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inorganic basic reducers, The antioxidant only inhibits or preventsoxidation for a time. The rate of reaction of oxidation after the effect of the inhibitant is lost is the same as before. There are several theories as to the action of these antioxidants. The writers’ results seem to confirm two hypotheses.
Vol. lS, No. 7
1-The antioxidant, being basic, combines with the acidic products of oxidation and prevents them from acting as autocatalysts toward oxidation. 8-The triple-bonded atom with two partial valences or elements with free valences forms intermediate compounds with the e a d y oxidized ethenoid carbon. 3-This temporary compound controls the rate of reaction for a definite, but limited period of time.
The Precise Determination of Aromatic Hydrocarbons in Gasoline’ By A. M. Erskine HAMILTONCOLLEGE,CLIXToX, hT. Y.
HE lowering of the critical solution temperature of gasoline and nitrobenzene by the presence of aromatic hydrocarbons has been found to be directly proportional to the percentage by weight of the latter. In the fractions 50-95’, 95-124’ C., and the residue above 124’ C. the lowerings in degrees by each per cent aromatic hydrocarbon are 1.18 for benzene, 1.12 for toluene, and 1.07 for xylene (*O.Ol), respectively, these factors applying exactly u p to 20 per cent by weight of t h e aromatic hydrocarbon. This principle has been applied in a method for the precise determination of the individual aromatic hydrocarbons in gasoline. The crude sample is fractionated and the aromatics removed by 98 per cent sulfuric acid essentially as in Tizard and Marshall’s “aniline point” method.
.. HE usefulness of the change in the critical solution temperature of a pair of liquids by the presence of small quantities of a third substance as a delicate test of the purity of a liquid was first emphasized by Crismer.2 Chavanne and Simon3 have developed a method for the determination of the gross percentage of aromatic hydrocarbons in gasoline based upon the lowering of the critical solution temperature in aniline produced by the aromatics and have applied it to the study of various types of gasolines. This method involves the determination of the solubility curve of the gasoline in aniline both before and after removal of the aromatics by mixed acid. Tizard and Marshall4 have simplified the procedure greatly in their “aniline point” method, in which they measure the solubility temperature of equal volumes of aniline and gasoline, this proportion giving the approximate critical solution temperature when the amount of aromatics present is small. Their method requires merely the determination of the solubility temperatures of two single mixtures, one before and one after the removal of aromatics by 98 per cent sulfuric acid, the difference between the two aniline points being taken as approximately equal to the aromatic content. Waterman and Perquin6 have extended this method to higher boiling fractions.
T
1
Received March 24, 1926.
PresentecP before the Division of Petro-
leum c h e m i s t r y at the 71st Meeting of t h e American Chemical Society, Tulsa, Okla., April 5 to 9. 1926. * Bull. SOC. chim. Belg., 9, 145 (1895); 10, 312 (1896); 18, 1 (1904);
ao,
294 (1906). a Comfit. rend., 168, 1111, 1324 (1919); 169, 70, 185, 285, 693 (1919); Bull. SOC. chim. Belg., Si, 331 (1922); see also Chercheffsky, C. A , , 16, 1805 (1921). 4 J . SOC.Chem. Ind., 40, 20T (1921). 6 Rec. Wan. chim., 41, 192 (1922): C. A , , 16,22.81 (1922).
Tubes containing equal weights (the critical proportions) of each fraction and of nitrobenzene are prepared volumetrically before and after the sulfonation. The solubility temperatures of these are determined within 0.05 to 0.10 degree, using a simple apparatus for agitation and temperature control. The rise i n critical solution temperature multiplied by t h e proper factor gives the percentage by weight of the individual aromatic hydrocarbons in the corresponding fractions from which the percentages in the total crude sample can be calculated. The method has the advantage of the critical solution temperatures being a t or slightly below room temperature and especially t h a t of considerable precision. A number of test analyses on synthetic mixtures gave values within a n average of 0.2 per cent of the known contents. The critical solution temperature of hexane and nitrobenzene has been shown by Timmermans6to be 19.2’ C. a t about 48 per cent by weight of hexane. Timmermans also investigated the effect of various phenols, aldehydes, acids, and nitro compounds in raising the critical solution temperature of the nitrobenzene-hexane mixture, but apparently no systems have been studied in which this critical solution temperature is lowered. Jones,’ in showing the important general value of ternary critical solution temperatures as criteria of liquid purity, mentions this binary system nitrobenzene-hexane. The work covered by this paper developed from the need of an accurate method for determining the small amounts of the individual aromatic hydrocarbons in natural gas gasoline. The general object was the study of the lowering of the critical solution temperature of nitrobenzene and gasoline due to the presence of aromatic hydrocarbons and the application of the relationship as a method for determining with a greater accuracy than by previous methods the individual contents of benzene, toluene, and xylene in gasoline. Preliminary Experiments From Timmermans’ study of the hexane-nitrobenzene system one would expect gasoline to give a critical solution temperature in nitrobenzene a t around room temperature. This was tried out by shaking together in test tubes three mixtures of a crude charcoal natural gas gasoline and pure nitrobenzene containing, respectively, 44.9, 48.75, and 52.1 per cent by weight of gasoline. The solution temperatures 6 Z. physik. Chem., 68, 129 (1907); Roozeboom, “ D a s Heterogene Gleichgewicht,” 1918, Vol. 11,pt. 2, p. 70; see also Hildebrand, “Solubility,” 1984, p. 143. T h e Chemical Catalog Co., Inc. 7 J . Chem. SOC.(London), 123, 1374, 1384 (1923). 8 See page 722, this issue.
INDUSTRIAL A-VD EJVGINEERING CHEMISTRY
July, 1926
(appearance of turbidity) were found to be l9.55", 19.55', and 19.3" C., respectively. Similar determinations were then made on small portions of the six pure hydrocarbons isolated by Anderson and Erskineg from natural gas gasoline and the critical solution temperatures were found a t approximately equal weights of hydrocarbon and nitrobenzene to be as follows: normal pentane, 24.5" C.; isopentane, 31.25' C.; normal hexane, 14.5" C.; isohexane, 24.05" C.; normal heptane, 11.5"C . ; and isoheptane, 18.05" C. By analogy from the work of Chavanne and Simon on gasoline-aniline mixtures, it would be expected that the presence of aromatic hydrocarbons would lower the critical solution temperature of gasoline and nitrobenzene. Theoretically, also, since aromatics are soluble in both paraffin hydrocarbons and nitrobenzene, the addition of the former should lower the critical solution temperature of the latter. That this is the case is shown by the data in Table I, which were obtained from weighed mixtures in sealed tubes of pure benzene, pure nitrobenzene, and the above gasoline after treatment with mixed acid to remove aromatics. Table I-Effect
of Benzene on Solubility Temperature of GasolineNitrobenzene Mixture
-PERCLXTACE
Benzene 0.00 0.98 2.02 4.83 10.26 14.67
WEIGHTS-
Gasoline 50,5l 49.67 49.84 47,69 44,76 43.51
Nitrobenzene 49.49 49.36 48.13 47.48 44.97 41.82
Solubility temperature O
c.
20,29 17.61 15.15
-
8.50
'L.7 -16.0
Preparation of Crude Fractions
The gasoline used in the following experiments was a straight absorption natural gas gasoline from the same source -Strong Plant, The Mars Company, Van, Pa.-as that used by Anderson and E r ~ k i n e . I~n ~ order ~ ~ to obtain crude fractions, each containing a single aromatic hydrocarbon which could be used to get aromatic-free fractions of the proper range for the various synthetic experiments and test analyses, the original sample, specific gravity 0.6721 (15.56"/ 15.56" C.), was separated into the three fractions, 50-95" C., 95-124" C., and the residue above 124" C. (760 mm.). The fractionating apparatus was a Hempel column of 2.5 cm. inside diameter filled to a depth of 56 cm. with glass Raschig rings about 0.6 cni. long. The column was wrapped with two layers of asbestos cord and had an adjustable reflux condenser a t the top and a side tube for the vapor exit and the thermometer similar to that in the column of Clarke and Rahrs.I1 It was found desirable to discard the gasoline distilling over below 50" C. in order to minimize composition changes due to evaporation losses in the lowest fraction and the synthetic mixtures prepared from it. Because of the low end point of the gasoline and the small volume above 124" C., it seemed best to use the residue remaining in the distilling flask after the cut a t 124" C. as the final fraction, instead of driving it over. Two portions of the crude gasoline, 2000 cc. and 3000 cc., respectively, were distilled in this manner, the fractions obtained having the following specific gravities at 15.56"/ 15.56" C. taken with a hydrometer: Fraction
c.
50-95 (760 mrn.) 95-124 Above 124
Sample A 0.6859 0.7263 0.7778
Sample B 0.6849 0,7269 0.7769
Removal of Aromatics by Sulfonation
The proof that the crude fractions contained small amounts of benzene, toluene, and m-xylene is reported in the paper following this 0118.8 It was necessary to remove these to obtain aromatic-free fractions for the subsequent experiments with synthetic mixtures of known aromatic content. Although the nitration method is ordinarily employed for this purpose, the use of 98 per cent sulfuric acid is preferable. That this is the most suitable reagent has been pointed out by Thole1*and it was also adopted by Tizard and 1Clarshall.4 Danaila, Andrei, and M e l i n e ~ c ubase ~ ~ a volumetric method for the determination of the gross aromatics content of gasoline upon the use of sulfuric acid of this concentration. They show that constant-boiling sulfuric acid, 98.33 per cent,14 and, in general, sulfuric acid from 97 to 100 per cent concentration absorb the aromatic hydrocarbons quantitatively a t ordinary temperature. According to the experience of the investigators cited, 98 per cent sulfuric acid is better than concentrated nitric acid or mixed acid because it has less action on the paraffin and saturated cyclic hydrocarbons. I n a number of preliminary experiments in which the effect of the acid treatment was followed by observation of the "aniline point," it was found that rejection of the material below 50" C. made possible extraction by 98 per cent sulfuric acid at room temperature without appreciable composition change in the 50-95" C. fraction due to volatilization. These experiments also showed a slight but negligible oxidizing action by the acid on the paraffin or other nonaromatic hydrocarbons in the 50-95" C. fraction. The somewhat greater effect on the two upper fractions will be considered later. Each of the crude fractions prepared above was sulfonated for removal of the aromatic hydrocarbon in it by shaking intermittently for one-half hour in a separatory funnel with two volumes of 98 per cent sulfuric acid. The acid layer was removed and the gasoline layer washed successively with two volumes of distilled water, one volume of 10 per cent sodium hydroxide, then three times with one volume of distilled water, and finally dried over calcium chloride. Comparison of Aniline and Nitrobenzene Methods
It seemed worth while to make a careful comparison of the exact relationships between solubility temperature lowering and aromatic hydrocarbon content in the cases of both aniline and nitrobenzene, with equal volumes and also equal weights of gasoline and the other liquid. Five synthetic mixtures containing 1 to 20 per cent benzene were made up by exact weighings from pure benzene, melting point 4.5" C. (Eastman), and the aromatic-free fraction, 50-95' C. (Sample A). Sealed tubes were then made up from these and also from the aromatic-free fraction itself using equal volumes (2 cc.) of gasoline and pure aniline, melting point -6" C. (Eastman). The tubes were prepared from ordinary 15-em. test tubes which had been thoroughly cleaned and dried. After drawing down the tube to a diameter of approximately 5 mm. a t a point about one-third the distance from the top, the aniline was added. The gasoline mixture was then introduced and the tube sealed off immediately, keeping the lower end in ice. The liquids were brought exactly to 20" C. (*O.l") in glass-stoppered Erlenmeyer flasks in a thermostat before measurement and the 2-cc. pipets (B. S. certified) were brought to the same temperature by standing in a large test tube immersed in the thermostat. -4similar series of tubes was prepared following J . SOC. Chem. Ind , 38, 39T (1919). Bul. chim. puvd aplicafa Soc. Romdnd Stiinfe, 26, Nos. 4-6, 3-49 (1923); C. A , , 18,3710 (1924). l 4 F o r a method of preparing this, see White and Holben, THIS JOURNAL, 17, 83 (192.5). 12
9
THISJOCRNAL,16, 263 (1924).
The material was kindly supplied by the United Natural Gas Co., Oil City, Pa. 11 THISJOURNAL, 16, 349 (1923).
695
13
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696
the same method, using pure nitrobenzene, melting point 5" C. (Eastman), and the same gasoline mixtures. A series of six sealed tubes was then prepared containing equal weights of the gasoline mixtures and aniline and also a like series with nitrobenzene. The liquids were brought to 20" C. in the thermostat and the volumes necessary to give 2.00 grams were measured into the tubes from a 10-cc. Mohr pipet (graduated in 0.05 cc.). An apparatus set-up consisting of the pipet, plain and tail stopcocks, and suction flask16was found very convenient in filling the pipet by suction and running out from it volumes with an accuracy of 0.01 to 0.02 cc. As the volumes of the synthetic mixtures on hand were too small for convenient density determinations, the densities were calculated from the composition on the as20
/8
iJ '4 lu ' 4
5h e
$m
P
k6 4
2
P€RC€NTAG€ W € / G N T //YDROCARBON
Figure
1
sumption that the law of mixtures holds accurately for these mixtures of aromatic and paraffin hydrocarbons. The error involved in this was negligible, Youngl6 having shown that benzene and normal hexane mixed in the ratio of 50 mol per cent each undergo a volume expansion of only 0.52 per cent. To determine how close to exactly equal weights the liquids actually were, each of the tubes in both series was weighed on the analytical balance, first when empty, then after addition of the aniline or nitrobenzene, and finally after addition of the gasoline and sealing. The difference between the percentage weights of the gasoline and the other liquid varied from a minimum of 0.18 to a maximum of 1.44per cent, with a n average of 0.61 per cent; that is, the average variation was only 0.30 per cent either side of 50 per cent each. The apparatus and method used for determining the solubility temperatures accurately and conveniently in these and succeeding experiments were as follows: The sealed tube was fastened by rubber bands t o another tube of the same size fused to a long glass capillary tube having a spiral of two turns near the top which was held by a clamp. A stiff wire rod attached t o the capillary below the spiral spring and connected t o an eccentric run by a variable speed electric motor gave a rapid and vigorous back-and-forth motion t o the tube. The tube was immersed in water in a 4-liter Pyrex beaker, which was very rapidly stirred by a four-bladed glass propeller run by the same motor. A Bureau of Standards certified thermometer graduated in tenths of degrees was immersed in the bath with its bulb in close proximity to the sealed tube. With a low Bunsen flame as the source of heat and the addition of small amounts of ice water for cooling in the case of the tubes containing nitrobenzene, i t was easily possible t o raise or lower repeatedly the temperature of the bath within the limits of 0.1 degree. For temperatures below 10' C. i t was found most convenient t o use an alcohol bath which could be readily cooled through the desired small range by the addition of small bits of solid carbon dioxide, rise in temperature being obtained merely by heat absorption from the room. Bur. Standards, Bull. 4,No. 4, 590 (1908). "Distillation Principles a n d Processes," 1921, p. 34. Macmillan & Company. 16
18
Vol. 18, No. 7
The uniform procedure adopted in the determination of the precise solubility temperature was to read alternately the temperature of disappearance of the turbidity on very slow heating and the temperature of its reappearance on very slow cooling four or five times. The mean value of the averages of the upper and lower temperatures was taken as the solubility temperature. This method when carefully carried out minimizes the slight error due to the conduction of heat through the walls of the tube and the consequent lag of the inside temperature behind that indicated by the thermometer outside. The turbidity appearance and disappearance are very sharply defined and with a little practice can be determined with a high degree of precision. On cooling, a marked opalescence gradually appears, which remains over several tenths of a degree, and this is followed by a sudden clouding due to the separation of the two phases, the latter being the true turbidity point. Estimating hundredths on the temperature readings with a magnifying lens attachment, it was found that the reading for either the upper or lower temperature rarely varied more than 0.02 degree. The difference between the averages of the upper and lower temperatures was alwayr less than 0.1 degree. For example, in twenty-two tubes of the four series the average difference between these averages was 0.06 degree. The mean value of the two thus gives a measurement for the solubility temperature which one can be certain is not in error by more than 0.05 degree. I n the case of temperatures below -5" C. for mixtures of high aromatic content with nitrobenzene, which were taken by a pentane thermometer on which only tenths could be estimated, the attainable accuracy was slightly less than this figure. Table 11-Comparison
of Aniline and Nitrobenzene Methods
I-ANILINE-GASOLINE 11-NITROBENZENE-GASOLINE Benzene in Solubility Lowering Solubility Lowering gasoline temp. temp. % Benzene ' C. (mean) % Benzene Per cent C. (mean)
-
Equal Volumes 0.00 1.00 2.01 5.12 9.96 20.15
-
15.82 15.03 14.00 11.16 6.53 3.9 Av.
0:?95 0.905 0.910 0.933 0.980 0.905
Equal Weights 111-ANILINE-GASOLINE IV-NITROBSNZENE-GASOLINE 0.00 64.15 16.96 1.00 63.12 1:oio 15.78 i:iio 62.43 0.856 14.66 1.144 2.01 1.148 0.965 11.08 5.12 59.21 5.27 1.174 54.16 1.003 9.96 20.15 43.64 1.018 - 6.8 1.181 Av. 0.974 Av. 1.165
The results obtained in the tubes prepared under the four series are summarized in Table 11. These data show the proportionality between solubility temperature lowering and benzene content in all four cases, but a wide variation in the value of the propQrtionality factor is indicated. It was decided to adopt the equal-weight nitrobenzene method, since it shows two distinct advantages. The first of these is the larger value of the proportionality factor and consequently the greater accuracy attainable by the method. For example, if one takes 0.05 degree as the limit of accuracy of the temperature determination, a lowering of this much corresponds to 0.04 per cent benzene determinable by the equal-weight nitrobenzene method but only 0.06 per cent by the "aniline point" (equal-volume) method. The second and more important advantage is in the lower temperatures which may be employed with nitrobenzene as compared with aniline. This is of distinct importance in the 50-95" C. fraction because it eliminates composition changes in the mixture due to volatilization losses. That these may be considerable using aniline is indicated by the fact that the critical solution temperature of normal hexane is 69' C., its boiling point, and that of isohexane is 73.8" C., or 12.8 degrees above its boiling point.3
July, 1926
697
That the equal-weight proportions of nitrobenzene and gasoline give a solubility temperature very close to the critical solution temperature, at least for small aromatic contents, is shown by Curve I, Figure 1, the solubility-temperature curve of nitrobenzene and a crude, unfractionated natural gas gasoline. This was determined from mixtures of a charcoal gasoline, specific gravity 0.6675 (15.56"/15.56" C.), and nitrobenzene prepared in sealed tubes. The necessary weight proportions were obtained with sufficient accuracy by measuring the calculated volumes wit,h the Mohr pipet by the method described above. The solubility temperatures were determined by the apparatus and procedure previously described. The critical solution temperature shown by the curve is 17.4" C. a t about 48 per cent gasoline by weight. Curve 11, Figure 1, which was plotted from the data of Timmermans6 for pure hexane and nitrobenzene, also shows the critical point a t 48 per cent hexane. I n both cases it is evident that the 50 :50 per cent mixture gives a solubility temperature approaching the critical solution temperature well within 0.1 degree. It may therefore be concluded that the lowering of the solubility temperature of the 50:50 per cent mixture due to aromatics present in the g:tsoline is essentially a lowering of the critical solution temperature, and
nitrobenzene were prepared from each synthetic mixture and also from the aromatic-free fraction using the volumetric method previously described. The specific gravities of the synthetic mixtures were calculated from their compositions as before. The solubility temperatures were determined by the method previously described, the average of each closely agreeing pair of tubes being taken as the critical solution temperature. Table 111-Comparison Lowering Mixture % Benzene
Av. per cent deviation from mean:
1.2
of Proportionality Factors Lowering Lowering 70 Toluene yoXylene 1.113 1.059 1.119 1.057 1.111 1.054 1.149 1.123
1.123
1.065 1.095 1.066
0.9
1.1
The result3 obtained in the three series of experiments are shown by the curves of Figure 2. The points for the first two mixtures are omitted on each curve. The closeness to an exact linear relationship in each case and also the appreciable difference in the values of the proportionality factors for different aromatic hydrocarbons are readily seen from these curves. How far above 20 per cent aromatic content the straight-line relationship holds is a question which it is hoped can be investigated soon. The values of the proportionality factors for each of the five points from which the curves were drawn are given in Table 111. The small values of the average percentage deviations from the mean indicate the accuracy of the method with respect to the relationship concerned. The mean values were adopted for use in the test analyses described below. Test Analyses
f k , CENr ARoNanc HYm?OCARffON Figure 2
the method will hereafter be referred to as the "nitrobenzene critical solution temperature method." It is of interest to note that Keyes and Hildebrand" found the critical solution temperature of the hexane-aniline system a t 48 per cent by weight of hexane. Proportionality Factors between Critical Solution Temperature Lowering and Aromatic Content
The exact relationship between lowering and aromatic content was worked out, for each aromatic hydrocarbon using synthetic mixtures prepared by accurate weighings from the aromatic-free fractions, 50-95", 95-121", and above 121" C., Sample B, and the corresponding pure aromatic hydrocarbon. The aromatic contents of these were in the neighborhood of 1. 2 , 5 , 10, and 20 per cent by weight. Eastman benzene, melting point 4.5" C., toluene, boiling point 109.5-110.5" C., and m-xylene, boiling point 138-139" C., were used. Duplicate sealed tubes containing equal weights of gasoline and 17
J A m Chem Soc., 39,2126 (1917).
4 number of complete analyses were made on synthetic mixtures containing accurately known amounts of aromatlc hydrocarbons in order to check the method. The data and results obtained in these analyses are summarized in Table IV. The known mixtures were made up by exact weighings from the previously prepared aromatic-free fractlons and the corresponding pure aromatic hydrocarbons. A volume of each varying from 20 to 35 cc. was sulfonated for the removal of the aromatic in it by the method previously described. The time of shaking for the benzene mixtures was from 10 minutes continuous to 20 minutes intermittent shaking. The mixtures containing toluene had 5 to 10 minutes and those containing xylene 10 minutes continuous shaking each. Sealed tubes containing equal weights of the gasoline and nitrobenzene were prepared in duplicate before and after the sulfonations for the determination of the rise in critical solution temperature due to the removal of the aromatic hydrocarbon. The temperature determinations were carried out in the manner previously described. The rise in critical solution temperature multiplied by the reciprocals of the factors found above then gave the per cent aromatlc hydrocarbon. I n the case of the upper two fractions it was found that the effect of 98 per cent sulfuric acid on nonaromatic hydrocarbons cannot be neglected as in an approximate method, and it was necessary to work out correction factors for this. The critical solution temperatures of the sulfonated synthetic toluene mixtures were consistently higher than those of the original completely aromatic-free fractions, indicating a removal of other hydrocarbons. 'According t o Sentke,'* normal octane is an exception to the general rule that concentrated sulfuric acid does not attack paraffin hydrocarbons a t ordinary temperature. The slight action observed may there18
Engler-Hofer, "Das Erdol," 1913, Vol I, p 231.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
698
fore be due to this hydrocarbon. The correction factor was determined by a blank sulfonation on a portion of the original aromatic-free fraction using 10 minutes continuous shaking and the other conditions exactly as in the test analyses. The critical solution temperature was determined before and after the treatment. Duplicate sulfonations in this way gave a rise in critical solution temperature of 0.58 and 0.62 degree, average 0.60 degree. This was then applied as a negative correction to the observed rise obtained in the toluene test analyses. A correction factor for this effect in the xylene range was worked out by a similar blank sulfonation with the same conditions as in the xylene test analyses and the rise of critical solution temperature found to be 0.52 degree. Table IV-Test Analyses Aromatic hvdrocarbon CRITICAL SOLUTION Rise Rise TBMPERATURE(obsd.) (cor.) Mix- known Final Degrees Degrees ture % Wt. Initial Benzene 0.89 6.04 9.46 15.09
15.57 9.22 5.21 -1.47
16.61 16.27 16.20 16.08
1.04 7.05 10.99 17.55
6
Toluene 1.03 1.13 1.06 9.61 9.55 9.54
12.21 11.94 11.77 2.38 2.19 2.21
13.76 13.87 13.41 13.77 13.52 13.19
1.55 1.93 1.64 11.39 11.33 10.98
(-0.60) 0.95 1.33 1.04 10.79 10.73 10.38
1 2 3 4
Xylene 1.16 4.87 10.41 14.88
18.64 14.69 8.72 3.79
20.05 20.04 20.07 20.13
1.41 5.35 11.35 16.34
(-0.52) 0.89 4.83 10.83 15.82
1 2 3 4
1 2 3 4 5
...
... .. ....
Aromatic hvdrocarbon
found % Wt. (Rise
XO ,845) 0.88 5.96 9.29 14.82 (Rise X0.890) 0.84. 1.18 0.92 9.60 9.55 9.24 (Rise X0.938) 0.83 4.53 10.16 14.84
Av.
Differ... ~~
ence Per cent -0.01 -0.08 -0.17 -0.27
-0.19 i-0.05 -0.14 -0.01 0.00 -0.30 -0.33 -0.34 -0.25 -0.04 0.15
The fraction taken for the xylene synthetic mixtures, Nos. 1 and 3, was yellow as a result of having stood in the sunlight for about 17 months. It was necessary to remove the unsaturated compounds causing this color, which was done by refluxing with an aqueous solution of mercuric acetate and alcohol, as in the method of Danaila.13 After steam-distilling the unattacked gasoline, which was perfectly colorless, the gasoline layer was washed successively with sodium hydroxide, sodium bisulfite, and distilled water until neutral, and dried by calcium chloride. This treatment gave a rise in critical solution temperature of 0.45 degree due to the removal of the unsaturated hydrocarbons. The resulting product was then used to prepare the synthetic xylene mixtures. Accuracy of Method
The following factors may be considered as the important ones affecting the accuracy of the method: (1) accuracy of the temperature determination, ( 2 ) reproducibility of the mixtures of critical proportions, (3) completeness of separation of the aromatics in the crude fractions, and (4) the sulfonation reaction. The first has been discussed under the comparison of methods. All the temperature measurements in the succeeding series of experiments have borne out the estimate given there. For example, the average difference in turbidity temperatures for forty-nine tubes used in the synthetic experiments was 0.06 degree, with a minimum of 0.04 and a maximum of 0.12 degree. That the 50:50 per cent mixture can be prepared very accurately by observing the temperature control and other necessary precautions was shown previously in the case of the weighed tube mixtures. The temperature checks on duplicate tubes serve as indications of the reproducibility of such mixtures. For example, in twenty-seven pairs of tubes in the synthetic experiments the duplicates were within an average of 0.06 degree of each other, varying from 0.00 to 0.12 degree. Considering factors (1) and (2)
Vol. 18, No. 7
together, the method is seen to be capable of giving results with an accuracy within 0.05 to 0.10 per cent (by difference), other factors being constant. As to the segregation of the aromatics in the crude fractions, even though a slight overlapping might occur, as claimed by Thole,I2 the cutting points a t 95" and 124" C . should minimize this error and the percentage by weight of aromatic in each fraction would not be affected appreciably because of the closeness in the values of the proportionality factors. To get information on these points it is hoped to carry out further tests of the method using synthetic mixtures containing all three aromatics, fractionating these, and determining the aromatic content of each fraction. The sulfonation reaction in the case of the upper two fractions probably introduces the largest single error in the method. The values of the correction factors applied for this error seem fairly satisfactory but may not apply to other types of gasoline. This general point is receiving further investigation. It should be pointed out that, as in the methods of Tizard and M a r ~ h a l l ,and ~ Chavanne and Simon,3 this method is applicable only in the absence of unsaturated hydrocarbons. These lower the critical solution temperature almost as much as do aromatics, a few synthetic experiments with amylene showing a lowering of 0 . 8 degree for 1 per cent olefin. The results on the xylene mixtures indicate, however, that these may be removed completely by mercuric acetate.13 Summary
The essential steps and conditions in the method as it would be employed in determining the aromatic contents of an unknown gasoline may be summarized as follows: The sample (300 t o 500 cc.) is fractionated, using the most effective column possible, into three fractions, 50-95', 95-124", and 124-150' C. (760 mm.) or the residue above 124' C. After removal of any olefins by a treatment with mercuric acetate, a 25- t o 50-cc. portion of each fraction is sulfonated with two volumes of 98 per cent sulfuric acid shaking steadily for 10 minutes, except in the case of the 50-95" C. fraction, where 20 t o 30 minutes are necessary for high benzene contents. The acid layer is removed and the gasoline layer washed first with distilled water, then with sodium hydroxide solution, and finally with water until neutral, after which it is thoroughly dried over calcium chloride. Tubes, preferably sealed, are made up from the gasoline both before and after the sulfonation containing equal weights (2.00 grams if test tubes are used) of gasoline and pure, dry nitrobenzene, the equal weights being obtained with sufficient accuracy by measurement of the required volumes from a pipet graduated t o 0.05 cc. with suitable temperature control. The critical solution temperatures of these are determined using any apparatus t h a t will give vigorous shaking and a controlled slow rise and fall in temperature. The rise in critical solution temperature after deducting 0.60 and 0.52 degree in the toluene and xylene fractions, respectively, multiplied by the factor, 0.845 for benzene, 0.890 for toluene, and 0.938 for xylene, gives the percentage by weight of the aromatic hydrocarbon in the corresponding fraction. The amount of each in the total crude sample may then be calculated from the ratio of the weight of each crude fraction to the total initial weight of the sample.
The advantages which can be claimed for the method are, first, the lower temperatures which are used as compared with the methods based on solubility temperature measurements in aniline; and secohd, the precision possible in determining the individual aromatic hydrocarbons, the test analyses indicating an accuracy represented by a percentage difference of *0.2 per cent. A Division of Trade Practice Conference has been created by the Federal Trade Commission. It will coordinate and facilitate all work incidental to holding conferences with representatives of industries t o aid them in the elimination of harmful or unfair trade practices.
