IKDUSTRIAL AND ENGIKEERIKG CHEMISTRY
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viewed against diffused light the lower edges of the sleeves and the meniscus of the mercury show up against a white background as sharply defined straight and curved lines. T h e n the sleeves are adjusted so that they seem to touch the tops of the mercury columns the absolute pressure is represented by the difference in height between the edges of the sleeves. (A gage with arrangements for reading to 0.1 mm. is manufactured by the Scientific Glass Apparatus Company, Bloomfield, Pi. J., U.S.Patent 2,075,326, March 30, 1937.) The reasons for choosing a comparatively wide diameter for the limbs of the manometer are threefold: 1. The mercury meniscus is independent of forces of capillary
attraction. 2 . The visibility is greatly improved, even if the inside of the glass becomes dirty after long use. The production of a film on the glass can be minimized by avoiding contact of the mercury with rubber, by using clean mcrcury free from other metals (b), and by t’rapping dust and mist by an appropriate filter. 3. Air, which after long use or by too sudden release of vacuum may get into the reference limb, will collect a t the t,op of the bend in C. If the air bubble has a diameter of 0.2 mm., a size clearly visible t o the naked eye, its volume would be 0.42 cu. mm. at 7.6 mm. of mercury (0.01 atmosphere) and 4.2 cu. mm. at 0.76 mm. of mercury (0.001 atmosphere). -445the cross section of B
VOL. 10, KO. 5
is 200 sq. mm., the height of the air layer would be 0.0021 mm. at 7.6 mm. of mercury, and 0.0210 mm. at 0.76 mm. The error caused in the reading would be 0.03 and 2.8 per cent, respectively. Even such small errors can be avoided by driving the air out of C in the following manner: The gage is tilted to the right until the mercury level in D approaches the bot’tom, and is connected with the vacuum line while in thie position. When the gage is evacuated, it is tilted to the left until the mercury flows over the top bend of C, pushing the air out into D. When the level in -4 approaches the bottom, the gage is put back in its vert,ical position. For correct reading it is necessary to have the gage in a perfectly vertical position. Other precautions to be observed, especially for pressures of 2 mm. and less, have been discussed repeatedly in the literature (1, 3).
Literature Cited (1) (2) (3) (4)
Burton, M.. IXD.EYG.CHEM.,Anal. Ed., 9 , 335 (1937). Easly, H. F., Ibid., 9, 82 (1937). Hickman, K. C. D., Reo. Sci. Instruments, 5, 161-4 (1934). Kechenberg, C. van, “Einfache und fraktionierte Destillation,” Miltitz hei Leipzig, Schimniel 8: Co., 1923.
RECEIVED February 12, 1938. Journal Series paper of t h e New Jeraey Agricultural Experiment Station. D e p a r t m e n t of Agricultural Biochemistry.
High-Vacuum Fractional Distillation without Gravitational Reflux G. VOh ELBE
*%DB.
R. SCOTT. Coal Research Laborator?. Carnegie Inqtitute of Technolog?, Pittsburgh, Pa.
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\ I I I SCHEhLlTIC REPRESESTATIOS O F A N .%PP.IR.ITUS FOR HIGH-VACUCM FRACTIOS l L DISTILLATIOX AS U 6 E D FOR S E P l R A T I o X OF
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HE usual method of fractional distillation involving gravitational reflux cannot be applied to the separation of mixtures of substances of low rapor pressure, since, to maintain a reasonably fast reflux rate, the vapor pressure of the components must, in general, exceed 1 mm. Furthermore, this method is not applicable to liquid volumes smaller than a few milliliters, since the holdup losses then become a significant fraction of the total input. K i t h the following apparcntly not previously described method, which is free from
gravitational reflux, the mixture to be fractionated is placed in a rather long, evacuated glass tube, along which for a certain distance a temperature gradient is maintained by a thermostat system. The mixture tends to accumulate a t the low-temperature end of the gradient, which is the coldest part of the tube; by pulling the latter slowly and uniformly through the gradient in the direction toward the warm end. the mixture can be made to distill continuously within the gradient and to separate more or less completely into its com-
MAI- 15. 1938
.%NALYTICAL EDITION
285
most identical, 139" and 138" C., respectively p-Xylene melts at 13.2", and below this temperature its vapor pre.swre is lower than t h a t of m-xylene, which melts at -51 '. With the heater a t temperatures below 13.2", it was possible t o follow the separation by visiial observation of t h e c r y s t a l s emerging from the liquid mixtures. Satisfactory distilling rates within the gradient were obtained if the temperature at the warm end was kept anywhere between 0" and - 2 5 " , anti a t the cold end - 50 " or lower. The sample, 0.1 to 0.01 cc., was contained in a small glass tube which wab placed in the distilling tube, which was mm. of mercury then evacuated to < and sealed. The sample tube 11-as then broken by thermal expansion by applying a small flame mommtarily. At the beginning of the process, the volatile I+ MV material was allowed to condense in the coldest zone of the tube, rvithin the cooler. The tube was then pulled, initially, a t a rate such that the substance remained within the gradient. Separation soon occurred, p-xylene cryst I ing at the n-arm end, a tube at the bottom of Figure 1. The pulling speed x-as then slo~vlyraised until the crystals left the gradient and enFIGURE2. THERMOST~TIC C O S T R O L F O R HEATER A S D COOLER tered the heater zone, which Tvas 35 cni. R , reservoir a n d regulating valve for toluene. T . b y a h i c h t h e temperature of heater or coolei. T r , (14 inches) long. The process was conmay be controlled. I the following procedure was evolved. This method was found to b e easy to carry out, and yielded a sturdy product of high sensitivity. Using 9-mm. Corning 01.5 glass, the parts shown in Figure 1, A , B , C, are prepared. A is formed by breaking off about half of a thin bulb blown on a section of 2- to 3-mm. tubing drawn from the 9-mm. stock. B is a short piece of 9-mm. tubing with an attached spindle to serve as a handle, and C is a section of the original tubing 10 to 12 cm. long. To prepare the electrode, B is heated to softening at the open end and attached lightly to A , following which C is heated and sealed to B , over A , as shown in D and E. The seal is then heated uniformly and gently blown out t o give F . The excess glass is cut off by a hot wire at the positions indicated by the dotted lines in F .
If the electrode is sufficiently sensitive, a spot of interference figures 9 to 16 sq. mm. in area ail1 appear in the diaphragm.
The thickness of the bulb blown in preparing A controls the diaphragm thickness for any final electrode size. It is not important or usually possible for the capillary to remain in the center of the diaphragm. The electrode is ready for use after a preliminary soaking in approximately 0.1 A' hydrochloric acid for at least 36 hours. As used in this laboratory, s ~ ~ ~ $ $ $ ~ ~ the electrode is rinsed with cell. distilled water, superficially dried with filter paper, and filled to the level indicated b y the arrow in F with a saturated solution of quinhyY drone in approximately 0.1 i hydrochloric acid. A bare p l a t i n u m w i r e is then ins e r t e d t o m a k e electrical contact. A convenient as- _ _ s e m b l y is s h o w n i n F i g ure 2. T h e asymmetry potential which always develops across the diaphragm FIGURE 2. E L E c T R o D E ASSEMBLY is determined on a solution of known pH value. The precision obtained withathis electrode used in conjunction with a Leeds 8: Northrup enclosed lamp and scale galvanometer and a potentiometer reading to 0.1 millivolt is better than 1millivolt or 0.02 p H unit. The advantages of this electrode are its mechanical stability, sensitivity (permitting its use with apparatus usually available), and ease of manufacture. Obviously, this type of construction can be modified to suit individual requirements.
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A simple procedure for making a very sensitive, durable glass electrode is given. Accurate measurements may be made with this electrode using a portable galvanometer with a sensitivity of the order of 40 megohms and an ordinary galvanometer.
: Literature Cited (1) Mouquin and Garman, IXD. ENQ.CHEM., Anal. Ed., 9,287 (1937) ~
FIGURE 1. PREPARATION OF ELECTRODE
RECEIVED February 28, 1938.
