Galen W. Ewing' N e w Mexico Highlands University Las Vegas, N e w Mexico
I
I
Modular h ~ t r u t n e n t c l hin Analytital Chemistry
The teaching of chemical instrumentation in its laboratory aspects has always presented a dilemma ( 1 ) . On the one hand commercial instruments can be utilized. This has the undoubted advantage that the problems of design have already been solved by experts, and need only be described. I t is nearly always a valid assumption that the manufacturer has hired a competent design engineer. A further advantage often cited hut open to question is that students acquire experience in the operation of the same instruments which they will encounter in later industrial employment. On the other hand the student will undoubtedly accumulate a deeper understanding of the principles of instrumentation if he has an opportunity of constructing his own apparatus. Neither of these alternatives works out ideally. I t is too easy to obtain excellent results on standard instruments by adherence to the instruction manual, and this may give both student and instructor a false feeling of confidence. Just as in driving a car, one can become a slcilled operator without understanding the detailed mechanism he is controlling. Econolnic problems are frequently encountered also. It is not usually possible to maintain a separate inventory of expensive apparatus solely for teaching, and scheduling may be difficult where one instrument is used both for teaching and for research. And research professors are justifiably reluctant to entrust their equip ment to students in courses, even under supervision. At the other extreme, it is unrealistic to expect a student to construct his own instruments from basic components. Clearly a middle-of-the-road compromise nus st be found. A series of instruments specifically designed for teaching is essential. The most fruitful path seems to lie in the direction of pa.rtial pre-assembly of basic instruments, such that the student can complete the assembly without the requirement of extensive training in construction techniques. Outstanding contributions have been reported along this line by Professors F. D. Tahbut of Reed College (S), E. N. Wise of the University of Arizona (S), and H. V. INalmstadt of the University of Illinois and his coworkers (4). I t is the purpose of the present paper to describe another approach which has been developed a t New Mexico Highlands University and implemented commercially by A. R. F. Products, Inc., of Raton, New Mexico, and Boulder, color ad^.^ 1 Present address, Seton Hall University, South Orange, New Jersey.
32
/
Journal o f Chemicol Education
The Design Concepf
The need for instruments designed for instructional purposes can he met by a series of modules, each intended to perform a single circuit function. The modules should be provided with uniform input and output connectors and a supply of patch cords so that the student can interconnect them in any desired manner. The modules should be completely constructed and wired by the manufacturer, so that the student's time may be spent in working with problems of instrumentation and chemistry, rather than in electronic construction or kit assembly. It is imperative however that the entire construction he open to detailed inspection, as by transparent cases or removable cover plates. To this end a series of eleven modules has been designed and are currently being manufactured. These are listed in Table 1,beneath a tabulation of the instrumental assemblies described in the manual which accompanies the modules. These are not exhaustive lists; many minor modifications can he made. It should he emphasized that no measures have been taken to prevent a student from assembling the modules incorrectly, except where dictated by safety considerations. Hence he must make his own decisions, and the excellence of his judgment can easily he reflected in the gracles given to his reports. A rather extensive laboratory guide has been prepared, in which the modules are described and guide lmes for their combinations are presented. The manual also includes 29 experiments, of which 17 illustrate the application of instruments to chemical problems. The other 12 are physical rather than chemical in nature, and present methods of calibration of instruments or measurement of instrument parameters. Sufficient theory is included on such matters as impedance matching and amplifier feedback, but very little theory concerning chemical phenomena. Thus the guide is not expected to replace a text in instrumental analysis. The manual is keyed to 16 current texts and manuals in this and closely allied fields, by frequent references. Speciflc Design Features
Since a major segment of experimental work in analytical chemistry is titrimetric, an important decision in
a The work carried out at the University was supported by a grant from the Coune Content Improvement section of the NSF, which is gratefully achowledged. The bulk of any profit resulting from s& of these instruments is equally divided by the stockholden of the manufacturer, on the one hand, and by Highlands University on the other. The contractual arrangement has been approved by the NSF.
Table 1.
Instrumental Assemblies
Modules used (see below)
Instrument pH-electrometer for glass electrode (com-
-
narisnnl -- .-..,
pH-electrometer for glass electrode (deflection) Potential meter for low resistance ce& (wmparison) Potential meter for low resistance cells (deflection) Potentiometric titrator for glass electrode (recording). Potentiometrlc titrator for low resistance cells (recording) Potentiometric titrator, constant current (recording) Chronopotentiometer Amperometrio titrator, constant potential Manual polarographic instrument Recording palarographic instrument ConduetoXetrie titrator Electrodeposition apparatus, unregulated (eravimetric) lated (&avimetriE)Electrodeposition apparatus, cathode-regulated (coulometric) Coulometric titrator, amperometric detec+inn..
Coulometric titrator, potentiometric detection Thermal andyeer (via cooling curves) Thermometric titrator Filter photometer, single or dual beam Prism spectrophotometer, single or dual beam Grating spectrophotometer, single or d u d beam
A., C.. D., E C, D, E ~
~
A, G C, D, G
.A-,R. -, c-, -n., T.B, C, D, L 6 D, a, L F, H, J, K, L D, I
-
-
0,1
D, I
The Modules A. B. C. D. E. F.
Voltage source Current source Indicating meter Operational amplifier Electrometer Wheatstone bridge
G. Potentiostat H. Thermistor probe I.
titration is readily instrumented through the constantcurrent source module. Figure 1 shows an assembly of modules suitable for potentiometric titration with a glass and calomel electrode pair. No specific recorder is supplied with the A. R. F. instruments. In the author's laboratory, a miniature deflection recorder of 100 microamperes full-scale sensitivity4 has been found satisfactory and is quite inexpensive.
Absorptiometer
J. Solution pump K. Timer L. Recorder (not an A.R.F. module)
In some instruments a choice can be made; e.g., in this coulometrio titrator, either the meter or the recorder can be utilized for reed-out; both are not needed.
the design of an analytical system must be concerned with the apparatus for titration. In the present system no special provision is made for manual titratiou; conventional burets leave little room for improvement. For recorded titrations, delivery of reagents must be uniform with time. This can be accomplished by means of a device such as a Mariotte bottle for maintaining a constant hydrostatic head, by a motor-driven piston pump or hypodermic syringe, or by a continuous action, positive displacement pump. The continuously acting pump was selectedSas being less bulky than the Mariotte bottle, more convenient than any alternative, and a t the same time inexpensive. The pump delivers 1.88 ml/min. Its reproducibility is about i 1% for times of several minutes, but it cannot serve reliably for times of less than about a minute, because of a slight pulsation in delivery. Titrations can be timed either by powering the pump through the seconds-timer module, or by reference to the time scale of the strip-chart recorder. Coulometric a Manufactured by Sigmamator, Inc., 68 North Main St., Middleport, N.Y.
Figure 1. Modular units required to form a pH-meter for titrationr. The central unit to which the glar, and cdomei electrodes are attached 1s the electrometer. Its outpvt is amplified b y the amplifier module and impressed upon the meler. The perirtaltk pump for constant-rate titrotion oppeors a t the l e f t
Special mention should be made regarding the designs of several modules which contain novel circuitry. The current source is built around a Darlington circuit which contains four transistors, three of them in the emitter-follower configuration (Figure Z).' It is calibrated to give currents of 0.25, 1.00, 10.0, and 25.0 ma, accurate to better than ly0within load ratings. The Wheatstone bridge, primarily for measurement of electrolytic conductance, operates on 60-cycle alternating current from the power lines. It has a
*
"5~3'31 a5
Figure 2. Constant-Current Source, srhemdc. CR-1 is o rilicon rectifier bridge, typical d those used in other modules; CR-2 and CR-3 ore Zener diodes for voltage regulation. Rerirtor Rconrirt. dfour precision reti+orr, one for each current level, selected by a panel switch.
built-in phase detector which has the advantage over a conventional rectifier that the meter will show whether the bridge is balanced or not, and if not, will also show in which direction it is unbalanced. The bridge is also applicable to thermometric titrations and other temperature effects, through the use of a thermistor probe. Manufactured bv Rustrak Instrument Co.,. Inc... 130 Silver St., Mauchester, N.H. 6 I am indebted to Professor A. A. Emus for suggesting this circuit. Volume 42, Number 7, January 1965
/
33
The dc amplifier module contains a commercially obtained operational transistor amplifier with a different,ial input,. Input and feedback circuitry permit a selection of several amplification ratios from unity to 1000. Capacitive feedback is also available, so that t,he unit can bc used as an integrator to produce a steadily increasing polarizing voltage for polarography or for service as a coulometer. A potantiostat nlodule is included, which operates by means of a silicon-controlled recti6er ( 5 , 6 ) (Figure 3) to regulate the potential between an electrolysis cathode and a calomel reference electrode. An adjustable reference potential is taken from the voltage source module. Positive control within 10 millivolts is obtained. The maximum output is 5 amp at 10 v. Optional manual control is provided, so that the instrument can be used in unregulated electroIysis.
I
I
i
I
Figure 3. Potenfio$tot, schematic. The frequency of orcillation of the vnijvnction tronrirtor Q ir controlled b y t h e v o l t a g e difference between the t w o input terminolr, and in turn regulates the firing point of the silicon controlled rectifier, SCR,within each cycle, and this determines the power d e livered t o the load through the output terminolr. Pmviaion i , m a d e for ~ . " " O I ~ont.01.
The potentiostat can be utilized in other olosed-loop control systems, wherever a voltage signal can be obtained from a current-actuated device. For example, as incandescent lamp can be regulated and controlled at any desired brilliance by monitoring with a photovoltaic cell.
Figure 4. Abnorptiometer, with related modules. The left rear unit is the regulated power supply for the incandescent lamp. The abrorptiometer is shown with only one photocell connected (front left1 t o the amplifier and meter modules. N o t e the + a m p a r e n t carer of amplifier and meter.
cuvet) or to be divided equally between the two. The photocells are each connected to a pair of binding posts, so that any desired one- or two-cell photometric circuit may be assembled, along with the dc meter and whatever other modules may be required. The wavelength control of thc monoch~omatorreads in arbitrary numbers, and must be calibrated (by the student) against emission lines of a mercury lamp, or against a didymium glass filter. This requirement of student calibration has teaching merit and permits a lower cost. Additional modules are planned as they can be developed. High on the list is a basic gas chromatograph. Literature Cited
(1) EWING,G. W., J. CHEM.EDUC.,33, 424 (1956). EDUC.,39, 611 (1962). (2) TABBUTT,F. D., J. CHEM. (3) WISE,E. N., J. CHEM.EDUC.,40, 73 (1963). H. V., ENKE,C. G., AND TOREN,JR., E. C . , (4) MALMSTDT, "Electronics for Scientists," New York. W. A. Benjamin Inc., 1962. (5) EWINQ, G. W., "Pittsburgh Conf. an Anal. Chem. and Appl. Speetrasc.," Feh. 29, 1960. F., AND DAVIS, J. B., Anal. Chem., 36,11(1964). (6) LINDSTROM,
Absorptiometer
In the interests of economy, the optical absorptiometric module is limited to the visible spectral region. I t is provided with an incandescent lamp and a voltageregulating transformer. The lamp and housing are removable, to permit substitut,ion of other sources, such as a mercury arc lamp. A choice of three optical paths can be made by means of movable mirrors. The light can be dispe~sedby a glass prism in a modified Littrow mount or by a plane reflection grating, or the monochromator can be bypassed and colored filters substituted. Figures 4 and 5 show the appearance of the absorptiometer assembled with other com~onents,and with cover removed. Two interchangeable photovoltaic and photoconductive cells are provided so that their properties can be commred. A beam s ~ l i t t e r~ e r m i t sthe whole light beam to fall on either cell (through a correspondyng
34
/
Journal of Chemical Education
Figure 5. Abrorptiometer with cover removed t o show the intern01 conrtruclion. Entrance and exit slits of the monochromotor are located in the dividing w a l l a t left center. Cuvetr are covered with square black covers. The p r i m a t the extreme right can b e replaced by a diffraction grating, not shown; in either cose the wovelength is selected by the micrometer, front right.
