It is seen that d.c. components are generated only by multiplication of two signals identical in frequency. Also, the d.c. component is proportional to the cosine of the phase angle between the two signals being multipled. Thus, measurement of this d.c. component (low pass filtering of the output of the multiplier) effects frequency-selective and phase-selective detection of the a s . signal. Frequency selectivity can be made sufficiently good that this approach represents an alternative to the use of tuned amplifiers (6). With the multiplication approach the resonant frequency of the instrument is determined by frequency of the multiplying signal. Because the cell alternatingcurrent signal and the multiplying signal are derived from the same oscillator, resonant frequency is always identical t o the frequency of the signal of interest. Thus, oscillator-frequency instability will represent little problem if multipliers are used as B means of detection. Electronic multipliers based on the quarter-square method or pulse heightpulse width modulation (%?) appear to have frequency response and accuracy sufficient for a c. polarography a t audio frequencies. Because operational amplifiers are usually an integral part of electronic multipliers, this approach represents another application of operational amplifiers to ax. polarography. ACKNOWLEDGMENT
The author is indebted to W. 13. Reinmuth under whose guidance the basis for this work was established. Thanks are due to D. D. DeFord and
E. H. Nagel for many valuable suggestions and discussions. LITERATURE CITED
(1) Bauer, H. H., J . Electroanal. Ch,em., 1 . 256 f1960). (2) ’Bauer, H.’ H., Elving, P. J., ANAL. CHEM.30, 341 (1958). (3) Bauer, H. H., Elving, P. J., J . Am. Chem. SOC.82, 2091 (1960). (4) Booman, G. L., ANAL. CFIEM. 29, 213 (1957). (5) Boonshaft and Fuchs, ‘:?igh Perform-
ance Feedback Controls. ADDlications Budletin 711A, Boonshaft ahd Fuchs, Inc., Hatboro, Pa., Jan. 1963. (6) Burington, R. S., “Handbook yf Mathematical Tables and Formulas, p. 18, Handbook Publishers, Inc., Sandusky, Ohio, 1948. (7) Cakenberghe, J. van, Bull. SOC.Chim. Belges 60,3 (1951). (8) DeFord, D. D., Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958. (9) DeFord, D. D., Nagel, E. H., Division of Analytical Chemistry, lUth.Meeting, ACS, Los Angeles, Calif., April 1963. (10) Erbelding, W., Cooke, W. D., Divmon of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1Q R l
-1--.
(11) Jessop, G., British Patent No. 640,768 (1950). (12) Kelley, M. T., Fisher, D. J., Jones, H. C.. ANAL.CHEM.31.1475 (1959): ,, 32., 1262 11960). (13) Kelley, M. T., Fisher, D. J., Jones, H. C., Division of Analytical Chemistry, ~
144th Meeting, ACS, Los Angeles, Calif., April, 1963. (14) Malmstadt, H. V., Enke, C. G., Toren, E. C., Jr., “Electronics for Scientists,” pp. 241-2, W. A. Benjamin, Inc., New York, 1962. (15) Milner, G . W. C., “Principles and Applications of Polarography,” p . 132-3, Longmans, Green, New Yor!,
1957. (16) Paynter, J., Reinmuth, W. H., ANAL. CHEW34, 1335 (1962).
(17) Randles, J. E. B., Discussions Faraday SOC.1, 11 (1947). (18) Randles, J. E. B., Somerton, K. W., Trans. Faraday SOC.48,937,951 (1952). (19) Reilley, C. N., J . Chem. Ed. 39, A853, A933 (1962). (20) Reinmuth, W. H., Paynter, J., Smith,
D. E., Columbia University, New York, N. Y . , unpublished work, 1961. (21) Rogers, A. E., Connoll T. W., “Analog Computation in 8;gineering Design,” pp. 25-8, McGraw-Hill, New York, 1960. (22) Ihid., p. 127. (23) Ihid., p. 429. (24) Ibid., p. 430. (25) Senda, M., Delahay, P., J. Am. Ch,em. SOC.83, 3763 (1961). (26) Smith, D. E., ANAL.CHEM.35, 610
(1963). (27) Smith, D. E., Division of Analytical
Chemistry, 140th Meeting, ACS, Chicago, Ill., September, 1961; D. E. Smith, Ph.D. thesis, Columbia University, New York, 1961; D. E. Smith, W. H. Reinmuth, Columbia University, New York, N. Y . ,unpublished data, 1961. (28) Smith, D. E., Northwestern Univeraty, Evanston, Ill., unpublished work, 1 QfiR
(29) Smith, D. E., Reinmuth, W. H., ANAL.CHEM.32, 1892 (1960). (30) Ibid., 33,482 (1961). (31) Tachi, I., Senda, M., Bull. Chem. SOC.Japan 28,632 (1955). (32) Takahashi, T., Niki, E., Talunta 1 , 245 (1958). (33) Tamamushi, R., Tanaka, N., Z. Physik. Chem. N . P . 21, 89 (1959). (34) Ihid., 28, 158 (1961). (35) Underkofler, W. L., Rhain, I., ANAL. CHEM.35. 1778 (1963). ( 3 6 ) Walke;, D. ‘E., ’Adams, R. N., Alden, J. R., Ibid., 33, 308 (1961). (37) Woodbury, J. R., Electronics 34, ‘ 56 (Sept. 196i). (38) Yu, Y . P., Ibid., 31, 99, Sept. (1958).
RECEIVEDfor review June 4, 1963. Accepted August 12, 1963. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, April. 1963. Work supported by the National Science Foundation.
A Practical Instrument Synthesizer CHARLES F. MORRISON1 Depurtment of Chemisfry, Washington State Universify, Pullman, Wash.
b Construction details are presented for the Generalized Analog Instrument Synthesizer Model ll (GAIS 11). Over 40 analytical techniques have been tested with this Instrument Synthesizer serving as the electronics for each. Through use of a special manifold, GAIS II makes available the versatility of the operational amplifier in a manner convenient for laboratory use. The manifold is programmed with plug-in resistors, capacitors, diodes, etc. to be the required instrument functions. The amplifiers are then interconnected with a mounted collection of instrument hardware to become the electronics for a given application. In addition to general laboratory service, GALS It has served as a 1820
ANALYTICAL CHEMISTRY
design prototype for specific instruments. It has taken the place of much of the equipment used for courses in instrumental analysis and chemical instrumentation. A bimodal, potentiometric, automatic titrator, and a recording, linear conductance bridge are presented as examples of instruments synthesized by this device.
ANY SPECIFIC USES of operational
amplifiers in analytical instruments have been reported and the virtues of this unique type of instrumentation have been extolled before our professional societies. The circuitry which performs the basic logical and mathematical opera-
tions in analog computing is obviously capable of replacing much specific chemical-in s t r u m e n t c i r c u i t r y . I t seems logical then that a n analog computor might serve as a very versatile instrument synthesizer. It could be programmed to be the desired electrical and electronic functions for whatever instrument was desired. Ewing and Brayden (2) have performed tests on the Heathkit computor as a laboratory instrument, and several multipurpose ana.log devices have been reported (6, 7, 14). The commercially available analog computor manifolds were not sufficiently flexible for our anticipated 1
Present
address, Granville-Phillips
Go., Box 1290, Boulder, Colo.
Figure 1 .
operational amplifiers which may be used simultaneously. Choice of differential amplifier, stabilized amplifier, and stabilized follower configurations, with or without current boosters, is made by inserting stabilizing amplifiers and boosters into their appropriate sockets. The complete manifold is designed for seven operational amplifier modules, which has proved to be a reasonable number for research and development applications. However, for instructional use a four-ampmer version would be adequate. The wiring of a single module is shown in Figure 1. The module is designed for use with Philbrick octal plug-in amplifiers, K2-W or K2-XA, =-PA, and K2-B1 (11). The amplifier configurations generated by this plug-in arrangement are conventional direct-coupled circuits. It should be noted that when the booster amplifier is not used, a shorting plug must be in its socket. The stabilized follower configuration suffers some instability when the signal frequency is near 60 or 120 cycles. This defect has presented no difficulty in our applications, and could be eliminated b y changing to the Deford (9, 10, 12) circuit. This would require a switching arrangement in addition to amplifier placement in order to give the follower mode.
r
Amplifier module schematic
uses, and they were generally too expensive for chemistry laboratory use. The Generalized Analog Instrument Synthesizer (GAIS) wits developed with the hope of creating a low cost, simple to build, high perforinance, multipurpose laboratory instrument. The GAIS still retains analog computing capabilities, but is stripped of those features seldom used in instrumentation. The manifold design makea possible a compact device with great flexibility. The complete synthesizer, which could incorporate an oscilloscope and recorder, if desired, can be howed in a portable relay rack. GAIS does not contain input transducers, as these are specific to each particular application; it represents only the electrical and electronic portions of the instrument. The two GAIS units built at Washington State University have been successfully tested in about 40 analytical spplications, both electrochemical and optical. They have served as the equipment for student projects and graduate research in areas which are not commercially instrumented (8). These devices have served as much of the instrumentation for courses in instrumental methods of analysis.
2 K POT
100K
POT
P
BRASS BUSHING SOWERS
Figure 2.
TO
GROM
FRONT
High resolution bias detail
CONSTRUCTION
Manifold. The manifold houses the operational amplifiers and their bias circuits. Provision is made for addition of stabilizinlq amplifiers and current boosters as they are needed, without decreasing the number of
Figure 3.
Manifold front detail VOL. 35, NO. 12, NOVEMBER 1963
1821
The manifold wiring contains shielded filament wires, and '/ginch copper tubing for +300 volt, -300 volt, and ground busses. The resulting instrument suffers no detectable amplifier interaction, and 60 cycle interference is 1 mv. or less peak-to-peak in the amplifier outputs. The bias supplies presented a problem in that both wide range and high resolution were required. Use of two inexpensive potentiometers as shown in Figure 2 makes possible resolution equivalent to a 50-turn potentiometer, Only when a t the limit of its range does the fine adjustment (front potentiometer) change the coarse adjustment (rear potentiometer). The mercury cells remain in continuous service for more than a year. The manifold front shown in Figure 3 uses 3/4-in~hspacing so that components mounted on double banana plugs may be used conveniently. It is important that considerable accuracy be obtained in the boring of these holes, as plug-in boards and boxes should fit over any desired group of amplifiers interchangeably. Color coding of the amplifier inputs and outputs is very desirable. The jacks on the manifold front are connected such that the heavy circle triangular array is the same as the operational amplifier triangle symbolinputs to the left, outputs to the right. The black jacks directly above and below the output terminals are grounds. All other unlabeled jacks have no internal connections and are for the support of plug-in components. Utility Panel. The utility panel mounts above the manifold and houses various hardware items within easy patch-cord distance. The circuitry is shown in Figure 4. All controls and switches are located on the front panel except for the calibration potentiometers on the precision source and voltage span. Power for the manifold filament transformers and +300-volt regulated d.c. power are switched a t the utility panel and carried to the manifold by a five-conductor cable. The high voltage is prevented from reaching the power switch until the filaments have heated for 60 seconds. The cooling fan operates only when the high voltage power is applied. The utility relays are closed by a current of 1 ma., so may be driven directly by the amplifier outputs. Because DPDT relays with this current sensitivity were not available, the sensitive relays operate 115-volt ax. relays which connect to the contact terminals on the auxiliary panel. The counter-timer uses a mechanical register which for clock action is run a t 10 operations per second by a synchromous motor driven switch. When the count-clock switch is in the "count" position the register records 10 operations per second so long as terminal P is connected to terminal C. This feature makes possible the accurate timing of events bracketed by relay operations. The timer has proved very useful especially when performing
1822
ANALYTICAL CHEMISTRY
o--o +
80
,
I
r4
:e
.-
3 L
coulometric titrations, or using a conh a n t flow buret. With the count-clock switch in the count position, the register will enter 1 count for each connection between tei-minals D.C. and C. This action has been used with digital titrators and integrators. The precision voltage bources have given excellent service; drifts of about 2 mv. in 6 months were notec . These sources were completely boxed to prevent pickup from the a x . voltages elsewhere in the auxiliary panel circuitry. The meter circuit is totally conventional. I t is sufficiently jensrtive to respond significantly to 1 mv. so that the amplifier biases may ke accurately set. The voltage span init is intended primarily for polarographic circuits, but finds other uses. Electrodynamic braking makes it possible to stop the span with great accuracy. The three voltage ranges are independently calibrated. The output iapedance of this source is too high to iise directly on a dropping mercury eleci rode. The span is intended to furnisk a signal to an operational amplifier for presentation to the electrode. Because this source is floating with respe:t to ground, it finds use in applicalions where the integrator type of ramp generator (3) cannot be used. This auxiliary panel design has been quite successful, adcitional switches and a potentiometer or two being the greatest needs. It I S recommended that several inches cff space be left between the auxiliary panel and the manifold. This spacc, fitted with a Plexiglas panel, makes it possible to see the amplifier configurations on the manifold. Auxiliary Equipment. I n the process of developing and testing GAIS 11,a number of items were constructed to increase the range of applications. The input current requirements of our amplifiers made use of glass electrodes impractical. Circuitry similar to that of Vanderechmidt (15) was chosen for an electrometer which plugged onto the face of the manifold covering two stabilized amplifiers. The
Q+soo v
P
GLASS
REF.
I
I
I ~ _ _ _ _ _ ~
~~~
Figure 5.
pH Meter attachment
schematic is shown in Figure 5 , All of the components shown in Figure 5 are mounted in a plug-in box. Insulated banana plugs mounted on the back of the box insert into the jacks on the manifold front, making connection to the amplifiers. (Shown as triangular symbols in Figure 5 and following. The overlapping triangles imply a stabilized amplifier.) The voltage source being measured is placed in the feedback loop of an operational amplifier] and in series with the CK5886 electrometer tube. The tube is used as a cathode follower, thus requiring a battery to operate the filaments. The second operational amplifier provides amplification and zero suppression for meter and output terminals. Input currents of approximately 10-13 amperes are drawn. The CK5886 showed drifts of a few millivolts per hour. Insulation of the glass electrode terminal was polystyrene. By attaching the glass electrode shield to the reference terminal rather than ground, the noise level is reduced to several
millivolts. The maximum sensitivity is 50 mv. full scale. The wide range of zero suppression makes it possible to use the maximum sensitivity anywhere in the pH scale. Because of the many titration methods used with GAIS 11, a constant flow buret was built. Figure 6 shows the wiring and gearing. Two motors are used, one for fast filling and flushing, the other for normal delivery of titrant. Electrodynamic braking makes possible accurate derivative end points. The buret and its circuitry are designed for use independent of GAIS, but may be controlled by the instrument synthesizer through me of the jumper terminals. For independent use these terminals are shorted; for control they are connected to a set of relay contacts on the auxiliary panel. Among the other items of auxiliary equipment are a mechanically-sequenced switch and a 2-ampere current booster which combines the circuit philosophies of Booman ( I ) and Kelley, Jones, and Fisher ( 5 ) .
7 Buret Valve
I1
hAnticipation
NE-2
FAST-DOWN C E N T E R - OFF
FAST- U P
0
R1NO
RETAINER
4
Figure
6'. Constant flow buret
Figure 7.
-
Bias /
G & x z - z z e
y
Basic potentiometric auto-titrator VOL. 35, NO. 12, NOVEMBER 1963
1823
h I
I
t
Buret Valve
I
t,
-L
et End-Point
Figure 8.
Voltage
f
Digital titrator
,
Set Mode Change
APPLICATIONS
The circuits which follow are presented as typical examples of applications which make use of GAIS 11. Complete description and evaluation of the methods and techniques related to these circuits is reserved for a later communication. The flexibility of this system of instrumentation is of special importance when a specific instrument is to be developed. The potentiometric titrator sequence gives an example of a typical development progression. The linear conductance bridge illustrates the ability of G.4IS I1 to perform instrumentation tasks that are difficult with more conventional circuitry. In the applications circuits the amplifiers, biases, and relays are part of the basic instrument synthesizer. Resistors, capacitors, diodes, and neon bulbs are mounted on double banana plugs and inserted in the appropriate jacks in the manifold panel. Connections to benchtop items such as titration cells and burets are made with patch cords. For often-used applications, or interconnections of some complexity, i t is convenient to permanently wire the configuration onto a plug-in board which can be removed intact to clear the GAIS console for other uses. Potentiometric Titrators. The Instrument Synthesizer is capable of becoming a wide variety of potentiometric titrators. A very simple automatic titrator was described by Kelley, et al. (4). The Kelley circuit used an operational amplifier without feedback as a voltage-sensitive switch to operate the buret. The circuit of Figure 7 shows a somewhat different philosophy. The titration cell is placed in the feedback loop of a stabilized amplifier. The output of this amplifier is equal in voltage to the cell voltage, has a drift of 100 pv. or less per day, and can furnish several milliamperes of current with creation of only 1824
ANALYTICAL CHEMISTRY
i -
Figure 9.
a trivial error. Current drawn from the cell is normally less than lo-* amperes, and can be reduced further by adding a current-compensatinginput. The null-sensing switch is a stabilized amplifier which adds an inverted end point voltage to the cell voltage, and drastically changes output as the sum of the voltages becomes zero. The first time derivative of the cell voltage is also included in the summation by virtue of the input capacitor. The presence of the derivative causes the titrator to anticipate the approach of the end point, and to stop the titration more and more frequently as the end point is neared. This type of titrator gives accurate results, but sometimes overshoots slightly because of the finite time required for mixing and buret turn off. The resistors, Rc.11 and &.I, are one million ohms each and should be the same resistance value to within 0.5%. The best value for the capacitor depends upon the magnitude of the deriva-
I
/\
/
Figure 10. bridge
-“z\
?
Enforced linear resistance
Bimodal titrator
tive of the titration curve. Several microfarads usually prove sufficient. The leakage resistance of the capacitor must be a t least 500 megohms. The diode must be able to withstand 400 volts peak inverse. The occasional overtitration led us to consider a digital approach to the titrator problem. The appropriate circuitry is shown in Figure 8. Instead of adding titrant constantly, the buret adds aliquots equal in voIume to the allowable volume error for the titration. The addition of an aliquot of titrant is triggered by the build up of the first time integral of the summation used in the previous titrator. The derivative signal is not used in this device. This titrator does not miss end points, but it is very noisy and considerably slower than the direct type. It has the possible advantage of using the mechanical timer rather than reading and refilling a conventional buret. The integrating capacitor is chosen to give cycle rates within the capabilities of the buret ~ to give valve, and then R D L .is~ chosen the appropriate “On” time for the desired aliquot size. Values of 0.01 mf. and 1000 ohms, respectively, proved should be practical in our case. RD,#~I, small compared to RC.II. The accuracy of the digital titrator and speed of the direct buret control can be combined in the bimodal titrator shown in Figure 9. This titrator maintains direct control until a cell voltage somewhat less than the end point voltage is attained. At this preset point, control of the buret is switched to the digital mode for the remainder of the titration. The speed of the titrator was limited for buret drainage considerations. The typical average deviation
PI K
I K C OSC
?
RESISTANCE B O X I
Figure 1 1. Enforced linear conductance bridge
for O.OZm1. aliquot rrize was approximately 0.01 ml. on runs of 10 replicate titrations of silver nitrate vs. sodium chloride. Bridge Methods. GAIS I1 is well suited t o bridge measurements in which two operational amplifiers are used to enforce the, desired bridge linearity. Figure 1C shows a resistance bridge in which the amplifiers guarantee constant current through the measuring arms giving a linear output with resistanco change over the entire output range of 1;he two amplifiers. For a bridge linear in conductance, the voltage across the mertsuring arms must be maintained constant and equal while the current through them is compared. Figure 11 shows a linear conductance bridge. Linearity s maintained so long as amplifier operating limits are not exceeded. For an alternating current source an operational amplifier if;programmed as a shorted inverting douhle integrator (13). Very good frequency and amplitude stability of the resulting sine wave were obtained. One kilocycle was chosen as the frequency for our experiments. The bridge shown in Figure 11 was used for
4
L
Figure 12.
Linear conductance bridge
recorded conductiometric titrations with linear results. Because the conductance change expected for a given titration is very nearly independent of the inert electrolyte level, provided that this level does not change during the titration, a conductimetric titrator was developed for use in strong electrolyte solutions. The circuitry is shown in Figure 12. To accommodate higher currents, the bridge is fully coupled with step-down transformers. These transformers allow the cell and reference to operate a t low impedance while the ratio arms pass
for high conductance solutions
only microamperes of current. The recorder monitors the difference between amplified, rectified, and filtered voltages which are proportional to the cell and reference conductances. The electrodes are platinum wires approximately 0.5 mm. in diameter, 1 cm. long, and 0.5 cm. apart. The electrodes are fused into glass and plated with platinum black. In order that the concentrated electrolyte does not suffer dilution during the titration, a preliminary balancing of electrolyte levels is required. The titrant is made more concentrated in inert electrolyte than any solution to be
-------
\
5 MY
RECTIFIER-
FILTER-ATTENWTCU
WcoRD$R
-
~
Figure curve
1 3.
Conductimetric
DETECTORCIRREMT AMPLIFER
M i
d
titration
Figure 14. monitor
Programming detail for basic conductance
VOL. 35, NO. 12, NOVEMBER 1963
e
1825
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,
170
M
UCH HAS BEEN WRITTEN
1826
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.
