ANALYTICAL CHEMISTRY
42
thenes to Ca aromatics, but it could not be published heretofore because of wartime restrictions. The method has certain limitations. It will not reveal such compounds as unsaturated naphthenes, since they are removed in the bromination and steam-distillation step. I t will not distinguish between the Cs aromatics, nor between naphthenes of the same boiling range, such as methylcyclohexane and ethylcyclopentane, nor between the various dimethylcyclohexanes. However, the presence of conjugated diolefins, which are troublesome in other methods of analysis, does not affect the analysis by this method.
the development of this procedure. Appreciation is expressed also to Humble Oil & Refining Company for permission to publish this report. LITERATURE CITED
Doss, M. P., “Physical Constants of the Principal Hydrocarbons”, 4th ed., New York, Texas Co., 1943. Faragher, W. F., Morrell, J. C., and Levine, I. M.,IWD.E m . CHEM., h A L . ED.,2, 18 (1930). Fischer, H. G. M..private communication (Nov. 28, 1940). Grosse. A. V.. and Wackher. R. C.. IND.ENG.CHEM..ANAL.ED.. 11,614 (1939).
Lewis, J. B., and Bradstreet, R. B., Ibid., 12, 387 (1940). National Bureau of Standards, Circ. C410, (1937). Schneider, V., Stanton, G. W., and Watkins, E., Petroleum Re-
ACKNOWLEDGMENT
The authors wish to thank R. E. Tannich, H. G. Schutze, J. Holmes, G. Wash, N. F. Chamberlain, C. bI. Floyd, and members of the routine laboratories of Humble Oil & Refining Company and of the Baytown Ordnance Works for valuable assistance in
finer, 18, 112 (1939).
Ward, A. L., and Kurtz, S. S., ISD. ESG.CHEM.,ANAL.ED.,IO, 559 (1938). PRESESTED before the Division of Analytical and Micro Chemistry a t the 109th Meeting of t h e AVERICAN CHEMICAL SOCIETY, Atlantic City,
iK.J.
Quantitative Determination of Paraffins and Naphthenes in Gasolines By the Lamp Method S. G. HINDIN
AND
A. V. GROSSE, Houdry Laboratories, Houdry Process Corporation of Pennsylvania, Marcus Hook, P a .
The lamp method for the determination of hydrogen in liquid organic campounds has been applied to the quantitative estimation of paraffins and naphthenes in gasoline fractions. The accuracy of the procedure is independent of isomeric composition of the fraction; it varies from *1.5 weight 7 0 paraffin in the CScut up to *3.0% in the CIZcut. The application and use of the hydrogen-to-carbon ratio in the solution of various analytical problems are suggested.
T
H E authors have recently described a lamp method (5) for the accurate determination of the hydrogen content of liquid organic compounds and mixtures, particularly in saturated hydrocarbons. This paper discusses the application of this technique to the estimation of paraffins and naphthenes in gasolines, after removal of aromatics and olefins by any of the customary procedures, such as absorption by sulfuric acid or a sulfuric acidphosphorus pentoxide mixture, by silica gel percolation, or by selective solvents. In view of the expansion of the various catalltic processes in the petroleum industry, the quantitative estimation of paraffins and naphthenes has been assuming ever greater importance-for example, a thorough analysis of the saturate fraction of the charge stock is particularly important in aromatization and isomerization processes. Up to the present time, the determination of paraffins and naphthenes in gasolines, as exemplified by the so-called “PONA” method for the determination of hydrocarbon class composition, has been based primarily on the measurement of physical properties, mainly density and refractive index, or on derived functions, such as molecular refraction or refractivity intercept (6-8) of the saturate fraction, after the aromatics and olefins have been separated or due allowance has been made for their presence. While such methods are adequate when approximate data are required, and are particularly useful when considerations of sample size and analysis time are important, they are not reliable when there is need for accurate data. The cause of this inaccuracy is twofold: (1) The various isomers of a compound of given empirical (and hydrocarbon class) composition show wide
variations in physical properties while average values are used for purposes of analysis, and (2) charge stocks and gasolines both show high concentrations of specific isomers rather than equal amounts of all possible isomers. As an illustration of the first point, the constants of the five hexanes and the nine heptanes and the major CS and Cr naphthenes are tabulated. Table I includes, in addition, the calculated values for the Lorentz-Lorenz specific refraction,
, and the refractivity !), the Eykman refract.ion, intercept, (n - %). As is evident, the calculated “constants” d
show appreciable deviations from the average, and the maximum percentage deviation is substantial. The physical constants of the higher hydrocarbons are not so accurately known, but it is possible and even probable that the maximum deviations from the average are even greater. USE OF HYDROGEN-TO-CARBON ATOMIC RATIOS
As pointed out some time ago ( 4 ) ,in a critical analysis of the methods used for estimation of paraffins and naphthenes, “the only unambiguous procedure at present should be based on the determination of the atomic ratio of hydrogen to carbon. The ratio follows directly from the empirical formula of a hydrocarbon and its interpretation involves no further assumptions.” Since all paraffins have the empirical formula C,HZ, t 2, and all monocyclic naphthenes, C,H2,, determination of the hydrogen con-
43
V O L U M E 19, N O . 1, J A N U A R Y 1 9 4 7
From the practical standpoint, highly branched paraffins are not likely to occur ng-1 1 nl-1 1 d in natural materials; in synthetic man - Hydrocarbon Density -n a 4-n 2 2 terials, attention should be paid to their Ca Paraffins possible presence, and the accuracy of 1,0452 n-Hexane 1.3749 0.6594 0.34706 0.76074 1.0449 2-1IeCa 0.76129 1.3714 0.6531 0.34750 the lamp method, as shown in Table 111 1.0444 3-1leCa 1 . 3 7 6 5 0.8643 0.34582 0.75818 1,0442 2,2-,\Ie?Ca 1.3688 0.6492 0.34740 0.76078 modified accordingly. 1,0442 2,3-1reci 1.3750 0.6616 0.34601 0.75840 Table I11 is a correlation of the hydroAv. 0,34676 0,00067 0,75988 1 0.00127 1.0446 0.0004 Xaphthenes gen-carbon ratios of ,the paraffin homo1,0354 1,4097 0.7486 0,33075 0,72873 Me-cyclo Ca logs in the gasoline range, the differences 1,0369 Cyclo Ca 1,4262 0.7786 0.32922 0.72724 A v . 0.32999 * 0.00077 0.72799 * 0 00075 1.0362 * 0,0007 in the hydrogen-carbon ratio for paraffins C7 Paraffins and naphthenes, and the accuracy of the 1,0459 0,75738 n-Heptane 1.3877 0.6837 1,0455 ?-Me Co 0,75764 1.3849 0 . 6 7 8 8 lamp method for each fraction. 1,0452 3-Me Ca 0.75559 1.3887 0.6870 1,0443 3-Et Cs 0.75195 Since the accuracy of the lamp 1.3934 0.6982 1,0453 2,Z-N e& 0,75808 1,3822 0.6739 method, a t its present stage of develop1.0445 0,75277 1,3920 0.6951 2,3-1\Ie?Cs 1.0451 0.75796 1.3816 0.6730 2.4-11 e2 Ca ment, is of the order of r0.0370 hydro1,0444 0.75291 3,3-11ezCs 1,3910 0,6933 0 7,5375 1.0445 2,2,3-31e~C4 1,3895 0.6900 gen, corresponding to *0.005 in the Ar. 0,75534 * 0 00221 1,0450 * 0.0005 hydrogen-carbon ratio, the accuracy in I . 0366 0,72826 1.4198 0 , 7 6 6 5 the analysis of the various cuts decreases 1.036 1.414 0.72972 0.755 from *1.3 weight 70with pentanes to 1,036 1,4222 0 . 7 7 3 0,72603 1,036 0,72928 1,4120 0,752 h3.0 weight % with dodecanes, as is 1.036 0.73058 1.4087 0 , 7 4 5 1,0384 1.4231 0 , 7 6 9 4 0.73088 shown in the last column of Table 111. Av. 0 33057 + 0.00077 0.72913 + 0 00133 1 0365 4- 0.0007 Actually, the accuracy in the higher ranges is probably better than that indicated here, since volatility losses are Table 11. Boiling Points of Paraffins at 760 Rfm. reduced; in the absence of pure compound data, however, this 1Iaximuni Minimum point is ambiguous. Table I.
Constants of Hexanes, Heptanes, and Naphthenes
f
B P.
Hydrocarbon
B.P.
Hydrocarbon
Overlap
98.42 125.63 150.74 174.04 195.84
n-Heptane n-Octane n-Sonane n-Decane n-Undecane
79.21 99.23 122.28 136.8 159.1
2,2-31e?Cs 2,2,4-1\IesCa 2,2,4,4-IIenCs 2,2,5,5-IIeiCa 2,2,6,6-11erC7 (unknown)
Sone Sone 3.3 13.9
c.
CI
Cs Co CN
Cil
f
c.
=
c.
14.9
tent or calculation of the hydrogen-carbon atomic ratio of a saturated hydrocarbon mixture of n carbon atoms gives the paraffin and naphthene content immediately. [Although, in theory, bridged rings may occur in the lower boiling gasoline fractions (0,1,3-bicyclohexane boils near S O T . ) , there is, so far, no evidence of their occurrence. Even in the upper end of the gasoline range, where decalin or its homologs may appear, none have, t o the authors' knowledge, been shown to be present.] Usually, the molecular weight of the paraffin is derived from the boiling range of the cut under investigation. ( I n the higher boiling cuts, overlapping may occur as indicated below.) The calculation of the paraffin-naphthene ratio from hydrogencarbon data is as follows: Let z = the experimentally determined H/C ratio of the paraffin-naphthene mixture a = the H/C ratio of the paraffins boiling in the same range, as calculated from the emuirical formula x-2 then -x 100 = Lveight roparaffins and
a-2 a-x
ayi
I
Boiling Point,'
X 100 = weight %naphthenes
Figure 1 indicates the relationship of the hydrogen-carbon ratio of paraffins to their boiling points: all monocyclic naphthenes have, obviously, a hydrogencarbon ratio of 2.0000 By the method of Francis ( 3 ) the boiling points of all paraffins in the gasoline range may be calculated. Thus, the region of overlapping boiling points for the higher paraffins is established: the maximum and minimum boiling points of the nonane, decane, and undecane isomers shown in Figure 1 are based on Table 11.
c.,dt 760 Mrn.
Figure 1
Table 111. Hydrogen-to-Carbon Atomic Ratios and .4ccuracy of Lanip 3lethod 3Iolecular Weight ( H = 1.0081 Para5ns Pentanes
Formula
C
12.010)
c ~ H ~ ~ 72.147
~~~~~~~s Octanes Nonanes Decanes Undecanes Dodecanes
=
C8Hl8 CoHm Ci:H?z CllH?, c ~ ~ H ? ~
86.173 100,200 114.226 128.252 142,278 156.304 170.331
Weight % H 16.767 16,378 16,097 15.886 15.721 15.588 15.479 15,388
H/C Ltomic Ratio 2.4000 2.3333 2.2857 2.2500 2.2222 2,2000 2.1818 2.1667
Difference in Ratio between Paraffins and Xaphthenes A(P - N) 0,4000 0,3333 0.2857 0.2500 0.2222 0.2000 . 0.1818 0.1667
Accuracy of Lamp Method of Analysis, Weight (Calcd.) 11.3
11.5 11.8 -2.0 12.3 *2.5 1 2 i 13.0
1
%
0
1
45
V O L U M E 19, NO. 1, J A N U A R Y 1 9 4 7 Ta,hle I V I\ as prepared to facilitate calculation of the paraffin and naphthene percentages in the gasoline range. The percentage of paraffins in a fraction boiling in a specified range is given as a function of the experimentally determined water number, w, \r-hric it' is weight in grams of water produced by combustion of 1 gram of the hydrocarbon being analyzed. It may be derived from the formula of the hydrocarbon, or vice versa. Thus, the a a t e r number of hexane ( C G H i 4 )is:
1L'
7
x mol. wt. of H,O - 7 X 18.0162 = 86.1734 mol. wt. of C ~ H U -
--
Table 1- may be used to convert water number t o hydrogencarbon atomic ratio (for hydrocarbons), and Table VI used to convert hydrogen-carbon ratio into Reight per cent hydrogen. 1 graphical representation of the relationship of xveight per cent paraffin to I\ eight per cent hydrogen is given in Figure 2. The
Table V. Conversion of Hydrogen-Carbon Ratios into Water S u m b e r a n d Vice Versa =
(ic"
I I C Ratio or ni
i
4
5
,j(
=
2
3
0 3737 0.4421 0.5093 0.5755 0.6405 0,7047 0.7679 0.8300 0.8913 0.9517 1,0112 1.0696 1 1274 1 1842 1.2402 1,2954 1 3499 1,4035 1.4565 1 5086
0 3806 0.4488 0 5160 0,5820 0 6470 0 7111 0.7741 0,8362 0.8973 0.9576 1,0170 1.0754 1.1331 1,1899 1.2456 1.3009 1 3553 1.4089 1.4617 1 5138
51
5"
53
54
55
R,1 10.2 15.3 20.4 25 5 30.: :35., 40.8 45 9
5 2 10 4 15 6 LO 8 26 0 31 L 36 4 41 6 46 8
5 3 10.6 15 9 21 2 26.5 31 8 37.1 42 4 47 7
5 4 10.8 16.2 21.6 27.0 32.4 37.8 43 2 48 6
5.5 11.0 16 5 22 0 27.5 38 0 38.5 44.0 49.5
2in a
1
0.3668 0.4353 0.5026 0,5689 0.6341 0 6983 0 7616 0.8239 0.8853 0.9457 1 0058 1,0638 1,1217 1.1785 1 2347 1 2900 1,3445 1,398'3 1.451:? 1.5034
0
ri
(18.0162)
12.010n
0.36 t o 1.55 and H / C Ratio = 0.50 to 2.50)
+ 1.0081m f o r CnHm: Table VI.
57
58
5 7 11.4 17.1 22.8 28 5 34 2 39 9 4.5 6 51 3
5.8 11.6 17.4 23 2 29.0 34 8 40 fj 46.4 52.2
56 5 11 16 22 28 33 39 44 50
6 2
8
4 0 6 2 8 4
-
1 0
1 1 1.2 1 3 1.4 1 5
1 6
1.7 1 8
1 9 2.0
2 1 ., .> 2.3 2.4 2 , ,5
__
i
58
I
5.8 11.6 17.4 23.2 29.0 34.8 40.6 46 4 52 2
i
I
1
>
3
4.028 4 795 ,5.550 6.293 7.024 I 744 8.453 9.151 9 839 10.516 11.184 11.840 12.489 13 126 13.755 14 375 14.986 15.588 16.182 16,767 17,345
4.105 4.871 5.625 6.366 096 i ,815 8.623 9,220 9,907 10.583 11 250 11,905 12,553 13.189 1 3 . 817 14 436 15.046 15.648 16.241 16 825
4.182 4,947 5.699 6.440 7,168 7,886 8,593 9,289 9 975 10 650 11.316 11 970 12.617 13.252 13.879 14.497 15.107 1.5 707 16 300 16,883
4 259 5,022 5.774 6.513 7 240 7.958 8 663 9.358 10.042 10.717 11.381 1 2 .035 12.680 13.316 13 940 14 558 15,167 15.767 16.358 16.941
-_
5 9 11 8 17.7 23.6 29 5 35 4 41.3 47.2 58.1
7.
60
_
6.0 12.0 18.0 24.0 30.0 36.0 42.0 48.0 54 0
_
6 0 12 0 18 0 24 0 30 0 36 0 42.0 48.0 54 0
6 1 12 2 18 3 24 4 30 5 36 6 42.7 48 8 54 9
9 0 4217 0,4892 0 5557 0.6211 0,6856 0 7490 0.8115 0.8730 0.9336 0,9934 1,0522 1 1102 1.1672 1,2235 1 2791 1,3337 1 387.5 1.4407 1.4931 1,5447
Av. A 69 68
66 65 64 63 62 62 61 ii0
59 58 57 56 55 55 54 53 52 52
'
~
63 6 3 12 6 18 9 25 2 31 5 37 8 44 1 50.4 56 7
62
6.2 12.4 18.6 24.8 31.0 37.2 43.4 49.6 55.8
84 8 4 12.8 19 2 J5 6 32 0 38 4 44 8 a1 2 57 6
65 6 5 13 0 19 5 26 0 32.5 89.0 45.5 52.0 58 5
66 6 6 13.2 19.8 26.4 33.0 39.6 46.2 52 8 59 4
67 6 7 13 -1 20.1 26.8 33.5 40.2 46 9 53.6 60.3
68 6.8 13.6 20,4 27.2 34.0 40.8 47 6 54.4 61 2
69 6.9 13.8 20.7 27.6 34.5 41.4 48.3 55.2 62 1
C = 96 0 t o 82 7 7 , HIC-Ratlo = 0 50 t o 2 50)
1 0 0 8 1 X 100 1 On81 12 010/,n
0
59
5.9 11.8 17.7 23 6 29 5 35.4 41.3 47.2 53.1
8 0 4148 0.4825 0.5491 0.6146 0.p91 0.,4?7 0 8053 0.8670 0,9276 0.9874 1,0463 1 1044 1.1616 1.2179 1 2735 1 3282 1 3822 1 4354 1.4879 1.5386
I
0 4080 0 4757 0 5425 0 6082 0 6728 0 7364 0,7990 0.8608 0.9215 0 9815 1 0405 1 0987 1.1558 1.2123 1 2680 1 3228 1 3768 1,4301 1,4826 1,5345
Conrersion of Hydrogen-Carbon Ratios to Per Cent Hydrogen and \ i c e Fersa r c n=
0.5 0.6 0.7 0 8 0.9
Proportional Parts 59 60 61
6 0.4011 0.4690 0,5359 0.6016 0,6663 0.7301 0.7929 0.8547 0 9156 0 9756 1,0346 1.0928 1 1502 1,2068 1.2624 1,3173 1.3715 1,4248 1 4774 1,5294
.itoniic weight C = 12,010 A t o m i c weight €1 = 1.0081
(11 = 4 0 t o 17 3'',
1I C R a t i o or ni n
5
4
_
61 6 1 12 2 18 3 24.4 30 5 36 6 42 7 48 8 54 9
~
63
64
6.2 12.4 18.6 24.8 31.0 37.2 43.4 49.6 55.8
6.8 12.6 18.9 25.2 31.5 37.8 44.1 50.4 56.7
6.4 12 8 19 2 25.6 32.0 38.4 44.8 -51.2 57.6
'Or
cnl"rL
4toinic ueight C = 12 010 ktomic ueight H = 1 0 0 8 1
Per C ent Hydrogen 5 4 336 4 413 5.098 5,174 5.923 5,848 6.587 6.6660 7.385 7.313 8 . 029 8,100 8.733 8 803 9.427 9 496 10.178 10.110 10.784 10.851 11,513 11.447 12.100 12,165 12,808 12.744 13.379 13.442 14.066 14.003 14.620 14 681 15.228 15.288 15.827 15.886 16.417 16.476 16,999 17,057 4
~
62
)I
65 6 5 13.0 19.5 26.0 32.5 39.0 45.5 52.0 5s 5
Proportional P n r t a 66 67 68 6.7 6.8 6.6 13.2 13.4 13.6 20.4 19.8 20.1 26.4 26.8 27.2 3 3 , 5 34.0 33.0 39.6 40.2 40 8 47 8 46.2 46.9 54 4 52.8 53.6 61 2 59.4 60.3
6
8
7
9
A
9.012 9.702 10.381 11,050 11,709 12.359 12.999 13 630 14.252 14 864
4.719 5.475 6.219 6.951 7.672 8.382 9.081 9.770 10.448 11.117 11.775 12.424 13.062 13.692 14 814 14.925
76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61
15.468 16.064 16.651 17.230
15.528 16.123 16.709 17.287
60 59 68 58
4 489 5,249 5,997 6.733 ,457 8.170
4 566 5,324 8.071 6 . 806 7 529 8 241
4,642 5.400 6.145 878 , ,600 8.312
8 873 9.565 10 246 10.918 11.578 12,230 15.872 13.505 14.128 14.742
8.942 9.633 10.313 10,984 11.644 12.295 12.935 13.567 14 190 14.803
15.348 15.945 16.534 17 115
15 408 16.004 16 592 17.172
,
69 6.9 13.8 20.7 27.6 34.5 41.4 48.3 55 2 62.1
70 7.0 14 0 21 0 28 0 35 0 42 0 49 0 56 0 63 0
71 14.2 21.3 28.4 35 5 42 6 49 7 56 8 63 9
72 7.2 14.4 21.6 28.8 36.0 43.2 50.4 57 R 64 8
6.
73 7.3 14 6 21 9 29.2 36.5 43.8 51.1 58.4 65. I
74 7 4 14 8 22 2 29.6 37.0 44 4 51 8 58 2 66 6
75 7 5 15 0 22 5 30.0 37 5 45 0 52 5 60 0 67 5
76 7.6 15.2 22.8 30.4 38.0 45.6 53.2 60.8 68.4
ANALYTICAL CHEMISTRY
46
An actual case may be quoted; a close cut, 208" to 215" F. (97.8" to 101.7".c.) ana!yzed 34.5% naphthene, by weight, using the intercept method ( I ) with average values for all C, paraffins and naphthenes. The lamp method indicated 37.3%naphthene, by weight. A later, very much detailed examination of precisely fractionated cuts of the samde showed it to contain more than 90% of n-hkptane plus methylcyclohexane and gave a value for thenaphthene content within 1% of the figure given by the combustion procedure.
//
Some typical gasoline fractions were analyzed by the lamp method and the results compared Kith those obtained by the refractivity intercept method (Table IX). .4n interesting observation made was that the intercept method apparently yields high values for paraffin percentage. This might well be further studied.
3
u
2 2-
3
LO -
APPLICATION OF LAMP METHOD IN ESTIMATION OF HEAT OF COMBUSTION -
0 , 141
In the determination of the heat of combustion of gasolines, the hydrogen content of the gasoline must be accurately known to correct for the weight of water formed. Table X indicates the accuracy and precision of the lamp procedure as applied to gasolines.
,
I*.
110
1.1
85,
160
Weisht Per Cent of Hydrogen
Figure 2
relationship between water number and hydrogen-carbon atomic ratio is, of course, linear. DISCUSSION OF EXPERIMENTAL DATA
Procedure. The procedure ( 5 ) consists in burning a weighed amount of sample (usually 1 to 5 grams) in an A.S.T.M. sulfur lamp, in a steam of dry air, and collecting the water formed in a desiccant, CaC12 P205. The % hydrogen is calculated:
+
%
weight of H20 collected = weight of sample burned X 11.191
APPLICATION OF LA\IP IIETHOD TO ANALYTICAL PROBLEhlS
The method may be used in conjunction with other procedures-for instance hydrogenation, bromination-to give novel solutions of many analytical problems. A few of these solutions are given here. 1. The degree of unsaturation of a hydrocarbon mixture may be determined by this means, before and after hydrogenation, as a substitute for the usual bromine absorption. In case of a pure hydrocarbon, hydrogenation is, of course, unnecessary. 2. The hydrogen-carbon ratio may be used to classify olefins as either straight chain or cyclic. To illustrate, the bromine numbers of methylcyclohexene and heptene are, respectively, 166.2 and 162.8. Aside from the usual problems encountered in hromination, the difference in the two values is of the order of
The precision and accuracy of the procedure, as applied to pure hydrocarbons, are indicated in Table VII. Analysis of Complex Mixtures. Even in highly complex mixtures such as motor gasolines, where the boiling points of components range from 24' to 215" C., no appreciable fractionation ocTable VII. Precision and Accuracy curs, because the entire body of the liquid rises up the wick and is % H H/C Ratio burned at, the tip. The nonfractionation may be easily shown by Actual Determined Determined .Ictual refractive index measurements (using the Na D line, a t 20' C.) on 2.286 2.281 1 0,002 16.07 I .01 n-Heptane 16 10 the original sample and on some of the same sample that has been 2.250 2.240 * 0.008 15.83 * .04 15 88 Iso-octane Cyclohexane 14.37 14.34 * .03 2,000 1.993 * 0.006 partially burned in the lamp. In Table I1 of a previous paper (5) Methylcyclohexane 14 37 14.34 I .02 2.000 1.993 * 0.004 are listed some gasoliues and synthetic blends, their boiling Cetane 15.13 15 13 i. 01 2.124 2.124 * 0 . 0 0 2 ranges, the approsimat,e spread in refractive indices of the components, the Table VIII. .4nalysis of Synthetic Xlixtures of n-Heptane and Rlethylcyclorefractive index of the original blend, and hexane the refractive index of the sample reSfLTnple % ' Paraffin Composition, maining in the flask after one third to From lamp From refractivity 7 Paraffins boy Weight Water No. H / C Ratio % H method intercept one half of the total has been burned. To test the accuracy of the procedure 30.8 1.329 i 0,003 2.081 * 0,005 14.875 1 0.035 29.0 1 2 . 0 35.3 47.0 1.3565 * 0.0005 2.132 1 0,001 15,18 1 0.01 46.8 =k 0.6 54.6 with mixtures, synthetic blends, by 64.0 1.382 * 0,004 2.180 * 0.01 15.465 * 0.045 63.3 * 2.6 72.7 weight, of n-heptane and methylcyclohexane were prepared and analyzed. Table IX. Analysis of Gasoline Fractions Table VI11 indicates the actual sample composition, the per Para5n Paraffin Gasoline by Lamp b y Intercept cent hydrogen on analysis, the composition as calculated from cut H Method Method the per cent hydrogen, and the composition based on the refracF. "0 % % tivity intercept4ensity relationship. 205-250 14.81 * 0.02 28.9 * 1.4 35.2 Although an objection to the use of average paraffin-naphthene 302-347 14.935 * 0 . 0 4 5 46.3 * 3.7 52.5 14.34 10.04 00 10.4 data on a selected mixture such as n-heptane and methYlcYcl0may indicate presence of bicyclio naphthenes, value for % This point is under investigation; it is of pafticular interest in,view of the hexane may be raised, the authors feel that the procedure is valid, ~ ; ; ~ t ; ~ n ~ l i a b Idea t a on the presence of biwclics in the high bolllng gasoline inasmuch &s closely fractioned cuts may very well show high percentages of specific isomers.
3461;~e210w
V O L U M E 19, NO. 1, J A N U A R Y 1 9 4 7 Table X.
Analysis of Gasolines by the Lamp Method % Hydrogen
Sample By lamp method Aviation gasoline A 13.71,13.74 Aviation gasoline B 13.65,13.64 >Totor gasoline A 13.08,13.01,13.10 M o t o r gasoline B 13.10,13.09 * These samples were not analyzed
combustion Levin-chri$ modification 13.6 f 0.1
12.9
d 0.1
Approximate Composition Weight % P 0 N A 35 10 25 30 40 10 20 30 40 10 30 20 45 10 25 20
47
Determination of aromatic plus olefin by sulfuric acid a b s o r p tion Determination of H/C ratio of the saturate fraction directly and of the aromatic and olefin fraction by calculation 6. Determination of hydrogen content could be used to analyze mixtures of mono- and dicyclic aromatics. If rings of the phenylcyclohexane or phenylcyclopentane type are encountered, determination of the H / C ratio, followed by analytical hydrogenation, followed by another determination of H / C ratio, could be used t o indicate not only the aromatics, but also the weight per cent of naphthene ring. ACKNOWLEDGMENT
magnitude of the error in the determination of bromine number. By analyzing for per cent hydrogen (methylcyclohexene = 12.58% H , H / C = 1.7143, and heptene = 14.37% H, H/C = 2.0000),there should be no uncertainty. This technique may be applied to gasoline analysis: Determination of H / C ratio Removal of olefins by the Bond ( 2 ) method Determination of H / C ratio of olefin-freematerial 3. The H/C ratio may be used to measure the diolefin content of an olefin. The advantage of using this procedure is that conjugated or nonconjugated systems n-ould be equally amenable t o analysis. 4. Direct analysis for per cent hydrogen would probably be the most direct and most accurate method of following hydrogenation and dehydrogenation reactions. 5. The usual POSA analysis might be varied in this manner: Determination of H / C ratio
The authors wish bo thank Jack Grider for assistance with t h e analytical work, and Jack Hughes for his help in the calculation of some of the tables. LITERATURE CITED
Am. SOC.Testing Materials, Procedure ES-45a. Bond, G., IKD.EYG.CHEM.,ANAL.ED., 18, 692-6 (1946). Francis, A.. IND.ENG.CHEM., 35,448 (1943). Grosse. A. V., Refiner Natl. Gas M f r . , 18, No. 4, 149-57 (April 1939). Hindin, S. G., and Grosse, A . V., IND.ENG.CHEM.,ANAL.ED., 17, 767-9 (1945). Kurta, S.S.,and Headington, C. E., IND.ENQ.CHEM.,ANAL. ED.,9, 21 (1937). Kurtz, S. S., and Ward, A. W., Ibid., 10, 559 (1938). Kurta, S. S., and Ward, A. W., J. FranklinZn-st., 222, 563 (1936).
Analytical Methods for Carotenes of lycopersicon - Species and Strains F. P. ZSCHEILE'
AND
J. W. PORTER
Department of Agricultural Chemistry, Purdue University Agricultural Experiment Station, Lafayette, Znd. Studies on the nutritional importance of tomatoes have necessitated the development of rapid methods for the determination of carotenoids having provitamin A activity. Quantitative absorption curves of the principal carotenoids have in Lycopersicon species are presented. Spectroscopic methods for the determination of @-caroteneand lycopene are discussed in detail and results compared critically with those obtained by a chromatagraphic procedure. Sampling and extraction procedures are given and errors of recovery and those due to solvent impurity and carotenoid isomerization are discussed from the viewpoint of practical analysis. The proper application of spectrophotometric methods offers a valuable tool for the rapid analysis of tomato carotenoids.
T
H E spectrophotometric analysis of tomato fruit carotenes has been of considerable importance in a program conducted a t this station for the development of improved strains of tomatoes, genetically constituted to contain much more p-carotene(pr0vitamin A) than the present commercial varieties ( 4 ) . This problem will assume greater significance as the nutritional importance of tomatoes in our national economy becomes more generally recognized and as efforts t o improve the vitamin content of tomatoes begin to materialize in commercial production. It is also important in the study of fundamental problems of carotenoid physiology in relation to pigment development in plants. The complexity of the pigment system has made the analytical problem considerably more difficult than is indicated by earlier methods applied to mixtures of (presumably) all-trans forms (6, 7 ) , partially because of the presence of cis-isomers in some tomatoes (11) and in part because of the presence of addi1 Present address, College of Agriculture, University of California, Davis, Calif.
tional pigments (IO). However, from the practical viewpoint, after the various components have been identified, the spectroscopic analytical situation with regard to content of 8-carotene in the presence of lycopene does not appear so difficult as first suggested by other workers (6). EXPERIMENTAL
The following procedure has been designed to be as brief and simple as possible, so that it can be applied in a routine manner to hundreds of fresh and cannrd samplcs per season. Sampling. Fresh unpeeled fruits are used. If the fruit and core are large, the core is removed before cutting the fruit into pieces and homogenizing in a Waring Blendor. After about 2 minutes the fruits will be blended into a thick liquid mass which may be accurately sampled by pouring. Extraction and Separation of Carotenoids. A 20-gram sample, weighed to the nearest 0.1 gram, is poured into a clean Blendor cup and 75 ml. of acetone are added, some of which may be used t o rinse the sample from the weighing vessel. Sixty milliliters of hexane (b.p. 65-67" C.) are added and the mixture is blended
