Methyl Esters of the Higher Fatty Acids Separation of Small Quantities by Fractional Distillation FLAVIUS W . WYRIAN
AKD
CHAS. BARKENBUS, University of Kentucky, Lexington, Ky.
T
HE separation of the higher saturated fatty acids by
It is seldom possible in oil analysis t o start with known mixtures for testing the procedure. Since the methyl esters of higher saturated fatty acids can be obtained in the pure state, mixtures of known composition could be readily obtained and the accuracy determined more satisfactorily than in most analyses of this nature. Because of its low holdup and high efficiency, the spinningband type of column, described by Lesesne and Lochte (6), was used for this investigation.
fractional distillation of the acids or their esters has been reported frequently in the literature. It has been shown that distillation of the esters gives more satisfactory results than distillation of the acids. Most of the analytical work on mixtures of higher f a t t y acids has been done b y fractionally distilling large quantities of the methyl or ethyl esters (I,%’, 3, 6,7). Hilditch (4) states that in the analysis of a mixture of fatty acids b y distillation of their methyl esters, the accuracy depends to a large extent on the amount of material used. According to Hilditch, accurate results may be obtained if 500 grams or more of a mixture are distilled, though he does not mention the efficiency of the column required to separate the esters. I n many oil analyses the amount of saturated acids available for fractional distillation is too small to give accurate results. The recent development of efficient fractionating columns with small holdups has offered a possible solution to this problem. The purpose of work reported in this paper was to determine the accuracy of separation, using small amounts of known mixtures of the methyl esters of the higher fatty acids.
H
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Packed
The spinning band n.as 38.5 cm. long and with the heat1 the column had about 20 theoretical plates measured with benzene and carbon tetrachloride at total reflux. I t was equipped for vacuum distillation as shown in Figure 1, and a 39-liter (10gallon) drum was placed in the system to keep the pressure more constant when changing fractions. The pot and column were heated electrically. The pot had a capacity of about 20 ml. and v a s connected to the column by a ground-glass joint. This joint and the three-way stopcock a t the head of the column which came in contact with liquid were lubricated by finely powdered graphite. The receivers for the very small fractions were made by cutting the ends from 7.5-cm. (3-inch) soft-glass test tubes and were attached to the delivery arm of the still by means of a piece of copper wire.
Purification of M-aterials llethyl palmitate and methyl stearate mere obtained from the technical grade of Eastman methyl stearate. The crude methyl stearate !vas fractionated through a 10-plate Podbielniak column to separate the methyl stearate and methyl palmitate. These two fractions were crystallized from acetone a t -10’ C. to separate any unsaturated material and were then refractionated. Only those fractions viith a constant index of refraction were used in this work. The methyl esters of caprylic, capric, lauric, and myristic acids were obtained from the Eastman Kodak Company and were fractionated through the spinning-band type of column. Only the fractions with a constant index of refrmtion w r e used.
Rearing
I
- -
cr, P .I T
FIGERE2. METHYL
658
a
I
-
I f
1.4250
0
FIGURE 1. DIAGRAM OF COLW
I
3.0 6.0 9.0 I20 150 GRAMS D I S T I L L € D
PURIFICATION OF METHYL CAPRYLATE, METHYLLAURATE,AND X I E T H Y L MYRIRTATE
CAPRATE,
ANALYTICAL EDITION
NOVEMBER 15, 1940
The graphs for these distillations are shown in Figure 2 .
A search of the literature failed to find the index of refraction listed for these methyl esters and, for the sake of record, they are collected in Table I. It was found that for a range of 1' to 2' the change in index with temperature was 4 parts in the fourth decimal place with each degree change in temperature.
659
Longer time should be allowed before taking off fractions which fall between the flats in the distillation curves. The size of the fractions removed varied from 0.020 to 0.200 gram, depending on when they were taken. Smallest fractions were taken between flats, so that a sharp rise in the distillation curve could be obtained. TJsually the column went
Procedure of Distillation Known mixtures of the purified esters were prepared and fractionally distilled and the composition of the mixtures was calculated from the fractions obtained. The fractionations were followed by taking the refractive index of each fraction and plotting this against the total grams distilled. The refractive index was taken with an Abbe refractometer a t 45" C. and the index was read on each fraction until checking results were obtained. I t is necessary to have the temperature controlled accurately during the reading of the refractometer, and several readings were taken over a period of a n hour to be sure that they were constant. In the case of a mixture of methyl palmitat>e and methyl stearate the difference between the indices of the two esters is only 0.0029. Because of this small difference, readings agreeing approximately to 1 part in the fourth place are essential in order not to introduce too large a n error. With practice, and with well controlled conditions, readings to less than 2 parts in the fourth plnce could be obtained. This unaToidable error is not so large for the other esterb. The analysis of such small samples by kteimining their index of refraction is a much better proreclui than determining the saponification value. The quantity of the 1~ix5uredistilled each time n a s from 1.0 to 5.0 grams, depending upon composition of the mixture. With binary and some tetnary mixtures 1 to 2 grams could be used, but with all four-component mixtures and some of the three-component, if a small percentage of any component was present, i t was necessary in most cases to use about 5 grams of sample. Fractionations were repeated on each mixture until a flat in the curve was obtained for each component. The total analysis of each mixture was calculated by assuming that the distillate coming over on the flat portions of the curve was a pure ester and that the distillate coming over b e tween two flats was a binary mixture composed of the pure esters represented b y the flats.
3
64320
0
.4D"
0
c.
1 4346 1 4317 1.1281
Esters Methyl laurate Methyl caprate Methyl caprylate
2.0
4.0
3.0
50
Ir,
w
h
/ 4315
P 14305
Q
s
3
'4295
2
2
14285
0
TABLE I. ISDEXOF KEFRXTIOX FOR ~ I E T H IESTERS L Esters RIethjl stearate Methyl palmitate Methyl iriyristate
10
/4325
1.0
0.5
n4Ds"" C.
1 4220
1 4161 1 4069
7
The operation of the still requires very careful technique.
It was found that the difference between the column and pot temperatures could vary over a considerable range, the minimum difference being about 10" and the maximum about 30". The column must always be maintained at the lowest temperature that will cause refluxing of the liquid. This was determined by allowing the liquid to reflux and then gradually lowering the column temperature until refluxing was barely observed. Even slight superheating will cause a poor separation. The temperature was held at this minimum until the column went dry and when this point was reached i t was best t o flood the column and again bring the temperatures back to the minimum. The column should also be flooded when the fractionation is first started and if the fractionation is stopped before it is completed, the column should be flooded when i t is started again. Fractions were removed at 1- to 2-hour intervals and occasionally longer time was allowed for the mixture to reach equilibrium, depending on the mixture being fractionated,
/4270
4 Icl I 4 2 5 5
a
z
1.4225
Q
l i
lu
a:
l.4210
0
/O GRAMS
FIGURE3.
20 D / S TIILL € D
FRAL'TIOSAL DISTILL.4TION
OF
TWO-COX-
P O S E X T hlIXTURES
Upper.. Methyl palmitate and methyl stearate Center. Methyl myristate and methyl palmitate Lower. Methyl laurate and methyl myristate
INDUSTRIAL AND ENGINEERING CHEMISTRY
660
dry a t the end of a flat and the temperatures had to be raised and regulated again before the fractionation was continued. The time required to complete a fractionation may be 36 to 48 hours, but during this time the column requires very little attention and the fractionation may be stopped a t any time and continued later.
OBTAISEDBY FRACTIONATION OF MIXTURES TABLE 11. RESULTS OF METHYLESTERS RIixture Used Grams 4.983
Actual Composition
2.161
2 . 708
4.910
/ 4305
14290 14275 20
/O
0
14325
I
I
I
I
1
39.65/c palmitate
60.4Yc stearate
35. 3Y0 myristate 64.776 palmitate
3 6 . 4 % myristate 63,6Yc palmitate 2 1 . S% laurate i 8 2 % myristate 11.9% myristate 10. palmitate 77.6YCstearate 2 6 . 4 % laurate 1 7 . 5 % myristate 56.1Sc palmitate 3.01Y0 caprate 7 047, myristate 4.97Yo palmitate 84.9753 stearate
2,084
/ 4320
Determined Experimentally
4 0 . 4 % palmitate 59.67, stearate
1,504
I4335
VOL. 12, NO. I I
21,5Y0 laurate 78.5Y0 myristate 11.37; myristate 10. S 7 0 palmitate 75. l:& stearate 26 4Yc laurate 17,8% rnyristate .j5,8Yc palmitate 1 .75yLcaprate 6.747, niyristate 4 . 79Y0 palmitate 56.725; stearate
The main source of error seems to be in the most volatile fraction. The long time necessary to remove any fractioii causes a greater loss t o appear in the more volatile component. An inspection of the results shows that the first and last fractions have the greatest error, the error in the high-boiling component being due primarily to the method of calculation. When the percentage composition of any component is small the error is much larger. KO mixtures containing less than 3 per cent of any component were used but it seems reasonable I 435 0
I4300 I 4 3 IO / 42 75
I4270
/ 4250
I4230 I4225
u
I
I
I
0
I
1
Q: 14200
1
1.5 GRAMS
I
1
1
3.0
DISTILLED
14/90
/4/50 I 4 3 50
DISTILLATION OF THREE-COYFIGURE 4. FRACTIONAL PONENT MIXTURES Upper. Lower.
Methyl myristate methyl palmitate, and methyl stearate Methyl laurata, ðyl myristate, and methyl palmitate
KO attempt was made to keep the pressure constant from one distillation to the next. The pressures used xere from 1 to 3 mm. of mercury except for the fractionation of the lower boiling esters. \Then methyl caprylate or caprate was present a pressure of about 30 mm. of mercury was maintained until they were removed, in order t o minimize the loss of the esters through the mater condenser a t the head of the column.
14310
I4270
Results of Fractionation Table I1 and Figures 3 to 5 shorn the results obtained for each mixture fractionated. The calculations Tvere based on the amount of the esters recovered with the exception of the highest boiling component. Since there was some holdup by the still, this component was obtained by difference after first proving its nature by the flat in the curve. This method of calculation places all the accumulated errors in the percentage composition of the highest boiling component.
0
lo
20
30
40
5.0
GRAMS DlSTILL€D
FIGURE5 . FRACTIONAL DISTILLATION OF METHYL CAPRATE,METHYLMYRISTATE,METHYLPALMITATE, A S D METHYL STEARATE h1IXTURE
Upper. Lower.
First fractionation Second fractionation
NOVEMBER 15,1940
ANALYTICAL EDITION
that, by increasing the amount of the mixture, analysis could be made where a component is present in less than this amount. None of the binary mixtures was particularly difficult to separate. With the three-component mixture, methyl myristate, methyl palmitate, and methyl stearate, it was difficult to obtain a flat for the methyl palmitate. The results given, however, check fairly close to the actual composition of the mixture. The mixture of methyl laurate, methyl myristate, and methyl palmitate was satisfactorily separated on the first fractionation. Only one four-component mixture was fractionated. V i t h this mixture the operator did not know which esters were present. The first fractionation, represented in Figure 5 (upper), served as a guide for the second fractionation shown in Figure 5 (lower). The first fractionation gave a flat for methyl caprate, methyl myristate, and methyl stearate and also a n indication that some methyl palmitate was present. The second fractionation gave a flat for methyl myristate, methyl palmitate, and methyl stearate. Combining the two fractionations it was evident that the mixture contained all tour esters. The analysis was calculated on the second fiactionation because it was believed that some of the lowlmiling methyl caprate was lost through the condenser a t the head of the column on the first fractionation. Although there was some loss of caprate in the second fractionation, it was not so great as that in the first. The saponification cquiralent of the mixture as calculated from the actual com-
661
position w s 285.6, and from the composition determined experimentally i t was 287.8.
Summary The methyl esters of caprylic, capric, lauric, myristic, palmitic, and stearic acids have been purified and their refractive indices determined. Small quantities of known mixtures of these esters have been fractionally distilled through a spinning-band column and their composition has been determined. K i t h the exception of the more volatile esters the analyses are fairly accurate, considering the difficulty connected with separating such mixtures. I n oil analyses where only >mall quantities of these acids are availalde this method offers for the first time a convenient and fairly accurate method of analysis.
Literature Cited (1) Armstrong, E. F., Allan, J., and Moore, Ind., 44, 63T (1925). (2) Elsdon, G. D., Analyst, 38, 8 (1913). (3) Haller, hl., Compt. rend., 146, 250 (1908).
C. W., J . SOC.Chem.
(4) Hilditch, T. P., “Fats and Waxes”, New York, D. Van Nostrand Co., 1927. (5) Lepkovsky, S. L., Feskov, G. V., and Evans, H. Chem. Soc., 58, 978 (1928).
M., J . Am
(6) Lesesne, S.D., and Lochte, H. L., IND. ENO.CHEM.,Anal. Ed., 10.450 (19381. (7) Uno; S.,And Iwai, M., J . SOC.Chem. I d . Japan, 38, Suppl.
bind., 603 (1935).
Determination of Carbon Monoxide Pyrogallic-Tannic Acid Method as Adapted to Standard Gas Analysis Equipment FRED COOK, Bituminous Casualty Corporation, Rock Island, Ill.
-4method is described for the determination of carbon monoxide in concentrations of 0.10 to 0.002 per cent. The apparatus is designed as an accessory to standard gas analysis equipment and requires a special set of color standards in addition to regularly available laboratory devices. The determination may be run either independently or simultaneously with other gas analysis. ITH ordinary gas analysis apparatus the lower limit of concentration that may be measured v i t h certainty is 0.1 per cent. I n the case of carbon monoxide this limit is not low enough to detect quantities that are dangerous to health, to permit folloving the progress of mine fires in sealed sections to their extinction, or to ensure the absence of carbon monoxide in industrial gases where small quantities of this substance are undesirable. The device described here is essentially the “pyro-tannic acid method” for determination of carbon monoxide in air as described by Sayers and ”ant (1) in 1927, incorporated as an accessory to standard gas analysis equipment. Several advantages are obtained by this modification. The method of Sayers and T a n t is based on the formation of a stable colored suspension by the addition of a small amount of pyro-tannic (equal parts of pyrogallic and tannic)
acid to a water-blood solution which has been partly or completely saturated with carbon monoxide. The procedure recommended involves collecting a 250-cc. sample of carbon monoxide-contaminated air through a soda-lime tube into a dry sample bottle. A small quantity of 20 to 1 blood solution is added to the bottle and shaken for 20 minutes to ensure equilibrium. The solution is transferred to a small tube, pyro-tannic acid added, and the blood saturation measured by comparison with color standards. The amount of carbon monoxide in the air is calculated from the blood saturation and the oxygen content of the air. This method is very well suited for field work where on-thespot information is desired as to the presence of small quantities of carbon monoxide in air that is known to contain between 19 and 21 per cent of oxygen. The method was devised as a test of the purity of air to be used for breathing and the authors claim a n accuracy of 0.005 to 0.030 per cent in concentrations of 0.010 to 0.200 per cent of carbon monoxide. Work in the writer’s laboratory corroborates this accuracy. For general application in laboratory gas analysis this procedure is objectionable because the sample must be collected in a dry bottle of limited size. The sample is rendered unfit for analysis of other gases by the necessity of opening the bottle in the laboratory, and the oxygen content of the sample must be known within reasonable limits. An inspection of Equation 2 will illustrate the importance of accurate data on the oxygen content. When used with the device described here a sample collected by any method, wet or dry, is satisfactory About a 100-cc. portion is needed for the carbon monoxide analysis,
