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V O L U M E 26, NO. 7, J U L Y 1 9 5 4 Apparatus for Sublimation with Small Temperature Differences. Lohr A. Burkardt, Chemistry Division, U. S. S a v a l Ordnance Test Station, China Lake, Calif.
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T IS occasionally desirable to recondense a sublimable ma-
terial a t a temperature close to that a t which the sample is being held. Such a situation may arise if it is desired to obtain good crystals of a given form of a polymorphic material Close temperature control and a small temperature gradient are required to produce good crystals by sublimation. For materials of low vapor pressure, particularly those which decompose a t higher temperatures, the apparatus described is suitable, as it a i l 1 operate for weeks with very little attention. The accompanying sketchillustrates a simple apparatus, requiring little attention, which has performed ATER-COOLE D satisfactorily in this laboratory. I n essence, it consists of a sealed tube, which mav he evacuated or filled with gas as may be desired, containing the material to EVACUATED TUBE be sublimed. This tube is placed in a larger tube containing a layer of mercury together with a liquid which boils a t the tem. B O I L I N ~ L,QU,D perature a t which the sublimate is to be conHEATING BATH densed. This latter tube is closed xvith a stopper equipped with a suitable condenser. T h e a p p a r a t u s is heated by means of a suitable bath a t a temperature a fern degrees above that of the boiling point of the condensing liquid. The mercur\T uDon which the inner tube floats transmits the higher outer gath' temperature directly to the material being sublimed, thereby providing the temperature gradient which causes the material to move to the slightly cooler end of the tube. The entire condensing surface is maintained a t the desired temperature, thereby ensuring that all the sublimed crystals in the case of a polymorphic material will be the same form. When sufficient material has been sublimed, the inner tube is cut open for removal of the contents. This apparatus has been used in this laboratory for the vacuum sublimation of T N T (melting point 80.9" C . ) which it was desired to condense a t 78 ' C., in order to obtain the high temperature form of this material. Ethyl alcohol, which boils a t 7 8 " C. a t the elevation of this laboratory, was used as the condensing liquid. The heating bath was maintained a t 82" C. P~~LISH with E Dt h e approval of the Technical Director, U. S. Naval Ordnanre Te-t Station.
Head and Fraction Cutter for Small-Scale Spinning Band Column. Irvin C. Dumas, Robert Perthel, Jr., and Robert h4. Silverstein, Stanford Research Institute, Stanford, Calif. band columns owe their popularity to 1017 pressure drop, low holdup, and high efficiency, which dictate their use in situations involving the fractionation of small amounts of material over a wide range of pressure. The introduction of the spinning band, however, complicates the problem of head design. The simplest possible head is the partial condenser type. Its principal disadvantage lies in the troublesome temperature conPIKXISG
trol necessary to regulate the take-off. The use of stopcocks to regulate takeoff is precluded because of contamination and holdup. The use of solenoid take-off for handling small amounts is frequently unsatisfactory because of the difficulty in maintaining a good leakproof seat. The conventional fraction cutter is unsatisfactory for use on a small column on two counts: The stopcocks are a source of contamination, and the manipulations involved in changing the receiver frequently result in significant pressure fluctuation within the column unless adequate ballast is interposed on the column side of the cutter. The multiple receiver unit is limited by the number of receivers. Losses of, and cross contamination by, the loxver boiling fractions over a period of time may become very troublesome.
\J \ 1
DRIP T I P
9nm OD.
TRUBORE
TUBE
0.2380 I.D. 10mm O.D.
Figure 1
The head design depicted in Figure 1 offers the following advantages : The magnetically coupled drive is mounted above the condenser. The drive follows the design of Foster and Green [Ax.~L.CHEM.,24, 1869 (1952)l. The lower end of the bearing housing is machined to fit a 19/38 joint. Vibration is practically eliminated by wrapping the joint with a strip of foam rubber and clamping it to the rack. The entire head is in line with the column, and both head and column can be conveniently vacuum-jacketed or externally heated (or, preferably, both). The reflux return is visible Leakproof seating of the solenoid-operated valve is attained by using a machined Teflon plug. The plug is screwed into the lower end of the glass tubing, whose enlarged up er end carries the iron solenoid core. The internal glass t h r e a g are made by
