ELECTRICAL MEASURING INSTRUMENTS FOR USE IN TEACHING ELECTROCHEMISTRY*
Electrochemistry is a comparatively new subject in the college cumculum. During the past few years there has been a gradual tendency to place the majority of such scientific subjects on a more practical basis than heretofore, and electrochemistry has been among the foremost of these subjects. The tendency has been not to eliminate the theoretical side of the subjects, but to include practical applications of the theory in illustration thereof. However, the introduction of practical applications has not yet fully extended to the electrical instruments which are used in the laboratory experiments. The tendency still appears to be to purchase and use instruments which are adaptable for several different services, with all external connections, in order to teach the fundamentals of the potentiometer, the Wheatstone bridge, etc., rather than instruments which are sufficiently practical to find actual use in the industries themselves. Potentiometers The so-called "Student's Potentiometer" can be used for temperature measurements with thermocouples, or for H-ion measurements, and is also adaptable as a Wheatstone bridge. It is probably on account of its adaptability that this instrument is one of the most frequently found instruments in the electrochemical laboratgries. This instrument, with the external connections required to make potentiometric measurements, is shown in Figure 1.' The "Type K potentiometer, a far more accurate instrument, is shown in Figure 2. In many electrochemical laboratory courses, a student is given one of these instruments, a buffer solution and two or three Hildebrand hydrogen electrodes, and is then expected to obtain checks out to approximately four places. It is quite evident, from the connections shown in these figures, that such an experiment consists in an exercise of making electrical connections, rather than in the practical use of H-ion measurements. There is probably no better method of teaching the principle of a potentiometer, but this should have been already learned by the students from previous courses in more closely related subjects. When a student has completed an experiment of this nature, he is generally skeptical about
* Presented before the Fifty-fifth General Meeting of the American Electrochemical Soeiety, held at Toronto, Canada, May 27, 28, and 29, 1929. ' Figures 1, 2, 3, 4, 5, 6, and 7 are reproduced by courtesy of Leeds & Northrup Co.. Philadelphia. 69
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STUDENTS' POTENTIOMETER
H-ion measurements in general, and is convinced that their proper place is anywhere but in an industrial laboratory. There are several other practical potentiometers in the market, which are adapted for industrial use. In Figure 3 is shown the "Portable Acidity Meter," which has a self-contained battery and standard cell. This instrument is provided with two scales, one reading from 0 to 1.6 volts and the other directly in pH, for use with the hydrogen electrode. In Figure 4 is shown the popular "pH indicator." This instrument has a self-contained standard cell, and has a voltage scale especially adapted for use with the quinhydrone electrodk In between these two instruments there is the "Hydrogen-Ion Potentiometer,'' which is direct reading
TYPE K POTENTIOMETER
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but not portable, and the "Portable Potentiometer," which is portable but not direct reading. In addition there is the "H-ion Field Kit" shown in Figure 5. This instrument is entirely self-contained, including all necessary chemicals, etc. It is direct reading in pH, for both the quinhydrone and the hydrogen electrodes. A description of the operation of this instrument has recently been publi~hed.~
Wheatstone Bridges The conventional meter slide wire Wheatstone bridge is too well known to require a description. This is another instrument whose use is more in the nature of a study of electrical connections than in the practical use of the measurements. The use of buzzer and telephone receivers makes this instrument unsuited for most industrial purposes. Several Wheatstone bridges are now on the market which make use of the 60-cycle lighting circuit as a source of supply, and an alternating Parker, Ind. Eng. Chem., 20, 676 (1928).
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current galvanometer as a detector. In Figure 6 is shown the face of a newly developed bridge, which is very convenient for working out new applications of conductance measureyents. It is called the mho-ohm bridge because it will measure in either of these units according to the position of the two reversing plugs. This bridge measures from 1 to 12,000 mhos. ohms or from 0.1 to 1 X In taking a measurement, the conductivity cell is attached to the posts marked "cell." Current is supplied to the posts marked "110 volts," 60 or 25 cycles. The two keys are depressed, and the galvanometer is brought to balance by shifting the single plug, and finally by turning the slide wire. The reading in either mhos or ohms is found by multiplying the dial reading by the proper ratio found at the base of the single plug. A reciprocal reading may be taken as a check by reversing the position of the double plug and rebalancing the bridge. The advantage of using a bridge which is direct reading in mhos, or relative conductance, is readily appreciated when it is mentioned that the relation between conductance and concentration is nearly linear for a large number of industrial solutions. In many cases these readings may be used directly as a measure of concentrations. Figure 7 shows another recently developed bridge, whose operation has
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not been described previously. This bridge was designed especially for measuring ash in sugar, but may be used for any other type of measurement as well. This particular bridge is suitable for use with a cell whose constant is close to 0.15 cm. -'and may then be calibrated to read conductivity (specific conductance) directly. It is also provided with hand temperature compensation, to reduce all readings to the conductivity a t 20% The temperature compensation dial is correct for solutions having a temperature coefficient of 2.19 per cent per degree C. This coefficient is about the average value obtained for dilute salt solutions a t room temperatures. All the measurements made a t 20°C. are correct, regardless of the temperature coefficient, and, if all measurements are made in the neighborhood of this temperature, no appreciable error will be introduced, even with solutions having a temperature coefficient differing considerably from the value of 2.19 per cent. The bridge is calibrated by introducing a solution of potassium chloride containing 0.581 gram per liter, into the cell. The conductivity of this solution, a t 20°C., is exactly 0.001, and this value is set up on the bridge. The keys are depressed and the bridge is balanced by turning the temperature compensation dial. The two screws found near the center of the dial are loosened, which releases the outer disc, upon which is cngravcd the
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cell constant and temperature scales. This outer disc is then turned, independently, to read the temperature of the standard solution, as indicated by the thermometer in the cell. The screws are again tightened and the bridge is calibrated. Subsequent calibration should be required not more frequently than once in six months. The cell constant may be read directly from the corresponding scale. Subsequent measurements are made by first turning the compensator to read the temperature of the solution in the cell, and then balancing the bridge by turning the slide wire and moving the plug. By reversing the single-pole switch, readings may be taken in conductance units, expressed in reciprocal megohms. These readings depend on the temperature and cell constant-factors wbich are eliminated in the readings of the conductivity. In Figure 8 is shown the bridge diagram of the sugar ash bridge. It
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,@a. Field 26u FIGURE 8.-CONNECTIONS OP THE SUGAR ASH BRIDGE will be noted that this bridge is so connected that, by moving the main slide wire, the resistance of only one bridge arm is altered. This permits the placing of the temperature and cell constant compensation dial in the bridge arm, c, while still permitting ratio coils in the opposite arm of the bridge. It would be possible to simplify the design slightly, provided a conductivity cell could be found which would retain its calibration indefinitely. However, the electrodes of such a cell would always be subject to accidental displacement, and cell constants are frequently found to drift over a period of years. Such changes in the cell constant can readily be compensated for, in the sugar ash bridge, by recalibration, while the other type would continuously give inaccurate readings. The bridge shown in Figure 7 is suitable only for solutions which can be
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FIGURE 9.-Tm YOUDEN HYDROGEN-ION APPARATUS measured with a cell having a constant of 0.15. This includes solutions from about 1 per cent salt concentrations to those representing an average grade of distilled water. Bridges for usa with other cells, having constants above and below this value, will undoubtedly be supplied as soon as the demand is created. Miscellaneous Instruments The kouden H-Ion Concentration Apparatus.-In Figure 9 is shown a newly developed, popular-priced instrument for measuring H-ion. concentrations. It is designed primarily for laboratory and field work, and in one form is made portable. Simplicity and rapidity of action are claimed for it. The wiring of the instrument is based on Hildebrand's method of using a galvanometer as a detector for balancing an auxiliary e. m. f. against that of the cell, and then measuring the voltage of the former by means of a millivoltmeter (range 0 to 300 mv.). By this method the voltmeter draws no current from the H-ion cell. This instrument is designed for use with two quinhydrone electrodes, one of which is used as the reference electrode. This latter cell is made up with a standard phthalate buffer solution with a pH of 3.98. The pH of solutions in the other cell is determined by measuring the voltage
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difference (plus or minus), and then reducing to pH values by referring to a table (Pinkhoff method). By the use of the special reference electrode described above, a calomel cell is eliminated. It is recommended that the reference electrode be made up freshly after about 20 measurements. The Palo Acid-Alkalimeter.-Another recently developed instrument is shown in Figure 10. This instrument is essentially a vacuum tube voltmeter, adapted to H-ion measurements. The connections have been improved considerably since Goode3first published a connection diagram, 3. Am. Chcm. Soc., 44,26 (1922)
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and the operation of the instrument is now entirely practical. It is being used for both plant control and laboratory work. Unlike the ordinary voltmeter, the readings obtained are independent of the internal resistance of the cell, and may be made a t a considerable distance from the cell. The instrument cannot readily be made portable, due to requirements in the way of " A and "B" batteries. It is probably best suited for making continuous readings as an indicator, since it is not necessary to turn a slide wire to obtain a reading, as i~ the potentiometer indicator. The final reading is taken on a voltmeter having a range from 0 to 1.5 volts, The same meter also acts as armilliammeter. The indicating meter has a special calibration, so that the instrument can be made direct reading in pH for any type of electrode, including the quinhydrone and hydrogen electrodes. The Haring Cell.-An instrument designed especially for use in the industrial electrochemical laboratory is shown in Figure 11. The Haring cell is essentially a practical instrument, but i t is also well adapted for teaching the fundamentals of such measurements as resistivity, throwing power, and polarization.' The auxiliary equipment required for these measurements are an ammeter, a variable rheostat and either a voltmeter or a potentiometer. One of the potentiometers previously described could readily be used in connection with this cell and, if desired, in a comparative measurement of polarization by the half-cell method.
Discussion The Harinz- cell, on account of its ada~tabilitvfor teachine" the funda-
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Haring, Trans. Am. Electrochen. Soc., 49, 417 (1926), and Haring and Blum, Ibid., 44, 313 (1923).
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mentals of electrochemistry, will doubtless soon find a place in every laboratory where this subject is taught. This will probably be one of the first scientific instruments which pupils will endeavor to have introduced into the technical laboratories. Some of the other instruments which have been described, for measuring H-ion concentrations and conductivity, are just as suitable for industrial measurements, and they are gradually coming into favor for such use. I t appears highly desirable that the pupils in electrochemistry become familiar with these instruments, in order to he familiar with their use when they enter technical lahoratories, as well a? to encourage their introduction. When instruments have been made to he direct reading in the desired units, and when their manipulation is simple, they are no longer laboratory playthings but practical equipment for the industrial laboratory.
