A SIMPLIFIED MAGNETIC ROTATION APPARATUS
0
THOMAS N. DODD, Jr. St. Peter's College, Jersey City, New Jersey M o m analytic laboratories are well equipped with modern instruments. Spectrographs, spectrophotometers, vapor chromatographs, etc., are now available even to undergraduate students. However, a few experimental measurements are still badly neglected because the necessary instruments are not obtainable and must be constructed by the user. For example magnetic rotation, which is as characteristic of a substance as density or refractive index, is seldom determined because the usual setup is difficult t o build and use. This paper describes a simplified apparatus which can easily be assembled from equipment available in most laboratories. This apparatus was d e signed for use in a senior course in Instrumental Analysis, and it gives satisfactory results when operated by students. It is hoped that this article will stimulate more interest in magnetic rotation measurements. THE FARADAY EFFECT
A detailed treatment of the Faraday Effect is given by Partington (1). Only a few fundamentals will be discussed here. If wire is wound around a polarimeter tube as shown in Figure 1, and direct current is sent through the wire to produce magnetic lines of force parallel to t,he tube axis, then any transparent
and parallel to the tube axis. The Faraday Effect will be positive (dextro) if the current flows clockwise around the tube when viewed by the observer as shown in Figure 1. Reversing either the current or the tube will produce negative rotation. This holds for all substances except those, such as iron solutions, which are paramagnetic. The field strength, H, is always proportional to the current that induces it. Thus equation (1) can be reduced to where a is the magnetic rotation in degrees, V is the Verdet constant of the substance in the tube, I is the amperes flowing through the coil, and K is a constant for the particular tube used. Its dimensions are Verdet units/ deg./ amp. (4). The tube constant can be determined by using water as a primary standard since V of water has been carefully measured and found to he 0.01307 min./cm. oersted at 25°C. using sodium light (6). Verdet constants vary considerably with the wave length of light used, but their change with temperature is relatively small. If the same current is always used during the measurements the Verdet constant of any liquid may be calculated as follows: V
=
0.01307 a / m x n o
(4
where a! is the magnetic rotation of the liquid a t 25'C. using sodium light, and orafl is the same quantity for water measured under identical conditions. THE POLARIMETER TUBE
+ Figure 1.
Pol-meter
Setup w i t h Wir. Wound Amvnd Tuba to pro. duce Magnetic Rotation
substance placed in the tube will rotate plane polarized light. This magnetic rotation during transmission is characteristic of the material in the tube. It was discovered by Faraday (8) and is known as the Faraday Effect. Even optically inactive liquids, such as water or carbon tetrachloride, show definite magnetic rotations; and optically active liquids, such as sugar solutions, show Faraday Effects in addition t o their natural optical activities. Faraday and Verdet (5) found the angle of magnetic rotation to be given by the equation x = V1H
A standard 200-mm. polarimeter tube having the dimensions shown in Figure 2 was selected for the experiment. The screw ends were removed, the cover glasses taken out, and a second rnbber washer was placed in each end cap in addition to the original one. A thin fiber washer was put over each end of the tube to support the coil as shown in the figure, and the caps were replaced. The fiber washers were spread apart until they rested against the screw ends. Then they were secured to the tube with plastic electrical tape. Finally the portions of the tube between the
(1)
where x is the rotation in minutes, V is the Verdet constant of the substance in the tube, 1 is the length of the tube in centimeters, and H is the average magnetic field strength in oersteds throughout the tube
Fi-
2. Detail. of the m-mm. P.,larimarim,t.
Tub.
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two washers were wrapped with a single layer of the same tape. The tuhe was now mounted on a lathe, one end cap being held in the lathe chuck and the other on a live center placed in the tailstock spindle. The tool post and compound rest were removed, and a spool of number 28 gauge plain enameled copper wire was fixed at the proper height on a vertical rod attached to the cross slide. The wire was wound on the tube by rotating the lathe spindle toward the operator; the reverse direction would have loosened the end caps. A very uniform coil was produced by the automatic feed which moved the spool of wire lengthwise along the tuhe during the winding process. About 2500 feet of wire were required, and 11,640 turns mere recorded on a counter attached to the spindle. The tube was removed from the lathe, the screw ends taken off, and the coil wrapped with plastic tape for protection. Then the leads were burned to remove enamel, and attached to an ordinary line cord secured to the roil with tape. Finally a cover glass was placed in each end cap in addition to the two rubber washers, and the thickness of the cover glass prevented the cap from touching the coil when screwed hack onto the tube. Since the coil had the same diameter as the end caps the tube could he placed into a standard size polarimeter. POWER SUPPLY AND MEASURING CIRCUIT
The polarimeter coil was measured with a Wheatstone bridge and found to have a resistance of 161 ohms a t 22'C. One may connect this coil to any source of direct current which can be accurately adjusted from &I15 volts and deliver 0-700 ma. The power supply and measuring circuit shown in Figure 3 can be assembled from equipment available in most laboratories. The constant voltage transformer provides a relatively stable current and isolates the system from the line. This transformer may be left out, hut its omission will make it impossible for one to ground the potentiometer. The powerstat delivers 0-135 a,-c. volts to the rectifier which in turn will deliver 0-120 d.-c. volts to the system. No filter was used to smooth the pulsating direct current because the lack of such a filter does not hinder the measurements. I n fact a filter would make the electrical system a little less stable and more difficult to adjust. The current flowing through the polarimeter coil can be indirectly measured with a potentiometer. If RI of Figure 3 is exactly 1 ohm then the amperes flowing through RI (and the tube in series with it) will be equal to the voltage developed across R,. This voltage, which is numerically equal to the tube current, is measured with the potentiometer, and the latter should be sensitive enough to detect an error of 0.1 mv. However the exact value of the current is unimportant. It is merely necessary t,o have a current which can be accurately reproduced during each measurement. Thus Rl may be an inexpensive 1 ohm resistor having a + ly0error even though the current is controlled to one part in a thousand. R should havc a 10-wat,t capacity so that the half watt sent through it will not heat it and increase its resistance. The vernier control, R2,is included to facilitate current VOLUME 34, NO. 9, SEPTEMBER, 1957
CONSTAKI VOUAGE TRANSFORMER
POTENTIOMETER SETUP
Parts List for Electrical Circuit A. Milliammeter 1CL750 ma. d.c.) F. Fuse (1 amp., 250 v.) GR. General Eleetrie Co. germanium rectifier stack, single phase bridge (input 140 v. ax.; output 125 v. d.c., 1 amp-) Complete potentiometer setup including standard cell and P. galvanometer eapahle of detecting a 0.1 mv. error PI'. Polsrised plug and rereptack R,. Resistor (1 ohm, 10 watts, wire wound, +I%) Rheostat (6 ohms. 25 watts, wire wound) R.. Ra. ~olaiimetkrtube (161 ohms at 22"C., see text) Rever~lngswitch (I).p.d.t. with interconnected jaws) S. Sola constant voltage transformer (output 115 v. a&., 120 TI. watts) T2. Pawerstat variable transformer (output 0-135 v. ax., 7.5 amp. )
adjustment, and the reversing switch allows both positive and negative rotations to be readily measured. OPERATION
The setup descrihed has two inconvenient features: In the first place two people are required to operate it; one person must adjust the current while the other reads the polarimeter. Of course an automatic current regulator would eliminate the need for a second operator, and several such regulators are descrihed by Lingane (6). In the second place there is no room for a water jacket on the tube, and the coil will heat up slightly when energized. Fortunately magnetic rotations vary little with temperature, but the current should not he allowed to warm the tube since heat will striate the liquids and make the polarimeter difficult to read. Thus the tube should be energized for only a few seconds at a time, and a waiting period should he allowed between runs. It is also helpful to have an electric blower circulating room temperature air over thr roil while in the polarimeter. These precautions will prevent striations in most liquids, and the electric current can always he reduced if there is still too much heat produced. 445
The setup was operated as follows: The tuhe was filled from a pipet so that no liquid could get into the coil and short the wires; then it was placed in the polarimeter. The vernier rheostat mas set with twothirds of its resistance in the circuit, the tube was energized, and the powerstat adjusted so that the desired current was shown on the milliammeter. This adjustment was made as quickly as possible to avoid heating the tube. Then the current was shut off by opening switrh S. Once the powerstat was set it seldom needed changing during the measurements. Now operator No. 1 set the potentiometer voltage numerically equal to the desired amperes. Then he turned on the tube current by closing the reversing switch, held down the potentiometer button, and manipulated the vernier to keep the potentiometer galvanometer centered. The galvanometer will fluctuate a little even though a constant voltage transformer is used. Also the tube resistance will increase and reduce the current. Therefore the vernier resistor had to be decreased gradually to hold the current constant. Operator No. 2 was able to adjust the polarimeter to *O.O1° within 6 seconds after the current was turned on, and operator No. 1 had the galvanometer well centered by this time. He could easily adjust the current with a precision of one part in a thousand. Then the current was turned off for 30-60 seconds before the next reading was taken. The zero point of the polarimeter was not determined because the most convenient value to measure was the difference between the positive and negative current readings. This value was actually 2a, a fact that should he remembered when K is calculated from equation (2) or (3). PERFORMANCE
room temperature of 22%. However the temperature was assumed to be 25°C. because the current may have warmed the liquids slightly. The current (amperes) was controlled to better than one part in a thousand. Thus the accuracy of the results depended mainly on the accuracy of the polarimeter readings. The 2a values given in the accompanying tables were obtained from eight separate polarimeter settings, each estimated to *0.0l0. Both positive and negative current readings were taken in the following order: -- -; then the average of the negative readings was subtracted from the average of the positive readings. A precision of better than one part in 400 was possible with water as shown in Table 1. This relatively high precision was obtained because water does not striate easily, and the polarimeter fields remained sharp and clear during the measurements. Table 2 lists the magnetic rotations of a few selected organic liqnids. The literature values in the table from reference (7) have been multiplied by 1307/1302 because the authors of reference (7) reported a low value for water (0.01302 deg./ cm. oersted) in a later article (8). I n the present experiment the purest commercial grade chemicals were used, hut no attempt was made to purify them further. In each case the observed Verdet constant, calculated from equation (3), deviated from the literature value by less than 1%. Organic liquids tend to striate more readily than water, and their magnetic rotations cannot he read as easily without a cooling jacket to keep the liquids a t constant temperature. In fact toluene and carbon disulfide striated so badly with 0.4 amp. flowing through the coil that they had to be measured with a reduced current. ACKNOWLEDGMENTS
Distilled water was used to standardize the tuhe as shown in Table 1, and the tube constant was calculated from equation (2). Sodium light was used for all the measurements, and they were made at a TABLE 1 Calibration of Tube with Distilled Waters
The author wishes to thank Messrs. Joseph J. Banas, Nicholas D. Delano, and Walter F. Jankowski for assisting with the readings and for winding the coil. LITERATURE CITED (1) PARTINGTON, J.
PnY i n deg. (Difference belween and ~eadinga)
+
++
++
I i n amps.
Tzrbe conslent K i n Verdel unils/deg./amp.
Mean
0.00446 *0.00001 " V ~ x n= 0.01307 rnin./cm. oersted at %6.,SOC. (5).
Pa2,"
i n den.
(2) (3) (4) ..
R., "An Adv~noedTreatise on Phyfiioal Chemistry," Longmans, Green and Co., New York, 1953, Vol. 4, pp. 592632. FARADAY, M., Phil. Mag., 28, 294; 29, 163 (1846). VERDET,E., Cmnpt. Rend., 39,548 (1854). STEINGISER. . S... AND H. HYMAN.J . Am. Chem. Soe... 60,. 2294
(1938). (5) "Internstiand Critical Tables," McGraw-Hill, New York, 1929, Vol. 6, pp. 425, 426. (6) LINGANE, J. J., "Electroan~lyti~al Chemistry," Interscience Publishers, Inc., New York, 1953, pp. 214-44; 381-86. (7) WARING,C., H. HYMAN,AND S. STEINGISER, J . Am. C h m . Sac., 6.3, 1985 (1041). (8) Zbid., 65,1066 (1943).
I
i n amus.
observed
litemture 25%".
Ethanol, absolute Carbon tetrsehloridc Nitrobenzene Toluene Rtmnobenzene Carbon disulfidr
"K 446
= 0.00446 Verdet
units/drg./amp. (Tabln I ) .
-
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