A Fractional-Distillation Microapparatus CARL TIEDCKE Laboratory of Microchemistry, New York, N. Y.
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NE of the most important operations in organic chemical
Craig (1) and Shrader and Ritzer (7'). The limitation of microdistilling equipment to purifications cannot be too strongly emphasized. Because as much as 5 to 20 per cent of the initial sample in distillations, for all practical purposes, can be considered lost, owing to retention of liquid by the surfaces of the apparatus, i t becomes obvious that microdistillation apparatus should be designed with minimum surface areas and minimum distance between the distilling flask and receiver. Only in this way can maximum recovery of sample and distillate be obtained. Unfortunately, some of the published designs do not meet this very important requirement. I n his widely diversified microchemical practice, the author repeatedly encountered need for improved microdistilling apparatus. It was apparent that suitable apparatus, to meet these needs and requirements, could not be built b y the simple expedient of reducing the dimensions of efficient macrodistilling apparatus. Consequently a new apparatus was designed in which minimum distance betn-een distilling flask and receiver was obtained by using an "inside receiver" and which also permitted collection of the arbitrary fractions without interruption of distillation. Because of the size and shape of the new apparatus, exact calculations are either extremely difficult or impossible, and proper proportioning of t h e parts was made solely on a trial and error basis in various experimental models. Citing the dimension of these earlier models is n-ithout value. For most purposes, the dimensions of larger or smaller equipment to purify materials with boiling points falling between 60" and 300" C. can be made proportional to those of the unit here described. If departure must be made from the proportions given, a very elementary principle of distillation should be rigidly adhered to-i. e., the volume of the distilling flask plus the volume of the condenser chamber and connections should be substantially less than the volume which the sample will occupy when transformed into vapor a t its boiling point. Unless this principle is observed, reflux action will prevent distillation, especially with high boilers or mixtures with high boiling components.
work is distillation. Several excellent working apparatus have been designed during the past 30 years for fractionating macroquantities of liquids under normal and reduced pressures, among which are those of Kubierschky ( d ) , Friedrichs (Z),W d m e r ( 8 ) ,Midgley ( 5 ) ,Othmer (6), and Jantzen a n d Tiedcke (3'). The latter, with its specially constructed receiver unit, is widely used as a standard apparatus for t h e separation of high molecular fatty acids i n the form of their esters.
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Apparatus FIGURE 1,
D I A G R A M OF
APPARATUS
The apparatus (Figure 1) consists of the distilling flask A with a capacity of 3 t o 5 ml. The lower section of neck B is 5 mm. in diameter; its upper section, 10 mm. in diameter, connects the flask with the chamber, C. This chamber has two openings; the one on top is 25 t o 30 mm. in diameter and into it is fitted condenser E with a ground-glass joint at c. (7 '/2 ground-glass joints, while not specified for the model which the author is currently using in his practice, will be used in future units.) The condenser is drawn out to a tip and is cooled by a stream of cold water which enters through the glass tube, i, filling the condenser and emerging at o. Since the effective condenser surface is small, tubes i and o are of a relatively large diameter (5 mm.) to permit very rapid replacement of cooling water. Tube D, 20 to 25 mm. in diameter, lies within C and is fitted with a groundglass joint at d. This tube has a 5-mm. hole a t p through which the distillate drops from the tip of the condenser into the receiver unit, F. This unit consists of 3 small beakers of 1-ml. capacity each, made of glass and attached to each other by fusion. It is placed in tube D through the opening which is closed during distillation with rubber stopper e. D has a slightly flattened floor to provide a better footing for the receiver. The two outer beakers of the receiver unit are fitted with glass buttons elevating the beakers 0.6 cm. (0.25 inch) above the flattened floor of D to insulate the beakers, so that reheating of the distillate is re-
Microdistilling equipment, however, has not received t h e same concentrated attention, despite the fact t h a t improvements in microchemical procedures generally have kept pace with the tremendous progress in t h e chemistry of vitamins, hormones, and other biologicals. This may partly be due to t h e fact t h a t an accurate microthermometer has not been developed. As a result, microdistillations are used primarily t o purify samples anti not for sharp separation of fractions as in macrodistillations. I n fact, much of t h e dissatisfaction with microdistillations often stems from attempts by operators to make quantitatively sharp fractionations with equipment (including t h a t described herein) which is obviously limited t o qualitative purifications. However, arbitrary fractions can be made as a first step to closer refractionation. Some microdistillation apparatus for making arbitrary fractions have been reported, noteworthy among others, those of
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tarded as much as possible when running high boiling point samples. The receiver is moved under the condenser tip by means of the glass rod, b, which passes through the bore of stopper e . ( b is not fused t o the receiver unit, as might appear from the picture. Adjustment of the receiver unit could perhaps be made easier at first by providing a glass loop on the unit to be engaged by a hook on the end of b.) Since the rod must fit tightly, it may be lubricated with glycerol if necessary. The apparatus is easily and quickly assembled and cleaned. Operation is very simple. The liquid to be distilled is placed in A through the top opening by means of a pipet. For vacuum distillations, a porcelain chip is added to suppress bumping. However, even if bumping occurs, the collected distillate is not likely to be contaminated, since the receiver is almost entirely shielded from splashed or entrained liquid. This protective arrangement, which also prevents the rubber stopper from coming into close contact with the vapors, is one of the principal features of the design. Heat is applied according to requirements, by direct flame, a constant-temperature bath, or electric hot plate. For best results heat should be slowly and carefully applied. All the distillations cited, including the acetone-ethyl alcohol mixtures, required a minimum of 15 minutes. Since microdistillations are usually run without temperature readings, a fair degree of temperature control is obtained b y the thermometer of the bath or a calibrated rheostat in the hot-plate line. The apparatus may be used for distillation under normal or reduced pressures. I t s efficiency has been tested with many liquids with boiling points from 60" to 300" C. as follows: Three groups of liquids were selected for tests with boiling points under 120' for distillation at normal pressure, 120" to, 200" for distillation at about 15 mm. pressure, and 200" to 300 for distillation at about 0.1 mm. For the first group, 3-cc. samples of acetone and ethyl alcohol mixtures were made in the ratios of 1 to 1, 1 to 2, and 1 to 3. On distillation of the 1 t o 1 mixture, the first fraction of about 1 cc. was found to be pure acetone, as evidenced by refractive index and odor. The third fraction, also of about 1 cc., was found to be pure ethyl alcohol. Similar good results were obtained for the other ratios. For the second group a mixture of benzaldehyde (boiling point 179") and benzoyl chloride (boiling point 198") was distilled at a pressure of about 15 mm. Carbon and hydrogen determinations showed the first and third fractions to be practically pure separations. For the third group a mixture of lauric acid (boiling point 225 ") and myristic acid (boiling point 250") was distilled at 0.1 mm. and gave almost complete separation of these two fatty acids. Separate refractionation of the first and third fractions then yielded pure fatty acids as determined by elementary analyses. The efficiency of separation of any distillation equipment is governed b y many factors. The one factor which imposes greatest limitation on a microdistillation apparatus is that of the range of difference in boiling points of components in a mixture, I n the equipment described, mixtures with components having only a 20" difference in boiling points are readily separated without interruption to collect the lower fraction. As the difference increases to about 40" continuous fractionation becomes increasingly difficult, and with a difference of over 50" fractions must be taken off separately. When continuous separation is attempted, the lower boiling component, depending on the vacuum used, will either be exhausted b y the vacuum system, or condensed, reboil, and also be discharged by the vacuum system. Thus for a mixture involving components with more than a 50" difference in boiling points, this apparatus can recover only one component continuously, though both may be recovered if the convenience of continuous operation is sacrificed. For materials with boiling points above 250°, fractionation becomes increasingly difficult because of bumping tendencies, larger losses due t o surface wetting, and the tendency for r e fluxing to occur a t higher distillation temperatures. Microdistillation apparatus is intended primarily for purifications b y means of distillation methods. Several practical cases encountered by the author in his practice illustrate its use.
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A research residue of about 5 cc. was submitted for purification and confirming identification. The sample was thought to be quinoline (boiling point 238") containing about 10 per cent of aniline (boiling point 184'). On distilling a 3-cc. sample, the last two beakers of the receiver (about 1 cc. each) contained pure quinoline as determined by elementary analysis. The fraction in the first beaker proved to be aniline. Another research residue consisting of 7-picoline (boiling point 143") containing about 20 per cent of a-picoline (boiling point 128") was separated at 15 nim. and the last two beakers (1 cc. each) yielded pure ?-picoline checked by refractive index. Ten per cent of the 4-cc. sample distilled was lost. In the above cases both components were recovered and identified. An example of wide differences in boiling points was presented by purification of a glycerol-water mixture submitted for identification of glycerol. Because a vacuum of 0.1 mm. was used, the water estimated to be about 10 per cent could not be collected, since at this pressure it passes into the vacuum system without condensing. Pure glycerol was obtained and checked by both refractive index and elementary analysis. Only 75 per cent of the glycerol was recovered. The comparatively high loss of 25 per cent was caused by heavier wetting films due to high viscosity. While these examples generally indicate use of this microdistillation equipment, it is difficult to state precisely the exact limitations of any microdistillation apparatus for purification work. Each problem requires individual handling and successful use is largely dependent upon the skill and judgment of the operator. Since the term "microquantities" is purely relative, samples smaller than 5 cc. are, in the author's conception from the standpoint of distillation, considered "micro" and are best distilled in the apparatus described. For samples larger than 5 cc. macro designs can be employed because in most instances losses of sample due to retention b y glass surfaces are of no consequence. The apparatus with the dimensions described in this paper is suited to distilling 2- to 4 c c . samples. The same design properly proportioned can be made for a n y microquantity down to 0.5 cc. For still smaller samples, the apparatus of Craig ( I ) is recommended, which collects the distillate as an adhering drop on the indented tip of the condenser. In the design of the apparatus described, the size of the receiver beakers is a critical factor for determining the other dimensions of the apparatus parts. If, for example, 1 cc. of liquid is to be distilled and three arbitrary fractions are to be taken, the total capacity of the three receiver beakers need not be larger than 1 cc., thus making the volume of each beaker about 0.3 cc. Such a receiver unit can be constructed much smaller than a receiver unit of 3-cc. capacity, and hence tube D, chamber C, and condenser E can all be of smaller dimensions. I t also follows that as the number of beakers in a receiver unit are increased, their individual capacity must be decreased in order to maintain their proper relation t o tube D, location of hole p , and the condenser tip.
Since parts can be easily made in proportion, an apparatus of proper size can be made for any microquantity from 0.5 to 5 cc., making i t for all practical purposes a truly universal microdistilling apparatus.
Literature Cited (1) Craig, L. C., IND. ENQ.CHEM.,ANAL.ED., 8,219 (1936); 9,441
(1937). (2) Friedrichs, 2.angew. Chem., 32, 340 (1919). (3) Jantzen, E.,and Tiedcke, C., J.prakt. Chem., 127,277-91 (1930). (4) Kubierschky, 2. chem. Apparatenkunde,3 , 212-16 (1908). (5) Midgley, T.,Jr., IND. ENQ.CHEM.,ANAL.ED., 1,813(1929). (6) Othmer, D. F., IND. ENQ.CHEM.22,322-5 (1930). (7) Shrader, S.A., and Ritzer, J. E., IND. ENQ.CHEM.,ANAL.ED., 11, 54 (1939). (8) Widmer, Helu. Chim. Acta, 7, 59 (1924). PRESENTED before the Division of Analytical and hlioro Chemistry at the 104th Meeting of the AXERICAN CHEXICAL SOCIETY, Buffalo, N. Y.
