Immersion Still Head for Low-Pressure Distillation of Organic Mixtures A . J. BAILEY, University of Washington, Seattle, Wash.
LOW-PRESSURE still head was designed and used
characterized by high boiling point (100" to 300" a t 1 mm. of mercury), viscous or tarry consistency, and an extreme tendency to bump, froth, and spatter. Frequently, the formation of a leathery skin on the surface added to the other difficulties. while antibump devices such as a n immersed wire coil heater could not be used because of the viscous medium. This immersion still head was used successfully on these mixtures, even those with a skin on the surface merely required a longer time of distillation. The explanation appeared to be that the flask did not have to be heated so strongly and hence the lower temperature differential caused less superheating. Inability conveniently t o trap all vapors of some mixtures led to the substitution of special mercury pumps and a water aspirator for the usual large-throated mercury pump and mechanical pump, thus avoiding fouling the oil in the latter. Mercury diffusion pumps which require a fore-pressure of 25 to 30 mm. and produce a pressure of 25 to 50 microns were described b y Kraus (I), Munch ( 2 ) ,and others. Two of these mercury pumps, of slightly different size, were used in series with a n aspirator as t h e forepump. This pumping system gave extremely satisfactory service in conjunction with t h e immersion still head dewribed above. Attempts at trapping all organic vapors n-ere successful only when elaborate apparatus was used; the aspirator and mercury pumps provided a simpler and more satisfactory solution, since the pumps were easily disconnected and cleaned.
successfully on exceedingly refractory organic reaction mixtures. These included high boiling liquids and tarry and resinous materials and were characterized b y an excessive tendency to froth, bump, spatter, and form a leathery skin on the surface. The success of this still head in promoting smooth distillation suggested that i t would be equally valuable in other laboratories.
Design of Still Head The usual minimum pressure at which the still head operates is of the order of 1 mm. of mercury, or with many solids, a somewhat lower value, approximating the action of a crude molecular still. The details of construction of the still head are apparent b y reference to Figure 1. If the distillate was liquid, the still head functioned as a condenser and vapor-lift pump; the drops collecting at the bottom of the condenser were lifted through the sniall central tube by the difference in pressure and allowed to run down through the liquidvapor separator into the sample bottle. In actual operation, the drops were lifted smoothly and uniformly, chiefly as a bubblefilm in the tube. With high melting solids it was necessary to allow the condenser to run hot to keep the distillate liquid. By grinding off the ends of the delivery t'ubes to an acute angle, drops were discharged uniformly from the extreme ends of the acute tips without a tendency of the liquid to back up in the tubr, bridge the walls, and block the tube with liquid. The still head, drawn accurately t o scale in Figure 1, fitted either a 2-liter (as shown) or a 1-liter standard Pyrex flask. Snialler sizes of still heads in smaller flasks were used with equal success. Some adapters accommodat'ed two or more sample bottles a t the same time, permitting taking several fractions without breaking the vacuum. A small loss of pressure occurred in lifting the liquid through the small tube out of the flask. By using: a small tube, the liquid bridged before a large drop collected, andascended as R .ingle bubblefilm. I t v-as usual to have only one but)ble-film in the tube at a time; the pressure drop from lifting \vas less than 1 mm. of mercury. There was no
Effectiveness of Still Head All viscous, tarry organic reaction mixtures encountered in this laboratory hare been successfully distilled with inimersion still heads. Previous attempts a t separation with other devices in\-ariahlg failed. Still heads of this type operate in the important gap between vacuum distillation and molecular distillation. Prior to the use of the immersion still head, it was customary t o remove all rolatile material in ordinary vacuum distillation apparatus, then put it directly into a molecular still. It was found, however, t h a t high-boiling oils invariably remained in the material and volatilized in t h e lon--pressure atmosphere of t'he molecular still, in spite of meticulous graduat,ion and adjustment of temperature and pressure, to such an extent that the ent'ire tubing and pumping assembly condensed a film of the volatilized material and limited the pressure of the entire system to the vapor pressure of this condensed film. K i t h the immersion still head, however, these residual, high boiling fractions were easily removed, SO that they did not) foul the molecular still system. Probably the most important advantage of the immersion still head over standard vacuum distillation apparatus is that the latter, even a t the end of distillation, are still refluxing high-boiling fractions on t'he walls of the flask. The immersion still head, 011 the other hand, by bringing the condenser to within a few millimeters of the heated material, easily remores t,hese fractions which usually reflux on the walls. T h a t this difference is important is emphasized and best illustrated by the facile and clean-cut, separation of the extremely refractory tarry and resinous mixtures noted above.
Other
drawback to using the same tube as vacuum line and distillate line. If the was solid, it was deposited on the condenser.,and periodicalk removed n-ith n solvent.
Operation
CENTIMETERS
FIGURE 1. IMMERSIOK STILL HE.\D A , Vacuum line 8.Vapor-lift tube, 3 mm. in
diameter
This typeof still n-as developed to effectseparation of experimental mixtures Tvhicli resisted all attempts a t separation in other types of apparatus. T h e s e mixtures were
Literature Cited (1) Kraus. C . A . , J . A m . Chem.Soc.,39,2183 (1917). ( 2 ) Munch, R. H.; Science, 76,170 (1032).
71
