Constant-Temperature Bath. for Molecular Stills 0. A. NELSON AND H. L. HALLER Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Washington, D. C.
T
HE desirability of maintaining accurate control of tem-
perature during distillation processes has been known for a long time. It is particularly important in processes involving fractional separation of two or more components from a mixture, regardless of whether the distillation is carried out a t atmospheric pressure, under slightly reduced pressure, or a t high vacuum. While it is true that if a perfect vacuum could be obtained a liquid would give off vapor a t any temperature, nevertheless for liquids of comparatively high boiling points the rate of distillation a t room temperature would be so low that no appreciable quantities could be obtained in experimental time. At elevated temperatures the rate of distillation is increased, but the rate may vary for different substances, and it is just this difference in rate of distillation that permits of separation of phlegmatic liquids from one another by the process of fractional distillation (1). Various methods have been proposed for the control of temperature during distillation in the molecular still. In most cases such devices are complicated, require considerable attention, or cannot be depended upon to give satisfactory control. An apparatus that provides a constant temperature with a minimum amount of attention is shown in Figure 1. It consists essentially of a boiler, B, containing a constant-boiling
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TO REFLUX CONDENSER
MOLECULAR
liquid, a h e a t i n g unit, a n d a s i d e t u b e f o r a reflux c o n d e n s e r . The b o i l i n g flask was made from a 750-cc. Kjeldahl flask by heating the glass in the bottom to softness and then drawing it in t o form a cup or cavity havi n g a diameter of about 7.6 cm. The d i a m e t e r o f the boiler is 5 em. and its height t o t h e lower surface of the cup is a p p r o x i mately 16.5 cm. A satisfactory heating ro HIQH coil was made from VACUUM PUMP No. 26 or 28 Nichrome wire of such l e n g t h as to give about 25 ohms resistance. A variable resistance was inserted in s e r i e s with the heater so as to permit control of the rate of r e f l u x i n g of the boiling liquid. The m n 1e c u l a r still ( B u r e a u o f EONSTANT Standards type, 2) BOlLlNQ used with this conLIQUID stant-temperature bath has a diameter of 6.7 em., and the s n a c e between it FIGURE 2. CONSTANT-TEMPERATURE aAd the wall of the BATH cavity is filled with a high-boiling mineral oil, which functions as a heat conductor between the vapors of the constant-boiling liquid and the material in the still. Another form of the same apparatus is illustrated in Figure 2. The still proper (Hickman t pe) is 3.8 cm. in diameter and the boiler is 5 cm. in diameter anlabout 16.5 cm. long.
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The principal difference between the two modifications is that in the apparatus shown in Figure 1 the heat from the vapors is conducted to the still by means of a high-boiling mineral oil, while in the one shown in Figure 2 the hot vapors strike the bottom of the still directly. The apparatus herein described has the advantages of simplicity, ease of construction, and reliability. I n addition, when a pure compound is used as a constant-boiling bath, the necessity of determining the temperature of the material in the still is eliminated. By so controlling the rate of refluxing that no apparent condensation takes place on the bottom of the cup or still, constant temperature can be maintained for any desired length of time. Both constant-temperature baths and stills have been used in this laboratory over long periods of time with satisfactory results.
Literature Cited (1) Hickman, K., J. FranXlin Inst., 213, 119 (1932). (2) Mair, B. J., Schicktani, 8. T., and Rose, F. W., J. Research Natl. Bur. Standards, 15, 557 (1935).
FIGVRE 1. CONSTANT-TEMPERATURE BATH
RECEIVED May 27, 1937.
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