1. S. Lippert and J. E. RiWer, Jr.' Mechanical and Aero-space Engineering Department University of Massachuetts Amherst, 01002
Parameters Involved in Zone Refining Organic Compounds
Zone refining has been found to be a most effective technique in the purification of organic compounds. Since organic compounds are notoriously impure, it is thought that zone refining will make a major contribution in establishing the true properties of these materials (1, 2 ) . The purpose of this paper is to report aninexpensive,easily constructed multistagezone refiner that has been used for quantitatively demonstrating the parameters involved in zone refining organic compounds and that could also be used in the research laboratory for obtaining pure organic compounds. The construction of an inexpensive single stage unit has previously been reported (5). Apparatus
A schematic sketch of the multistage zone refiner is given in Figure 1. The unit is housed in a 7 X 10 X
COPPER COOLING COILS
LASS SPACERS ALUMINUM STAND
TYGON TUBING
SPECIMEN GLASS TUBE HOLDER
Figure 1.
Schematic sketch of multistage zone nflner.
28-in. aluminum stand. The top, bottom, and backing of this stand were cut from a '/a-in. aluminum sheet and 6/8 X 5/gin. aluminum posts were screwed to the sides of the stand a t the four corners to provide rigidity and parallel alignment of the top and bottom. The front 1 At the time this workwas done, the writers were, respectively, graduate student and assistant professor of the Mechanical and Aero-space Engineering Department. I. S. Lippert is now process engineer, Polaroid Corporation, Wdthsm, Massachusetts. J. E. Ritter, Jr. (to whom requests for reprints should be addressed) is now associate professor of the Mechanical and Aerospace Engineering Department.
650
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Journal of Chemical Education
and sides of the apparatus are enclosed with Plexiglas to eliminate temperature fluctuations caused by drafts. A Synchron electrical motor with a pulley attached to the drive shaft is screwed onto the top of the stand. This motor-pulley arrangement can either raise or lower the specimen by winding (or unwinding) a nylon thread that is secured to the sample tube. The rate of travel of the sample may he easily modified by varying the size of the pulleys and/or speed of the motor. For this study the pulleys varied from '/&-in.to 1-in. diameter and two motors, 1 rpm and 2 rpm, were used. A glass tube (16 mm 0.d. X 18 in. long) is attached to a fiberboard sheet and is used as a guide for the sample tube and as an aid in positioning the heaters and cooling units. Small glass cylinders in. high) fit concentrically over the glass tube and separate the ten heating elements from the ten cooling units. The cooling units are made by coiling '/,in. 0.d. copper tubing 6L/2times around the glass tube and are connected to one another by Tygon tubing. It was. found necessary to circulate cold tap water through the cooling coils to insure sufficient cooling between the heaters. The ten Nichrome wire resistance heaters in. rectangular cross section) are fitted in. X tightly around the glass tube and are electrically connected in parallel by screwing them to two thin copper strips on the back of the fiberboard. By supplying power to the heaters through a voltage stabilizer, Variac, and transformer, a close control of temperature can be maintained. Procedure
Test hatches of napthalene doped with either methyl red or benzoic acid were prepared by thoroughly mixing the ground powders of the materials. The sample tubes for the napthalene-methyl red system were glass test tubes, 70 mm long X 10 mm diameter, and for the napthalene-benzoic acid system Teflon tubes, 3 in. long, a/8 in o.d., and bored out to 5 / ~ 6 in. i.d. The sample tubes were filled with the test mixture and placed in a constant temperature bath to allow the mixture to melt. The temperature of the bath was kept just a few degrees above the melting point to avoid any decomposition of the organic compounds. Since the melted material would take up approximately '/a of the space of the ground powder, more powder had to be added. After the sample tubes were filled, the glass tubes were sealed with a cork and the Teflon with a threaded Plexiglas stopper. The samples were then ready to be zone refined. After zone refining, a concentration profile, impurity level versus distance along the longitudinal axis, was obtained for each sample. The changes in concentra-
tion of methyl red in napthalene was measured with a color meter (Hunterlab D25). The "a" scale of the color meter was used since this scale measures redness, ranging from +99.0 for maximum redness to 0 for no traces of red. The negative side of this scale measures greenness from 0, no traces of green, to -99.0 for maximum green. Five measurements were taken a t a given position by rotating the glass test tube about its longitudinal axis. The average of these measurements and the 80% confidence limits were then computed for each position along the length of the sample. The concentration of benzoic acid in napthalene was measured with a Differential Scanning Calorimeter (Perkin-Elmer Model DSC-1B). Teflon sample tubes were used because they could be sliced into thin sections, thereby permitting a few milligrams of the sample to be obtained for analysis in the DSC a t various positions along the length of the test tube. The DSC measures the difference in power required to maintain the sample and a thermally inert reference a t equal temperatures as the sample is melted; thus, it gives a measure of the latent heat of fusion. Since the benzoic acid a t the concentrations used in this study is contained entirely within the eutectic structure, a measure of the amount of benzoic acid for a given sample is obtained by measuring the proportion of the heat of fusion due to the eutectic. As the amount of benzoic acid increases, the amount of eutectic and its contribution to the total heat of fusion will also increase. Therefore, by determining a calibration curve from samples of known benzoic acid concentrations, the concentration in unknown samples could be determined. Results and Discussion
As has been previously reported (8, 4, napthalene doped with methyl red proved to be an ideal system for demonstrating zone refining. I n addition, concentration profiles were easily obtained with the aid of the color meter (Fig. 2). Table 1 summarizes typical experimental results obtained on testing some of the experimental variables involved in zone refining organics. I n column 5 the value of redness (or the concentration of methyl red) a t the midpoint of each sample is reported for a given set of conditions. This value was obtained by taking the weighted average of the nearest points on each side of the midpoint. These values may be compared to an average reading of 4.4 for the initial concentration of 0.03 wt % methyl red.
Table 1.
$
SAMPLE A, AVERAGE VALUE WlTH 80%CONFIDENCE LIMITS
4
SAMPLEB.AVERAGE VALUE WlTH LIO%CONFIDENCE LIMITS
NORMALIZED DISTANCE (DISTANCE ALONG LONGITUDINAL AXIS OF SAMPLE DIVIDED BY TOTAL LENGTH OF SAMPLE) Figure 2.
Concentration pmflie for nopthdene initiaily doped with 0.03
Column 6 gives the difference between the averages in column 5, i.e., the differences in concentration of methyl red a t the midpoints of two samples tested for the indicated experimental parameter. Using a t' test (6),the null hypothesis that the average values a t the midpoints are the same is tested a t the 95% significance level. The results of the t' test are given in column 9 as not rejecting or rejecting the null hypothesis. Rejection of the null hypothesis means that a statistical difference of redness values exists a t the 95y0 level of significance. From Table 1 it is seen that reproducibility is good and that purification of methyl red in napthalene increases as the number of zone passes increases. This increase in purification would be expected from both theoretical considerations (6) and previous experimental observations (1,7,8). Also, Table 1 shows that greater purification is attained on slower zone travel speeds. This is generally attributed to the slow crystal growth rate of organic compounds coupled with the relatively long time required for impurities to diffuse away from the advancing solid-liquid interface into the bulk liquid (1, 8). It is interesting to note that with a large number of zone passes, 10, the slower zone speed did not produce a significant difference in purification. This is probably because both samples approached the ultimate separation. Finally, it is seen that zone
Experimental Results on Zone Refining Naptholene Initially Doped with 0.03 wt
Experimental Variable Tested Reproducibility Effect of Number of Zone Passes Effect of Rate of Zone Travel Effect of Direction of Zone Travel
Mean Number Zone Travel Direction Redness rtt of Speed of Passes ( i b ) Zone Travel Midpoint 1 1 1 10 1 1
10 lo 1 1
0.606 0.606 2.0 2.0 2.0 0.786 2.0 0.786 0.606 0.606
UP UP Down Down Down Down Down Down UP Dawn
0.758 0.380 3.08 -0.626 3.08 1.18 -0.626 -1.18 0.758 -1.63
Difference in Redness at Midpoint
O/o
Methyl Red
Test of Null Associated Hypothesis at 1' Degrees of 95% Signihcance Statistic Freedom Level 10.56
Not Reject
0.378
0.421
3.706
6.28
9.12
Reject
1.90
2.34
8.99
Reject
0.554
1.04
10.94
2.388
3.07
7.45
Not Reject Reject
Volume 46, Number 10, October 1969
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651
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Table 2.
Experimental Variable Tested
8,
,,
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Initial Concentration Number of Benzoic Acid of ( w t %) Passes 20.3 20.3 20.3 20.3 10.4 10.4 10.4 10.4
Effect of Number of Zone Passes Effect of Rate of Zone Travel
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1 1 1 3 1 2 2 2
Z
o
SAMPLE A
5a: 5W
A
SAMPLE B
0 1.4-
A 0
U
A
0.606 0.606 0.606 0.606 1.21 1.21 1.21 0.606
A
0 0
5
-E
0
2
4
6
8
10
NORMALIZED DISTANCE (DISTANCE ALONG LONGITUDINAL AXIS OF SAMPLE DIVIDED BY TOTAL LENGTH OF SAMPLE) Figure 3. Concentration proflle for nopthalene initidly doped with 10.4 wt % benrois ecid. Somple A rone reflned downword for 2 posro* at 0.606 in./hr and rompla B rone refined downward for 2 panes at 1.21 in/hr.
refining downward produces better purification. This is due to the density difference between the fluid in the hulk of the zone and a t the freezing interface (caused by both a temperature and concentration gradient) being greater when zone travel is down, hence, mixing in the zone is greater (1). Concentration profiles for the napthalene-benzoic acid system could he ohtained with the aid of the DSC (Fig. 3). Table 2 gives some results obtained with this system. Column 5 gives the concentration of henzoic acid a t the midpoint of the sample and column 6 the difference in concentrations between paired specimens. I n column 7 this difference is expressed as a percentage of initial concentration for comparison with the repro-
652
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Difference at Difference as Concentrations of Zone Travel Concentration of Speed Bensoic Acid at Benzoio Acid at Midpoint Yoof Initial Concentration (inh) Midpoint (wt %) (wt %)
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A
A
A
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Exuerimentol Results on Zone Refining Downward Noutholene Doped With Benzoic Acid
Reproducibility
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Journal of Chemical Education
22.9 22.0 22.9 20.6 12.3 11.8 11.8 8.91
0.9
4.44
2.3
11.3
0.5
4.8
2.89
27.8
ducibility value of 4.44%. As with the napthalenemethyl red system, greater purification was achieved as the numher of zone passes increases and as zone travel speed decreases; however, it is important to note the pronounced effect that the zone travel speed has on purification. This sensitivity of purification on zone travel speed is probably the reason that the expected increase in purification with numher of zone passes was not appreciable at the high zone travel speed (1.21 in./hr). In summary, an inexpensive, easily constructed multistage zone refiner was used to quantitatively demonstrate the effect of some experimental variables (number of zone passes, zone travel speed, and direction of zone travel) on the effectiveness of zone refining. Concentration profiles were ohtained for the napthalene-methyl red system with the aid of a color meter and for the napthalene-benzoic acid system with a DSC. Acknowledgments
This study was based on a thesis submitted by I. S. Lippert for the Master of Science degree in Mechanical Engineering a t the University of Massachusetts, Fehmary, 1969. The support of NSF Grant No. GY-4709 for part of this work is gratefully acknowledged. Literature Cited
WILCOX,W. R., FREIDENBURG, R., AND BACK,N., Chem. Rev., 64, 187 (1964). PFANN,W. G., Sei. Am., 217, 62 (1967). HINTON, J. F., MCINTYRE, J. M., AND AMIS,E. S., J. CHEM. EDUC.,45, 116 (1968). CHRISTIAN, J. D., J. CHEM.EDUC.,33, 32 (1956). BOWKER, A. H., AND LIEBERMAN, G. J., "Engineering Statistics," Prentice-Hdl, Inc., Englewood Cliffs, N. J., 1961. PFANN,W. G., "Zone Melting" (2nd Ed.), John Wiley & Sons, Inc., New York, 1966. JONICH, M. J., AND BAILEY,D. R., Anal. Chem., 32, 1578 (1960). (8) BA& J: S., HELM,R. V., 30, C36 (1958).
AND
FEHRIN,C. R., Refining Eng.,
