JOURNAL OF CmMICAL EDUCATION
ZONE REFINING JOHN D. CHRISTIAN Monsanto Chemical Company, Texaa City, Texas
ZONErefining (or zone melt refining) is a highly efficient purification technique which consists of the gradual transport of a molten zone, produced by external heating, along an elongated ingot of the impure material. The slow solidification of the liquid material a t the cooling end of the molten zone constitutes a recrystallization step. This process can be of considerable utility in the laboratory not only as a means of purification but also for demonstrating phase relationships to students. This technique was developed by Pfann' as an improvement on a fractional freezing process reported by Schwab and Wichers2 in 1944. Although the previous work was directed toward the purification of benzoic acid and acetanilide, Pfann was concerned with the production of very pure germanium for electrical applications. Zone refining has since been used for the purification of ~ i l i c o n ,tin,4 ~ antipony, and various organic materials? Numerous other applications and a general account of the phase principles involved have been discussed by G o ~ d m a n . ~ A mathematical treatment of zone-melting processes has been presented by Reiss.' The writer has carried out zone-refining experiments with organic materials in Pyrex tubing 3, 5 , 8, 10, and I
PFANN, W. G., J. Metals, 4 , Trans. 747 (1952); C. A . ,
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2 SCRWAB, F. w., AND E. W I C ~ R S ,J. Research ~ a t l Bur. . Slandadards, 32,253 11944); C.A,, 38, 4846 (1944). 3 ANON., Chem. Week, Deoember 25, 1954, p. 62. TANENBAUM, M., A . J. GOSS, AND W. G . P ~ J. ~ ~ 6v AIME Trans. 20, 762 (1954); C. A,, 48, 8 1 4 5 ~(1954). 6 . A-.N.-. ~ .N ~~~~~~~h iT,ondm). 7. 465 (1954). . ~-~~ ..,, , 6 GOODMAN, C. H. L., ibid., 7, 168 (i954j. 7 R m s s , H., J . Metals, 6 , 1053 (1954). ~~
12 mm. in diameter. Using a 1-meter length of 3-mm. diameter tubing, the entire sample undergoing purification was only about 2.5 grams. Flexible laboratory heating tape wrapped once or twice around the tubing is very satisfactory in creating the molten zone when organic materials are used. When length permits the tape can be arranged to create a second or third molten zone on the same tube. If this is done, care should be taken to preclude mixing of the liquid zones. The sample tube can be mounted in any convenient position. A horizontal arrangement, however, was found to have some convenience. Colored p-dichlorobenzene cakes, sold commercially as deodorizing blocks, were found to be very satisfactory as a crude feedstock for zone-refining demonstrations. Noticeable decolorization could be obtained (10-mm. tubing, one-in. heated zone) even when the heating coil was manually advanced about one-half inch every half hour. A constant gradual movement of the molten zone a t about the same rate with a clock mechanism, however, was considerably more effective than the sporadic manual manipulation. Naphthalene containing a t,race of methyl red was also found to be an excellent feedstock for demonstrations. The removal of fluorescent impurities ( e . g., a trace of anthracene in naphthalene) makes a very impressive demonstration but requires an ultraviolet lamp and darkened room for the examination. Thermal recrystallization, either through zone refining or fractional freezing, completely avoids solvent ~ t problems. The process, however, is obviously not adaptable to materials which are unstable at temperatures slightly above the melting point or which tend to form supercooled liquids on solidifying.
