A SIMPLIFIED CONSTRUCTION FOR VACUUM DISTILLING APPARATUS RAY WENDLAND North Dakota State College, Fargo, North Dakota
C o m R o m m vacuum distillation with provision for taking several fractions is one of the most valuable methods available for the isolation of pure compounds from distillable mixtures. All too often in the college instructional laboratory the technique is neglected, or performed with a variety of primitive contraptions.' The writer has fabricated apparatus of sufficiently simple constmetion that it can be assembled by a person of average ability in glass blowing-presumably the college instmctor or one of his skilled students (See Figures 1 and 2). The apparatus constructed was worked out with several essential considerations in mind, namely (1) the apparatus should be all qlass with standard taper (or spherical) joints, thus eliminating all possible cont,amination from rubber connections, ( 2 ) the apparatus should operate under continuous vacuum such that the distillation does not suffer from interruptions inherent in the usual fraction cutters and the Noonan apparatus (see footnote), (3) a short fractionating column should be incorporated, and (4) the column, condenser, and fraction distrihutor ahould be of one-piece constmctiou to eliminate leaks as much as possible and to simplify setting up. The last consideration was based on the premise that vacuum disFigure 1. Conrtr"cti0n Dntaile fol vacuum Distilung IIpparatu. tilling is essentially a more complicated operation than most elementary laboratory processes; hence The outcome of these studies is the apparatus picdistilling apparatus should be permanently mounted and be ready for immediate use when the occasion tured in Figure 1. Figure 2 shows a similar device intended as a fraction cutter for use on a fractionating arises. column. ' A somewhat standard operation, whieh is still in vogue Items required for the construction of both pieces after some sixty gears, employs a Claisen Hask attached t o a regular distilling flask as s. receiver, the latter being chilled by a of apparatus are described below. m~andemlgstream of water and evacuated through the side arm vhich is canneoted first to a, trap and then to a water pump. [Several standard texts describe the method, e. g., the laboratory manuals of Adam8 and Johnson (Macmillan Co., 1949), Boord, Brode, and Bossert (Wiley, 1949), and Fioser (Heath Co., 1941).] The grand sum of one contaminated fraction can he collected during a single performanc-provided that one's neighbor does not cause the water pump to hack up by conducting a suction filtration. More complex assemblies employ fraction cutters, the design oi which has evolved into something of a fine art in research laboratories. An ingenious one is that of Noonan [ I d . Eny. Chem. Anal. Ed., 10, 34 (1938)l which combines all necessary functions through a single stopcock, which is however of intricate construction and beyond the reach of the average glass blower. Noonan'n's apparatu8 suffers from a basic weakness, namely that during the separation of a.given fraction and evacua, tion of the next receiver, the distilling f a & is temporarily disconnected from the pump; inevitably there is an increase in pressure in the system and interruption of the distilling process.
Item
Description"
A, B . . . . . . . 29/42atandard taper joints C . . . . . . . . . . 35/25 hall joint D... . . . . . . . Reducing adapter for thermometer 10130 to 29/42 E . . . . . . . . . . West type condenser (200to 300 mm. long) F . . . . . . . . . . Claisen flask "T" conneotion taken from a standard 50C-ml. Claisen Hask G . . . . . . . . . . Dintrihutor tuhe, 8 mm. O.D. If... . . . . . . Connecting tubes (4), 11 mm. O.D. spaced 90 degrees around Bask I I . . . . . . . . . . 500-ml. Hssk with 35/25 hall joint J.. . . . . . . . . 29/42 distilling flask K .. . . . . . . . 10/30 standard taper thermometer 0-250°C. L . . . . . . . . . . Connector tuhe for vacuum pump, 8mm. M... . . . . . . Large bore stopcock
All Darts are Dvrex.
To the expert, construction of the apparatus will
JUNE, 1951
follow readily from the figures. However, the writer, after several bad experiences, considers it worth while to give a few detailed directions for the benefit of the amateur or occasional glass blower. Of all the connections shown, the one most troublesome is likely to be the rounded "T" joint F in Figure 1. After several attempts I gave up the idea of fabricating my own from large diameter tubing, and instead used a section of a 500-ml. Claisen flask as noted in the table. After taking off the side arm of the Claisen flask the upright tube was sealed to the thermometer joint A. The full length of this joint A is about 130 mm. below the ground surface; the combination of the two makes a fractionating column of convenient length. At the upper end of this column the condenser is sealed, as shown. The use of the spherical (ball and socket) connection on the flask I is a novel feature. By its use the flask can easily be rotated to collect several different fractions and at the same time make perfect connection into the delivery tubes. These delivery tubes could a t the same time be attached to standard taper joints with drip tips. This feature would be desirable, but of course adds to the cost and complexity of the system. It should be mentioned that a spherical flask such as I instead of the commonly used flat Erlenmeyer is an improvement in safety of operation, for the flat vessel will have much less structural resistance to collapse under atmospheric pressure than the sphere. Furthermore, the spherical joint and flask are commercially available as a single unit-an added advantage. Further improvements of the apparatus can be envisioned. In a separate model, the writer provided a sealed and evacuated insulating jacket around the fractionating tube above F (Figure 1) which is then silvered for better heat control. The increase in glass fabrication difficultiesis notable, and for some may not be considered worth the hazards involved. USE OF A m C T I O N A T I N G COLUMN
Since many types of fractionating columns have come into common use (e. g., the Widmer spiral and the helix packed column) vacuum distillation through such columns requires a special take-off device or "fraction cutter." Again with the object in mind of designing an easily constructed device that permits uninterrupted distillation, the writer has built the apparatus shown in Figure 2. The fraction arising from the column passes into the Claisen type side arm and comes into contact with the vertical condenser, from which the condensate passes through a dropper and falls into the stopcock tube. The collection and distribution of the fractions are made by exactly the same means as shown in Figure 1. The top of the condenser is sealed by means of 10-mm. tubing to the spherical male joint which supports the collection flask, this tubing at the same time being opened through a "T" for connection to a manometer and then to the vacuum pump. Thus the pump pulls simultaneously a t the top of the condenser and
through the receiver to provide uniform pressure distribution. The apparatus described has been in service in this laboratory for the past year, both for student instruct ~ o nin vacuum distilling technique-for which it was specifically designed-and for research purposes, for which it is entirely adequate. As an operational feature it cannot be argued too strongly that good distilling of this type requires a motor-driven vacuum pump, unless an unusually high-capacity aspirating system is available. All too often the ordinary desk aspirator fails at the critical moment. When one uses a standard motor-driven pump the usual precautions for protecting the pump from water and corrosive vapors should of course be duly observed. The advantage gained in providing students with first-rate dist,illingequipment was considered worth the time required, and the results are recommended for other instructors. I t may he added &ally that single installations of such equipment when kept in constant use will accommodate the needs of a large number of students in rapid sequence, who would otherwise be spending a great deal of time setting up the cumbersome and crude devices mentioned earlier.
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