C. C. GravattLand Paul M. Gross
Duke University Durham, North Carolina 27706
Apparatus for the Purification and Crystal Growth of Organic Compounds
I n connection with numerous solid state investigations over the past two decades greatly improved apparatus and techniques have been developed for the production of metals, alloys, and inorganic compounds of very high purity. I n spite of their effectiveness these methods seem to have found only limited acceptance among the routines of most chemical laboratories, possibly because of the apparent complexity of many of the pieces of apparatus described in the l i t e r a t ~ r e . ~I n connection with work on organic semiconductors3 requiring compounds of high purity, two relatively simple pieces of apparatus have been assembled from readily available standard materials. The first of these has proven highly satisfactory for the purification of organic solids melting between 25°C and 300°C by zone refining and the second for the growth of organic single crystals from solution. Zone Refiner
Details of the zone refiner are shown in Figure 1, the design following the principles given by Herrington.' The copper sleeves are cut in 1-cm lengths from copper plumbing tubing of 10.5 mm i.d. The heaters consist of fine Nichrome wire wound over a single insulating layer of asbestos paper wrapped around the copper sleeves. Additional layers of asbestos paper over the Nichrome provide thermal insulation. All heater units are wired in series and the temperature regulated by an autotransformer. The cooling sections are made by coiling small copper tubing around the copper sleeves. Tap water passed through the coils provides sufficient cooling for all compounds melting above 25'C., The alternate heating and cooling units are separated from each other by 3-mm thick asbestos hardboard, and the entire system is aligned with four threaded rods which pass through the asbestos boards. The refiner has four heating and cooling sections, although this number is arbitrary. A decrease in the number of passes required for ourifieation would result from a lamer - number of sections. The compound to he refined is melted into the Pyrex tube which is then sealed off under a nitrogen or helium atmosohere. The tube is moved noward throueh the refinerat a rate of 2 cm/hr by a cord attached to a'&lley on the shaft of a synchronous 110 V clock motor. A microswitch (G, Fig. 1) turns the unit off when the tube Presrnt addras: Bell Telephone Laboratories, Murray Hill, New Jermv. Z S e e for example: HANNAY,H. B., (Editor), "Semieonductars," Reinhold Publishing Co., New Yark, 1960, Ch. 3. GIWVATT, C. C., A N D GROSS,P. M., J. Chem. Phys., 46, 413 (1967). Also BASHAW, JOHN, AND GROSS,P. M., J . Am. Chem. Sac.; 90, 3120 (1968). H13nnlNam~, E. F. G., "Zone Melting of Organic Compounds," John Wiley & Sons, Inc., NewYork, 1963.
completes a pass, and the spring balanced plunger of the switch is no longer held in the closed position when the bottom of the rising glass tube clears the plunger. The heaters are regulated to produce a molten zone approximately 1 cm in length a t a temperature 15-20°C above the melting point of the material, and the coolers maintain a solid zone of the same length. Ten to fifteen passes are usually sufficient to give 5 g of a high purity sample which is taken from the upper one-fourth of the material in the tube. As examples of the purification attainable, the melting point of reagent grade naphthalene was increased from 77.9-78.8"C to 80.2-80.5°C and that of dimethyl terephthalate increased from 139.5-139.8"C to 140.6-140.9°C. Crystal Growth Cell
The crystal growth cell is shown in Figure 2. The unit is 25 cm in length and the large sections are 2.5 cm in diameter. One section of the cell is heated with a 2-mm wide Nichrome band fastened directly to the glass with epoxy cement and the second side is thermally insulated. The lower cross arm is cooled by copper coils. Both sections of the cell are capped with glass stoppers which hold thermometers. The second cross arm, always above the liquid level, allows pressure equilization and the introduction of controlled atmospheres. A fritted glass basket is placed in the heated side and is filled with crystals of the compound. The tube is filled with a saturated solution of the compound in the solvent selected a t the temperature chosen, the crystals in the basket serving as a reserve to maintain saturation and as nutrient for the seed crystal growth. Seed crystals are fastened to fine glass rods with epoxy cement or otherwise suspended in the unheated section. The
w Figure 1. Zone Refiner. A, copFigure 2. Crystal Growth Cell. per sleeve; B, Nichrome wire; C, A, Nichmme bond heater; B, asbestos; D, copper cooling coils; copper cooling coil.; C, thermomE, orbettos hardboard spacers; 0 , pressure equdizotion eter; F, threaded rod; G, misroswit~h; crorr arm with stopcock; E, fritted H, Pyrex tube, oppmximataly basket containing nutrient; F, three-fourths filled. cry$tal.
Volume 46, Number 10, Odober 1969 / 693
temperature differential in the cell can be controlled by adjustment of the flow rate and temperature of the cooling liquid in the coils, and therefore a wide range of crystallization rates are possible. Large plates of
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dimethyl terephthalate, 1 cm in length and 3 mm thick, have been grown from acetone a t GO0C and with a differential of less than 1°C in this cell.
