The Use of Semimicro Technic in Organic Chemistry-VI* Semimicro Fractional Distillation NICHOLAS D. CHERONIS and NATHAN LEVIN Chicago City College, Chicago, Illinois
T
HE object of the present work was to develop apparatus for teaching fractionation to students in organic chemistry using semimicro quantities and a t the same time to develop procedures which could be used in the organic laboratory for efficientfractionation of volumes of from 1 to 10 ml. of liquid. The various apparatus for micro and semimicro fractionation which have been described in the literature '-lo were found too elaborate or expensive for student use, though some give very good results. Two reasons were responsible for the further development of the semimicro fractionation apparatus previously described." First, i t was found oedaeoeicallv desirable to retain the traditional trea'tmeg i f frictionation by means of a fractionating column; second, the results obtained in actual student work with the type of column shown in Figure 1 indicated that considerable improvement was needed. The adoption of the type of column shown in Figures 1 and 2, which is essentially a traditional fractionating column diminished in size, was dictated by the lack of an accurate and inexpensive microthermometer. In most aooaratus for semimicro or microdistillation the temperature a t which fractions. are taken is estimated from the temperature of the bath, a procedure which cannot be asked of a beginner. Other types of microdistillation apparatus are designed for making arbitrary fractions. Such microthermometers as are avail-
able leave much to be desired. The graduations are in 2' or more, and the price for a set of three thermometers required for the interval of 20' to 300' is about $15 to 518, a figure which makes their use by students prohibitive. An efficient thermometer for micro or
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Presented before the Division of Chemical Education of the American Chemical Societv.. 107th meetinr. Cleveland. Ohio. Apr. 5. 1944. FRIEDRICHS. 2.Angm. Ckem., 32,340 (1919). COOPER ~mFASCE, Ind. Eng. Chem.. 20,420 (1928). a MIDGLEY, Ind. Eng. Chem., Anal. Ed., 1, 86 (1929). 'JANTZEN ANDTIEDCKE, J.prakt. Chem., 127,277 (1930). "ma. Ind. Eng. Chem., 22,322 (1930). wEs~oN,Ind.Eng. Chem., Anal. Ed., 5,179 (1933). PARXES, ibid., 18,100(1935). "LARD AND HERMANCE, Mikrochcmie, 18,289 (1935). 'CRAIG,Ind. Eng. Chem., Anal. Ed., 8, 219 (1936); 9,
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(1027\ ,*"".,.
lo RERNHAUER,"Einfuhmn~ in die Or~misch-chernkheLaboratoriurntechnik," J. Springer, Berlin, 1938,pp. 55-8. SHRADPR AXD R ~ r z m Ind. , Eng. Clzem., Annl. Ed., 11, 54
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" GEITLERA~FINE, ibid., 11,469 (1939). la
semimicro work a t a reasonable price is highly desirable, but has not yet been developed. The setup shown in Figure 1 uses an eight-inch test tube for the boiling kettle. In Figure 2 the kettle consists of a 25-ml. pyrex round-bottom flask. The present investigation dealt primarily with the d e c t on semimicro separation of liquids by fractional distillation,
BARKER, BARKEBUS, AND ROSWELL, ibid., 12,468 (1940). TIEDCKE, ibid., 15.81 (1943). GETTLER,NIEDEIU,AND BENEDEITI-PICHLER, Mikrochemie,
11,174 (1932).
" CHERONIS, PEAKES. aid.. 18, 100 (1935). J. CHEM. EDUC., 20,431 (1943); "Semimicro and
Macro Organic Chemktry,,, New York, 1942, p. 74.
FIGURE 1.-APPARATUS FOR SEMIMICRO FRAWIONATIO~
Y . Crowell Company,
85
and emphasizes the following factors: (1) the d e c t of height of the column; (2) the effect of the packing; (3) the effect of the rate of vapor withdrawal, or rate of distillation; (4) the effectof insulation. 1 . Effed of Height of Column. Although the efficiency of separation usually increases with increase in the height of the column, in semimicro work the greater holdup which results as the height of the column increases acts as a limitation. Various lengths in the semimiao column were tried from 120 to 300 mm. From results obtained in a large number of fractionations i t was decided to limit the height of the column T
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FIGURE 3.-PACKINGS
TABLE 1
S&MIUICPO F E A ~ O N A TCOLUMNS INC Dinrcnrions*
Number
FOR
SEMI-
COLUMNS. NUMBERS REFER ro DESCRIPTION G I ~ N IN TABLE 1
to 160 mm. and increase the efficiency of fractionation by improving the packing of the column. 2. Effed of Various Packings. The original packing used in this type of semimicro column was a single or a double wire spiral of nichrome wire. When this became a critical war material, iron and also zinc wires were used. Though these packings could be used by
4 8 12 16 Volume of Distillate in Milliliters
20
Nolurc of Packing
(Lenglh 'urn.)
X
MICRO
0
beginning students, the efficiency was not high, and it was considered advisable to investigate a large number of packings in order to find a material which would give high efficiency, and a t the same time be free from cor-. rosion. The various types of packing used are summarized in Table 1, and shown diagrammatiqally in Figure 3.
XI
140
XI1
140
XIIIt XIV
160 160
XV
160
XVI*
160
Single wire spiral: aluminum wire 115 em. in lenrthand 0.05 mm. in diameter: 65 aoirals * Double wire spiral: outer same as in I; inner I00 em. in length; 80 =pirals Wire helk on g l a s tubing; tube: 160 X 4 mm.: inner wire 40 cm. in 70 spirals; outer 36 em. in 18 spirals Glass capillaries 0.4 to 0.6 mm. in diameter and 140 mm. in length Solid gl- rods of 0.4 to 0.6 mm. in diameter and 140 mm. in length G I ~ S Syarn 2 mm. in diameter; length 25 em.; 15 rpirala Podbielniak Heli-grid packing Square gl- rod 140 mm. io length; 4.5 mm. in diameter; twisted to form 9 spirals Glass yarn 1mm. inthicknew 25 em. inlmgth; 40 spirals loose fitting within eoiumn Glass yarn 1.3mm.X 25em.; woundon 140 X 4mm. &ss rod; 20 spinls; just fits within the column Glass yarn 0.8 mm. X 30 cm. on glass d;45 spiral.; fits vaylomely Same as X; diameter of rod 3 mm.; length of glass yarn 30 em.; 25 spiriralr; fits easily same a. No. V I I Glass yarn 1.3 mm. X 45 em.; wound on a 160 x 4-mm.md; 24rpirals: fits easily Glass yarn 1.3 mm. X 50 cm.; mound on n 3-mm. glassrod; 30spinls: fits earily Podbielniak Hdi-grid packins
* Dinmeter of column unless othvrise specified; O.D.
I.D. = 6 to 6.5 mm.
t
= 8 to
8.5 mm.;
Diameter outside 6.5: iaside4.5 mm.
To measure their efficiency, various types of packing mixtures of methanol-water, acetone-ethanol, and benzene-toluene were fractionated, using the same conditions throughout but changing the type of packing. The results obtained with the methanolwater mixture are summarized in Figures 4 and 5 in which the volume of the distillate is plotted against the temperature. In this manner the 'packings which showed efficiency were selected. These were the glass yarn spirals and the Podbielniak Heli-grid p a ~ k i n g . ' ~ Since the Heli-grid packing cannot be removed and inserted these columns were specially c o n ~ t r u c t e d . ~ ~ Inspection of curves IX and X in Figure 5 shows that even with the same type of glass yarn spiral the contact of packing with the walls of the column is of importance. I n column number IX the glass yarn spiral fits loosely and hence vapor ascends easily upwards, while in column number X the vapor ascends through the space left between the glass yarn spiral and the walls of the column, and hence has a greater contact opportunity. Comparison of curves VII and X indicates that a properly constructed glass yarn spiral com-
(Rate of distillate withdrawal, 0.5 ml./min.) Column: Length 140 mm.; O.D. 8 mm. Packing: Roman numerals as shown in Table 1. l8 PODBIELNIAK. I d .Eng. Ckem., 13,639 (1941). 19 Podbielniak MC-706 "Micro-Col" distilling tube, Podbielmcuns SEMIMI MICRO FRACTIONATION OF 20 ML.-MIXTURE OF niak Centrifugal Super-contactor Co.. Chicago, Illinois. VOLUMES METHANOL AND WATER EQUAL
pares favorably with the Podbielniak type of packing. This is also shown in Figure 6, which represents a &omparison of the efficiency of fractionation of 10 ml. methanol-water mixture in three of the columns selected on the basis of previous tests. Curves I1 and I11 are for glass yarn spirals of 30 and 24 turns, respectively. By means of these three packings i t was possible to obtain 4.5 to 4.7 ml. of 95 per cent methanol from a 10-ml. mixture of 50 per cent methanol in a single distillation. 3. Effect of Distillation Rate. Comparison of curves VII and XI11 in Figure 5 shows that no matter how efficient the packing may be, if the rate of distillation is rapid, conditions for equilibrium become less favorable with consequent drop in the efficiency of the separation. The rate of vapor withdrawal must be so con'trolled that the necessary equilibrium conditions are established and heat interchange takes place in the column. High reflux causes flooding and the slugs of liquid reduce the effective area of packing. Extreme conditions of flooding are indicated when the liquid accumulates in the upper part of the column just below the thermometer with vapor bubbling through the liquid. The effect of the rate of vapor withdrawal on the efficiency of separation is strikingly illustrated by the results of three fractionations summarized in Figure 7. If the rate of distillate withdrawal is maintained to about 0.5 ml. per minute the separation is poor. If, on the other hand, the rate is dropped to 0.2 ml. per minute, the separation is very good for the type of column and packing, since in a single fractionation it is possible to obtain 4.8 ml. of 96 per cent methanol from a 10-ml. mixture containing 5 ml. of pure methanol. 4. Effect of Imulation. If there is considerable cooling as the vapors ascend the column very little equilibrium can beestablished, and consequently poor separation will result: To prevent cooling on the walls of the column two tmes .. of iacket were used. One is easily constructed by the student from asbestos paper; the other was a Podbielniak MC-705 "Micro-Col" vacuum jacket with multiple metal radiation shield^.'^ The effect of insulation is shown in Figure 8. Using the same column, 10-ml. mixtures of equal volumes of methanol and water were fractionated, first without any jacket, and then using the asbestos and vacuum jackets. The curves show that the efficiency of the vacuum jacket is highest; the asbestos jacket, when the cost is taken into consideration, compares favorably with the vacuum jacket. Theoretical Plate Value of Semimicro Columns. The apparatus shown in Figure 2 was used. A capillarv , orovided with a sto~cockwas sealed on the flask about 15 mm. from the bottom. The cauillarv was bent downward a t right angles 50 mm. from the seal. The liquid within the flask was above the so that it was possible by opening the stopcock to withdraw ra~idlv'a ' , few droos of the liauid within the flask. The column was provided with a vacuum glass jacket! and the cork and thermometer replaced by a stopper
4 8 12 16 Volume of Distillate in Milliliters (Rate of distillate withdrawal, 0.5 ml./min.) Packing: Roman numerals as shown in Table 1. 0
FIGURE 5.SEmMICRO FRnCrIONATTON OF 20-ML. MIXTURE EQUAL VOLUMES METHANOL AND WATER
20
OF
holding a microcondenser. A mixture of n-beptane and methylcyclohexane was placed in the flask and heat was applied by means of a small flame of a microburner, so that gentle refluxing took place. At intervals of one hour 3 or 4 drops of liquid were withdrawn from the kettle and from the top of the column into separate small tubes. The latter sample was collected by removing the microcondenser and collecting the drop of liquid accumulated a t its tip. The refractive indexes of the samples were determined. When two successive samples gave the same reading equilibrium was established, and these readings were used to calculate the number of theoretical plates. The Podbielniak MC-706 microcolumn and the glass yarn spiral loo
J
4 6 8 10 Volume of Distillate in MI. I = Podbielniak Heli-grid Packing; I1 = Glass Yarn Spiral No.11; 30 spiralr. 111 = Glass Yarn SpiralNo. 111; 24 spirals. 160 mm.; O,D, mm, Column: Averaee rate of vamr withdrawal 0.2 ml./min. -0
2
On lo-ML.MIXTURES OF FUX~E ~.-SEMIM~CRO FRA~TIoNAT~ON VOLUMES METHANOL AND WATER EQUAL
Take a piece of glass rod 4 mm. in diameter and 200 mm. in length. Heat the rod in a burner 30 mm. from the end and draw i t out slightly so as to reduce the thickness of a small section to 2 mm. Repeat the process a t the other end. With the file, cut the main section 140 to 145 mm. in length. This provides a rod of 4-mm. thickness whose ends are tapered off to 2 mm. Obtain a piece of glass yarn 300 to 400 mm. in length, and 1.3 mm. thick (Glass Yarn No. 50).=0 Place oae end of the yarn close to one end of the rod, and tie it securely to the rod by means of an ordinary thread. Hold the glass yarn by one hand and rotate the glass rod by the other so that the yarn is wound into a spiral around it. Space the glass yarn evenly and a t such distance that 25 to 30 turns are completed by the yarn. Tie the other end of the yarn to the tapered end of the rod, and cut off the excess. Prepare a very small Volume of DistiIIate in MI. amount of "Sauereisen ~ement"~0 by mixing the filler Column: Length, 100 mm.; O.D. 8 mm. with the liquid, so as to obtain a mixture which is Packing: Glass Yarn Spiral No. 111. not too thick. By means of the microspatula place a small amount of the cement on the tapered ends of the rod so as to cover the junction of the yarn and rod. Avoid placing the cement too thickly as i t will be imcolumn No. 3 required 5 to 6 hours of refluxing to ar- posSlble to insert the packing into the column; one rive a t equilibrium conditions. The' Podbielniak end of the rod may be covered with cement so as to column gave tests for 14 theoretical plates, and the glass form a knob, which will prevent the packing slipping down the column. The rod is washed, after the cement yarn column 11 theoretical plates. Construction of Semimicro Columns and Packings. has dried, in hot water to remove the soap contained in The glass column is about 180 mm. in length. The up- the glass yarn. It is allowed to drain and dry and then per length of 40 mm. has a diameter of 28 to 30 mm. i t is ready for use. The packing can be heated with to permit insertion of the thermometer. 'The c o l u q cleaning solution of aqua regia without affecting the proper used for packing bas a length of 160 mm. and yarnor cement. Operation of Micro Column. The packing is inserted 8 to 8.5 mm. outside diameter with 6 to 6.5 mm. inside diameter. These glass columns are commercially into the column from above. The glass yarn should fit available.20 The glass yarn packing is made as follows: snugly into the glass column; loose fitting will result in poor fractionation. If possible the packing is wetted 'Wilkens-Anderson Company. Chicago, Illinois. with the liquid to be distilled, and then inserted from above. If the amount of liquid is sm'all, then the packing is inserted in place, the jacket adjusted, and the column fitted into the boiling flask or tube. The kettle is then loosened so as to provide an escape for air and the liquid to be distilled is poured slowly through the top of the column into the boiling vessel. The cork holding the column is inserted securely into the opening of the boiling vessel and heat is applied. When boiling begins and the vapors reach the upper part of the column flooding may occur on the upper part just below the thermometer. In such case the flame is momentarily removed and adjusted, so that distillation starts a t a very slow rate. When the boiling points of the components of the mixture are high as, for example, in the fractionation of a mixture of aniline and nitrobenzene, there will be considerable flooding in the beginning. To offset the heat loss, as soon as the boiling begins a flame is directed against the asbestos Volume of Distillate in MI. jacket, particularly the upper part, until any flooding Column: Length, 160 mm.; O.D. 8 mm. that may have started has disappeared; and the flame Packing: Glass Yarn Spiral No. I11 is moved over the jacket so that distillation proceeds FIGURE 8.-Eamcr OP JACKET DURING S E ~ ~ CF R XA O~ O N Aslowly without flooding. The amount of liquid which lTON OF 10-ML.MIXTURE OP EQUAL VOLUD~ES OF METHANOL AND remains in the column (holdup) is 0.5 to 0.7 ml. WATER
Construction of Asbestos Jacket. Asbestos jackets for semimicro columns are easily constructed from asbestos paper about 1 mm. in thickness. Cut three or four pieces 150 mm. in length, and 75 to 90 mm. in width. Place one piece on a table and moisten it thoroughly with dilute sodium silicate solution (equal parts ,of water and commercial sodium silicate solution). Place the other pieces on top and saturate both with sodium silicate solution. Wrap the wet asbestos sheets tightly around a piece of 8-mm. glass tubing 200 mm. in length. By means of a string or wire tie the jacket a t each end and a t the middle, and dry in a warm place. When the form of the jacket has set, remove the glass tube and allow to stand overnight. The string or wire may be removed, after heating the jacket to dry it. By means of a sharp knife cut the asbestos jacket to the required length, which is about 135 mm. It should fit tightly against the upper part of the column, and on its lower edge against the cork by which the column is attached to the boiliug vessel. Apparatus for Fractionation of I to 2 MI. of Liquid. Figure 9 is a diagram of an apparatus for the fractionation of 1 to 2 ml. of liquid. The bulb is 15 to 20 mm. in diameter, but it can be made smaller for samples of 1 ml. Similarly, the length of the column from the boiling vessel to the side tube may be varied from 70 to 140 mm. The longer the column the greater the amount of holdup. The column can be constructed of 6- or 8-mm. glass tnbing. For the former, glass yarn of 0.8-mm. thickness (No. is used to construct the spiral. The rod around which the yarn is wound is of 3-mm. thickness. The part of the column above the side arm is flared to pexmit the insertion of a thermometer with a piece of rubber tubing around it. The column connects to the boiliug flask, either by a glass joint or by means of rubber tubing. In the latter case the bulb is first blown on the glass and then cut a little above the point shown in Figure 9. This apparatus is useful in the fractionation of small amounts of liquids in routine analytical work.
The authors wish to acknowledge the help of Dr. W. J. Podbielniak and H. R. Kaiser for suggestions
and materials in the determination of the theoretical plate values of semimicro columns; of Mr. Walter Buriischer of Wilkens-Anderson Company for supplying the major part of the packing materials used in the semimicro columns.
There is power in a larterfall, in a B24, in on acorn. But there is a greater p o w in education which teaches men how t o conhdl the forces of nature and changes the thoughts and the actions of nurn himself.
WHAT MAKES A NATION GREAT? Not its land, not its mines, not its rivers, not its forests, not its m o n e y i m p o r t a n t as these things are. Only people make a rurtion truly greet. Natuml resources are of vital coneern to a nation but they must be detteloped and used by p o p & . Our country is richly blessed with the gifts of ~ t u r e . Science and technology--strietly m a n m a d e a r e producing synthetic substitutes for many naturnl resources that we do not have. Thefutureis bright with hope because educated men and women con perform these mnders.
