P U B L I S H E D BY T H E A M E R I C A N CHEMICAL SOCIETY
W A L T E R J. M U R P H Y , EDITOR
Spectrophotometric Determination of Alpha-Eleostearic A c i d in Freshly Extracted Tung Oil Determination of Extinction Coefficients in Oil Solvents R. T. O’CONNOR AND D. C. HEINZELMAN, Southern Regional Research Laboratory, N e w Orleans, La., A. F. FREEMAN AND F. C. PACK, Tung Oil Laboratory, Bogalusa, La.
AND
determination of the acid in tung oil. A photoelectric spectrophotometric procedure which would provide such a rapid and accurate means for the determination of this acid in freshly extracted tung oil is indicated by the photographic spectrophotometric studies of Bradley and Richardson (3) and Jamieson and Rose (13). Such a procedure would be bssed on a measurable physical property, quantitatively related to the triene unsaturation of the acid, and would provide means of rapidly evaluating the quality of large numbers of samples as encountered in such investigations as those on the production of tung oil in the Gulf States. The inherent advantages of the spectrophotometric method are enhanced by studies of the absorption spectra of tung oil (Figure I ) , and of the remaining constituents of the oil. The double bonds of the a-eleostearic acid molecule occur naturally in a triene conjugated arrangement and, consequently, no chemical pretreatment, such as the alkali isomerization used for determination of linolenic acid by spectrophotometric measurements ( H ) , is required. The triene conjugation of the a-eleostearic acid in tung oil can be measured directly on the crude oil. Of the remaining constituents of tung oil, none shows any appreciable characteristic absorption of ultraviolet radiation above 2200 8. (14) and, therefore, no corrections for overlapping absorption by diene or tetraene conjugated constituents (5) are required. Furthermore, the large proportions of triene conjugated material present in the tung oil, together with the rather high value of the extinction coefficient of a-eleostearic acid, necessitate measurements from very dilute solutions, thus reducing any general “background” absorption of the oil to negligibly small amounts and eliminating any need for mathematical corrections often required for precise spectrophotometric measurement of fatty acids ( 4 ) . Dingwall and Thomson ( 7 ) have found that they can determine relative amounts of the (Y and @ isomers of eleostearic acid in mixtures on the basis of slight differences in the positions of the absorption bands. Analyses of freshly extracted tung oil, reported widely in the literature, however, indicate that only the a isomer of eleostearic acid is elaborated in the oil of the common species (11). @-Eleostearicacid, accordingly, would seem to occur in tung oil principally as a result of isomerization of the alpha form brought about after extraction through the action of light or suitable catalysts, such as sulfur and iodine. The determination of eleostearic acid in such samples would be complicated by the fact that the equilibrium between the alpha and beta forms is continually changing and by the fact that this isomerization is usually accompanied by a polymerization which alters much of
A study has been made of the procedure for estimating a-eleostearic acid in freshly extracted tung oil by direct spectrophotometric measurements. Extinction coefficients for the pure acid in oil solvents are reported. A possible explanation of the discrepancies in the value of this coefficient in ethyl alcohol-i.e., partial deterioration during storage-has been given some confirmation b y storage experiments. The extinction coefficients in hydrocarbon solvents have been used to determine the a-eleostearic acid content of some tung oil samples. The spectrophotometric procedure used is more direct, much simpler, considerably more rapid, and very probably, because of these factors, more accurate than chemical methods.
HE characteristic component of tung oil is its some 70 t o
Tsos,
of a-eleostearic acid, practically all of which is in the glyceride form. This triene conjugated fatty acid accounts for the outstanding desirability of tung oil as a drying oil. It is not elaborated in appreciable quantities in any other common vegetable oil available in commercial quantities. Consequently, most tests of quality of tung oil samples are methods for estimating a-eleostearic acid content. I n common use are: Toms’ method (83)by Wijs iodine number and by bromine vapor values; Kaufmann’s method (16) from thiocyanogen and bromine numbers; Bolton and Williams’ method (2’) from so-called “instantaneous iodine numbers”; the diene method of Kaufmann and Baltes (16); and Ellis and Jones’ method (8, 19) from maleic anhydride values. All these methods are indirect and require very time-consuming chemical operations. Their accuracy suffers from weak assumptions involved in their calculations. Direct determinations of the a-eleostearic acid content of tung oil include the estimation of “polymerizable matter” in Bolton and Williams’ method ( 1 ) based upon assumptions which have been shown by Gardner (9) to be unreliable, and the direct determination method of Ku (17), based on relative solubilities of the fatty acids in tung oil and requiring corrections for the solubilities of palmitic, stearic, and oleic acids by methods which are admittedly only approximations. SPECTROPHOTOMETRIC M E T H O D
An outstanding physical property of a-eleostearic acid is its characteristic absorption of ultraviolet radiation due to the triene conjugation of the double bonds occurring within the molecule. The direct spectrophotometric measurement of this characteristic absorption and comparison with measured extinction coefficients of pure a-eleostearic acid in a suitable oil solvent would appear to offer a simple, rapid, and direct method for the 467
INDUSTRIAL AND ENGINEERING CHEMISTRY
468
the acid. Eleostearic acid content of such oils could be best obtained by use of the spectrophotometric method for binary mixtures. Since tung plant researches are concerned wholly with tung oil freshly extracted from the fruits shortly after harvest, the method described in this paper contemplates only freshly extracted oil samples, which contain only the alpha form of eleostearic acid, and in which polymerization by heat and any other means has been kept a t a minimum.
Vol. 17, No. 8
man spectro hotometer, is equal to lo I,,/I, where Io is the intensity of ligxt after passage through t f e cell containing the solvent and Z the intensity of light after passage through a matched cell containing the solution; c is the concentration of the solution expressed in grams per liter; and 1 is the internal length of the cell in centimeters. This nomenclature has been used throughout this paper.) Van der Hulst (12) finds the extinction coefficient to be 189, a value confirmed by later work of Miller and Kass (20). However, more recent observations tend to confirm the lower value of Dingwall and Thomson. Thus, Bradley and Richardson (8) report that the lower value “--seemed best in the light of our experience with many oils”. Kass (14) reports that attempts to determine the true extinction coefficient by collaborative work in independent laboratories have “tended to confirm the results of Dingwall and Thomson, but the discrepancies in the reports of the collaborators as to the location of the peaks-made this part of the studies inconclusive”. Kass further reports that repeated examinations of freshly prepared and carefully purified samples of a-eleostearic acid in his laboratory tend to confirm the higher value. All these coefficients were obtained in ethyl alcohol, an excellent solvent for the pure a-eleostearic acid, but unsuitable for the crude tung oil samples.
For direct measurement of the tung oil samples, one of the hydrocarbon solvents frequently used for ultraviolet absorption measurements is most desirable. The extinction coefficient of a-eleostearic acid in a hydrocarbon, however, may differ appreciably from the value in ethyl alcohol. For this reason, and because of the confusion between the two values reported in the literature for the coefficients in alcohol, samples of a-eleostearic acid were prepared, carefully purified, and dried, and their absorption was studied.
z
-I-
0
o
z l-
X W
WAVE Figure 1
I
in my
LENGTH
Ultraviolet Absorption
of Tung Oil
Containing approximately 80% a-rlrortearis acid, In cvclohexane
From these considerations the spectrophotometric determination of a-eleostearic acid in a tung oil is seen to consist of simply the direct measurement of the optical density of a suitable concentration of the crude oil, dissolved in an appropriate oil solvent, at the position of maximum absorption of the triene conjugation and an equally simple calculation of concentration by means of Beer’s law. In spite of these advantages, little attention seems to have been given to the use of spectrophotometric methods for the testing of tung oil quality by the drying oil industry. This lack of interest has been due, undoubtedly, to the fact that the photographic methods for measuring optical densities in the ultraviolet region of the spectra, until recently the only methods generally available, entail need of both expensive equipment and specially trained technicians. Xow, however, the commercial availability of photoelectric instruments for measuring intensity of absorption in the ultraviolet has placed such measurements within the reach of most testing and research laboratories. AS a knowledge of a-eleostearic acid content is very desirable in connection with tung oil investigations within this bureau, studies were made of the spectrophotometric procedure. EXTINCTION COEFFICIENTS
Three independent measurements of the extinction coefficient of a-eleostearic acid, all in ethyl alcohol, have been found in the literature. Dingwall and Thomson ( 7 ) report a value of alpha = 168. (Extinction coefficient has been converted to the usual value alpha, a = E:.[&,, defined from the Beer-Lambert law as: a = E / c l where E, the optical density read directly from the Beck-
The pure a-eleostearic acid samples were prepared by the method of Nicolet ($2’) modified by the addition of a third crystallization from Skellysolve F and the use of an Abderhalden dryer with Skellysolve F for finally drying the compound. The gure white crjstals obtained in this manner had a melting point etween 47.5 and 48.0” C. Extinction coefficients were measured in ethyl alcohol (99%), cyclohexane, iso-octane, and heptane. The alcohol was selected to permit a comparison with values from the literature and, if possible, to throw some light on the confusion between the two values reported for the coefficient in this solvent. The alcohol was purified by usual methods and had a transmission at 2350 A. of approximately 85%, measured against distilled water’ through 1-cm. matched cells. I t w m redistilled through a short fractionating column just before using. The cyclohexane and iso-octane were selected, as experience has shown these two hydrocarbons to be the most desirable for ultraviolet absorption measurements. They were purified by methods previously described (10). As samples of heptane from petroleum ether were available they were included, at first, as another suitable solvent. However, inconsistent results were obtained with the use of different samples of this solvent at different times, confirming the conclusions of Carter and Gillam (6) and of Zscheile and Beadle (24) that petroleum ether fractions are not suitable for precise measurements of absorption constants. Results obtained with the use of this solvent were, accordingly, discarded. Portions of the freshly prepared a-eleostearic acid, accurately weighted with a microbalance, were dissolved in the selected solvents and diluted to give a final concentration of approximately 0.005 gram per liter. The optical densities of these solutions were measured in a Beckman quartz spectrophotometer, in I-cm. cells, with wave drum settings at each millimicron from 265 to 275 mp, and extinction coefficients calculated from Beer’s law equation in the form alpha = optical densitylcl.
Table
I.
Extinction Coefficients of Pure a-Eleostearic A c i d
Ethyl Alcohol 99% Iso-octane Wave Extinction Wave Extmction length, coefficient, length, coefficient, m+, of u,a t me, of 01, a t position position position position of maxiof maxiSam- of maxi- of maxim u m a b - m u m a b - m u m a b - rnumabple No, sorption sorption sorption sorption 270 169.5 1 270 183.0 270 169.8 2 270 183.5 270 170.1 3 270 183 8 ~. Av. 270 183.4 271) 169.8 ~
Cyclohexane Wave Extinction length, coefficient, mp, of a: a t position posltlon of maxiof maxim u m a b - mumabsorption sorption 271 168 8 271 168.6 271 168.3 271 168.6
August,
ANALYTICAL EDITION
1945
The wave-length position of maximum absorption and the corresponding extinction coefficients from independently prepared samples and measurements are shown in Table I. These values confirm rather well the higher value of van der Hulst (It?) and of Miller and Kass (20) and show there is an appreciable differencein the value of the extinction coefficient in ethyl alcohol and in the hydrocarbon solvents. A spectral curve of the whole absorption of the pure a-eleostearic acid is shown in Figure 2. I n an attempt to ex lain the lower values for the extinction coefficient obtained by singwall and Thomson ( 7 ) and by Kass’ collaborators ( I d ) , samples of the a-eleostearic acid were stored for further studies. I n one experiment samples were kept in clear glass vials a t room temperature and under ordinary room lighting. These samples underwent visible change within 24 hours. The white crystals had partially liquefied and had become pale yellow. In a few days this change had resulted in a dark yellow resinous material. Because of these obvious changes no further measurements were made on these samples. In a second experiment the freshly prepared acid in a glass vial was placed in a mailing tube and stored in a refrigerator at about 0” C. A t the end of 24 hours no visible change had occurred in this sample, but absorption measurements showed that the extinction coefficient in ethyl alcohol now averaged only 170. Further storage in this manner for a few days still resulted in no visible change, nor was the value of the extinction coefficient after this first decrease appreciably changed. Extinction coefficients in the hydrocarbon solvents were correspondingly lower at the end of the 24-hour period, and again no further appreciable change occurred during the next few days. Comparison of the value of the extinction coefficient after storage with the value reported by Dingwali and Thomson (7) affords a possible explanation for the lower values and confirms the suggestion made by Ksss (14) that the lower values are due to partial deterioration during storage.
Table
11.
a-Eleostearic A c i d Content of Tung
Refrec- Wijs tive Iodine Sam- Index, Value ple 28’ C. (18)
1 2 3 4 5
1.5183 1.5180 1.5177 1.5174 1.5173
164.7 164.7 163.9 163.3 162.8
Browne Heat Test, Min, 10.0
9.5 9.75 10.5 9.5
79.1 78.5 77.7 77.2
79.1 78.4 77.8 77.2
111.
a-Eleostearic A c i d Content of Tung Oil Admixtures” Approximate Composition Tung Mineral oil oil
Sample
80.1 81.8 80.7 79.8
80.7 81.9 80.5 78.9
80.4 81.8 80.6 79.4
%
% 99 98 96 92 90 88 84 80 70 50
%
%
% 0.0 -0.6 0.0
-0.3 4-0.4 -0.5 +0.2 -0.1
-0.3 -1.0 containing
* lo ^
150
il;i
=- t w
w 0 0
As suitable coefficients for a-eleostearic acid in oil solvents were now available, they were tested by the measurement of a few tung oil samples for the a-eleostearic acid content. Table I1 shows the results obtained, together with corresponding values by use of the chemical method of Ellis and Jones (8, 19).
The agreement between the values obtained from measurcnients in iso-octane and in cyclohexane proves the self-consistency of the reported extinction coefficients in these two solvents. Ah maleic anhydride values are only claimed to permit “fair approximation of the per cent tung oil” or to “serve as a means for detecting and roughly estimating adulteration of tung oil” (19), no attempts have been made to explain the differences betwee11 the per cent of a-eleostearic acid obtained ffom these values and those obtained by the direct spectrophotometric methods de>c.ribed. To test the accuracy of the method over a range of a-
a-Eleostearic Acid Deviation Found by found from spectrotung oil In photometric present in mixture measurement mixtures
eleostearic acid concentrations which might be encountered in various fresh tung oil samples, admixtures of tung oil and mineral oil were prepared and spectrophotometrically measured and the a-eleostearic acid content was determined. Results, listed in Table 111, illustrate the accuracy with which the per cent of acid can be determined by the rapid spectrophotometric method, and indicate that admixture of more than about 4% can be detected by the lower values of a-eleostearic acid content.
SPECTROPHOTOMETRIC D E T E R M I N A T I O N
The spectrophotometric values were obtained by dissolving an accurately weighed sample of the tung oil in the selected solvent and dilutin to a concentration of approximately 0.005 gram per liter. %he optical densities of these solutions were read directly in the Beckman spectrophotometer in 1-cm. cells, at 270 mp if the solvent was iso-octane, at 271 m/r if cyclohexane, and the concentration of the a-eleostearic acid in the tung oil was calculated from the equation: optical density 100 Per cent concentration = alpha el
Oil and Mineral
80.6 80.6 1 79.9 79.3 2 2 78.1 78.1 4 3 74.4 74.1 4 8 72.6 73.0 10 5 70.8 70.3 12 6 67.2 67.4 16 7 60.4 60.3 20 8 50.8 50.5 30 9 33.8 32.8 50 10 Mixtures prepared volumetrically f r o m two pairs of solutions approximately equal concentrations of tung or mineral oil. 1
Oil Sampler
a-Eleostearic Acid Content, % Spectrophotometric From Maleic Values Anhydride Values Iso- Cyclo1 2 Av. octane hexane Av. 78 4 78.4 78.4 78.6 78.8 78.7
79.1 78.3 77.9 77.2
Table
468
WAVE Figure 2.
LENGTH in m)r
Ultraviolet Absorption of a-Eleostearic A c i d in Iso-octane
While experiments described here indicate the extremely unstable nature of pure a-cleostearic acid crystals, both cheinical and spectrophotometric evidence seems to indicate that i n the tung oil the acid is considerably more stable. Within the meaning of freshly extracted tung oil as used here it is not required that the oil be examined the day it is extracted. However, it is recommended that the oil be examined as soon 3s practical. Isomerization and/or polymerization of the acid i n tung oil s t v r n s
470
INDUSTRIAL AND ENGINEERING CHEMISTRY
to be more dependent upon the conditions of storage than upon the actual time of storage. Exposures t o light, heat, and oxidation are the most important factors causing these changes in the oil. ACKNOWLEDGMENT
The authors acknowledge the assistance given them by L. E. Brown who made all the microweighings, E. B. Schuman for the preparation of the drawings, and M. E. Jefferson for counsel throughout the spectrophotometric work. LITERATURE CITED
(1) Bolton, E. R., and Williams, K:A., Analyst, 51,335 (1926). (2) Ibid.. 55,360 (1930). (3) Bradley, T. F., and Richardson, D., IND.ENG.CHEM., 34, 237 (1942). (4) Bricc, B. A., and Swain, M. L., presented at 29th meeting of Optical Society of America, New York, October 1944. (5) Brode, W . R., Patterson, J. W . , Brown, J. B., and Frankel, J.. I N D . ENG. CHEM., ANAL. ED., 16,77 (1944). (6) Carter, G. P., and Gillam, A. E., Biochem. J . , 33, 1325 (1939). (7) Dingwall, A., and Thomson, J. C., J . Am. Chem. SOC.,56, 899 (1934). ( 8 ) Ellis, B. A., and Jones, R. A,, Analyst, 61,812 (1936).
Vol. 17, No. 8
(9) Gardner, H. A., “Physical and Chemical Examination of Paints, Varnishes, Lacquers and Colors”, 6th ed., p. 660, Washington, Institute of Paint and Varnish Research. 1933. (10) Graff, M. M., O’Connor, R. T., and Skau, E. L., IND.ENG. CHEM.,ANAL.ED.,16, 556 (1944). (11) Hilditch, T. P., “Chemical Constitution of Natural Fats”, pp. 132-3, 221-2, New York, John Wiley & Sons, 1941. (12) Hulst, L. J. N. van der, Rec. trav. chim., 54,639 (1935). (13) Jamieson, G. S., and Rose, W. Gordon, OiE & Soap, 20,202 (1943). (14) Kass, J. P., “Protective and Decorative Coatings”, ed. by J. J. Matiello, Vol. IV, Chapter 12, New York, John Wiley & Sons, 1944. (15) Kaufmann, H. P., Ber., 59B, 1390 (1926). (16) Kaufmann, H. P., and Baltes, J., Fette u . Seifen, 43, 93 (1936). (17) Ku, P. S., IND. ENG.CHEM.,ANAL.ED.,9,103 (1937). (18) McKinney, R. S., and Freeman, A. F., Oil & Soap, 16, 151 (1939). (19) McKinney, R. S., Halbrook, N. J., and Rose, W. G., Ibid., 19, 141 (1942). (20) Miller, E. S., and Kass, J. P., presented at A.C.S. meeting, St. Louis, April, 1941. (21) Mitchell, J. H., Jr., Kraybill, H. R., and Zscheile, F. P., IND. ENG.CHEM.,A N . ~ LED., . 15, 1 (1943). (22) Nicolet, B. H., J . Am. Chem. SOC.,43, 938 (1921). (23) Toms, H., Analyst, 53, 69 (1928). (24) Zscheile, F. P., White, J. W., Jr., Beadle, B. R., and Roach, J . R . , Plant Phys., 17, 331 (1942).
Determination of I-Trichloro-2,2- bis(p-chloropheny1)ethane in Technical
DDT
STANLEY J. CRISTOL, ROBERT A. HAYES, AND H. L. HALLER U. S. Department of Agriculture, Beltsville, Md.
Bureau of Entomology and Plant Quarantine,
Technical DDT consists essentially of a mixture of 1 -trichloro-P,Pbis(p-chloropheny1)ethane (p,p’-DDT) and 1-trichloro-2-o-chlorophenyl-2-p-chlorophenylethane (o,p’-DDT), with small amounts of by-products. A method is described for the determination of 1-trichloro-P,P-bis(p-chlorophenyl)ethane, which i s the most effective insecticidal component present, in technical grades of DDT as well as in dusts containing DDT. The method involves crystalliration from a saturated solution of 1-trichloro-P,P-bis(p-chloropheny1)ethane in 75% aqueous ethanol and is apparently reliable tosithin 1% when a small empirical correction is added.
and Dcvelopment contracts. A detailed collaborative report of these investigations is in preparation for publication.
With chloral that is essentially pure a technical D D T is obtained that contains approximately 70 to SO% of p,p’-DDT and 15 to 25% of o,p’-DDT. When dichloroacetaldehyde is present in the starting material, the product may contain dichlorodiphenyldichloroethane (DDD) isomers. The p , ~ ’ - D D T is more toxic to most insects than the o,p’ isomer or any other component of technical DDT, and it is therefore necessary to have a method for its determination when these other materials are present. (The entomological testing of these compounds was carried out in laboratories of the Bureau of Entomology and Plant Quarantine.) N E of the most promising developments in the field of new Gunther (4) and Hall, Schechter, and Fleck ( 5 ) have proposed insecticides is the use of the synthetic organic chemical methods for the determination of DDT. These methods, which commonly known as DDT (1,3, 6). This designation, which is measure either hydrolyzable chlorine (4) or total chlorine (6), derived from the generic name dichloro-diphenyl-trichloroethane, are not suitable for the evaluation of samples of technical D D T represents the product obtained by the condensation of chloral for their p,p’-DDT content, since most of the impurities are inwith chlorobenzene in the presence of sulfuric acid. Technical terfering substances. Fleck and Preston (2) have described a D D T consists essentially of a mixture of two isomeric dichlorosetting point-composition diagram for the system o,p’- and p,p‘diphenyltrichloroethanes-1 - trichloro - 2,2 bis(p - chlorophenyl) DDT, which they have suggested may be used to estimate the ethane, hereinafter called p,p’-DDT (I), and 1-trichloro-2-0, approximate p,p’-isomer content of technical D D T samples. chlorophenyl-2-p-chlorophenylethane,hereinafter called 0 , ~ ’ Schechter and Haller (?) have shown that the o,p’- and p , p ’ D D T (11)-together with small amounts of by-products and reD D T isomers give different colors when their tetranitro derivaaction products of impurities in the starting materials. tives are treated with methanolic sodium methoxide, and they have suggested that these results may form a basis for the deter/ \ mination of p,p’-DDT in the presence of its o,p’ isomer. “ ~ ~ > , H C C l(I)~ C1a\CHCClr (11) The authors have investigated a method for the estimation of p,p’-DDT by recrystallization from saturated solutions of pure c1p,p’-DDT in aqueous ethanol. The most reproducible results c1 on mixtures of known p,p’-DDT composition were obtained by Studies of the composition of technical D D T have been made the Drocedure outlined below, which is satisfactory for mixtures in the Bureau of Entomology and Plant Quarantine and in the of this at least 40% of ‘ p , P t - ~ ~ ~hfodifications . laboratories of Paul D. Bartlett of Harvard University, Nathan to allow for analysis of products containing less L.Drake of the University of Maryland, and ~ ~s. N~~~~ l ~ basici procedure ~ than 40% of p,p’-DDT or of dusts have also been investigated. of the Ohio State University, under Office of Scientific Research
0
-
a/
