Frederick D. TabbuR
Reed College Portland, Oregon
Experiments and Equipment for Teaching Chemical Instrumentation
chemical instrumentation is becoming a standard course in the undergraduate curriculum ( 1 ) ; in fact, the Committee on Professional Training of the American Chemical Society has recommended it. The author was concerned about the purpose of laboratory work in such a course and had already initiated the projrct which is to be described. Previously an attempt was made to give the student experience with as many instrumrnts as time and available equipment would allow. Students were stimulated by using the instruments but displayed little real understanding of how the instruments worked. Despite efforts t o the contrary, a t the completion of the course students remembered commercial instruments as devices which, upon turning knobs, yielded dial readings which were unquestioned. This is not the purpose of laboratory work of such a course, for it is very often necessary to modify equipment and even invent it to perform specific research duties. Students still lacked that familiarity with instrumentation which they inevitably mould have to learn. This is not intended as a criticism of commercial instruments-instruments which are designed specifically for analysis must consider functional design and compactness, but unfortunately these considerations very often complicate an understanding of the instrument. Another type of instrumentation is also needed. These are instruments built specifically for teaching. These instruments should require a fundamental knowledge of the theory before they can be operated. These instruments need not be precise, but they should require some student effort to work them and perhaps as a consequence of this, they should be less expensive. The following is a progress report of an instrumental analysis laboratory which has been developed for the past two years a t Reed College. The laboratory work described is a part of a onesemester course with three lectures and three labGiven in part as a paper to the Division of Chemicd Education at the Meeting of the American Chemical Society, Washington, D. C., March 22, 1962. Given in part as a paper at the Instrument Society of America. Charleston. West Virsinis. . May " 2.. 1962. 1 ARONSON, M. H., "Electronic Circuitry for Instruments and Equipment,'' Instruments Publishing Co., Pittsburgh, 1960. BAIR, E . I., "Int~oduction to Chemical Instrumentation," McGraw-Hill, 1962, pp. 12&341. Ewmo, G . , "Instrumental Methods of Chemical Analysis," McGraw-Hill, New York, 1960, pp. 361-302. LINGANE, J. J., i'Electmanalytical Chemistry," 2nd ed., Interscience, New York, 1958, pp. 9-35. M~LLBR, R. H., GARMAN, R. L., AND DROZ, M. E., "ExperimenC. N., AND tal Electronics," Prentiee-Hell, 1942. REILLEY, S a w r ~ n , D. T., "Experiments for Instrumental Methods," H. A., McGraw-Hill, New York, 1961, pp. 289378. STROBEL, "Chemical Instrumentation," Addison-Wesley, Reading, Milssachusetts, 1960.
oratory periods per week. Nearly three weeks of lectures are devoted to electricity and electronics. Useful books' for introducing these topics are those by Aronson, Strobel, Reilley and Sawyer, Ewing, Miiller, Garman and Droz, Lingane, and Bair. Students are sufficiently proficient by their junior and senior year that they can be referred to the literature for the chemical details of the determinations. As the result of a three-way compromise between available student time, expense, and relative importance, the laboratory work introduces five basic components used in instrumentation today: the potentiometer, the vacuum tube, the servomechanism, the monochromator, and the photocell. An attempt isalso made to maintain a balance between inetrumentation and chemistry. A component is not introdxed for its own sake, but for a chemical measurement. Equipment is spread out and mounted on plastic panels so that it is accessible and the components are easily traced and examined. It is simple enough that the components can be assembled for use in the course by assistants using only written instructions, and it is sufficiently inexpensive that each student can have his own unit. The Experiments
The student is first introduced to the potentiometer. He becomes familiar with it and the Poggendorf method for measuring potentials by calibrating his own potentiometer with a commercial Poggendorf apparatus. He then assembles his own Poggendorf apparatus using a microammeter as the null detector. The Poggendorf is standardized with a Weston cell made by the student and then is used for a potentiometric titration. He is given a choice of four determinations: an acid-base titration using the quinhydrone electrode ( 2 ) ; a complexometric titration of bismuth, cadmium and calcium with EDTA (3); a redox titration (4); and a precipitation titration ( 5 ) . With the aid of the appropriate literature reference the student devises his own plan for carrying out the determination. The effect of internal resistance of the potential source on the Poggendorf method is pointed out to the student. Furthermore it is implied that this limitation can be overcome by the vacuum tube. Thus the transition to electronics is made. The student measures the characteristics of a vacuum tube which are used in applying it as an amplifier, because this is probably its most important use in chemical instrumentation. Plate current is measured by the student as a function of grid voltage and load resistance. The tube, a Tungsol 7851 electrometer, is examined as a triode and as a tetrode. The student learns the concept of negative feedback by determining the effect Volume 39, Number 12, December 1962
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of cathode resistance. He then assembles a potentiometer which uses the electrometer and microammeter as a null indicator. With a grid current of 10-'I amp this electrometer-potentiometer is capable of measuring potentials with rather high resistances. The student makes a glass electrode and uses it with his electrometer-potentiometer to make pH measurements. The instrument is calibrated with standard buffers, and the pK's of phosphoric acid are determined by titration (6). The same electrometer-potentiometer can be used as a current amplifier by using the I R drop through a known resistance as the signal. The student devises his own circuit for doing this and then uses the current amplifier for a polarograph. A polarographic determination is undertaken (7, 8). The student is then introduced to the servomechanism. He receives a chopper amplifier, the circuit of which he traces and explains. He receives a reversible servomotor and a chart drive. These components are assembled with the potentiometer to make a recording potentiometer. The student is able to do this on the basis of schematic explanation of servomechanism theory. The student then chooses one of four determinations which use the servomechanism: 1. He can record titration curves by using a syringe which is driven a t a constant rate in conjunction with his recording potentiometer. The closed syringe allows the use of titrants which are unstable in air. For example, the titration of iron (111) with chromium (11) is performed (9). 2. The servomechanism can be used to maintain a variable a t some fixed value. For example, the electrolytic separation of cadmium from zinc is carried out by maintaining the mercury cathode a t a fixed potential (10). By this method 10" parts of cadmium can be removed from solution and the remaining one part of zinc determined polarographically. The polarograms are recorded with the recording potentiometer. This is accomplished by using the chart drive to turn the potentiometer which provides the applied potential for the polarographic cell. The ZR drop produced by the diffusion current through a known resistance serves as the input to the recorder. 3. Electrolysis current is also a variable that can be held constant by the servomechanism in a manner quite similar to the potentiostat. A constant current coulometric determination (11) is then possible. 4. The recording potentiometer can also be used to record the potential of a working electrode during a
Figure la. Potentiometer assembly: I l l mercury batteries, I21 mercury switch tap key, 131 ten turn h e l i d potentiometer, (4)standardizing potentiometer.
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Journal of Chemical Education
constant current electrolysis as a function of time. The chronopotentiograms thus obtained can be used to analyze the solution (12). The student is then provided with an Ebert monochromator (IS). The monochromator uses a plane reflecting grating and reflecting spherical mirrors. The grating is rotated by a constant speed drive (the same one used for the syringe in the titration mentioned ahove) which is connected to a revolution counter. The student adjusts the optics to bring them into focus. He examines the dispersion in each order of the grating, using the mercury lines in a sun lamp as a source. A photoelectric detector is attached to the exit slit which enables a quantitative measurement of intensity. The student calibrates the position of the grating relative to wavelength using the same mercury lines. The monochromator can be used to examine emission spectra; or, by placing a continuous light source and curvette before the entrance slit, it can be used for absorption spectra. By placing a film strip in place of the exit slit, the monochromator is converted into a spectrograph. Since the signal from the photoelectric detector is read out with a potentiometer, the output from a single beam measurement can be rerorded. Equipmenk
Design and Performance
Poggendorf ApparatusZ
Potentiometer. A ten-turn helical potentiometer is mounted on a 5- X 5-in. panel of a/16-in.thick Lucite as shown in Figure l a . I t is wired by the student as shown schematically in Figure l b . The 1000-ohm potentiometer draws a little more than 1 ma from the mercury battery, which allows a small bat,tery (2400 ma-hr lifetime) to last a t least a semester. Galvanometer. A 0-20-microamp ammeter is mounted in a 5- X 5-in. panel of Lucite. A 5OOK potentiometer is wired in series with the microammeter for protection when large currents flow through it. The plastic panels which hold the components fit on a rack with 5-in. tracks, thereby allowing a number of components to be assembled on the same track. The Poggendorf apparatus assembled on such a rack is shown in Fiaure 2. A power supply which provides 2 All components have been constructed by assistants unless special mention is made to the contrary.
d Fig. 1b.
Potentiometer arsembly.
6.3 volts ac, +I40 and +280 is part of the rack. This is used later in the electronic circuits. The potentiometer which is most conveniently adjusted to 1.250 or 2.500 volts if two batteries are used can be read to 0.5 part per thousand of full scale. For cells with low resistance the uncertainty in the measured potential is less than the scale readability.
9 volt mercury battery is an adequate B+ supply for the tube. Glass Electrode. Each student requires about I/, hr to make the glass electrode shown in Figure 6. (Corning 015 is a soft glass and is easily worked in a Meker burner flame.) The electrode should be supported from the coaxial wire. No.4 Rubber Stopper on PM Motw Shaft
\
PM Motor
/
Clear
Piastic Sheet
50 A
E
Potentiometer
.
Figure 2.
Poggendorf
Apporrrt~r.
.--
,
Figure 40.
Magnetic stirrer con#rudion.
Figure 4b.
Magnetic rtirrer circuit.
PM Motor
Figure 3.
Student's Westoncell.
The student Weston cells (Figure 3) which determine the accuracy of these measurements vary within a range of 3 mv. Magnetic Stirrer
'
An inexpensive magnetic stirrer is indispensable for potentiometric titrations, particularly the recorded ones. Figures 4a and 4b are sketches of the construction and electrical circuitry for the stirrer used in the work. The titration cell is set on an asbestos sheet or some other non-magnetic material which is held by a ring support.
pH Meter. The previously mentioned components are assembled as shown in Figure 7. The pH meter has an accuracy of 0.05 pH units. Polarograph. A precision resistance is inserted in place of the glass electrode and reference electrode of the pH meter. The resistance is of sufficient size to produce about 0.5 volt IR drop. In the case of closely spaced waves, it is advisable to use smaller resistances to minimize the variations in applied voltage to the polarographic cell. Current can easily be read to 1% accuracy. Servomechanism
Chopper Amplifier and Power Supply. The amplifier shown in Figure 8 is aesembled on a 5- X 10-in. sheet
Electrometer Tube
The 7851 is mounted upside down in a jar on a 4- X 5-in. sheet of clear Lucite, as shown in Figure 5. The surface of the electrometer can be kept free of moisture (a necessary requirement for high impedance measurements) by placing a desiccant in the jar. A
Dryjnp~g~t--po-hI
Figure 5.
Electrometer arrembly.
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of Lucite. It is powered by the voltage doubler Dower suoolv 9 which is attached to - shown in Figure the rack.. Servomotor. The servomotor is a reversible motor mounted on a 5- X 5-in. piece of l/rin. aluminum.
Figure 9.
Figure 6.
Power supply.
Recording Potentiometer. Figures 10a and lob show the manner in which the servomotor, potentiometer, and chart drive are linked mechanically by the student for use as a recorder. I'igure 8 shows the electrical circuit. The response of the pen can be quickened by using a larger diameter spindle on the servomotor shaft (some students used large corks). The linearity, span, and mechanical reproducibility depend on the sleeve of the helical potentiometer shaft. The dial cord should be more firmly linked (with more turns) on the potentiometer than the servomotor sleeve. Slippage on the latter is less important. The speed of the chart drive can be varied by small amounts by changing the diameter of the rubber tubing roller. For larger changes a different motor must he used. One revolution per minute is the most useful speed although 5 rpm is necessary for the chronopotentiograms. The chart drive will accept all sizes of roll chart paper up to 11 in. as well as 8- X 11-in. chart paper. The recording potentiometers built by students are sensitive to from 1 to 5 mv. This means that a signal of this size is required before the pen moves. The full scale deflection can be expanded to take advantage of this sensitivity. 100 mv full scale is about the limit of scale expansion and this is uspd when recording polarograms.
Student's glass electrode
Conrtont Drive Syringe
Figure 7.
The basic part of this device is a lead screw driven by a reversible synchronous motor. The details of this are shown in Figures l l a and l l b . This is the most difficult component for the student assistants to build of all the components used in this program. The major problem in the construction is the reduction of backlash. This is particularly troublesome when the lead screw is used to turn the grating for the monochromator. When a 20-ml syringe is used with a 1 rpm motor the delivery rate is about 1 drop/6 see.
pH meter.
rJiF
/Tcw[r 8.eKR25MFD 1 1_ 1 2.zfi*%4Fn
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Figure 8.
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I I ---UECHANICIL ----------- ----i L l A I I G E BETWEEN AND POTENTIOMETER 4
CHOPPER
Recording potentiometer circuit.
Journd of Chemical Education
AMPUFIER
MOTOR
The monochromator is housed in a thick plywood box as shown in Figure 12. The mirrors, M, and Mz, are mounted on tripodal mirror mounts which in turn are supported in sections of 3-in. drain pipe. Single edged razor blades serve as slits. The
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Figwe
1 0 .
detail of the grating mount is shown in Figure 13. The lead screw is bolted to the bottom of the monochromator and rotates the grating by pushing the bar attached to its base. The advantage of such a linkage is that the linear motion of the nut on the lead screw produces a nearly sinusoidal motion of the grating, which means
.
Recording potentiometer conslrudion (lop view).
Figure
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.
c"a*,
Figure 1 Ob.
13. Detail of grating mount.
that the wavelength a t the exit slit is directly proportional to the rotational position of the lead screw. Calibration of wavelength thus is simplified. The monochromator is aligned and focused by tracing the image of the entrance slit with a white card through the optical path and adjusting the source and M I as to full illuminate MI, the grating and Mz. M 2 is then adjusted for optimum resolution.
--,..
Recording potentiometer conrlrudion [ride view).
Photocell
The photoelectric detector uses the circuit shown in Figure 14 and is assembled as shown in Vigure 12.
,,,.~, Figure
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Leod screw [top view).
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Figure 1 1 b.
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Lead screw [ride view).
Figure
Figure 12.
Ebert monochromator.
14. Photometer deledor.
Note that the photocell, Tungsol 8049, is being driven by the potential built up between the cathode and grid of the electrometer, Tungsol 7851. When combined in this way the sensitivity is comparable to that of a photomultiplier tube. The spectral range of the 8049 is between 3000 and 6000 A. When used as a spectrophotometer the precision of the readings is better than 1% over this range with a slit width of Volume 39, Number 12, December 1962
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less than 5 mfi. However the detector is apparently not linear a t high values of per cent transmission. Beer's law is only obeyed a t lower values of per cent transmission using rectangular cells. Equipment:
Construction and Cost
If a ''homemade'' instrument requires the skill of a machinist to build it, the availability of that instrument to many institutions is greatly reduced. This has been kept in mind throughout the construction. All the equipment which has been described was built ready for classroom use by student assistants who had no previous experience in a shop. The only power tools were a circular saw and a drill, and the instructions were written. A key factor in this construction was the Liberal use of paper templates or patterns. These patterns were made by the author exactly to scale and simply laid out on the work and the drilling and cutting points marked through them. The wiring which the assistants did (e.g., chopper amplifier) was done from written instructions similar to those which accompany radio kits. Some of the machined parts which have been described are neither available commercially nor can they be made by inexperienced students. I n such cases they were made by a local machine shop. A summary of the prices and construction times is given in the table. Cost and Time Totals Item Rack Power aupply for rack Potentiometer assembly Galvanometer assembly Weston eel1 Calomel electrode Glass electrode Magnetic stirrer Electrometer assembly Chopper amplifier Servomotor assembly Chart drive Recording potentiometer Linkages Automatic polarograpb Lead screw Constant speed syringe Monochromator housing Mirror mounts and mirrors Grating, mount and drive Photometer housing and slits
Cost of parts (1)
Cost of pbs
Construetiontime (hoursjb
$ 3.95
$ 3.95
9.99 23.50 8.12 2.45 1.30 0.25 6.53 5.66 24.04 22.49 15.74
9.00
2.67 0.75
(10)"
2.45 1.30 0.25 5.71 4.57 23.34 14.74 15.74
... . .. ... 0.75 0.75 1.5 1.33 0.75
(3.00) 2.95 .. . 10.76 9.66 0.5 15.10 15.10 3.15 4.66 4.66 5.89 5.89 3.5 24.84 24.84 2.33 42.46 42.46 2.00 9.39 9.39 2.00 The first column represents the cost of parts purchased for a single unit. The second column represents the cost per unit if purchased in quantity of 10 units. 'This column represents the average time per unit for assistants making 10 units.
-
Conclusions
The progress to date is encouraging. Because each student has his own instrument, there is 'more independence of thought, more originality, a n d more imagination. There appears to he a genuine interest on their part in chemical instrumentation. I n fact, many of the students have undertaken extensions of some of the experiments that were carried out in the course. Moreover, when commercial instruments are described and demonstrated to. them, their questions are much more penetrating than they had been in the past. The low cost is appealing. Equipment for ten students, each with his own set of instruments, totalled less than the cost of one commercial manual spectrophotometer. Of course, components are used- interchangeably, so a student cannot have all of his instruments a t the same time. Acknourledgment. The support of this work by the Course Improvement Section of the National Science Foundation is gratefully acknowledged. Copies of the experiments, plans, sources of material, and prices are available from the author for those who are iuterested for $2.50. Literature Cited (1) Symposium on Problems in Teaching of Instrumental Analysis sponsored by the Division of Chemical Education a t the 128th meeting of the American C h e m i d Society, Minneapolis, September, 1955. J. CHEM. EDUC.,33, 422-441 (1956); Symposium on Educationd Trends in Analytical Chemistry sponsored jointly by the Division of Anslvtied Chemistrv and Chemical Education at the 136th meeting of the Gerican Chemical Society, Atlantic City, N. J., September 1959. J. CHEM. EDUC.,37, 276-292, 337-343, 400 (1960). MORGAN, L. R., LAMMERT, 0.M., A N D CAMPBELL, M. A., Trans. Electroehem. Soe., 61, 405 (1932). REILLEY,C. N., SCHMID,R. W., A N D LAMSON,D. W., Anal. Chem., 30, 953 (1958). . , KOLTHOPP.I. M..LAITINEN.H. A,. AND LINGANE.J. J.. J. Am. hem. s&.,59,429'(1937).' (5) SHINER,V. J., A N D SMITH,M. L., Anal. Chem., 28, 1043 (1956). (6) R E I L ~ YC. , N., AND SAWYER, D. T., "Experiments for Instrumental Methods," McGraw-Hill, New York, 1961, pp. 17-25. (7) SHREVE, 0.D., AND MARKHAM, E. C., J. Am. Chem. Soe., 71, 2993 (1949). J. J., A N D KERLINGER, H., Ind. Eng. Chem., Anal. (8) LINOANE, Ed.,13, 77 (1941). (9) LINGANE, J . J., Anal. Chem., 20,797(1948,. (10) LINGANE,J . J., "Electrosnalytical Chemistry," 2nd ed., Interscience, New York, 1958,pp. 43G432. W.N., A N D KO, R., Anal. Chem., 23, 1019 (1951). (11) CARSON, R. H., Anal. (12) REILLEY,C. N., EVERETT,G. W., AND JOHNS, C h . , 27,483 (1955). , G., J.Opt. Sac. Am., 42,641 (1952). (13) F a s n ~W.
Royal Institute Monograph for Teachers Number 6 "Principles of Titrimetrie Andyais" by E. E. Aynsley and A. B. Littlewo3d is the- sixth in the series of monographsfarteaehers(price4~.Gd.,$0.65). (SeeEditorially Speakingonpage 537 of the November 1961 issue.) The d~scussion,though excellently written, is very general, strictly classical in approach, and samples rather than covers the subject. The most serious shortcoming is the absence of solved numerical problems. This and others in the series are avsileble from the Royal Institute of Chemistry, 30 Russell Square, London WC1, England.
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