1256
ANALYTICAL CHEMISTRY
working the glass down on a screw under an inert atmosphere. The butt end of the Teflon plug is threaded to fit, and the tip is machined to a 10' taper to seat circumferentially in the 3-mm. internally ground, take-off hole. The tapered tip extends through the hole, so that the area of contact is a narrow ring with a high unit area loading to take advantage of the slight deformability of Teflon. Holdup is minimal. The novel fraction cutter (Figure 2) permits changing of receivers without contamination and without cutting off the column from the pump. Its operation involves the rotation of the b o b tom plate so that the receiver can be successively pre-evacuated by an auxiliary pump, used, and removed. There is no holdup.
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TOP PLATE
m BOTTOM PLATE
SECTION A-A' ASSEMBLY
Figure 2
Two sight glasses (Corning, Code No. 694900, 4 I / e X 9 / ~ 2 inch, Cir. Fl., M F 74) are used for the plates. A hole to accept a l/, inch bolt is drilled in the center of each plate. A 15-mm. hole and two 3-mm. holes are drilled in the top plate equidistant from one another on a 2.75-inch circle. Three similarly placed 15 mm. holes are drilled in the bottom plate. The upper plate is provided with a 28/15 socket joint sealed around the large hole, and with two 3-mm. capillary taps sealed around the small holes. The central pivot holes are fitted with a nut, washer, bolt, and spring device to provide the proper compression for good contact and ease of rotation. A clamp on the central tube (sealed to the upper plate) furnishes convenient sup ort. The upper plate is flabground on the underside, and the Power plate is flabground on both sides. The receiver lips, of course, are also flabground. All ground surfaces ark finished with No. 600 Carborundum. Dow-Corning high vacuum grease is a satisfactory lubricant between the plates and on the receiver lips.
Improved Recrystallization Tube. Charles E Blades' and Wolfgang SchonigeP, University of Wisconsin, Madison, Wis.
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ONICAL centrifuge tubes have long been used for the recrystallization of small quantities of material, the mother liquors being removed by decantation or by means of a fine pipet. In order to improve the method for removing the surface layer of liquid from the crystallized material, Skau (3) and Craig (1)have designed filter tubes in which this supernatant liquid is separated by centrifugation through a filter septum. These tubes have found considerable use, but require a transfer of the crystallized 1 Present address, Research Laboratories, Air Reduction Co., Inc., Murray Hill, N. J. 9 Present address, Med.-Chern. Institut und Pregl-Laboratoriurn, Universitat Grar, Graa, Austria.
mixture from one container to another, thus incurring some mechanical loss and inconvenience. A modified Craig tube has been reported ( 2 ) in which the crystals are retained in the recrystallizing compartment, during and after separation from the mother liquor, by a simple inverting technique. The tube described in this paper accomplishes tmhesame purpose without the necessity for inverting prior to centrifugation. It may serve as a receptacle for successive recrystallizations without intermediate removal of the solute. This has particular value in the preparation of analytically pure material when only small amounts are available. The details of the tube are given in Figure 1.
A piece of 14-mm. tubing, A , 5 to 6 cm. in length, waa sealed to a 1.5- to 2.0-cm. piece of 2-mm. capillary tubing, B, the proximal end of which had been constricted, C, to an inside diameter of 0.1 to 0.2 mm. as estimated by means of a close-fitting stainless steel wire. The length of this constricted region was kept to a minimum to avoid plugging during use and particular care was taken to avoid forming an annular trough a t the seal. A suib able length of l/c-inch Teflon (trade-name of E. I. du Pont de Nemours & Co., for polytetrafluorethylene) rod was turned on a lathe to give a 2-mm. rod, F , the pointed end of which n-ould fit snugly into the capillary and thus stop er the tube. The beveled inner wall, D,of the constricted capieary was ground with fine grinding powder and a suitable glass rod to provide a rough surface for holding the plug in place. For use, the tube was suspended in a 15-ml. conical centrifuge tube with a rubber washer, G, as a cushion. In this manner the mother liquor was collected in the bottom of the centrifuge tube. A cork, covered with aluminum foil, served to allow the preparation of linefree crystals. For recrystallization of a solid, the tube with plug in place is suspended inside the centrifuge tube. The solution is introduced directly from a filter and may then be concentrated, if necessary, by heating the naked tube (Teflon plug still in place) in a steam bath. By careful manipulation this process may be carried out without a stirring rod, owing to the peculiarity of the inner surface which serves as a builbin boiling tube. The recrystallization tube is then replaced in the centrifuge tube and cooled for recrystallization, and after removal of the plug is spun in the centrifuge to separate the two phases. This occurs rapidly and without evaporation of the mother liquor from the surface of the crystalline material. The operation may then be repeated for further urification by simply replacink the Teflon plug, adding fresh soyvent, and repeating the cycle in the same tube. Washing of the solid is unnecessary, as practically no mother liquor adheres to the crystals ( 4 ) ,and a more efficient purification is thus possible. In addition, recrystallizations may be carried out in solvents in which the solute is very soluble, since small volumes of solvent are handled readily ( I , 4). The tube is adaptable for use with most crystalline solids, if the formation of fine precipitates is avoided. The size described was found to be optimal; a larger model, designed for use with a 50-ml. conical tube, developed critical unbalance in the centrifuge as filtration progressed. This recrystallization device is simple in construction, can he made from readily available materials, and can be used with equipment already on hand in the laboratory. ACKNOWLEDGMENT
The authors are indebted to A. L. Wilds for many encouraging discussions and to F. Peters for technical advice and rassistance. LITERATURE CITED
(1) Craig, L. C., IND.ENG.CHEM.,ANAL.ED.,12,773 (1940). (2) Craig, L. C., and Post, 0. W., Ibid., 16, 413 (1944). (3) Skau, E. L., J. Phys. Chem., 33, 951 (1929). (4) Weissberger, A., "Technique of Organic Chemistry," p. 464,
New York, Interscience Publishers, 1950.
