Vacuum Distillation Apparatus for Microquantities 11. J. BABCOCK' Northwestern L'niversity, Evanston, Ill.
may be replaced or circulated by using an aspirator to draw off the excess as cold water is introduced. After the rubber tubing has been disconnected, the fractions from A , B , and C are removed by capillary pipets or capillary siphons, or by drawing up directly into capillaries to be used in various tests. A convenient device is a tube 3 mm. in diameter, drawn out to a curved capillary a t one end and with a 15-mm. bulb a t the other end. The bulb is warmed and gradually draws up the liquid as it cools.
CHNEIDER (3) has recently reviewed apparatus for disS tillation of microquantities. Other forms have been described by Gould et al. Though there is frequent need for (1).
purification of small quantities of liquids in identification work, such apparatus is not widely used because of difficulty in construction. The apparatus described below may be readily constructed from a test tube, a piece of glass tubing, and two rubber stoppers by a person unskilled in glass blowing.
The small surface area and absence of ground joints minimize holdup due to the formation of a film over the glass surfaces. As the second fraction does not pass over a condenser or fraction cutter used for the first fraction, it is not contaminated hy traces from the first fraction. Because the fractions are not formed by dropwise transfer from a condenser ( I ) , there is no theoretical lower limit to the quantity that can be distilled. Heat rising between the tubes from the oil bath prevents oxcessive heat loss from the column. The all-glass construction without joints avoids the possibility of leakage or contamination with stopcock lubricant. The apparatus is quickly set up and may be readily cleaned by filling with solvent. The apparatus would appear to be useful for sealed tube reactions a t moderate pressures followed by distillation (without losses due to transfer of the sample). The apparatus effects a good separation of substances having fairly close boiling points.
APPARATUS
The sequence of operations in construction of the distillation tube from glass tubing of 5-mm. inside diameter is as follows: 3ulb A is blown and pressed in slightly a t the lower end. A rubber stopper cut to 8-mm. length is placed between A and B , which is a lopsided blister, blown on one side of the tube. Bulb C is blown. A hole just larger than C is blown in the bottom of a 25 X 200 mm. test tube. A 5-mm. hole is made a t position H by pressing a wire through the softened test tube. The apparatus is then assembled as indicated in Figure 1. It is helpful in judgin the size of fractions to mark the bulbs with paint at the 0.1- an8 0.2-ml. levels (with the tube held at a 45" angle) before assembling. VACUUM
EXPERIMENTAL
The efficiency of the distillation apparatus was tested by distillation of high-boiling, nonazeotropic binary mixtures a t pressures of approvimately 1 mm.
25 X TES
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Three distillations were made with each of the following mixtures. 49% p-cymene (boiling point 177" C.)-51% n-caproic acid (boiling point 204" C.) and 50% diphenylmethane (boilin Eoint 266" C.)750% dimethylphthalate (boiling point 284" Cf amples weighing approximately 0.15 gram were separated into a volatile fraction, a middle fraction, and the residue. These fractions were removed with capillary pipets and weighed.
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CENTIMETERS
The total material thus recovered from the apparatus amounted to 88 to 94% (average, 91Y0) of the original samples. The size of the volatile fractions ranged from 22 to 43% and the residues ranged from 20 to 4970 of the total recovered material in the different distillations. The composition of each sample ww determined from its refractive index on the assumption of a linear relationship. With the f i s t pair (27" difference in boiling points a t atmospheric pressure) both the volatile and residual fractions had purities greater than 99%. With the second binary mixture (18' difference in boiling points) the volatile fractions were 98% pure diphenylmethane and the residual fractions were 96% pure dimethylphthalate in each test. The efficiency of the apparatus is probably due in large part to the slow rate of distillation. Rose ( 2 ) has shown that with an unpacked column 6 mm. in diameter and 30 cm. long the efficiency rises from 2 to 17 theoretical plates as the throughput rate is decreased from 1 to 0.17 ml. per minute. In the above experiments approximately 0.1 ml. was distilled during 10 to 30 minutes, an average of 0.005 ml. per minute. As distillation of 10 ml. a t this rate would require 33 hours, a microdistillation may be preferred even when larger samples are available, if it is desired to purify only enough material to permit identification. No bumping occurs in the microdistillation because a t the low distillation rate the liquid evaporates rather than boils.
w Figure 1. Diagram of Apparatus
Distillations are made with the tube a t a 45" angle and C heated by a mechanically stirred oil bath. The sample is introduced into C with a pipet made from 3-mm. diameter tubing. Ice or other suitable coolant is placed in the chamber surrounding the tube above A . The tube is inclined to a 45' angle with B on the top side, the distillation tube is then connected to the vacuum system and the more volatile material is distilled into A by slowly heating the oil bath. When sufficient distillate has collected in A , the lower end of the tube is raised out of the oil bath to an angle of about 30" with the horizontal and heating of the oil bath is discontinued. Ice water from a wash bottle is promptly introduced through H to condense vapors between B and A . After about a minute the tube is carefully rotated on its longitudinal axis until B is on the bottom side. Cis then careful1 lowered into the bath and distillation continued until sufficient c h i l l a t e has collected in B. The distillation is stopped by raising C out of the bath until the tube is nearly horizontal and then releasing the vacuum. If a distillation is prolonged until the condensing water warms appreciably, the water
* Preaent addrese, Rutgers Univereity, New Brunswiok, N. J 632
V O L U M E 21, NO. 5, M A Y 1 9 4 9
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Table 1. Distillation of Limonene-Aniline-Benzyl Alcohol Mixture
Volatile fraction Middle fraction Residual fraction
Redistillation of Middle Fraction Weight, Refractive gram index 0,008 1.5770 0.012 1.6814 0.013 1.5761
First Distillation Weight, Refractive gram index 0.215 1 .'ii82 0.053 0.148
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The usual function of distillation is to separate a reaction product from other substances, some of which may have higher. others lower, boiling points. This use is illustrated by distillation of a ternary mixture containing by volume 25y0 &limonene (boiling point 177.8' C.; n*O 1.4743), 50% aniline (boiling point 184.35" C.; nno1.5863), and 25% benzyl alcohol (bqilin point 205.5" C.; nZo1.5396). A 0.435-gram sample was distillecfat 10-mm. pressure and a rate of 0.01 gram per minute, and 0.045 gram of the middle fraction was then redistilled under the same conditions. The data are given in Table I. The presence of either limonene or benzyl alcohol as impurity in the aniline would lower the refractive index. Preliminary tests showed that aniline solutions
containing less than 10% by volume of limonene, benzyl alcohol, or both gave refractive indexes within 1% of values calculated on the assumption of linear relationships between refractive index and composition by volume. The distillation data indicate that the middle fraction from the first distillation contained 83 to 9370 aniline, and that from the redistillation contained 90 to 96y0 aniline by volume, depending on whether the impurity is assumed to be benzyl alcohol or limonene. A second distillation not only serves to purify the material further, but gives an indication of the homogeneity and the trend in the physical constants as the substance is purified. Thus, in the above euample, the refractive index data show that the middle fraction from the first distillation was not homogeneous and that the impuritie- were substances of lower refractive index. LITERATURE CITED
(1)
Gould, C. W., Jr., Holaman, G., and Niemann, C., ANAL.CHEM., 20, 361-3 (1948).
(2) Rose, A,, I n d . Eng. Chem., 28, 1210-12 (1936). (3) Sohneider, F., "Qualitative Organic Microanalysis," New York, John Wiley & Sons, 1946. RECEIVED May 28, 1948.
Buret for Precise Measurement of Small Volumes of Gases S. S. BURKE, JR., University of Wisconsin, Madison, V i s .
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