Oxidation of Peppermi ROBERT H. REITSEMA AND FREDERICK J. CRAMER A. M . Todd Co., Kalamaaoo, Mich.
T
H E aging of peppermint oil can be measured by several methods, the most convenient of which are viscosity and oxygen uptake. These methods were used to give indications of the effect of factors, such as source of the oil, storage conditions, and contaminants on the stability of the oil. Nordihydroguaiaretic acid was found to be a very effective antioxidant. No precise method has been available for measuring the changes which take place in peppermint oil during storage, and consequently the odor, color, and change in the U.S.P. chemical assay have been relied upon. However, the study of some critical problems of peppermint oil, such as the best storage conditions, the differences among oils from various producing areas, and the effectiveness of antioxidants, requires a more exact measure of the aging process. Isolation of the individual products of aging is not feasible, since peppermint oil is a complex mixture. Many investigations on the nature of autoxidation, hydroperoxide formation, and polymerization of related materials are available (7). HowSfver, in order to simplify the present study, three basic reactions have been postulated-isomerization, polymerization, and oxygenation. Each method used in this work measures one or more of these three types of reactions particularly well, and by consideration of the importance of each reaction, the selection of the best assay methods should be possible. Simple isomerization of terpenes is assumed to be of minor importance in the present problem. Approximations of any change of this type can best be made by comparisons of optical rotations. Polymerization will increase the average molecular weight and reduce the number of molecules by the usual processes of chain initiation and propagation. It can be indicated by the increase i n viscosity, molecular weight, or the amount of nonsteam-distillable residue. An increase in peroxide content should anticipate polymerization. Oxygenation of the oil is undoubtedly the most important phase of peppermint oil aging from a flavor standpoint. This justifies the use of the term “oxidation” to describe the over-all process. Oxygenation results in the formation, for example, of acids, alcohols, esters, and ketones. Specific instances of these changes are known from studies of autoxidation of individual components of peppermint oil. Thus pinene (6),limonene *(4,6),and menthofuran (3)all produce oxygenated compounds by autoxidation, and with the pure components it was possible to isolate the resulting products. Many approximations of the extent of oxygenatiw are possible for mxtures and the oxidation products need not be isolated. A change in ester, free acid, or ketone content could be utilized. The uptake of oxygen is a
direct memure of oxygenation. An increase in peroxides should show at least the initiation of autoxidation. Viscosity also can be used, since, in general, oxygenated compounds have significantly higher viscosities because of molecular association. SELECTION OF AN ANALYTICAL METHOD
The various methods suggested above have been studied individually. The increase in “menthol” content has been used 88 a method of measuring oxidation (1). The change in the chemical assay of oil stored at room temperature for 3 months in partially filled brown sample bottles, and of oils warmed a t 50” C. for 7 days is shown in Table I. There is a lack of consistency in the relative change in the various values. For instance, in examples 1 and 2 the increases in specific gravity are similar, but only in example 1 is there a significant increase in the apparent menthol content. Example 3 gave a large increase in both the menthol and ester assays, whereas example 2 gave a considerable increase in ester assay only, and example 1 gave a greater increase in the total menthol assay. The ketone assay, calculated as menthone, showed an increase of 3.5% in example 4 and 2.2% in example 6,whereas the increase in esters and alcohols in example 4 was much greater than example 6. The data in Table I are typical of a great number of assays which were available as a result of control work. It appeared that, during severe oxidation the alcohol content approaches a maximum, although it is obvious from other measurements that oxidation is continuing. Fundamentally, only the oxygenation of the oil is shown by the chemical assays and there is no one value which shows a consistent change during oxidation. The change in free acid content has been a convenient measure of aging in fats. The free acids in peppermint oil can be determined by titration and it was found that there is a measurable increase in acid content during storage as shown in Table 11. Free acid determinations are of limited value aa they indicate only that phase of oxygenation which produces acidic groupg, and the magnitude of change is significant only after severe oxidation. The chemical assay for esters would indicate indirectly the free acid content of the oil in gross instances. The extent of polymerization can be shown roughly by measuring the amount of material which is not distilled with steam. For example, the amount of residue in one sample increased from 0.43 to 7.6% after having been heated 30 days in an oven at 50” C. Other samples had increased to 8.1 and 3.1%. Unfortunately, small changes in the oil cannot be detected by this method. No typical curve could be prepared for the increase in residue, partly, at least, because of the difficulty of controlling experimental conditions.
TABLE I. CHANQE IN ASSAYOF PEPPERMINT OIL 3 Months at Room Temperature Zxample No. 1 2 3 4 5 6
dpeoifia Gravity Init. Omid. Change 0,8977 0.9003 +0.0026 0: 8993 0.9020 +Q. 0027 0.8987 0.9060 4-0.0087 0.8990 0.S985 0.8873
.O.QlOO O.9lM 0.8992
+0..0110 4-0.0122 4-0.0019
aD
Init. -29.10’ -25.65’ -25.10’
Oxid. -28.55’ -24.90’ -24.00O
Change -0.65’ -0.75’ -0.90’
-24.12’ -25.2 -25.50
Warming 7 Days at 50° C. 5.11 -26.50’ -2.4:’ 5.53 -26.5’ -1.3 4.39 -24.50 -1.00
176
Init. 6.10 5.39 4.73
Ester, % Oxid. Change 6.38 f0.28 6.27 +0.88 6.93 +2.20
Alcohols as Menthol, % Init. Onid. Change 59.50 60.32 +0.82 58.04 58.10 +0.06 54.62 57.58 +2.96
10.23 9.90 6.22
57.00 56.38 48.69
+5.12 +4.37 4-1.83
6d.54 69.98 58.09
4-12.54 +13.60 4-1.40
January 1952
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
TABLE 11. CHANGEIN ACID CONTJBNT OF PEPPERMINT OIL Stored in Stoppered Bottles Three-Quarters Full for 6 Months a t Room Temperature M1.0.05 N Sodium Hydroxide per 5 M1. Oil Sample Initial After oxidation 0.50 5932 1.90 5942 2.40 0.43 5941 1.33 0.40 5961 0.90 0.45
for the most part. The area in which the oil waa produced wa8 found t o be quite significant and comparisons had to be made with due regard for the source of the oil. Three areas were defined-Midwest, Western (Oregon and western Washington), and eastern Washington. 35
Stored in Sunlight for 18 Daya a t Room Temperature 6150 0.75 9.75 2153 0.60 7.50 6012 6168
Stored in Oven a t 50° C. for 7 Days 1.30 3.35 1.10 3.05
177
30
$ 25 a
d 20
D I l5
3 Both polymerization and oxygenation are often peroxidecatalyzed reactions and the measurement of peroxides was studied. Because of the interference of other readily oxidked compounds and of ole6ns which add halogens readily, most methods for determining peroxides were of limited value. Reasonable results were obtained by shaking the oil with ferrous sulfate solution, separating the aqueous layer, and titrating it with ceric sulfate. It was found that freshly distilled oil contained from 0 t o 0.03% peroxide oxygen, whereas one sample heated in a bath at 35" C. for 35 days contained 0.1%. The amount of variation is very small and the relative accuracy of the method used is inadequate. The amount of oxygen absorbed by the oil under various conditions has been measured. E'igures 1 and 2 give some typical curves for the uptake of oxygen from samples a t 35" C. Slight inhibition periods are evident for some samples. Viscosity data should indicate polymerization and, t o a lesser degree, oxygenation. It is easy to obtain this value and a great number of determinations can be made on a small sample if the specific gravity of the sample is neglected. Results were reproducible with acceptable accuracy. Typical curves obtained by this method are given in Figures 2 and 3. Induction periods are often observed, although in many cases oxidation had begun during the short interval of storage before the sample wasavailable. Viscosity and oxygen uptake were the most useful values and provide a good over-all summary of the aging process, because they do not measure only a very specific type of change. Oxygen uptake was the most sensitive method, but after the first part of the aging there was little difference between oxygen uptake and viscosity results. The relative order of stability did not change on prolonged oxidation and, therefore, most experiments were carried out for long periods of time to show the variations more clearly. There was no specific correlation between the magnitude of the changes of viscosity and oxygen uptake as shown in Figure 2 and by similar experiments. Actually this is to be expected, since fundamentally, viscosity measures polymerization and air uptake measures oxygenation. Both results are significant, however, as the two processes occur simultaneously. The viscosity method was used more often because of its greater flexibility. Instead of holding the oil in constant temperature baths in special containers, one can store the oil in different ways and obtain results by viscosity. Thus, the changes in oil stored in open or closed bottles or in drums, under nitrogen or air, over periods of weeks or years, and a t any temperature are not readily measurable by oxygen uptake, but can be determined by the change in viscosity. EXPERIMENTAL
Individual samples of oil react quite differently to the varying conditions used in this work. Consequently, it was necessary t o run triplicates on each sample of oil and t o run as many different oils as possible. Results given in this paper are average values
s"
10
5
2
8 10 12 Days at 25O C.
6
4
14
16
18
20
Figure 1. Absorption of Oxygen by Typical Peppermint Oils
110 100
90
10
20
3
10
2
0
1 2
4
6 8 Days at 35O C.
10
12
14
16
Figure 2. Comparison of Oxygen Uptake and Viscosity Increase I. Western Washington-Or-n 11. Midwest oil 0 Inarelue in visoosity 0 Uptake of oxygen
oil
Oxidation rates are increased by numerous catalysts and only specially cleaned equipment was used. Containera were cleaned with cleaning solution, alcohol, and distilled water. Every effort was made to use only choice oil as soon as possible after steam distillation of the plants and immediately after removal from the drums.
OXIDATION CONDITIONS.Oxidation of the oils is slow under normal conditions, and usually it was desirable to increase the rate of oxidation by warming at 30' to 50' C. In one experiment it was found that sunlight was an efficient promoter of the change but comparison of various experiments using this method would be difficult. The use of ultraviolet li ht has been pro osed (g). An oven was suitable for heating t%e samples if tie, were placed in a tray of water, but otherwise the local heating a t the sides of the oven gave misleading results. A stirred, constant
INDUSTRIAL AND ENGINEERING CHEMISTRY
1’28
temperature water bath held a t =!=0.1” C., in which the samples were completely immersed, was the most suitable. Containers were closed to thebair only with a loose plug of glass wool. Similarity in appearance to naturally aged oil was observed especially with the samples promoted by heating. Light gave more yellowing than is usually found, but the odor was typically “oxidized.” 35 30
25
P t
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every other day, and a t the time it was made certain that fresh air was present above the samples. VISCOSITIES (8). Ostwald tubes used for this work required 3 ml. of oil. The time varied greatly between the tubes, but after calibration of the tubes against water the accuracy of checks with different tubes a t 25’ C. was 0.1 millipoise, which is the same accuracy obtained by repeating the run in the same tube, Since viscosity is very dependent upon tem erature, the bath temperature must be closely controlled. A d e p p t e r viscometer was not used for this work because of the larger Sam le needed and the difficulty of cleaning thoroughly to a v o i a adding oxidation catalysts. For several reasons the determination of viscosity has been simplified in this work by elimination of the density of the oil in the calculations. To convert the viscosities reported to absolute viscosity in millipoises, the values given have to be multiplied by the specific gravity of the oil.
20 Y
k
.* * 8 15 B 6
where ~H,O is the time in seconds for water to pass through the viscometer; t o i l , the time in seconds for oil to ma through the viscometer; and &I, the specificgravity of the 08-all a t 35 C. O
10 STABILITY AND GROWING AREA 5
2-
4
6 8 Days at 35O C.
10
I2
14
Figure 3. Increase in Viscosity of Typical Peppermint Oils
ASSAYS. The methods of the U. S. Pharmacopoeia XI11 were used, generally on a semimicro scale. ACIDVALUES. A 5-ml. sample of the oil was diluted with 10 ml. of alcohol. The solution then was titrated to phenolphthalein with 0.05 N alcoholic sodium hydroxide. Conversion of the value to an acid index would provide no additional information and resuIta are, therefore, expressed as ml. of 0.05 N alkali used by a blank. NONDISTILLABLE RESIDUE. A standardized procedure was difficult to obtain for this messurement. Steam wm passed into a flask containing 5 ml. of oil. A minimum of 125 ml. of water was codistilled to remove all oil. Occasionally a maximum of 200 ml. of water distilled before all oil was removed. The residue in the still ot was then evaporated under reduced pressure from a water gath a t 50’ C. and the weight of the residue determined. PEROXIDES. Various methods for determining peroxides were studied. In most cases the readily oxidized materials in the oil itself made the t e s b invalid. The method finally used was an adaptation of the ferrous sulfate method in which the excess ferrous sulfate was separated from the oil before titration with ceric sulfate. To 5 ml. of oil were added 10 ml. of petroleum ether and 10.0 ml. of 0.1 N ferrous sulfate. The air above the mixture was displaced with nitrogen, the mixture was shaken, and the aqueous layer was separated. The oil was washed twice with 5 mi. of water. The combined aqueous layers were titrated with ceric sulfate, prepared by adding 35 grams anhydrous ceric sulfate to 500 ml. of water containing 28 ml. of concentrated sulfuric acid and diluting to 1 liter with water. Diphenylamine, used as the indicator, was prepared by dissolving 0.1 gram in 10 ml. of concentrated sulfuric acid and diluting to 100 ml. with glacial acetic acid. OXYGEN UPTAKE.In preliminary work the consumption of oxygen was measured using a gas buret and a 50-ml. flask equipped with a magnetic stirrer under an atmosphere of oxygen The data given in Figure 2 were obtained using a simplified apparatus. In a 125-ml. ground-joint Erlenmeyer flask, fitted with a ground-joint stopcock, were placed 50 ml. of the oil. The flask was warmed in a constant temperature bath in which local heating was avoided by using a heater in a separate bath and circulating the hot water through copper tubing a t the bottom of the bath, Readings were taken daily using a gas buret. One flask contained 50 ml. of water to remove any errors due to atmospheric pressure or temperature changes. Air was used rather than oxygen for convenience, and since it was changed frequently it should not create a serious error. Oxygen consumptioii readings were taken daily and the samples brought back to atmospheric pressure. Viscosities were run on the samples a t least
It had been observed empirically that, as a rule, oil from the Northwest and particularly from the eastern Washington area undergoes change more rapidly than oil from the Midwest. In Figure 4 is given a summary of one experiment giving the average of six oils from the Midwest and western WashingtonOregon areas. Table I11 shows some typical results of the viscosity increase of Midwest and eastern Washington peppermint oil; no exceptions were found t o the lesser stability of eastern Washington oil indicated in Table 111, despite greatly varying experimental conditions.
OF MIDWEST AND EASTERN WASHINQTON TABLE 111. OXIDATION PEPPERMINT OILS
Midwest 28.5 22.0 0.9 3.6 38.8
Expt.’No.
Increase in Viscosity Eastern Washington 42.3 28.5 12.7 5.0 52.7
The order of stability Midwest>western>eastern Washington was found the general rule. It is of importance t o note, however? that all oil, irrespective of origin, underwent oxidation at readily
2
4
6
8
IO 12 14 16 18 20 22 24 26 28 Days at 50° C.
Figure 4. Comparison of Stability of Midwest and Western Washington-Oregon Peppermint Oils
measurable rates and that individual samples of western Washington-Oregon oil were found which were more stable than certain Midwest samples. Figure 2 shows that the same order of stability can be shown by viscosity or by oxygen uptake. Table IV indicates that the relative order of change is the same a t 50" C. or a t room temperature, but that the differences are less striking under accelerated oxidation. AND WESTERN TABLEIv. OXIDATION OF MIDWEST WASHINGTON-OREGON PEPPERMINT OILS
Midwest Oil Lot No. 1038 1058 1060 1071 1110 1121 Average Western WashingtonOregon Oil 971 973 976 1078 1087 1089 Average
Viscosity Increase 14 months at room temp. 6 days at 50° C. 0.9 9.5 15.3 7.2 6.3 2.2 6.3 7'1 .9.2 3.3 11.8 3.8 5.3 8.4
19.1 16.9 14.0 1.5 2.6 6.0 10.0
the results of the test using triplicates of four separate runs comparing natural and redistilled oils. The differences are much less than might be expected, although the samples of redistilled oil tend t o oxidize more slowly on the average. The area in which the oils were grown is a t least as important as the fractionation; and for this reason Table VI represents only mixtures with the same proportion of Midwest and western WashingtonOregon oils STABILITY AND CONTAMMANTS
The measurement of oxidation by viscosity has been used to study the effect of various contaminants on oil stability. Small amounts of acids, such as oxalic or hydrochloric, were found to rapidly produce large increases in viscosity; change in relative viscosity from 55.6 to 58.1 millipoises was found within 1 day at room temperature in a 50-ml. sample treated with 1 drop of concentrated hydrochloric acid. Dilute alkali on the other hand failed to produce any significant change.
12.1 7.5 9.2 11.3 10.0 8.6
10.0
/
8e 25 . STABILITY AND STORAGE CONDITIONS
20 -
The rate of oxidation of the oil can be varied merely by altering t h e temperature of the oil as shown in Table IV. Very little change takes place in oil stored properly in full containers in the dark and a t temperatures below normal room tempeatures. After 5 years, for instance, one sample stored i n a full drum showed no change in specific gravity or refractive index, 0.2% change in ester assay, and 1% change in alcohol assay. Table VI11 gives other examples of stability during a 2-year storage period. The effect of storage conditions is given in Table V where the same sample of oil was treated in three different wags. A full container or a nitrogen atmosphere was able t o prevent any aignificant viscosity increase.
TABLE V.
1'19
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January 1952
EFFECT OF
.-2 8 15.
2
4
6
8 10 Days at 3 5 O C.
12
ALCOLLC
14
16
18
Figure 5. Comparison of Antioxidants for Peppermint Oil
I n order to choose the proper container for the oils more intelligently, various metal salts were studied as catalysts. STORAGE CONDITIONS ON OXIDATION Little effect was observed with inorganic salts, possibly because RATE of their insolubility in the oil. The oleate salts were prepared Viacosity Midwest Eastern Washington and added to the oils in 0.1% concentration. The average in oil Oil values for triplicates is given in Table VII. 47.9 41.4
Original sample After heating a t 50° C. In partiolly filled container In full container Under nitrogen in partially filled container
58.2 50.0
65.1 41.7
49.0
42,3
STABILITY AND FRACTIONATION
The removal of the first fractions of peppermint oil by steam distillation should remove the major proportion of labile material~such as limonene, pinene, and aldehydes. Samples of natural oil and redistilled oil were compared; Table VI shows
TABLE VI. EFFECT OF FRACTIONATION ON OXIDATION RATE Increase Experimental Conditions Natural oil 25 Days at -35O C. 47 2 21 Days at -35O C. 35.7 25 Days at -36O C. 23 14 Days at -30' C.b 55 6 The second figure represents a grade of oil with moved. b Frequent change of air above sample.
in Viscoaity Fractionated oila 44.9 43.2 33.3 37 2 11 7 8 53 38 more light fractions re-
TABLEVII.
EFFECT OF METAL SALTS ON PEPPERMINT STABILITY
Untreated oil Oil, zinc oleate Oil, iron oleate Oil, copper oleate
Viscosity after 30 Days at 35O C. 95.2 98 2 75.5 76.3 97 5 90 8 97.7 95.7
92.0 86.8 106 2 97 6
Average 95.1 79.5 98.2 97 .O
Copper and iron salts appeared t o have little effect on the oxidation rate throughout the aging. The oil treated with the zinc salt was consistently lower than the other samples. Since this particular experiment was run in triplicate on only one oil, any conclusions about the effectof zinc salts would be premature. Tests were made also on oil which had been stored in duplicate under normal conditions for over 2 years in various containers filled to the top with samples from the same lot of oil. Specific gravities, rotations, and refractive indexes all were identical within experimental error. Table VI11 shows t h a t the viscosities of the samples stored in the glass containers were significantly lower than the others.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE VIII.
EFFECT OF NATURE OF CONTAINER ON OXIDATION OF PEPPERMINT
Container Glass Galvanized iron Stain!ess steel Aluminum
Sample A 53.72 54.55 54.26 54.33
’
Viscosity Sample B 53.54 54.70 54.17 54.71
Average 53.63 54.63 54.22 54.52
TABLE IX. EFFECT OF ANTIOXIDANTS ON OXIDATION OF PEPPERMINT OIL
Untreated oil (3:;pAlcolec N.D.G.A.
Viscosity Increase 8 days 15 days 5.1 9.9 4.6 8.3 3.0 7.2
Cumulative Air Uptake in M1. 8 days 15 days 65.7 103.4 53.7 86.3 44.5 83.1
Table X indicates that 0.1% is a favorable concentration of nordihydroguaiaretic acid.
TABLE X. EFFECT OF CONCENTRATION OF ANTIOXIDANTS Concn. of N.D.G.A., 0.1 0.01 0.005 None
%
antioxidants obtained from naturally occurring materials. From these nordihydroguaiaretic acid (N.D.G.A.), Alcolec, and e tocopherol were selected for further testing. A summary of the results using 0.1% of each as compared with a control is given in Figure 5. The curves are average values of triplicate runs on five different oils. The efficiency of nordihydroguaiaretic mid is evident from the graph. Alcolec and a-tocopherol are similar i n efficiency and only about one half as effective as nordihydroguaiaretic acid. The same effect can be observed by use of the air uptake measurement as shown in Table IX. The measurement of air uptake is more sensitive and more suitable in the early stages of oxidation. However, it was observed that the course of oxidation was regular enough so that later stages of oxidation could be taken as representative.
Viscosity 6 days a t 35O C. 20 days a t 35’ C. 48.6 50.6 49.9 65.0 49.5 69.3 52.5 73.5
An attempt made to find a useful synergist for nordihydroguaiaretic acid showed that neither ascbrbic acid, a-tocopherol, nor citric acid was effective.
STABILITY AND ANTIOXIDANTS
A study was made of potential antioxidants for peppermint oil. Hydroquinone has been reported to be useful (1). In this present work screening tests were made on several commercial
Vol. 44, No. 1
ACKNOWLEDGMENT
The technical assistance of Robert Clement and William Schipper.who obtained some of the data reported is gratefully acknowledged. The support of Winship Todd and the permission of the A. M. Todd Co. to publish this work is appreciated. LITERATURE CITED
(1) Baldinger, L. H.,Ellis, N. K., and Fawoett, K . I., J. Am, Pharm. ASEOC., Sci. Ed., 33, 41-3 (1944). (2) Morton, R. A.,Perfumery Essent. Oil Record, 20,258-67 (1929). ( 3 ) Schmidt, H.,Chem. Ber., 80,538-46 (1947). (4) Ibid., 82, 11-16 (1949). (5) Simonsen, J. L., “The Terpenes,” 1st ed., Vol. 11, p. 120,London, Cambridge University Press, 1932. (6) Strausz, H.J., Perfumery Essent. Oil Record, 38,260-3 (1947). (7) Swern, D.,Scanlen, J. T., and Knight, H. B., J . Am. Oil C h i s l a ’ SOC.,25, 193-200 (1948). (8) Swift, L. J., and Thornton, M. H., IND. ENCI.CHEM.,ANAL.ED., 15,422-3 (1943). RECBIVED June 16, 1951.
Compressibilities of Mixtures of Hydroken and Nitrogen above 1 Atmospheres CARROLL 0.BENNETT1 AND BARNETT F. DODGE Chemical,Engineering Department, Yale University, New Haven, Conn.
T
HE properties of gases at high pressures are of interest and importance from both a theoretical and a practical standpoint. I n the field of pressure-volume-temperature relations, the behavior of pure gases below 1000,atmospheres and especially below about 300 atmospheres is known over a considerable temperature range for many of the common gases. The compressibilities of some of the most common gases are also known in the range of loo0 to 3000 atmospheres. Many industrial processes involve mixtures of gases at high pressures, and design calculations are of course facilitated by a knowledge of the behavior of the mixtures. It is also of importance to know the P-V-T relations of gaseous solutions in order to perform certain thermodynamic calculations, such as those involving chemical or phase equilibrium. In making the calculations mentioned above, it would be desirable to be able t o predict the behavior of gas mixtures from the properties of the individual constituents, thus enormously reducing the amount of 1 Present address, School of Chemical and Metallurgical Engineering, Purdue University, Lafayette, Ind.
experimental data required. Several methods have been proposed for doing this, but to test them, data, of course, are needed on a certain number of actual gas mixtures under various oonditions of pressure and temperature. The compressibility data for gas mixtures are known for several system up to 500 atmospherea and for a few as high as about 1700 atmospheres. The purpose of the present work is to extend the range of pressures for which the behavior of gas mixtures is known in order to test the various methods available for the prediction of this behavior and, in the process, provide some information which may be used to shed some light on such physical topics as the intermolecular attractive forces. Since it was desired to study a gaseous solution at higher pressures than had been reached previously, it was desirable to choose a system for which data already existed up to a fairly high pressure. In order to test the methods of prediction, it was also desirable to choose a system whose components had a knowh behavior in the range to be investigated. On the basis of these considerations it was decided to study the P-V-2’ relations of
