t a c k a t K1 open. As the vent fully opens, the power to the motor is interrupted by a cam-switch arrangement a t the motor. However, K1 and K2 remain energized as they are supplied by the voltage leading to the cam-switch circuitry. When the door is closed, this voltage is also removed and K1 is de-energized. The 110 VAC contacts close a t K1 and remain closed at K2 because of the charge on C1; thus, K3 is energized. The pair of contacts a t K3, paralleling the manually operated start switches, are closed for a time period determined by C1 (approximately 1second), and a new cycle is initiated.
Switch S1 in the circuit allows selection of the automatic repetitive temperature programming, or, when in the manual position, disconnects K3 so that programming is restricted t o one cycle only.
ACKNOWLEDGMENT The authors thank Mrs. Janet Chapman for preparing the circuit drawing. Received for review January 12, 1973. Accepted July 27, 1973.
Simple Method for Continuous Monitoring of Electrode Rotation Rate Ira B. Goldberg, Richard S. Carpenter I I , and W. F. Goeppinger Science Center, Rockwell Internationd, Thousand Oaks, Calif. 97360
Rotating electrodes are a valuable tool for the study of electrode processes ( I , 2 ) . A rotating ring-disk electrode assembly, similar to those discussed in the literature ( 3 ) , was set up in this laboratory. In these cases, rotation rates were calibrated separately from the actual experiment. The experiments were done using a motor-tachometergenerator, which feeds into a speed control unit that regulates the voltage to the motor. A ten-turn potentiometer is used to regulate the rotation rate. After several calibration runs on various days, we have found that for a given potentiometer setting on the speed controller, the rotation rate may vary by as much as 5%, even though the rotation rate is constant to within 0.2% over a period of an hour or more. This suggests the need to monitor the rotation rate continuously or at least to calibrate the rotation rate prior to each experiment. Another alternative would be to use a precise frequency generator and power amplifier to control the rotation rate. Little attention has been given to the continuous measurement of rotation rate. Sonner et al. ( 4 ) use a photoelectric pick off on a reflective surface on the shaft of the electrode to measure the rotation rate. The time interval is monitored by a frequency counter. Hintermann and Suter ( 5 ) mounted a disk with holes on the shaft of the electrode. A light was placed under the disk and a phototransistor was placed over the disk so that a number of light pulses proportional to the rotation rate were detected and counted with a frequency counter. Other mechanical devices were also devised to given an electrical impulse per revolution (6). The goals which were kept in mind for the electrode assembly used in this laboratory were that the rotation rate could be monitored on an inexpensive frequency counter or timer, the electrochemical system could be easily interfaced with a computer for interactive control, the electrodes could be easily interchanged, and the entire assembly should be compact enough to be easily transported. Adams, "Electrochemistry at Solid Elecrrodes." Marcel Dekker. New York. N . Y . . 1969. (2) W. J. Albery and M . L. Hitchman, "Ring-Disc Electrodes." Clarendon Press, Oxford, England, 1971 (3) V. J. Puglisi and A. J. Bard, J. Electrochem SOC..119. 829 (1972). (4) R. H . Sonner, 8. Miller, and R . E. Visco, Anal. Chem.. 41, 1498 (1) R . N.
(1969).
(5) H. E. Hintermann and E. Suter, Rev. Sci. Instrum., 36,1610 (1964). (6) J. Woitowicz and 8 . E. Conway, J. Electroanal. Chem., 13, 333 (1967).
n
I
Figure 1. Schematic diagram of synchronous for the measurement of rotation speeds
pulse generator
EXPERIMENTAL A Pine Instrument Co. (Grove City, Pa.) rotating ring-disk electrode was connected to an Electrocraft (Hopkins, Minn.) speed controlled Model 550 motor, using a stainless steel collet. Ten nonreflective strips were painted on a 5-mm strip of reflective tape which was equal in length to the circumference of the collet. The tape was carefully placed around the collet so that the light of a Skan-a-matic Corp. (Elbridge, N . Y . ) Model S-351 subminiature reflective scanner illuminated the tape. Ten light pulses are obtained a t the phototransistor for each revolution of the rotating electrode. The small scanner actually permits the use of more than ten stripes to be painted on the reflective tape if desired. Although the transistor of the scanner is rated at up to 30 V, it was convenient to use a 5-V input. The low voltage side of the phototransistor was connected to a resistor R I N(Figure 1) and to the negative edge trigger imput of a Motorola SN74121 monostable multivibrator. R I Nis adjusted so that the voltage maximum and minimum at this point are about 2 to 0.8 V to enable the multivibrator to trigger. Since these voltages vary among these components, R I Nmust be determined empirically. In our case, 1 K Q was sufficient for the device to trigger even when the photodetector was used in room light with no particular shielding. The pulse duration was set for 90% of the minimum pulse rate (about 1 msec) by adjusting Rr and CT given by the tables in the manufacturer's literature. The monostable multivibrator provides a
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
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Figure 2.
Measurements taken with sensor 1.4 cm from rotating
band. Upper curve: Output of monostable multivibrator (1 V/div]: the lower dashed line is the 1-msec ouiput. LOW curye: input to monostable (0.5 V/div)-base line is + o s v. Time base, 1 rnEec/cm: rotation rate, 5400 rpm
from the phototransistor. In normal use, much better waveforms me ohtained when the sensor is about 3 mm from the hand: however, this photograph shows more dramatically hou, the waveform is improved by the multivibrator. The spikes from the phototransistor shown in Figure 2 do not trigger the multivibrator. Good output waveforms were obtained at rotation rates from less than 0.1 to 5000 rpm. The latter speed is faster than would normally be used in electrochemical studies. At slow speeds where only a few pulses per second are observed, an event timer can be used to measure the rotation rate. At the slower rotation rates, the output of the phototransistor was not sharp enough to trigger the frequency counter. If a n event timer is to be used, then one stripe could be painted on the Eflective tape and smaller values of CT and RT (Figure l) can be to Obtain a pulse' This also be desirable for computer interfacing. If a sine wave generator and power amplifier are used to Dower a ." svnchronous motor. hichlv stahle rotation rates can be obtained. However, we have found that the system described here conveniently provides a high degree of accuracy for the value of the rotation rate. The speed controller alone provided a constant rotation rate, but it was not highly reproducible. Another advantage of this system is that the uniform pulse shape is easily interfaced with a computer so that the rotation rate could be stored with electrochemical variablesif desired. I
normal output in which a 5-V output is triggered from
a base line output is and an inverted output in which triggered from B 5-v base line. We have arbitrarily chosen to use the inverting m t to measure the frequency. -i n. .~
about
RESULTS AND DISCUSSION Figure 2 shows the resulting waveform from the sensor when the sensor is 1.4 cm from the rotating striped hand, and also the output from the multivihrator. An inverted pulse is delivered from the monostable multivibrator which is triggered on the declining portion of the signal
1
"
Received for review June 25, 1973. Accepted August 15, 1973.
Device for the Accurate Electronic Measurement of Microliter Sample Volumes L. R. Layman and G. M. Hieftje Department of Chemistry, Indiana University, Bloomington, Ind. 47401 In several analytical techniques, such a nonflame atomic absorption spectrometry, small (microliter) volumes of sample solution must be accurately measured and evaporated before a determination can be made. The measurement and transfer of these small volumes by micropipet or syringe often contributes an important fraction of the total error in the determination ( I ) . In this report, a method is described that minimizes these difficulties by indirectly measuring the mass of liquid after it is placed in the apparatus. This measurement is based on a determination of the time required to evaporate the liquid using a constant rate of heat input. To determine this time, a suitable support for the liquid (a wire filament in this case) is heated by a constant current, with changes in the support temperature monitored as a drop of sample is placed on the support and evaporates. The simple apparatus necessary for this measurement is shown in Figure 1 and consists of a 0.010-inch diameter tungsten or platinum wire filament (or other heated evaporator) enclosed in a glass tube and surrounded by a controlled atmosphere, a constant current supply (OAl and the booster amplifier), a n operational amplifier derivative circuit (OA2 and OA3), and either a digital timer or strip chart recorder. In the procedure, the filament is heated by a 1- to 2ampere constant current to a temperature below the boil-
(1)
322
C. F. Ernanuel,Anal. Chem., 45,1568 (19731
ing point of the solvent. A microliter sample placed on the warm filament then produces a rapid drop in filament temperature which is reflected in a corresponding decrease in filament resistance. This reduction in resistance is registered as a drop in voltage and converted to a negative voltage peak by the operational amplifier derivative circuit, as shown in Figure 2. As the liquid evaporates, the filament remains a t this lower temperature, allowing the voltage derivative to return to zero. When the last of the liquid has evaporated, the filament quickly returns to its original temperature, causing the filament voltage to rise and producing a positive peak in the derivative circuit. The time between the negative and positive derivative peaks is measured either from a strip chart recorder output, or directly on an electronic timer. The volume of the sample is then directly proportional to the measured time. Of course, for this proportionality to hold, the gas composition and flow rate, the current, and the nature of the solvent must be held constant. In this method, the greatest precision was obtained using an automatically triggered electronic timer. This timing system, shown in Figure 1, consists of two comparators and an electronic clock. The comparator reference voltages are set such that the first comparator triggers on a negative voltage level, while the second responds to a positive voltage. The derivative circuit output is therefore converted to fast-rising square pulses which are used to start and stop the clock.
ANALYTICAL CHEMISTRY, VOL. 46. NO. 2, FEBRUARY 1974
