Nicci 1. Rossley and Gibson W. Higginsl
hexpensive Research Quality
Memphis State University Memphis, Tennessee 38152
Student Polarograph
In a recent article ( I ) the "abundance" of polarographic instruments in the typical laboratory was referred to. Although the practice of polarography is both widespread and increasing in relative analytical importance (2% apparatus available is either obsolete or else is research quality in terms of price, availability, and ease of operation. Little attention has been paid to production of equipment which is up-to-date in design and performance, and at the same time inexpensive to construct and very simple to operate; that is, the kind one would like for an undergraduate student. to be able to use. In an introductory experiment the student rarely gets past the point of scanning a portion of an energy region to produce a spectrum (in this case a polarogram). Little if any need exists for various offsets, compensation functions, and bipolar scans. The more of these available, the more complicated and expensive the apparatus becomes. In addition, use of polarographic experiments is frequently limited by scheduling problems, due to the amount of time per student required on the (typically) one availahle instrument. With these thoughts in mind, we have constructed a polarograph which is extremely inexpensive ($33) and simple to operate. It may he assembled in a "bare minimum" configuration, or extra features may he added at the user's discretion. The design is one of the standard three-electrode configurations currently in widespread use (3%the principle difference being that integrated-circuit operational amplifiers (model 741) costing $0.80 each were used as the active circuit elements. The three-electrode configuration has been proven to minimize uncompensated resistance in solution, thus making possible polarographic studies with mixed and nonaqueous solvents. Such experiments are well within the range of the undergraduate student, provided that the apparatus is available. I t is recognized that one must still use a good quality power supply, plus some means of reading out the signal, such as a voltmeter or recorder. Since the circuit has a normal electrical ground and the output does not float, i t may he used with any available potentiometric recorder having a sensitivity of 1-10 mV full scale. Multipurpose recorders are usually available and may be borrowed for the experiments. Both strip-chart and X-Y recorders have been successfully used. The power supply must he a dual regulated *12-20 V supply capable of furnishing at least *60 mA per polarograph. Since most commercially available dual regulated units give a few hundred milliamps output, several polarographs may be operated from a single supply. A breakdown of costs is presented in the table. Not included is the time required for assembly. This may be done fairly rapidly by an experienced person, or it may he assigned to individual students as an electronics project. Construction and testing of one unit should take about
Costs of Components Used in Bare Minimum Polarograph
Quantity RCA C A 3741 operational amplifier 4 4 8ICS plug-in sockets 6 potentiometers 1 1P12 throw rotary switch 1 3P7 throw rotary switch 6 ' / z W, 1%tolerance, resistors 1 10 f l oil-filled capacitor 1 lPlT toggle switch 5 W, 10% tolerance resistors 1 vectorboard section (optional) 1 Cu-2109A metal box
3.20 2.76 6.00 3.00 5.50 4.20 3.50 1.00 0.50 0.50 3.25 33.41
25-35 hr total, for an inexperienced person. The basic circuit is given in Figure lA, as the "bare minimum" configuration with four amplifiers. One prominent text (4) gives another circuit for the same purpose. The major difference is that amplifier V is eliminated, meaning that it is not possible to monitor the potential of the working electrode at the terminal labelled E..c. This is an advantage, particularly when an X-Y recorder is used, and therefore most workers prefer a four-amplifier configuration. With either circuit i t is possible to scan cathodically from zero volts versus the reference electrode potential. Switch selected values for RI allow adjustment of the output voltage of the current follower to match variations in cell cur-
C
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B.
Figure 'To whom correspondence should be addressed.
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1. Bare minimum three-dectrade
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polarograph. A) General sch?-
matic: 6 ) control loop switching diagram.
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Figure 2, initial potential ~ontrolcircuit. A ) Bare minimum: B) calibrated. direct readout.
rent and readout sensitivity. The cell switching circuit, omitted from Figure 1A for clarity, is detailed in Figure 1B. The value of R3 is not critical, but should be in the range of 1-10 kQ. The R3 resistor labelled D is the dummy cell. For precision recorder calibration this resistor should have 1% tolerance or better. Additional circuit functions may be added as desired. These may be either of the "bare minimum" variety or more luxurious, depending on the needs of the user. For example, the initial potential control can be either a one turn potentiometer with no calibration as in Figure 2A, or i t may be a multiturn device with calibration so that it reads to the nearest 1 or 2 mV, as in Figure 2B. The simple version costs about $2, the deluxe about $30-50, depending on choice of components. Other than elimination of "extra" features not absolutely required to obtain data, there is no compromise in component quality. The use of 1% tolerance resistors in the current follower loop allows faithful scaling of the output current to within the range of normal polarograpbic uncertainty. The advantage of the model 741 amplifier is that it contains built-in frequency compensation, meaning that no effort is required of the builder to achieve stability of the control loop current and voltage. The service hook-up for each amplifier is exactly as designated in the data sheet supplied by the manufacturer. This includes provision for balancing (see below). The noninverting input is hooked to the ground except in the voltage follower, in which case i t is the input. Since the entire polamgraph only draws about 60 mA, several units may be operated from one supply, thereby achieving great overall savings, and a t the same time eliminating the "bottle neck" that often occurs in student laboratories. If more than one polarograph is operated from the same power supply, each one should be decoupled with two large (50-100 pF) electrolytic capacitors. These should be placed as near as possible to the amplifiers, and should be attached so that one leads from
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Figure 3. Polarogram 014 X
352
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S.C.E.
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F lead in 1 F KN03.
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the ground to each of the power supply leads. Each amplifier must be "balanced," that is, adjusted to give zem output when its input signal is zero. In keeping with the bare minimum philosophy, no switching provision is included for carrying this out directly, so it must he done indirectly as follows. Switch to the Standby position. Attach the readout device (voltmeter or recorder) first to the potentiostat (E.,t) and turn its balance control until 0 V is read on the most sensitive scale. Attach the readout to the current follower (I& and repeat the procedure for the current follower with its gain set to the highest resistance. Switch to Dummy Cell and balance the voltage follower as in the step above. Switch back to Standby and the instrument is ready to use. Additional savings and wiring simplicity are realized by this approach to balancing. The solid-state amplifiers are reasonably stable to drift after a short warm-UD~ e r i o d .obviating the need for extreme and expensive measures such as ccopper stabilization. In addition, for simple student experiments. balancing is relative, so that either the recorder zero o; amplifier zero will suffice. If switch selected balancing is desired, the reader should consult the article by Enke (3). Results
At a power supply level of 15 V, each amplifier is capable of supplying approximately +13 mA, so that the cell current may vary between zero and the limits +13 mA, although the actual cell current will be determined by the total voltage applied a t point S (and measured a t Eout), and the contents of the cell. This amount of output is sufficient not only for polarograpby, hut also for rapid scan voltammetry, and even coulometry under certain conditions. The control loop was found to be linear within these limits, to the extent that a given input voltage gave the same output voltage a t Eo,t to the nearest mV, and the current through a resistive load (dummy cell) obeyed Ohm's law. A slight curvature was noted in the ramp produced by the integrator, but it was so slight as to cause negligible voltage error even with the strip-chart recorder. A series of lead in potassium nitrate solutions was polarograpbed in the three electrode mode with an agar bridge SCE (5) and a platinum wire counter electrode. A representative polarogram is shown in Figure 3. A plot of net diffusion current versus concentration is given in Figure 4. A plot of log (Id - I ) / I versus E is given in Figure 5. These data prove the validity of the use of the inexpensive, integrated-circuit amplifiers. Additional tests were run with the polarographic cell
Figure 4. Plot of diffusion current versus concentration for lead(l1) in potassium nitrate.
E ~ i g u r e5. plot of log(ld
VERSUS
SCE.
- 1 ) / 1 versus Potential.
described above. A small variable square wave of W0.2 V was introduced a t Ern and the resulting response was observed with an mcilloscope connected to the current follower output in place of the recorder readout. Figure 6A shows the cell current pattern seen on the screen when the potential is set so that no reduction was occurring. As expected, the current rises rapidly and then decays to zero as charging of the electrical double layer is completed. Figure 6B shows the cell current pattern when the potential is set in the limiting current region of the dc polaromam: The double layer charging current decays, leaving only the Faradaic component. In order to actually perform square wave polarography, it would be necessary to use an electronic device to sample the net current at some fixed point such as b, when the charging current has vanished. These waveforms amee with expectations up to about 1 kHz, depending on t h e square wave magnitude, meaning that the instmment is suitable for "routine" square wave polarography as constructed. By inference, i t is also suited for ac polarography using a sine wave of the same characteristics. since the band Dass requirements for a sine wave are lessstringent. A hig6 gain amplifier (factor of 1&100) and a full wave rectifier (diode bridge) could be connected to lour to allow recording of the ac current, or an ac millivoltmeter could be attached directly to lout. The - - - ~effect - - - ~ of resistance added in series with the reference electrode was measured. Values greater than 350 Q caused oscillation of the control loop when the square wave was applied. In conventional dc polarography, no ef-
Figure 6. Response t o 5 0 0 Hz S q u a r e wave. A) E = - 0 . 1 0 V versus S.C.E. (background): B) E = -080 V versus S . C . E . (limiting current region).
fect was noticed even when as much as 10 kQ was used. The slope of the log plot for the 1.00 x 10-3F lead solution was 0.0279, giving a value of -2 for the number of electrons. The instrument was also o ~ e r a t e din the two.--. electrode mode by shorting the reflrence and counter electrodes. E x ~ e c t e dbehavior was observed, including the decrease in slope of the rising wave as the uncompensated resistance was made larger. These experiments prove that very inexpensive solidstate operational amplifiers can be used to construct good quality routine instrumentation, and that the total cost can be made very nominal and the operation very simple by elimination of unnecessary functions. Literature Cited ~~
(1) Nichoinon, R. S., A w l . Cham., 44 (5). 478R (1972). (2) Flata.J.B.,Anol. Cham.. 44 (11),75A(1972). (3) Enke. C. G., andBauter. R.A.. J . CHEM. EDUC., 41 (4). 203 (19M). (4) Ewing. G. W., "Instrumental Methods of Chemical Analysis.'' 3rd Ed., MeGrawHiliBoak Co.. New York, p. 293. ( 5 ) His@is, G. W.,J Eieetroehm. Soe., (inp-).
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