titrated. The bridge reference conductance (resistance box) is set to give bridge balance when the cell contains a solution with conductance equivalent to that of the inert electrolyte in the titrant. The solution to be analyzed is placed in the conductance cell, and concentrated electrolyte is added until the bridge passes through balance and beyond t o full scale output as shown by the recorder. This electrolytic balancing titration does not have a critical end point and is performed very rapidly. The titration is then run with constant flow buret, and is recorded. A typical titration curve is shown in Figure 13. The titrant was usually about 10 times as concentrated in reactive species as the solution analyzed. Satisfactory titrations have been performed on 10-4M acids and bases in the presence of up to 3.5M electrolytes. Temperature control and constant speed stirring would be required to extend the technique further. Because of the number of components involved, and the bulk of the transformers, the conductiometric titrator was built into a plug-in box which covered four adjacent amplifiers.
Figure 14 shows pictorially how GATS I1 is programmed for a particular application. The circuit shown a t the bottom of the figure is that for a linear conductimetric titrator for use with dilute solutions. Above this, a portion of the manifold is pictured with the required components inserted. .4 plugin box is made up for the oscillator, and the rest of the circuit is assembled from components mounted on double banana plugs. Patch cords connect the bench top experiment to the manifold and recorder. The cut-away a t the top s h o w the position of the amplifier- hehintl the manifold. LITERATURE CITED
(1) Bo()man, G. L., ANAL. CHEM. 29,
213 ( 1957).
W., Brayden, T. E., Ibid., ( 2 ) Ew ing, G...35, 1826 (1963). (3) Kelley, ?*I.T., Fisher, D. J., Jones, H. C., Ibid., 32, 1262 (1960).
(4)Kelley, M. T., Fisher, D. J., Jones, H. C., Maddox, W. L., Stelzner, R. IT., I.S.A. Instrument-Automation Conference, Sew York, 8ept. 1960. Reprint S o . SY 60-52. ( 5 ) Krlley, &I. T., Jones, H. C Fisher, D. J., AKAL.CHEV.31, 488 (1959).
( 6 ) Lauer, G., Schlein, H., Osteryoung, R . A., Ibid., 35, 1789 (1963).
(7) Malmstadt, H. V., Enke, C. G., 144th Meeting, ACS, Los Angeles, Bpril 1963. (8) Pekema, R. M., Ph.D. thesis, Washington State Uniwrsity, Pullman, Wash., 1962. (9) Philbrick Researches, Inc., George A., “Analog Computor Techniques Applied to Industrial InstrumentationPart 11,” Boston, 1958. (10) Philbrick Researches, Inc., George A., “Application Bulletin 12-19-57,” Boston, 1957. (11) Philbrick Reseaches, Inc., Georgr .4., “Applications Manual for Philbrick Octal Plug-In Computing Amplifiers,” Boston, 1956. (12) Philbrick Researches, Inc., George A., “Lightening Empiricist,” Issue No. 6, Boston, 1958. (13) Philbrick Researches, Inc., George A., “UPA-2 Technical Data,” Boston, 1961. (14) Underkofler, W. L., Shain, I., ANAL. CHEM.35, 1778 (1963). (15) Vanderschmidt, G. F., Rev. Sci. Inst. 31, 1004 (1960). RECEIVEDfor review June 17, 1963. Accepted September 10, 1963. This work was supported in part by grants from the Course Content Division of the National Science Foundation and the Research Committee of Washington State University. Division of Analytical Chemistry, 144th bIeeting, xes, Los AngelPs, April 1963.
The Heath Analog Computer as a Versatile Analytical Tool GALEN W. EWING and THOMAS H. BRAYDEN, Jr.1 New Mexico Highlands University, las Vegas, N. M. b Two standard models of Heath analog computers and a hybrid modification have been evaluated for possible use as a versatile laboratory tool in electroanalytical and optical instrumentation, particularly for instructional purposes. Since these computers consist of collections of operational amplifiers with the requisite power supplies and auxiliary circuitry, they can b e applied wherever such amplifiers are called for, within the limits set by their electrical specifications. Experiments show that the larger Heath computer is suitable for many different circuit configurations is where overall precision within acceptable. It i s especially useful in a teaching laboratory because of the ease with which a student can assemble a variety of analytical instruments from a single piece of apparatus. Applications to conductornetry, polarography, and colorimetry are described,
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UCH HAS BEEN WRITTEN
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ANALYTICAL CHEMISTRY
on the utility of operational amplifiers in chemical instrumentation ( 3 ) . Since an analog computer is essentially
a collection of operational amplifiers together with appropriate power supplies and auxiliary control circuits, it is reasonable to suppose that such a computer would be highly useful in the analytical laboratory, both for routine and research analytical applications and as a teaching tool. The expense of commercially-available analog computers, together with the well-deserved popularity of Heathkits, suggested a study of the Heath computers to determine their suitability for this application. EXPERIMENTAL
Apparatus. The Heath Co. currently produces two models of selfcontained computers and a n assembly of five amplifiers for experimental purposes. The latter assembly contains amplifiers only with no auxiliary equipment other than a regulated power supply. Regarding this item, the authors have had no experience. The smaller of the two Heath computers is the Model EC-1 which contains nine two-stage amplifiers; it will be referred to hereinafter as the small Heath. The larger computer, which has apparently not been assigned a model number, has 15 amplifiers, each
with five stages, and will be referred to as the large Heath. The current prices of these two computers, in kit form, are approximately $200 and $950, respectively, without recorders or any other read-out devices than the built-in panel meters. For the analytical applications envisaged, nine amplifiers seemed t o be a more-than-adequate number, but the gain and stability to be expected of the amplifiers of the small Heath did not seem to be sufficient. It was therefore decided to purchase the kit for the small computer, and to modify it by adding stages to the amplifiers to make them the equivalent of the Model ES201 amplifiers provided with the large Heath. In Figure 1 is shown the circuit diagram of this amplifier, which is seen to be of a conventional single-channel design with astarved input stage. There is adequate space on the chaqsis for adding two more tuhes and one gain-adjusting potentiometer t o each amplifier. Such addition makes it necessary, however, to increaqe the size of the main poaer cupply. Lye were able t o ohtain through surplus Present address, Chemistry Department, Louisiana St,ate University, Baton Rouge, La.
Figure 1. Heath Model ES-201 operational amplifier
channel? a rectifier unit whicli only needed minor rewiring 1 o fit the present needs admirablv. Thc auxiliary supplies (intended for initial-condition potentials, operation cf the repetitive relay, etc.) as provided in the kit could be retained 1 s mounted on thl: chassis, these nine amplifiers are immcdiately adjacent to carh other, nithout shielding, hut a~ no high-frequency work is involved, this has gilen no dif?culty. It is con1 enient to mount the computer itself in a cabinet with extra panels for additional patch-cord facilities and a synchronous timer. 1 second rack contains the power supply and recorder. For most of the work to be described recording was performed with an ElectroInstruments Model 505 X-Y recorder. The output of any amplifier of the computer can be led directly to the appropriate input of the recorder by means of n patch cord. Measurements of Computer Performance. The accuracy of operations carried out witE an operational amplifier iq limited by two factors: the gairi of the amplifier, and the qtitbility or lack of d Aift. According to the literature from the Heath Co., the gains claimed for the amplifiers of the two models of computer are 1000 and 50,000, respectively. The gains of the aniplifiers in the present modified computer were mfasured by a con\ entional method. They can be varied in qiin from about 1.h x lo5 to twice that v a l u ~ by varint on of the gain tontrol. This is to brb compared with 5 x I O 4 conservatively llaimed bY Heath. The drift was m~?asured for an
Figure 2. Circuit for constant-potential source
amplifier by impressing its output onto the recorder and providing a constant input from one of the initial-condition supplies. The chart shows that for 12.5 minutes of operation two types of random fluctuations are present in the output from the amplifier: a lowfrequency component (less than 3 c.p.m.), and a high-frequency noise. Auxiliary experiments indicate that the former, which is about +15 mv. in magnitude, is derived from the power line, while the latter ( z t 2 mv.) originates in the amplifier. The initial-condition supply alone shows no random fluctuations or drift greater than +0.1 mv. after a 30-minute warm-up period. Similar stability tests were performed on a large Heath computer, built in accordance with the kit instructions. Both types of fluctuations were significantly less than in the unit here described. In neither computer was measurable drift observed in a 12.5minute observation following 30 minutes of warm up. To test the accuracy of amplification of an input voltage by an amplifier with a known ratio of feedback to input resistors, both static and dynamic tests were made. The governing relation is
where ei and e, ard the input and output voltages, Rt and R, the input and feedback resistances, respectively. In static tests the relation held to within *lye for any values of voltage from 0.1 to 100, for equal resistance values of 106, lo5, and lo4 ohms (all resistors have a 1% or better tolerance.) Considerable deviation, because of overloading the amplifier, occurred for resistances as low as lo3ohms. These results were verified for dynamic tests, wherein a ramp function was applied to the input, and the output plotted on the recorder as a function of time. The plots were truly linear, as compared to a straightedge, for resistance values of IO4 or higher, but somewhat nonlinear for lo3. The nonlinearity was only slight, not objectionable for many applications, for R , = 103 and R, = lo4 ohms. Another series of dynamic tests was run, in which the feedback resistor was replaced by a 104-ohm potentiometer driven by a synchronous
motor with gearing sucxh that the range was covered in approximately 1 minute. The input voltage was fixed a t each of several values from 5 to 50 volts, R , = l o 4 ohms in each case. All traces were accurately linear by the straightedge test. Linearity was also tested with alternating current of 1000 c.p.s. and found t o be equally good. The first series of dynamic tests described above testifies to the linearity of the ramp function, since the time axis built into the recorder was utilized for plotting. The ramp function was produced in the conwntional manner in which an amplifier with capacitive feedback integrates a constant vo1t:Lge against time. The circuit of Figure 2 was set up on the computer to test its performance as a constant-potential source. The battery maintains a potential E between the summing junction and the output of the amplifier. Since the summing junction is a t virtual ground, the output must be E volts removed from ground, and this voltage appears across the load resistance, R. Since the input to the amplifier draws essentially zero current, the battery is not subjected to any loading effects, which makes it possible to draw significant current from the output xithout producing any change in the voltage. For the purposes of this test, the battery was replaced by a connection to one of the initial-condition supplies of the computer. The results show that the output voltage is constant to better than 1% for levels a t least up to 50 volts, provided that the load resistance is large enough that not more than 9 ma. is drawn from the amplifier. For comparable tests of the computer as a generator of constant current, the circuit of Figure 3 was set up. Analysis of the circuit shows that the current through the load is solely determined by the setting of the voltage source (I.C. supply) and the value of the series resistor, R. For very small currents, this series resistor should be made correspondingly larger. This circuit was tested by momentarily shorting out the load, which drastic treatment resulted in about 1% change in current. This was equally true a t currents of 1 ma. and 10 ma. APPLICATIONS
To test the computer under actual conditions to be encountered in analytical work, it was set up successively in
Figure 3. source
Circuit for constant-current
VOL. 35, NO. 12, NOVEMBER 1963
e
1 8 n
-Figure 4. ductance
ANALYTICAL CHEMISTRY
Circuit for voltammetry and polarography DME = Dropping mercury electrode SCE = Saturated calomel electrode
Circuit for measurement of electrolytic con-
suitable configurations for two types of electrochemical and one photochemical analysis. Conductometric Titration. In Figure 4 is shown a two-amplifier circuit applicable to the measurement of electrolytic conductivity. The first amplifier, with its T networks for both input and feedback impedances, will be recognized a s the double integrator described by Reilley (3) as a convenient sine-wave generator. The frequency of its output depends upon the setting of the potentiometer; as used in the present experiments it was approximately 1000 C.P.S. The oscilloscope showed this to have an excellent wave form. The second amplifier in Figure 4 is a multiplier such that its output (RMS) is equal to the voltage produced by the oscillator multiplied by the ratio 104/R, where R represents the resistance of the electrolysis cell. This of course means that the voltage will be proportional to the conductance of the sample, as desired. The crystal diode rectifies the output for application to the recorder. A number of conductometric titrations were performed. Titrant was delivered from a Sargent motorized buret a t a rate of 1 ml. per minute. The recorder was operated with a time inch per minute. The base of graphs obtained showed two straight lines with slight rounding a t the intersection. The end points obtained by extrapolation were reproducible and in agreement with visual indicator changes within 1%. Polarography. A voltammetric circuit was created by combining a ramp generator with a simple voltage multiplier (Figure 5 ) . Polarograms taken in a two-electrode H-cell, on a n air-saturated solution containing 0.5 mg. of Cd+2 per ml. in 1 . O M KCI, were conventional in appearance. Five successive polarograms taken under identical conditions gave an average half-wave potential of -0.635 volt (Meites (a): -0.64) with a range of less than 10 mv. The measured diffusion currents showed poorer agreement, with a relative standard deviation of about =k2.5%. This could presum1828
Figure 5.
bFigure 6. Cornparison circuit for barrierlayer photovoltaic cells
ably be improved by careful attention to chemical details, such as electrolytic purification of the supporting electrolyte and by temperature control. Photometric Titration. As pointed out by Reilley (S), operational amplifiers are ideally suited for photometric measurements employing almost any type of photoelectric transducer. For ow present purposes conventional barrier-layer photovoltaic cells were selected. These were connected in the difference circuit shown in Figure 6. An acid-base titration was carried out in an ordinary 250-ml. beaker in the presence of phenol-red indicator. Two beams of light from the same incandescent lamp traversed this beaker a t approximately 90” angles, as previously described ( I ) . Two test tubes containing samples of the same indicator, one at high pH, the other a t low, were used as filters in the two beams, respectively. Random fluctuations of the order of 0.1 volt were noted in the output, which auxiliary experiments show must originate in the photocells. These fluctuations, however, were far less in magnitude than the 2-volt sharp break observed a t the end point for the 0.1M solutions employed. CONCLUSION
From the studies reported here, one can conclude that the larger Heath computer, which employs the amplifiers described in this report, is an excellent tool for educational use in analytical chemistry. For the applications tested reproducibility within about 1% wa3
-
observed, and similar results can be expected for a large variety of other analytical instruments which can be duplicated with this one unit. For manual experiments, the built-in panel meter gives adequate read-out; for automated experiments, it is necessary to add a constant delivery buret (or some equivalent, such as a Mariotte bottle) and a recorder. The utility of the computer for special or unusual situations is limited principally by the ingenuity of the operator. The computer can also be employed to advantage in many calculations and theoretical studies involving the solution of differential equations. This phase of its utilization is outside the scope of the present paper. ACKNOWLEDGMENT
The authors express their appreciation to A. A. Kraus for many helpful discussions, to Zachary A. Coles, Jr., and Martin S. Ewing, who obtained parts of the data, and to Pieter W. Knight, who built the main power supply. LITERATURE CITED
(1) Ewing, G. W., “Instrumental Methods of Chemical Analysis,” 2nd ed., p. 64, McGraw-Hill. New York. 1960.
(2) Meites, L.; “Polarographic Techniques,” Interscience, New York, 1955. Reilley, A933c. (1962). N.$ J - Chem* Educ* 39, (3)A853,
RECEIVEDfor review June 14, 1963. -4ccepted September 23, 1963. Presented a t the 144th xfeeting, ACS, Lo5 Angelee, Calif., April 1963. This work was supported by a research grant (G-12487) from:the National Science Foundation.
