C A R L T IE D C K E
Laboratory of M icrochemistry, New York, N. Y.
O
NE of the most important operations in organic chemical work is distillation. Several excellent working apparatus have been designed during the past 30 years for fractionat
ing macroquantities of liquids under normal and reduced pressures, among which are those of ICubierschky (4), Frie
drichs (2), Widmer (8), Midgley (5), Othmer (6), and Jantzen and Tiedcke (S). The latter, with its specially constructed receiver unit, is widely used as a standard apparatus for the separation of high molecular fatty acids in the form of their esters.
Figure 1. Diagramof Apparatus
Microdistilling equipment, however, has not received the same concentrated attention, despite the fact that improve
ments in microchemical procedures generally have kept pace with the tremendous progress in the chemistry of vitamins, hormones, and other biologicals. This may partly be due to the fact that an accurate microthermometer has not been developed. As a result, microdistillations are used primarily to purify samples and not for sharp separation of fractions as in macrodistillations. In fact, much of the dissatisfaction with microdistillations often stems from attempts by opera
tors to make quantitatively sharp fractionations with equip
ment (including that described herein) which is obviously limited to qualitative purifications. However, arbitrary fractions can be made as a first step to closer refractionation.
Some microdistillation apparatus for making arbitrary frac
tions have been reported, noteworthy among others, those of
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, it 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.
In 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 by the simple expedient of reducing the dimensions of efficient macrodistill- ing apparatus. Consequently a new apparatus was designed in which minimum distance between distilling flask and re
ceiver 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 the parts was made solely on a trial and error basis in various experimental models. Citing the dimension of these earlier models is without 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 at its boiling point.
Unless this principle is observed, reflux action will prevent distillation, especially with high boilers or mixtures with high boiling components.
A pparatus
The apparatus (Figure 1) consists of the distilling flask A with a capacity of 3 to 5 ml. The lower section of neck if 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 to 30 mm. in diameter and into it is fitted condenser E with a ground-glass joint at e. ( f ‘/s 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 ground- glass joint at d. This tube has a 5-mm. hole at 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 dis
tillation 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
82 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 15. No. 1 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 c. (6 is not fused to 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 com
ing into close contact with the vapors, is one of the principal features of the design. Heat is applied according, to require
ments, 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 micro
distillations are usually run without temperature readings, a fair degree of temperature control is obtained by the thermom
eter of the bath or a calibrated rheostat in the hot-plate line. The apparatus may be used for distillation under nor
mal or reduced pressures. Its 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 to 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 separa
tions.
For the third group a mixture of lauric acid (boiling point 225°) and myristie 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 by 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. In the equipment described, mixtures with com
ponents 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 dif
ference of over 50° fractions must be taken off separately.
When continuous separation is attempted, the lower boiling component, depending on the vacuum used, mil either be exhausted by 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 be
comes increasingly difficult because of bumping tendencies, larger losses due to surface wetting, and the tendency for re- fluxing to occur at higher distillation temperatures.
Microdistillation apparatus is intended primarily for puri
fications by means of distillation methods. Several practical cases encountered by the author in his practice illustrate its use.
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 mm. and the last two beakers (1 cc.
each) yielded pure 7-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 iden
tified. 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 micro
distillation equipment, it is difficult to state precisely the exact limitations of any microdistillation apparatus for puri
fication 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 by glass surfaces are of no consequence. The apparatus with the dimensions described in this paper is suited to distilling 2- to 4-cc. samples. The same design properly proportioned can be made for any microquantity down to 0.5 cc. For still smaller samples, the apparatus of Craig (1) is recommended, which collects the distillate as an adhering drop on the indented tip of the con
denser.
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 ar
bitrary 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. It 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 to 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 it for all practical purposes a truly universal microdistilling apparatus.
L iteratu re C ited
(1) Craig, L. C., I n d . E n o . Chem., A n a l . E d ., 8, 219 (1936); 9, 441 (1937).
(2) Friedrichs, Z. angew. Chem., 32, 340 (1919).
(3) Jantzen, E., and Tiedcke, C., J. prakt. Chem., 127, 277-91 (1930).
(4) Kubierschky, Z. chem. Apparatenkunde, 3, 212-16 (1908).
(5) Midgley, T., Jr., I n d . E n o . Chem., A n a l . E d ., 1, 86 (1929).
The application of the polarographie method of an
alysis expands steadily. Some of the analysis being made with the Heyrovsky Polarographs now in use include the analysis of brass; of steel and iron; of lead, magnesium, nickel, and zinc alloys; of metallic impuri
ties in aluminum; of lead and zinc in paints; of major constituents in plating solutions; and the differentia
tion of waters. Accuracy; rapidity; the possibility of detecting and identifying minute quantities and of making simultaneous determinations of several com
ponents; small sample requirement; preservation of sample; and permanent photographic recording of
every analysis are some of the reasons why the Heyrovsky Polarograph is becoming so widely accepted.
The procedures established thus far by no means de
fine the field of polarography— the perfected instru
mental system of the Heyrovsky Polarograph creates unlimited possibilities for analytical and research ap
plications.
A bibliography of more than 700 papers dealing with the polarographie method of analysis and a booklet discussing the Polarograph and polarographie analysis are available without charge on request.
E. H. SARGENT & CO., 155-165 E. Superior St., Chicago, Illinois
M ich ig a n D ivision: 1959 East Jefferson, D e tro it, M ich ig a n
S A R G E IV T
S C I E N T I F I C L A B O R A T O R Y S U P P L I E S
H . R E E V E A N G E L & C O ., IN C . 7-11 Spruce St., New York, N. Y.