JOURNAL OF CHEMICAL EDUCATION
0
ELECTRONICS INSTRUCTION IN THE INSTRUMENTAL ANALYSIS COURSE' H. W. SAFFORD University of Pittsburgh, Pittsburgh, Pennsylvania
Tm
need for electronics instruction in the instru- one three-hour laboratory period per week. I t is esmental analysis course is a controversial issue, and one sentially a course at the senior-graduate level, and prior for which widely differing opinions have been ex- training in organic chemistry, quantitative analysis, pressed. I n the writer's opinion even in introductory and physical chemistry is normally prerequisite. The work some electronics instruction is a must, for teach- topics covered include photoelectric photometry, visible ing a "push-button" course in instrumental analysis is and ultraviolet absorption spectroscopy, conductomeneither sound nor proper pedagogy. One should recog- try, potentiometry, polarography, coulometry, polarnize, realistically, that most of the instruments a stu- ized light microscopy, and refractometry. Except for dent encounters involve electronics in their construction the last two topics, in all of our introductory work the and operation. Certainly to profit most from his study of specific commercial instruments is held to a training, this same student should be given some in- minimum; rather there is stressed, both in the classtroduction to basic electronics and experience in the room and in the laboratory, the fundamental principles use of electronic techniques. The extent to which he underlying the operation of various classes of instruwill be able to solve his future problems by and recog- ments, the actual construction and study of general innize the advantages and limitations of a particular in- strument types wherever possible, treatment and instrumental method will he governed in no small way by terpretation of deta, and an evaluation of the advauthis knowledge that he gains. tages and limitations of each instrumental method conThere is no implication here that the instructor must sidered. present an exhaustive laboratory course in experiEarly in the first semester a three-way approach is mental electronics; such an approach is neither desir- made toward interweaving electronics instruction with able nor feasible. On the other hand, an effective com- the classical topics under discussion. This approach promise can be made without distorting the normal in- involves (1) extensive use of lecture demonstrations; strumental analvsis course. and without surrenderine (2) group experiments, demonstrations, and discussions any of the convictions an instructor may have concern- at the beginning of certain laboratory periods; and ing the presentation of the fundamentals of instrumental (3) actual construction and testing of basic instruanalysis. ments by the student. At the University of Pittsburgh the introductory course in instrumental analysis carries two credits for LECTURE DEMONSTRATIONS each of two semesters and consists of one lecture and If the instructor first dissects relatively simple circuits into their component parts and treats each of 'Presented as part of the Symposium on Prohlems in the Teaching of Instmmentsl Analysis before the Division of Chemi- these separately, later integration and study of the cal Education a t the 128th Meetine of the American Chemical whole becomes a relatively easy task. I n the four lectures we devote to basic electronics, the selection of inSooiety, Minneapolis, September, 1%5.
VOLUME 33, NO. 9, SEPTEMBER, 1956
dividual components and elementary circuits for study is influenced greatly by the frequency of appearance and utilization of these units in a large number of commercial analytical instruments. At each step of the way, schematic diagrams are drawn to illustrate the constructiou and circuitry of each component, the actual devices in cut-away form are handled by the student, and a lecture demonstration is performed. The equipment required for lecture denlonstrations need in no way be costly. A 5-inch oscilloscope, a regulated power supply, and a vacuum-tube voltmeter can he constructed from commercially available kits a t a total cost of approximately A projection meter to fit a standard slide-projection lantern is almost essential, and is available a t a reasonable price.3 Add to these such miscellaneous items as batteries, resistors, condensers, tubes, tube sockets, and hook-up wire, aud the instructor has equipment a t his disposal for demoustration of a wide variety of fundamental electronic circuits. Much can be left to the wishes and ingenuity of the instructor as to the actual lecture demonstrations to be performed. Many helpful suggest,ions and introductory experiments are to be found in the book"E1ements of Radio"' by Marcus and Marcus. For more advanced work the instructor will wish to consult "Experimental Elect,roni~s"~ by Muller, et al., and other references dealing wit,h the principles of electron tubes and basic electronics. For all of our lecture demonstrations, a baseboard and panel combination has been constructed and fitted with various tube sockets, variable resistors, toggle switches, and the like. Fahnestock clips are provided for easy connection so that any simple demonstration circuit can be arranged. Voltages are measured with a kit-type vacuum-tube voltmeter, while the plate current output of any tube under study is fed to a projection meter. All manipulative operations during a demonstration are described to the students and carried out in their full view, while the enlarged image of the meter scale projected on a screen permits them to follow the exact course of each exercise. The lectures on elementary electronics are prefaced with a brief review of the "Edison effect" and early suggestions and explanations concerning thermionic emission. Beginning a t this point, and throughout all discussions, the student is introduced to electronic symbols and nomenclature in a logical, stepwise fashion. His introduction to two-electrode tubes, or diodes, begins with an examination of the schematic representations of various types. Tubes with the envelopes cut %Heath Company, Benton Harbor, Michigan; Electronic Instrument Company, 84 Withers Street, Brooklyn 11, New York; Allied Radio Corporation, 100 N. Western Avenue, Chicago 80, Illinois. Wentrill Scientific Company, Chicago 13, Illinois. ' MARCUSAND MARCUS,element^ of Radio," Prentice-Hall, Inc., New York, 1953. MULLER,GARMAN, AND DROZ,"Experimental Electronics," Prentiee-Hall, Inc., New York, 1943.
437
4
J F i g ~ r s1.
Nson-lamp Sawtooth Oscillator
away are provided, for lengthy verbal descriptions are a poor substitute for his actually seeing various anode and cathode constructions and interelectrode distances. With the aid of the projection meter we demonstrate the unilateral conductivity of a diode and iuvestigate certain diode characteristics. It seems to be much more satisfying to the student to have data, for example, for a plate current-plate voltage curve actually obtained in his presence. Discussions of space charge, saturation current, and cathode emission hold his attention and interest much more firmly than when similar data are merely taken from the literature. A brief review of the differences and similarities between alterna,ting and direct currents and voltages has not been found to be a waste of time. For example, not many students know exactly hat voltage it is in an alternating cycle that a conventional voltmeter is indicating-nor that these "root-mean-square" voltages are approximately 0.7 less than the peak voltage available in a cycle. Since in the analysis of simple circuits it is often desirable to show the students an actual picture of the cyclic variations or waveforms of voltage and current a t various points, we introduce them very early to a cathode-ray oscilloscope. The instrument can be presented very simply as comprising a cathode-ray tube, amplifiers, and a power supply. Upon being shown the schematic diagram of a cathode-ray tube, the student sees the same principles involved as those underlying the simple diode. The exact arrangement of the electrodes and the deflecting plates between ~vbichthe electron beam passes before impinging on the fluorescent screen are observed by him in a cut-away tube. By applying various d.-c. and a.-c. voltages to the deflecting plates, many waveforms are shown and described so that the student will come to look upon the oscilloscope as a useful electrical curveplotting system. We try to make no mystery of the nature of the sawtooth voltage which is applied to the horizontal plates to bring about the left-to-right linear w e e p of the electron beam. To demonstrate a simple sawtooth oscillator one needs only a d.-c. source of somewhat over 60 volts, a radio-type variable resistor, a condenser, and an ordinary 110-volt neon lamp (Figure 1). When the condenser has been charged to approximately 60 volts, the neon lamp "fires" or glows, and then goes dark until the condenser is charged once again and the cycle is repeated. ' B y varying resistor R and selecting different
438
values for capacitor C one can illustrate a wide range of saw-tooth or sweep frequencies. Further, if one connects the output of this circuit to the vertical plates of an oscilloscope, the student is able to see the waveform of the saw-tooth voltage.
JOURNAL OF CHEMICAL EDUCATION
vised to tap off any desired voltage up to the maximum available from the power supply. We indicate that this can be accomplished easily by connecting resistors in series across the output terminals of the power supply. Further, the equivalence to a d.-c. battery of each resistor in such a voltagedivider system is emphasized by considering the polarity characteristics of the resistor and the voltage drop occurring across it. We are now ready to consider the triode, a threeelectrode tube, and the effect that its grid has on the electron stream moving toward the anode or plate. The standard circuit for investigating triode characteristics is used here to determine the amplification factor of a typical triode. This is the ratio of the increase in plate voltage to the decrease in grid voltage required to produce the same increase in plate current. This leads us to consider the possibility of using a triode in assembling a simple titrimeter (Figure 2). We point out that if one were to titrate a strongly oxidizable species, M+, with some standard oxidant, a t the onset of the titration the indicator electrode, perhaps platinum, should be quite negative, making the grid of the tube correspondingly negative. Under these conditions the current through the tube will be small and The use of a diode in converting alternating into di- meter M will give a low reading. At the end point the rect current is considered next. Since most electronic indicator electrode will suddenly become less negative equipment requires for proper operation voltages much and the current will increase sharply through the tube, greater than the available line voltage, we indicate that causing a corresponding increase in meter reading. diode rectifiers are usually operated in conjunction with From a plot of meter readings against volume-of-tistep-up or power transformers. The different circuit trant, one would hope to get the familiar curve shown arrangements and diode types necessary to produce a t ( B ) . half-wave and full-wave rectification are outlined. Here, however, the student is urged to exercise cauThe next components studied are those that may he tion in his thinking, for if the indicator electrode which used in improving the character of the pulsating direct is hooked directly to the grid becomes too positive, the current produced by the diode rectifier, that is, in grid may burn out. Further, the same thing might smoothing out the pulses and in stabilizing the output happen a t the start of a titration if the reference elecagainst input voltage fluctuations. First, cold cathode trode were inadvertently hooked to the grid. The diodes or gaseous, voltage regulator tubes receive brief warning to keep the grid negative a t all times and the treatment. The student learns from actual observa- various ways in which this may be accomplished are tion why the output voltages of these tubes remain sen- given full discussion. sibly constant even though input voltages vary over Before finally assembling the devices studied into a wide limits. As in all other cases, the waveforms of the workable analytical instrument, we outline briefly the voltages and currents before and after regulation are dis- use of an electron-ray or so-called "magic-eye" tube as played on an oscilloscope screen. a null instrument to replace the plate current meters Since for proper vacuum-tobe operation still smooth- previously discussed. (Since this device finds wideer direct current is required, the functioning of a con- spread use in many instruments, a brief review of its denser or capacitor and its role in reducing the ripples functioning may be in order here.) As illustrated in of current from a rectifier are now considered. After Figure 3, the student learns that an electron-ray tube is cut-away condensers of several types are examined for a combination of a conventional triode and a simple their construction and composition, the charging and cathode-ray tube, both built inside the same glass endischarging of a capacitor is demonstrated with the aid velope. In the upper section, the anode or target is a of the projection meter mentioned earlier. The ap- funnel-shaped piece of metal coated on its inside surplication of this charging-dischai.ging process is then face with a fluorescent compound. With a positive illustrated in a simple filter circuit. potential on this conical target, electrons from the The student, consciously or otherwise, thus far has cathode strike it and cause it to fluoresce all over with a been considering components and circuits that may be green color; we say that the "eye" is closed, a condition integrated into a simple power supply. One more item represented a t ( C ) . remains to be added: to furnish various voltages to the Examination of ( B ) shows that between the cathode different parts of a circuit, some means has to be de- and target on one side only, a grid or "ray-control elec-
VOLUME 33, NO. 9, SEPTEMBER, 1956
trade" consisting of a thin vertical piece of metal is mounted. If this grid has the same potential as the fluorescing target, it will have little influence on electrons going from cathode to target. If, however, it becomes negative with respect to the target, it will repel electrons which leave the cathode on that side. Since that portion of the target behind the ray-control electrode is shielded from electrons, it does not glow, and in effect the electrode casts a shadow on the fluorescing target; this is the open "eye" condition shown at (D). Thus the device can be used as a voltage indicator. GROUP DEMONSTRATIONS IN THE LABORATORY
This second step in supplemental training is in no way intended as a substitute for individual laboratory work. One example will serve t o illustrate this approach in bridging the gap between lectures and laboratory work. I n the lecture discussions on absorption spectroscopy there are considered the theories of radiant energy absorption, the Beer-Bonguer law, nomenclature, treatment of data, errors, basic elements of instrument construction including radiant energy sources, dispersing elements, photometric measuring systems, etc. With the aid of lantern slides, schematic diagrams showing the optical and electrical construction of all the instruments t o he used are presented and discussed a t the beginning of the first laboratory period. Then, as a group, the students inspect each instrument and become acquainted with the physical counterparts of the components they have seen in the diagrams. The cases of many instruments are deliberately removed so that the student may actually see the individual components and have a t least the essential features of the circuitry pointed out t o him. Thus he will be able better to understand later what is happening when the operating directions specify that the knob labeled "D" be turned. It may be argued by some that "a little knowledge of electronics is a dangerous thing" and that unless an exhaustive treatment of all details of the circuitry of an instrument is made the student would be far better off and less confused not t o be exposed a t all to a somewhat superficial treatment. Student response and performance seem t o warrant the procedure suggested here. Of course, some discretion must be used, but the alert instructor, without getting the students or himself involved too deeply, can do much to remove many of the unnecessary mysteries surrounding "pushing button No. 2 in the lower left-hand corner of the little black box." For this and other group demonstrations the time required is no more than an hour and a half, so that considerable laboratory time remains in each of the periods for individual experimentation. The disadvantage of working in larger groups for these brief sessions seems greatly outweighed by the opportunity of considering borderline topics and expanding lecture presentation of material that might otherwise have to he omitted.
CONSTRUCTION AND TESTING OF BASIC INSTRUMENTS
The most snccessful aspect of the three-way approach under discussion is undoubtedly practiced by most instrumental analysis instructors. This involves the actual construction and testing of basic instruments by the student in the laboratory, No condemnation of commercial instruments is intended. Indeed, they must be used in certain instances where student-constructed instruments would be neither desirable nor practical. However, the student who actually assembles and studies an instrument is much better equipped to cope later with the many different commercial forms of the same instrument. In our course, while the lectures on introductory electronics are beginning the students are kept busy in the laboratory performing classical experiments in potentiometry. Using individual items of apparatus rather than an enclosed commercial potentiometer, they gain review practice in following a wiring diagram while assembling the familiar Poggendorff compensation circuit. Next to be constructed is a potentiometer-voltmeter in which a standard cell is not required and the use of precisely calibrated slidewires is avoided. To shorten the time required for the assembly of this apparatus, the radio potentiometers, battery circuit switch, and tapping key are previously mounted on a baseboard and panel by the instructor. The students complete the assembly and wire the apparatus according to standard schematic diagram. Typical precipitation and redox titrations are then carried out to check the operation of the instruments.
I n the meantime, the lecture-study of fundamental electronic components and their incorporation into basic vacuum-tube measuring circuits is nearing completion and the student is ready for the laboratory construction of an electron-ray tube titrimeter (Figure 4). Under other circumstances he would be confronted with a difficult assembly job and would have t o follow what would seem to him a confusing and perhaps incomprehensible schematic diagram. Such a procedure would
JOURNAL OF CHEMICAL EDUCATION
440
R ~ - ~ o , 0 0 0 n 2 .1 R,-
I O , O o o n . ZW,
R4.Z70.OOO~, IW
C2- 2 0 MFO.. C L E C . , 450 K O.C.
''
F 6 .I 3L
former, the toggle switch, and the potentiometer "eye" control are mounted on the baseboard and panel and are fitted with Fahnestock clips. Necessary wires, condensers, and fixed resistors are laid out ready for use. Pairs of students, following the schematic diagram of Figure 4, wire the titrimeter to produce a completed setup. Even with the simplificat,ion in circuit design just mentioned, the titrimeter performs admirably when used as a continuous-reading instrument with the ordinary low-resistance electrodes employed in redox and precipitation titrations. I t has a further distinct advantage in its use as a null-point indicator. When coupled to a conventional aotentiometer. the titrimeter with its electron-ray taube replaces the usual galvanometer and allows one to make exceedingly rapid e. m. f. measurements for graphical interpretation. The simple coupling circuit is shown in Figure 5. The regular tapping key of the potentiometer is bypassed or shorted out and a single-pole, double-throw microswitch is used in its place. With the electronray tube or "eye" adjusted to an almost closed position, it is safe to make the initial balance of the potentiometer simply by holding down the microswitch tapping key and adjusting the slidewire until approximate balance is indicated. ' This procedure vould of course, be unthinkable when using a galvanometer as the null instrument. Final balance is indicated by minimum, or no, blinking of the eye when the mirroswitch is depressed and released. Tests of the titrimeter both in its direct-reading and in its potentiometer-coupled form are made by carrying out conventional titrations. Although in the laboratory the ficld of electrometric titrations has seemed especially suitable for an experimental examination of the ideas expressed herein, me try wherever possible to have the student see the similarities in the circuitry and operation of what appear to be unrelated instruments. Thus, before undertaking his rather fundamental studies with a Beckman DU spectrophotometer, we have him assemble a one-tube amplifier (actually a slide-back vacuum-tube voltmeter) as a photometric measuring circuit for the output of the vacuum phototube of a diffraction-grating spectrophotometer. During its construction he finds himself encountering the same simple components, arranged in essentially the same way as he by now is accustomed to finding them. Further, his feeling of confidence and understanding seems to receive impetus when he finds that he is able to make perfectly acceptable and useful measurements with a very simple circuit. Somewhat of an anticlimax are his experiences in assembling and wiring the circuits and apparatus he uses for his work in polarography, conlometry, and so on. ~~
Fzgure 4.
Electron-rag Tube Titrimeter
have no pedagogical value and would better be avoided entirely. Instead, however, he finds an integrated, useful assembly of elements that are now familiar to him. The utility of the diodes, triodes, capacitors, voltage dividers, etc., that he has been studying now assume a real meaning. Our selection of an electron-ray tube titrimeter for construction in student hands is not accidental. The rectifying, amplifying, and measuring circuit-segments involved are those to be found in many other instruments used in analysis. The titrimeter circuit involved here comprises.a direct-coupled d.-c. amplifier in which an electron-ray tube is employed to indicate potential changes a t the indicating electrode of the titration cell. No attempt has been made to design a circuit that would have all of the desirable features to be found in many commercial instruments. Rather, we eliminated complicating refinements aud aimed a t produring a circuit that n a s elementary from the electronic standpoint, yet would familiarize the student with the use of vacuum tubes in electrometric titrations and other operations. Just as for the potentiometer-voltmeter apparatus, and in the interests of time economy, some assembling is done by the instructor before the students come t o the laboratory. The four tube sockets, the filament transMICROSWITCH\
1.0 MFD., 4 5 0 V . D.C /
POTENTIOMETER GALVANOMETER TERMINALS
f '
TITRIMETER ELECTRODE TERMINALS
~
~
VOLUME 33, NO. 9, SEPTEMBER, 1956
We know that in the instructional time at our disposal we are not doing the most effective job possible, but much has been learned to point the way toward further improvement. We are not attempting to turn out electronics experts, but we do believe our students are gaining a better knowledge of the fundamentals of instrumental analysis than mould be the case if we dodged the electronics issue completely.
44 1
ACKNOWLEDGMENT
I t is a pleasure to acknowledge the assistance of Drs.
J. Daniel Bode and David F. Westneat, former students of the writer and laboratory instructors in the instrumental analysis course. Both men were the source of many helpful suggestions, and especial credit is due Dr. Bode for his design of a number of circuits for demonstration and laboratory work.
