Eugeniusz T. Knypl and Kmsimierz ~ i e l e k k i
Oiwiecim Chemical Works Oiwiecim, Polond
Automatic Apparatus for Zone Refining of Organic Substances
Zone refining1 is one of the numerous zone melting2,' techniques which was described by W. G. Pfann as a "powerful tool for manipulating impurities in crystals." The essence of zone melting is taking advantage of the different solubilities of an impurity in the solid and liquid phases of the material being refined, i.e., crystallization of a pure substance from its liquid phase and not from a solvent, as in classical methods of refining. This purification is obtained by passing over the ingot of impure material some melted zones which "gather" and carry impurities toward one end of the ingot, according to the equilibrium segregation coefficient. This is frequently a slow process, for the impurity must move by a diffusion process. For zone refining an adequately slow motion of the zone (usually from 1to 20 cm/hr) a t a constant rate for the length of the ingot must be assured; a rapid return and the repetition of the same slow upwards motion is also required. Normally the automatic operation of such apparatus is obtained by rather expensive means: using sets of small electric motors, suitable gears, etc. Figure 1 shows the principle of constructioo and operation of a refiner which may be built of simple laboratory equipment and give satisfactory use. The essential element is the floating disc, which floats upwasd with precision while water rises in the main vessel. In our experiments a wooden pipet stand base impregnated with paraffin to prevent the soaking up of water is used. When the water level reaches the highest bend of the siphon tube, the water will pour off to the initial level, as it would in a Soxhlet extractor. The zones then begin repeated migration. The heaters are made from resistance wire ("canthal") wound 50 times around a 3-mm thick glass ring. and they are supplied with 220 v ac through a transformer. A temperature of 350°C is easily obtained in the center. The heaters are shielded with asbestos rings to focus the radiation. The asbestos ring laid on the stopper of the floating disc protects the water vapor equilibrium from disturbance when the disc approaches the lowest heater. Heavy copper wire P u n , N. L., "Zone Refining and Allied Techniques," G. Newness, Ltd., London, 1960. PFAKN, W. G., "Zone Melting," Jalm Wiley and Sons, New Ywk, 1958. PFANN, W. G., Science, 135,1101 (1962).
352 / Journal of Chemical Education
heated a t one end by a Bunsen bnrner may also be used as a heater.& A millimeter scale may be glued to the wall to measure the rise of the water level and thus measure the rate a t which the melted zone moves. A special application of this zone refiner would he a micro- or multi-refiner. A multi-refiner can be built in such a way that more tubes with purified substances are fitted on the floating disc; thus refining of mixtures of different concentrations may be observed and phase changes may be investigated. To demonstrate this apparatus, naphthalene with an added dye such as methyl red or methyl violet has been found to be best. We have used hromocresol purple (about 0.1%): The result of the refining was very striking. There was no visual trace of the dye after two passages of the melted zone (1 em width) at a velocity of 2.3 cm/hr on the 8 cm length from the initial position of the heaters. Only transparent, crystallized, ultra pure naphthaleoe mas left. HERINGTON, 33.F. G., School Sci. Bet.., 149, 33 (l!Jtil).
r(/lube
Figure 1.-Automatic zone refiner.
uilh puntied rnaiwoi
