JOURNAL OF CHEMICAL EDUCATION
THE RECORDING POTENTIOMETER' Its Use in the Analytical Chemistry Laboratory GALEN W. EWING Union College, Schenectady, New York
Tmm is a long-standing dispute among teachers of instrumental analysis over the relative merits of commercial and home-made apparatus. With a commercial instrument it may be possible for the student to conduct a valid analysis without an adequate appreciation of the design principles of the instrument, whereas if he has assembled his apparatus from its ultimate components he will acquire such an appreciation. On the other hand in a course limited to one day of laboratory per week for a year, many instructors feel that the construction of apparatus by the student consumes too much time which might better be spent in actual chemical work. It is also felt that a course of this type has definite service functions; students must be instructed in the intelligent use of a number of the modern instruments which they are likely to encounter in their suhsequent work, both academic and industrial. The best procedure is to steer a middle course--to give the student some experiments where the emphasis is placed on assembly and testing of apparatus, others which depend on the use of commercial instruments. Presented as part of the Symposium on Problems in the Teaching of Instrumental Analysis before the Division of Chemical Education a t the 128th Meeting of the American Chemical Society, Minneapolis, September, 1955.
It is the purpose of this paper to point out the value of a recording potentiometer in both of these two types of student experiment. The modern strip-chart recording potentiometer is one of the most versatile instruments available. With simple external rircuitry, it can be made to respond to changes in any of the numerous physical properties of substances commonly utilized for analysis. One can record directly not only potentiometric titration curves but also curves of conductometric, amperometric, photometric, and thermometric titrations, current-voltage curves, as in polarography and related techniques, absorption spectra, and cooling curves. I t can also be applied in mauy mays which do not yield direct chemical information, for example in the study of circuits and components, the response of photocells, etc. One advantage to be gained through the use of a recorder is the obvious saving of time and relief from the tedium involved in the manual recording and plotting of data. Even more important, a n automatic recorder rannot exercise judgment: it cannot overlook or fail to record potentially significant data. Hence the inexperienced student has a better chance of making a valid iuterpretation of his results than he
VOLUME 33, NO. 9, SEPTEMBER, 1936
would if his manually-recorded data were taken a t too great intervals of the independent variable, or were faulty in some other respect. Likewise the instructor has an easier time in assisting the student if he can inspect the whole record of the experiment. Not to be overlooked is the advantage in becoming familiar with the recorder itself, an instrument which is becoming increasingly important in industrial laboratories. TYPES OF RECORDERS
Electrical recorders fall into two general classes: deflection and null instruments. The deflection meters, which may be galvanometers, voltmeters, ammeters, wattmeters, etc., are generally less complex in design and can be much the faster in response, following variations of input up to perhaps 100 cycles per second. However their accuracy is generally inferior to null types and they are restricted to narrow recording paper, in many instances with curvilinear coordinates. A major drawback to deflection instruments is the loading effect on the system being measured. I t requires a considerable amount of power to operate the deflection system, and this must be provided by the external circuit connected to the meter. Thus a deflecting millivoltmeter draws a signifirant current, and a deflecting milliammeter causes a significant voltage drop in its circuit. Deflecting recorders which are especially designed for high speed of response are known as recording oscillographs. They are particularly useful in engineering measnremeuts on dynamic system, vibration studies, and the likc, and can also he applied to biological and clinical work, including encephalography and electrocardiography. They are seldom selected for chemical work. Null recorders may be typified by the self-halancing potentiometer shown schematically in Figure 1. The unknown potential, Ez,is connected in series opposition with a potential E, taken from a slide-wire potentiometer. A vibrating chopper is so connected that it throws the two potentials alternately on to the input of an a,-c. amplifier, a t the frequency of the power line. The output of the amplifier energizes one winding of a two-phase motor, the other winding of which is connected to the power line. The motor controls the moving contact on the potentiometer slide wire. At the point of balance, E, = E,, and the amplifier receives no GO-cycle signal from the chopper, hence supplies no 60-cycle output, and the motor is idle. If Ez becomes larger than E,, a proportionate signal is observed and amplified, and the motor turns in such a direction as to increase E, to rebalance the circuit. If E, becomes smaller than E,, a similar actiou takes place, but the output of the amplifier is shifted in phase so that the motor turns in the opposite direction to attain balance. The motor shaft is mechanirally linked to the recording pen causing it to move a distance proportional to the angular displacement of the sliding contact. This is therefore a measure of the unknown potential, E,.
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The working current through the slide wire must be adjusted so that the deflection of the pen agrees with the calibration of scale and paper. This is generally accomplished by means of a standard cell and rheostat. Standardization may he automatic or semiautomatic. Automatic standardization has the advantage of convenience, but in many models the recorder shows a dead spot of several seconds occurring a t regular intervals of from 30 to 60 minutes when the standardization takes place. This is a disadvantage,
Figure 1.
Self-balancingPotentiorn.te.;
Typical Schemetic
as the dead spot may occur a t an important point in the experiment. and give false readings. In the semiautomatic system, standardization with its associated dead spot can be accomplished a t any time by manual control. This is the preferable arrangement except for prolonged experiments of several hours or days. The details of the servo loop and its components vary with the manufacture, but all are similar in principle. It is also possible to construct a null recorder which balances currents rather than potentials, or which balances a Wheatstone bridge rather than a potentiometer. Since these models are for special purposes and are not as flexible a s the potentiometer, they will not be discussed in detail. Circular-chart recorders are not convenient for present purposes and should be avoided. The majority of recorders utilize paper supplied in rolls, either 10 or 11 inches or from four to six inches wide. Such instruments are most commonly operated a t a constant paper speed and hence produce a graph of the input magnitude as the dependent variable against time. Other recorders accept standard graph sheets, in 8.5 X 11- or 11 X 16.5-inch sizes. Two similar servo mechanisms are then required, oue to operate the pen across the X-axis, the second to move the pen (or paper) to control position on the Y-axis. Certain roll-chart recorders also are supplied with two servos, and are no longer time dependent. Instruments which permit external control of both variables are known as "function plotters" or "X-Y recorders." We shall consider first the circuitry needed to match t o the recorder input a number of electrical quantities which are of importance in analytical chemistry, and then the choice of time or other quantities to be taken as the independent variable.
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Figure 2.
Potential Divider.
THE DEPENDENT VARIABLE
Potential. Direct potentials can be recorded with a minimum of intermediate circuitry. Since full-scale deflection of the recorder will usually be only 10 millivolts (or less), and the e. m. f. to be recorded may be many times as great, some provision must be made for bringing the quantity to be measured onto the scale of the instrument. Ideally this should be accomplished by internal changes in the recorder by which the voltage span of the slide wire is varied. This method retains the high input resistance of the recorder, which is the grid-to-cathode resistance of the first vacuum tube, usually greater than 108 ohms. Many models of recorders have provision for this type of range selection, with a series of interchangeable range resistors. It is, however, not usually convenient in standard recorders to effect such changes during the course of a n experiment, although certain models are designed especially to facilitate range control. Another method of adjustment is provided by a precision voltage divider, or "v~lthox."~A voltage
F i w r . 3.
Zero Adjusting Cixuit.
divider can be constructed in either of two ways, as shown in Figure 2. The choice between these circuits depends on a number of factors. I n (a) the resistance seen by the recorder is constant, while the resistance presented to the potential being measured is variable. I n (b) the opposite is true. For proper operation of the recorder, the resistance connected to it should not exceed a few thousand ohms. This condition obviously is met by circuit (a), and it is scarcely a limitation in (b) because the ultimate potential source is necessarily a low-resistance device (electrode cell, thermocouple, photovoltaic cell, etc.) and the overall effective resistance is the ~arallelresistance com2 H ~F. ~K., ~"Electrical ~ ~ Measurements," , John Wiley & Sons, Inc., NewYork, 1952, p. 160.
posed of the ultimate source and the divider. Glass electrodes and other high-resistance sources require preamplification. To minimize loading effects, the current drawn by the divider must be held to a minimum. The circuit, of Figure 2b permits much smaller current drain for the higher sensitivity ranges than does (a), whereas in (a) the current is less for the less sensitive ranges than in ( b ) . Since loading is apt to be more objectionable on high sensitivity ranges, the author feels that circuit (b) is preferable; a total of 100,000 ohms for the divider is satisfactory. If one wishes to record small variations in a relatively large potential, it is desirable to use a divider setting such that the changes in potential are adequately displayed on the chart. This requires provision for balancing out the constant portion of the potential or, in effect, the suppression of the zero-point. Such zero suppression can be achieved either by suitable modification of the potentiometer circuit itself, or by means of a subsidiary battery circuit such as those of Figure 3. The advantage of (b) over (a) is that it provides both coarse and fine zero adjustments. Either will permit positioning of the zero at any point on or off scale, limited only by the voltage of the battery. Zero suppression circuits are built into some recorders, generally the same models provided with range selection. Cuwent. Direct current is easily recorded by means of the potential drop produced in a series resistor. Shunt resistors of a few thousand ohms with a recording potentiometer of 10-millivolts scale permit measurements down to a few microamperes. This provides more than adequate sensitivity for polarography and allied analytical procedures. The range can be varied either by selecting the value of the shunt resistor, or by means of the volt-box described above. The zero suppression circuit of the preceding paragraph will perform the function of the bias control of a polarograph, to permit for example the study of a reduction wave without interference from the wave of a more easily reducible species in the solution. Recorders can be designed to balance an unknown current directly against a measured current drawn from the slide wire through a known dropping resistor. This is probably t o be preferred if the recorder is to be used solely for current measurement, but is not as versatile a laboratory tool as is the basic potentiometer. Current recording can also be employed with harrierlayer and other photocells, and can he used for resistance measurement in conjunction with an external potential source. Resistance. The most direct manner of recording resistance is by means of a self-balancing Wheatstone bridge. This is another specialized instrument. The potentiometer can be adapted for resistance work either by means of a current shunt and external source of potential or by a Wheatstone bridge used in an unbalanced condition. The latter arrangement is especially convenient when small changes in resistance
VOLUME 33, NO. 9, SEPTEMBER, 1996
are being studied. The Wheatstone bridge, which may be a self-contained portable unit or may be fabricated from components, is adjusted near to its balance point manually, using the recorder as a galvanometer. Then the unbalance potential is recorded as an indication of any changes in the resistance of the sensitive element. Such measurements are of interest for following temperature changes with a resistance thermometer (e. g., a thermistor) in photometric studies with photoconductive cells, in moisture determination, in gas analysis by thermal conductivity, etc. For electrolytic resistance (or conductivity) the bridge may be energized with alternating current (for example at 60 or 1000 cycles per second) and the output rectified for application to the recorder. A germanium diode or small copper oxide rectifier followed by a stage of RC filtering is the simplest type of rectifying circuit, and is satisfactory except at low voltages (less than one volt) where this kind of rectifier loses its effectiveness. For conductometric titration this simple arrangement has been found satisfactory if the bridge is kept unbalanced and only changes recorded. For applications requiring precise balancing of the Wheatstone bridge, a phase-sensitive detector must be substituted8 (Figure 4). The a.-c. signal to be rectified, &, is impressed between the center taps of a transformer secondary and of a fixed resistor. An auxiliary a.-c. voltage is applied to the primary of the transformer, and the recorder is connected across the resistor. Two identical rectifier units complete the circuit. The instantaneous polarity of the voltage developed in the transformer secondary will determine which of the rectifiers is operating a t any instant. The signal potential aids the transformer potential on one half cycle, opposes it on the next, according to its phase. Since the output of an a.-c. bridge changes vhase as the bridee vasses through balance, the d.-c. potential deliverez tb the record& will pas's through zero at the balance point. The auxiliary voltage must be supplied from the same source aa the bridge itself to ensure proper phase relations. It must be considerably larger than Ex in magnitude. I n Figure 5 , curve a shows the approach to zero at balance for a simple dry rectifier, curve 6 the response of the phase detector. Both curves give the response of the recorder as the variable arm of the bridge is driven linearly through the balance point. THE INDEPENDENT VARIABLE
Time. Time is the most convenient axis against which to display the behavior of physical quantities, as one need only select the proper speed of a strip chart recorder and let it operate, while concentrating one's attention on the remaining variable. However, in the majority of analytical methods the quantity which logically would be selected as the independent variable is not time, but volume (in conventional titration), quantity of electricity or other electrical parameter, or possibly temperature. It is therefore fre-
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quently necessary to take special steps to cause some quantity which is the logical independent variable to vary over its whole range a t a constant time rate. There are important situations where time is the logical independent variable. This will often he true in applications of reaction rates to analytical problems.' I n some instances transitory phenomena of analytical interest may occur following an abrnpt change in experimental conditions and representing a gradual attainment of equilibrium through some relatively slow process, such as diffusion. An example is the chronopotentiometric method.$*'
Volume. Following the course of a volumetric titration is one of the most fmitful applications of the recorder. The ordinary buret, however, cannot be used conveniently because diminishing hydrostatic head slows down the rate of delivery too greatly. One way to avoid this difficulty is to use a syringe type of buret driven by a motor and suitable lead screw. Another way is to provide a Mariotte bottle as constant head device to ensure constant flow of reagent.' s I n neither of these arrangements can volumes be recorded per se, and standardization is best accomplished by an over-all calibration procedure. Thus a weighed -See, far example, LEVY.G.B., Anal. C h . , 23, 1089 (1951). REILLEY. .- .. . C.... N., . ..G . W. EVERETT, AND R. H. JOHNS, Anal.
C1210(1955,, h e ~ ; $ ~ ~ ~ i ; " , !W,. AND
,
.
SHAFIRO, I,.,
AND
G, SCRIBNER, Anal, Chem,, 2,,
W. W. BRANNOCK, Anal. ~h:hem.,27,725
(1955).
TAYLOR, J . K.,
1576 (1949).
AND
E. ESCUDERO-MOLINS, Anal. Chm~.,21,
JOURNAL OF CHEMICAL EDUCATION
CELL
Eg"n6. T-pad cizcuit for current s..nning
Values are selected for 0-20 milliampere current range.
sample of primary standard material titrated with a given solution (its concentration need not he known accurately) a t a given rate of flow or motor speed is found by experiment to produce a deflection on the recorder of so many centimeters. Then unknowns titrated under similar conditions will produce deflections proportional to their active content. Pedagogically this is a favorable situation, for it emphasizes the point that our measuring instruments and techniques actually function to compare unknowns with primary standards. Quantity of Electricity. Volumetric titration can in many instances he replaced to advantage by conlometric titration where the chemical reagent is generated electrolytically. The end point of the titration is still identified by observation of one of the usual physical
Fig"..
7.
Schematic Drawing of Complete circuit for the Potentiometer Adet,t,ter
quantities: color, pH, diffusion current, etc. A recorded titration curve should be displayed against J I dt as independent variahle. This is convenient only if the current is held constant and the chart operated a t constant speed. If the current cannot be held constant, the recorder can be used to advantage to plot current as a function of time. The desired integral is then the area under the curve from the heginning of the reaction up to the end point, and may be determined graphically. I n some systems the eauivalance ~ o i n will t be evident as the uoint where the electrolysis current becomes zero or negligible. Otherwise, if a titration curve is .desired in addition to
the coulometric plot, two recorders will be required, or better a single recorder with two channels so that both curves are obtained plotted against time on the same chart. Current or Voltage. In voltammetric analysis (of which polarography represents a special case) one may wish to plot either the current through a cell as a fnnction of the applied potential, or conversely the potential developed across the cell as the current passing through it is ~ a r i e d . ~ The linear variation of applied potential presents no great difficulty, as a simple motor-driven slide wire across a source of fixed potential suffices. A precision 10-turn potentiometer is well suited to this application. Linear scanning of current involves slightly greater complexity. The simplest method is to employ a motor-driven rheostat in series with a dry-cell source; this will be satisfactory if the internal resistance of the cell under study changes hut little. Change in the effective resistance of the cell, however, is generally the parameter being measured; this simple circuit may not be satisfactory. This fault is best eliminated by incorporating into the circuit a combination of three identical variahle resistors on a single shaft, known as a T-pad attennator (see Figure 6).1°, l 1 The three units are so connected that as R1 increases, R2, and R3 decrease. The input resistance is R1 Rz and the output resistance is R1 Ry; each of these quantities remains constant as the shaft is rotated. The output voltage varies with R,, and as it is working into a circuit of constant resistance, the result is a uniform variation of current with time, if the unit is driven by a constant speed motor. The variations of the resistance of the measuring cell are completely negligible in this circuit. Optical Properties. Many variables other than electrical can be driven a t constant rate. An important example is wave-length scanning in a spectrophotometer or spectrograph operating in any region from X-rays to infrared. A general purpose recorder can be utilized, for example, with the Beckman DU Quartz Spectrophotometer by means of Beckman's special recording attachment.12 The recorder can also he adapted to serve as a. densitometer for measuring the optical density of spectrograph plates. A motor-driven stage or plate holder and a simple optical system and photocell are required as accessories.
+
+
STUDENT APPLICATIONS
Of the 33 experiments detailed in the author's
* DELAAAY, P., "New Instrumental Methods in Electroehemistry," Interscience Publishers, Inc., New Yark, 1954, Chap. 1. TERMAN,F. E., AND J. M. PEWIT,"Electronic Moasurernents," McGraw-Hill Book Co., Ine., New York, 1952, p. 641 El. Available, for example, from Technology Instrument Corp., Acton, Mass., also Shallcross Mfg. Co., Collingdille, Pa. "Beckman Division, Beokmm Instruments, Ino., Fullerton, Calif., Bulletin 380.
"
VOLUME 33, NO. 9, SEPTEMBER, 1956
t e ~ t h o o k , 'the ~ recorder can he used directly in 12,14 plus two more if the Beckman recording accessory he availahle. Its application to the experiment,^ on elertrometric methods needs no further elaboration. The recorder has been particularly valuahle in t,he experiment on coulometrir titration to monitor the amperometric end-point system. I t can also he used to advantage with the thermal conductivity apparatus for gas analysis. I t should prove possible to devise a simple filter photometric assembly to monitor the cohalt and nickel effluent from the ion-exchange column. The recorder is also useful in the college lahorat,ory in various experiments uot primarily analytical. I t is ideal for the study of the properties of electrical circuit,^. The long-term stahility of an amplifier, for example, can he determined by recording the output a t constant input with a slow chart speed over a period of hours or days. Fatigue in a harrier-layer photocell is easily demonstrated, as is the effectiveness of halanced two-cell circuits in its elimination. Its applicability in the physical chemistry laboratory in kinetic studies and in phase-rule work requiring rooling curves is evident. EXTERNAL CIRCUIT PANEL
three-posit,ion switch het,ween the bridge ratio arms determines the multiplyiug factor. The phase-sensitive det,ector for a.-c. bridge measurements is out of the cirruit when not in use. The voltage divider for sensitivity selection is composed of a bank of seven resistors t,o provide sensitivity factors of 1, 0.5, 0.1, 0105, 0.01, 0.005, 0.001, as well a s a shorting position. I t is import,ant t,hat all resistors he precision made, with lowtemperature coefficients aud minimum aging effect,^. This uuit may conveniently he mouuted in a small met,al rahinet and bolted permanently to the rase of the recorder. Figure 8 shows this unit in operation during t.he recording of a poteutiometric titration.Is COMMERCIALLY AVAILABLE RECORDERS
Following is a tahulation of all American ma14~1farturers of recording self-halancing pot,entiomet,ers (circular-chart instrument,^ are not considered). A brief description of earh is given; further informat,ion should be obtained from the respective firms. Applied Physics Co~poration, Pasadena, Cal
