsenic vapors passing through the flame. The optimum sweep rate can be found by experiment. Admittedly, air-acetylene is not the flame of choice for the arsenic determination. A cooler, less absorbing flame (in the far ultraviolet) such as hydrogen-argon-entrained air ( 4 ) would be preferable. The latter flame is being used in further work. The main advantage of the gas-sampling technique is its suitability for trace analysis. The element is isolated from the matrix and the total quantity is used to produce a sharp, absolute signal. In addition, interferences often associated with (4) H. L. Kahn and J. E. Schallis, Atomic Absorption Newsletter, 7 (l), 5 (1968).
solutions, such as the solids effect, and light scattering as well as chemical interferences are virtually eliminated. Although the technique is described for the analysis of arsenic, it can readily be extended to other elements. Any element amenable to atomic absorption that can be converted to a gaseous or a volatile compound can be analyzed this way. Possible examples include silicon tetrafluoride, boron trifluoride, nickel carbonyl, and the hydrides of antimony and bismuth. ACKNOWLEDGMENT The author thanks Robert Ginell for his valuable suggestions. RECEIVED for review April 24, 1969. Accepted July 1, 1969.
Motor-Speed Control for Rotating Disk Electrode Systems J. D. E. McIntyre and W. F. Peck Bell Telephone Laboratories, Incorporated, Murray Hill, N . J . USEOF FORCED-FLOW techniques permits the kinetics of chemical processes to be studied under steady-state conditions as a function of the rate of mass transport of reaction constituents to or from the reaction zone. For investigations of electrochemical reactions at solid electrodes, the rotating-disk electrode (RDE) system has proved to be of particular utility because the normal flux of a given species at the electrode surface is directly proportional to the square root of the angular velocity, w , of the disk ( I , 2). To obtain accurate values of the mass transport and kinetic parameters which characterize the reaction rate, the rotational speed must be closely controlled (to within ca. 1 %). Study of the relative degree of control of the overall reaction rate by kinetics and mass transport requires that the diffusion boundary layer thickness (a w - l i 2 ) be variable by a factor of ten or more. This communication describes the design of a simple motorspeed control for RDE systems which has been employed successfully in this laboratory for several years. The disk rotational speed can be varied continuously from 30 to 20,000 rpm without interchange of gears or pulleys. This range is generally adequate for most electrochemical investigations because near these limits, deviations from theoretical laminarflow behavior occur, due to natural convective mixing at the one extreme and cavitation or turbulence at the other. Speed-Control Circuit. A block diagram of the control circuit is given in Figure 1. The basic component of the system is a commercially available modular speed-control unit (Advanced Development Corp., Gardena, Calif., Model 10615-3) which employs two silicon-controlled rectifiers in a full-wave connection. The feedback voltage from a dc tachometer generator (TG) coupled to the motor shaft, either directly or aia a belt-and-pulley arrangement, is compared with an internal reference voltage by a differential amplifier. (A tachometer generator such as Model No. SA-757A-2, with 7 V/lOOO rpm output, manufactured by Servo-Tek Products Co., Hawthorne, N. J., is satisfactory. The nominal (1) V. G. Levich, “Physicochemical Hydrodynamics,” Prentice-
Hall Inc., Englewood Cliffs, N. J., 1962. (2) A. C. Riddiford, “Advances in Electrochemistry and Electrochemical Engineering,” Vol. 4, P. Delahay, Ed., Interscience Publishers, New York, N. Y., p 47.
SERIES MOTOR
IK
1
1
I l l
bh
t 1 5 V -15V
w I
b
b
115 V, 60
-
Figure 1. Motor-speed control for rotating-disk electrode systems A. PhilbrickiNexus Research P75AU (or equivalent) operational amplifier TG. Servo-Tek SA-757A-2 tachometer generator SC. Advanced Development Corp. 10615-3 control unit RI-R4. Series resistors, selected to give calibrated meter readings in rpm
maximum speed of this unit is 12,000 rpm. It may be safely employed at higher speeds with decreased service life and increased output voltage nonlinearity. Neither effect is of concern in the present application.) The output signal from this amplifier controls the firing angle of the SCR’s in such a VOL. 41, NO. 12, OCTOBER 1969
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I
0 ROTATIONAL SPEED ( R P M I
Figure 2. Speed regulation of a direct-drive motor and RDE assembly way as to regulate the energy supplied to the motor windings and provide constant-speed operation. The magnitude of the internal reference voltage can be varied over the range 0.05-3.2 volts by adjustment of an externally-connected 1-kQ potentiometer. With no attenuation of the T G feedback voltage, the speed can only be adjusted over the range 15-250 rpm; further, the speed regulation is unsatisfactory. By the simple expedient of attenuating the feedback voltage from the tachometer generator, the rotational-speed range can be greatly expanded and the degree of regulation significantly improved. When the degree of attenuation is increased, the motor speed increases together with the TG output voltage to restore the system to balance. Conversely, when the feedback-voltage attenuation is decreased, the motor speed also decreases. Regulation is improved because of the attenuation of noise and ripple produced by the tachometer generator; below 1000 rpm, this noise level is approximately constant (ca. 100 mV peak-topeak). Capacitative filtering cannot be used to reduce the noise level because the resulting phase shift of transient voltage signals causes the control to oscillate. Best regulation in the low speed range is obtained when the 50-kQ ten-turn potentiometer is initially adjusted to give close to maximum attenuation of the TG output voltage. The rotational speed is then set approximately by varying the internal reference voltage level with the 1-kfi potentiometer. Final speed adjustment is made by small variations of the attenuator setting. Above 1000 rpm, the method of adjustment is not critical. In Figure 1, a unity-gain voltage follower is shown connected as a buffer between the tachometer generator and the speed-control module. (A voltage follower with gain may be considered for use with higher level reference voltages. Tests of the latter circuit showed, however, that the noise level and regulation were worse than those obtained with the circuit in Figure 1.) In the low rotational-speed range, use of a follower minimizes nonlinearity in the meter reading caused by the input current (ca. 10 PA) to the differential amplifier. Values of the series resistors, R1, Rz, R3, and R1, are chosen to give full-scale meter readings corresponding to rotational 1714
ANALYTICAL CHEMISTRY
speeds of 100,1000, 10,000and 20,000 rpm, respectively. The meter can be used to set or measure the rotational speed with an accuracy of a few per cent. Accurate speed measurement and meter calibration are accomplished with a General Radio Co. Type 1536-A photoelectric pickoff and a Hewlett-Packard Model 5214L preset electronic counter. Power for the pickoff is provided by a simple 20-volt dc supply mounted on the speed-control chassis. Using an internal 100-kHz time base as reference, the time required for a preset number of shaft revolutions to take place is measured with the counter. Other speed-measuring devices of the noncontacting type should serve equally well. The components of the speed control are mounted on a standard relay-rack panel and an attached chassis. In order to minimize long-term speed drifts, a Rotron “Whisper” Fan (Rotron Manufacturing Co., Woodstock, N. Y . ) is also mounted on the chassis to cool the control module and provide rapid thermal equilibration. Fuse F1 (-1 A) in the ac input line is a safety device; it is selected to burn out and prevent motor runaway should the TG feedback loop be opened accidentally. Motor and Electrode Assembly. In the RDE assembly currently in use in this laboratory, the electrode shaft is driven by a ‘/a-hp Precise Super 50 Power Quill (Precise Products Corp., Racine, Wis.) with rated maximum speeds of 45,000 rpm (no load) and 24,000 rpm (full load). [A LabLine high-speed series motor (Lab-Line Instruments, Inc., Chicago, Ill., Cat. No. 1285) has also been employed successfully. This motor is more difficult to mount, align, and couple to the RDE and tachometer generator than the Precise unit.] The power quill is rigidly supported in a vertical position with a Precise Plane Mount (Cat. No. 9050) which is bolted to a rectangular supporting frame constructed from 3/4-inchthick aluminum sheet. A Rotron ‘‘Whisper” Fan is also mounted on this frame to cool the motor housing and preqent ovcrheating at low speeds. The frame is attached to a bracketand-lock assembly located on the 3-inch diameter vertical column of a drill-press stand (Clawing Div., Atlas Press Co., Kalamazoo, Mich.). A gas-tight precision ball bearing assembly, in which the ‘/Anch diameter precision-ground
stainless-steel shaft of the RDE rotates, is mounted in line with the power quill. (A detailed description of the design and construction of the rotating disk and ring-disk electrode systems and the electrical contact and bearing assemblies will be given elsewhere. Further details concerning the mounting arrangement described here are available from the authors upon request.) The RDE is attached by means of an insulating flexible neoprene coupling (PIC Design Corp., East Rockaway, N. Y . ,Cat. No. Tll-3) to a short l/&ch diameter rod which is secured in a Precise Type P collet by the parallelgrip collet chuck of the power quill. A Servo-Tek Model No. SA-757A-2 tachometer generator is mounted on a Precise Air Filtration Unit (Cat. No. 8400s) which is fitted to the top of the power-quill housing; the T G shaft is directly coupled to the quill. The entire motor, TG, and RDE assembly can easily be rotated, raised and/or lowered by means of a positioning mechanism (Clausen Cat. No. 1613) mounted on the drill press column. This facilitates positioning of the RDE in an electrochemical cell which is partially immersed in a constanttemperature bath. The direct in-line drive arrangement used in the present system eliminates the need for precision spindle or collet assemblies. Because of the wide speed range and excellent regulation characteristics of the speed-control unit, the use of belt-and-pulley systems is not required. A loss of regulation at high speeds due to belt slippage or vibration and the inconvenience of belt-changing are thus avoided. System Performance. Figure 2 illustrates the performance obtainable with this system under actual operating conditions. Rotational speeds were accurately measured with a General Radio Type 1536-A photoelectric pickoff and a HewlettPackard Model 5214L preset counter as previously described. The number of shaft revolutions was preselected for each speed to give an integration time of one to two seconds. The digital output signal from the counter was converted to a proportional analog voltage by means of a Hewlett-Packard
Model 580A digital-to-analog converter and recorded with a Varian F-80 recorder. High resolution was obtained by recording the counter digits of least significance. For each speed setting, deviations from the mean value were monitored over a 12-minute interval. The observed fluctuations were considered as noise; long term drifts were insignificant. From Figure 2, it is evident that for rotational speeds in the range 30 to 20,000 rpm, the percentage rms deviation is less than 0 . 5 z . In the range 1000 to 10,000 rpm, the regulation is excellent; the rms deviation is less than 0.1 %. Below 30 rpm, the shaft motion is noticeably stepped and the regulation is degraded. Discussion. The circuit described above is useful for controlling the speed of universal (series) motors, rated up to horsepower and operating from a single-phase ac line, over a very wide speed range. High torque output and close speed regulation are maintained at very low speeds, well below the usual operating range of universal motors. While the motorand-electrode assembly described here employs a direct drive system, the control circuit should be generally useful as well for other RDE assemblies requiring off-axis drives. Note Added in Proof. The motor-speed control described above has recently been modified to permit the angular velocity of the RDE to be automatically programmed. This was accomplished by interposing an inverting adder (Philbrick/Nexus Research P65A operational amplifier with 100kQ summing and feedback resistors) between the voltage follower output and the speed-control module. For operation in this mode, the output signal from the adder is fed to terminal 2 of the speed-control module; terminal 1 is grounded (cf. Figure 1). Any desired speed-control program can be attained by introducing the appropriate voltage signal (e.g., steps, linear, or square-root scans) via the external input of the adder. RECEIVED for review April 18, 1969. Accepted June 13, 1969.
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