J. F. Hinton J. M. Mclntyre and E. 5. Amis
A Zone Refining Apparatus
University of Arkansas Fayetteville, 72701
for Organic Compounds
-
Z o n e refining has been found to be a very effective kchnique in obtaining high purity organic compounds. The outstanding advantage of this technique is that the compound to be purified does not come into contact with any other chemical or solvent. The application of zone refining to most research purposes generallv involves the construction of a suitable apparat,us (1-5) since those commercial instruments which are available are quite expensive. We wish to report an apparatus that is easily const,ructed, from readily available materials, inexpensive (i.e., approximately W5), and that can be used wit,h a wide variety of organic compounds.
brass block (H)about 1 in. from one end, and another hole a / , in. in diameter is dnlled in the other end. The block is screwed onto in. in diameter, threaded on each the drive shaft and a rod (I) end is placed through the -in. hale in the black and bolted to the top and bottom of the housing. This rod provides for better alignment and increased rigidity. The heating-cooling unit (J) is clamped onto the brass block with two */,sin.bolts, and the sample tube ( K )is placed through the hole st the other end.
The Apparatus
The zone refining apparatus reported here (Fig. 1 diagram and photograph) is housed in a. Gin. X 5-in. X 17-in. aluminum two way minibox ( A ) ,available a t most electrical supply stores, fitted rpm) located a t the top and with t,wo Cramer 117 P motors bottom of the housing (B). This is the type motor nsually found on strip chart recorders. A pulley (C), 0.51 in. in diameter, is attached to the drive shaft of each motor. A '/?in. threaded rod (D), 32 thre;tds/in., both ends of which me fitted with a pulley (E), 1.57-ill. in diameter, is employed to drive the heating-cooling unit (J)in an upward or downward direction. Both sets of pulleys are so aligned that they may easily accommodrtte a pulley belt ( F ) . The drive shaft (D) is attached to the housing through a ball hearing assembly (G) placed a t each end, holes being cut in eaoh end to allow the shaft to sit in the bearing cups. It was found that the bearing assembly taken from a. bicyole wheel works very well and is of the appropriate size. A '/Fin. hale is drilled and threaded in a 3-in. X I-in. X '/pin.
,
C-SUPPORT RINGS D-COOLANTCOILS E -ASBESTOS WASHERS F -RESISTANCE WIRE G-METALWASHER H-SAMPLETUBE
C
I -BRASS
J -ASBESTOS Figure 2.
Figure 1.
116
/
Complete Zone ReRning Apporotur.
Journal of Chemicol Education
BLOCK CORD
Heding-Cooling Element.
The heating-cooling unit is shown in Figure 2. The unit is supported and attached to the b r a s block with two plates of sheet. metal ( A ) . Holes are cut in the plates to accommodate the sample tube and the attachment bolts ( B ) . A metal ring ( C ) was glued (epoxy glue) to one sideof eachplate for additional support.. The cooling element (D) is made from two pieces of '/& i.d. copper tubing formed around the sample tube with enough excess to attach Tygon tubing. A cooling coil (D) is glued (epoxy glue) to each of the sheet metal plates on the side opposite the metal ring. hbestas washers ( E ) out from asbestos paper are glued to each cooling coil providing approximately I/? in. of insulation. A piece of No. 18 B and S Chrome1 resistance wire (F) covered with high-temperaturespaghetti is formed around the sample tube. The loop, with enough excess to connect to a variable power supply, is glued to a metal washer ( G ) which is just slightly larger than the sample tube. The heating element is then sandwiched between the oooling elements and secured with epoxy gloe. For additional insulation, a piece of asbestos cord (J)is glued around the heating element. Tygon tubing is attached to the coaling coils with epoxy glue. The resistance wire is connected to a variable power source and the complete heating-cooling unit attached to the brass block (I).
Operation
From a hole cut in the top of the housing, the sample tube is lowered through the heating-cooling unit and held in place a t the bottom of the housing by a rubber posit,ioner. "0" rings were used to connect the pulleys and a three-way switch was used to control the two motors. A liquid coolant or air may be passed through the copper cooling coils. The part of the sample charge containing the concentrated impurity or impurities may be separated from the purified material simply by melting the impure section with t,he heating unit and withdrawing it using a pipet. The purified material is then removed in the same manner. Obviously the same procedure may be follol~edif the purified material is a t the top of the sample charge. An alternate method involves breaking the container tube at the appropriate spot in order to obtain a given quantity of highly purified material. The parameters of most importance with respect to the achievement of the highest degree of purification of any compound using zone refining techniques are the rate of zone travel, direction of zone movement, zone size, container dimensions, temperature gradient, and gradient control (I). Since the purification per zone pass increases as the zone rate of travel decreases, it is most important that a zone refining apparatus should have the capacity for slow zone movement. For the system described here, the rate of zone travel was 3 mm/hr. The rate of travel of the heating-cooling unit on the threaded bolt was found to be very constant thereby affording uniform movement of the zone melt. The rate of zone movement may be easily modified with this apparatus by varying the size of the pulleys. Due to the presence of gas in the samples, small amounts of insoluble material or unfavorable sampleimpurity density ratios, an upward or downward movement of the heating-cooling unit is dictated depending upon the particular situation encountered. From Figure 1 it can be seen that, by having a top and bottom mounted pulley motor, provision is made for both upward and downward movement simply by using the appropriate pulley system for the direction of movement desired. The size of the zone can be controlled by proper regulat,ion of the heating-cooling unit. Since a hightemperature gradient a t the freezing interface generally provides for better mixing and therefore results in greater purification, the control of the temperature gradient is essential (I). This implies that the temperature differential between the heating and cooling units should be large so that the division line between the
melt and solid is very sharp. By using a constant voltage transformer in conjunction with the resistance heater, the temperature of the heating unit can be maintained within the desired limits. The cooling unit is constructed so that a coolant from a constant temperature bath may be circulated above and belonthe heating unit thereby producing a well-defined temperature gradient. The apparatus may be protected from fluctuations in ambient temperature caused by drafts by enclosing it with the front and side cover. The size of this zone refiner is small enough so that it may be conveniently used in a "cold" room if necessary. With a sample of approximately 20 cm in length, zone melt lengths of 7.5 rnm and 10 mm were easily maintained in the zone refining purification of p-dichlorobenzene and N-methylacetamide, respectively. A test sample of p-dichlorobenzene "doped" with napthalene (0.308 wt %) was subjected to three zone passes and the purified portion of p-dichlorobenzene analyzed for napthalene by gas chromatography. A 5-ft column of 20% SE-30 on firebrick was used for the analysis of the p-dichlorobenzene sample. The data obtained showed that the amount of napthalene present after three zone passes had been reduced to less than 0.003 wt yo. Long container Pyrex tubes of 20 mm i.d. were used for all purifications carried out in this laboratory with compound charge lengths of approximately 10 to 20 cm. The closed end of the sample tube was sealed to a short piece of the same diameter tubing to position the sample tube a t the proper height with respect to the heatingcooling unit. The time required to traverse the entire length of the charge is rather long for cases in which a large quantity of material is used; therefore if the time factor is critical the time required to make a number of zone passes through the charge can be considerably shortened by using more than one beating-cooling unit. In summary, a moderately priced, easily constructed zone refining apparatus has been described and its effectiveness tested against the criteria discussed in the literature (1) and in the purification of p-dichlorobenzene and N-methslacetamide. Literature Cited
W. R., ~ ~ I E D E N B ER., R GAND , BACK,N., Chem. Rev., 64, 187 (1964). (2) CHRISTIAN, J. D., J. CHEM. EDUC.,33,32 (1956). R., AND SCBHAMM, C. H., J. CHEM. (3) ZIEF, M., HOLLISTER, EDUC.,40,351 (1963). (4) . . KNYPL.E. T.. AND ZIELENSKI. . K.., J. CHEM.EDUC.. , 40.. 352 (1963). ' (5) PFANN,W. G., "Zone Melting," John Wiley & Sons, Inc., NewYork, 1958.
(1)
WILCOX,
Volume 45, Number 2, February 1968
/
1 17
