JOURNAL OF CHEMICAL EDUCATION
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APPARATUS FOR FRACTIONAL DISTILLATION UNDER REDUCED PRESSURE JOHN B. BOWEN University College of Wales, Aberystwyth, Cardiganshire
Tm essential features of the apparatus are shown in Figure 1. The hot vapors from the distilling flask enter the apparatus through the side tube AC. They are condensed in the narrow annular space between the "cold finger" F and the outside wall ED,whence the distillate drains through TI into the receiver which is attached a t R. TI is a two-way stopcock of straight, wide hore. Te is a three-way stopcock whose key has a wide right-angle bore and a tail-leak L. Moreover, the shape of the key handle coincides with the direction of the right-angle bore. The limb Y of Tz is connected to the vacuum pump through suitable traps. The stopcocks are shown in the distilling position. To remove the receiver, T I is first given a quarter turn. Tz is then given a quarter turn clockwise, thereby admitting air into the receiver via the tail-leak L, and simultaneously connecting X and Y. Distillation thus continues via the path XY, the distillate accumulating temporarily a t D. After removing the old and attaching the new receiver, Tz is turned back to its original position, whereupon the new receiver is evacuated and distillation is then continued by reopening TI.
The apparatus is made of Pyrex glass. TIand T zare large stopcocks of good vacuum quality and wide bore (8 mm.) and both are operated from the same side of the apparatus. The hore of L can, of course, be very narrow. The "cold finger" F ends in a blunt taper carrying a sharp dripping point. (The outside wall ED is 25 mm. in bore so that the annular condensing space is 1.5 mm. wide.) The actual diameters of F and ED are not critical quantities provided that the annular space is not more than two millimeters. The dead space D-wherein the distillate accumulates during the changing of receivers-need not be greater than what is normally unavoidable in the constmction of the apparatus. The water leads Wl and WI are connected on the side opposite to that from which the stopcocks are operated and have a wide diameter so as to ensure a rapid upward flow of water past the condensing surface. A and R are standard4aper joints. The most notable feature in this apparatus is the incorporation of an efficient Schirml type of condenser in the device for separating the fractions. This obvi1 SCHIRM, E.,
Z. angel". Chem., 25, 1225 (1912).
APRIL. 1949
ates the necessity for an unwieldy setup involving a separate condenser leading into one of the numerous fractionating devices such as that of Thome.% The design of the condenser prevents external condensation of moisture on the apparatus. Again, because of its clean drainage, it can be successfully used for the vacuum distillation of quite small volumes of liquids. In this connection, the true alignment of F with respect to ED is important, in order that the annular space may be uniform. Otherwise liquid lenses are formed which refuse to drain properly. It is very frequently the case that a volatile solvent has to be distilled off a t atmospheric pressure before a residual product can be distilled under reduced pressure. This apparatus has the advantage that both of these operations can be accomplished consecutively in it without dismantling, thus saving time. Even large quantities of ether can be conveniently dealt with. THORNE, L. T., Bey., 16, 1327 (1883).
The side tube AC enters midway along the 20cm. condensing surface in order that efficient condensation may be maintained during the operations of changing the receiver. The apparatus is fitted with standard interchangeable ground joints for convenience. But these are not indispensable and the apparatus can be readily used in conjunction with an ordinary Claisen distillation flask, the side tube of which is fitted into A by means of a rubber bung. If the Claisen side arm is inserted to within about 1 cm. of the condenser surface, the efficiencyof the latter protects the rubber from appreciable attack by hot vapors. In so far as the author is aware, the first attempt t,o combine a condenser and vacuum fractionating device into one compact instrument is that due to Lmnell and P e r k h 3 The water-jacketed triangle described later by Cloke4is, in all essentials, very closely similar to it except for the fact that Linnell and P e r k i used the Thorne stopcock system whereas Cloke uses a less convenient one involving four two-way stopcocks. Several good stopcock systems for use in vacuum fractionation are already known. Among the best examples are those of K O ~S;~~e i g h t ;Tome, ~ Young and Eby;' or N ~ o n a n . ~A brief study of these systems shows that, the condenser arrangement described here could readily be combined with any one of them to produce a useful condensing fractionator. The author prefers the construction in the present communication since only one special stopcock, Ti,is required, which is of a simple design and very simple in operation. Moreover, it is considered that the Schirm condenser possesses advantages both of construction and efficiencyover the Liebig types suggested by T o m e , Young, and Eby and by Cloke. The dimensions specified above need not be adhered to strictly, provided that the annular condensing space be always kept narrow. For instance, a similar apparatus with a shorter (15 cm.), narrower condenser and smaller stopcocks was found to give very satisfactory general service in the organic teaching laboratories. However, when low-boiling distillates are dealt with (e. g., 35' to 60°C.) it is advisable to use the longer condenser. The author is much indebted to Mr. W. J. Nelson, glassblower to this College, for his elegant constructions of fractionating apparatus of this t y ~ e . 'LINNELL,W. H., College of the Pharmaceutical Society, Univ. of London. Private aornrnnniention. ~ - - ~ ' CLOKE..I. R.. Ind. f i n . Cbm ~~
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' TOWNE, R. S., E. E. YOUNG, AND L. T.EBY, Ind. Eng. Chem., Anal. Ed., 13, 626 (1941). NOONAN, E., I d . Eng. Chem., Anal. Ed., 10, 34 (1938).
