V O L U M E 25, N O . 1, J A N U A R Y 1 9 5 3 Table I.
Removal of Ethylene from Oxygen by Silica Gel
Lengths of Blackening, M m . Ethyl- AdsorpLeakCorrected ene tion ~ . , I1 Procedure I Values series c o n c n . , T ~ ~ Procedure I1 I S o . P.P.M. C. Sample Blank Sample Blank tion 47.4 48.6 567 9.3 51.8 0.4 2.8 1 0.082 -78 5.0 18.5 1.0 15.5 16.5 20,s 1.0 2 0,019 0 8.6 22.8 16.0 1.0 14.2 14.0 3 0.0035 -78 1.0
,:Ee,.
In such a procedure, it would be necessary to provide for efficient drying of the sample before its contact with the silica gel. Ethylene concentrations as low as 0.01 p.p.ni. have been observed to show rpinastic effects (9, 14). Are the data of Table I compatible with existing information on adsorption of ethylene on silica gel? Sorption isotherm data for ethylene on silica gels have been obtained a t ethylene pressures above 1 mm. of mercury (3-6',8,IO). If a linear adsorption isotherm below 1.9 mm. of mercury is assumed, the lowest point on the 0" C. adsorption isotherm of Lewis et al. (4), one finds mole adsorbed per gram of silica gel per millimeter of 63 x mercury ethylene pressure. Similarly, a value of 8.6 X is found from the data of Kalberer and Schuster (3) a t 4.1 mm. From the data of Table I for adsorption a t 0' C. a value of a t for this quantity is found. This discrepancy could least 5 x be due to nonlinearity of the isotherm below I-mm. ethylene pressure or to a difference in properties of the silica gels reflecting different methods of pretreatment. With regard to the high rate of adsorption, Steacie and Stovel (IO) noted that equilibrium a t 24" C. was reached practically instantaneously. Comparative sorption isotherm data for various hydrocarbons on silica gel (5)indicate that ethylene is doubtless not exceptional in its adsorptive behavior at very low pressures. Indeed it can he predictrd from present knowledge of sorption equilibria that
183 only nonpolar gases more volatile than ethylene would probably be less adsorbed than ethylene a t very low partial pressures, so that adsorption of trace amounts of many gases and vapors should be feasible. It is probably more questionable whether conditions for quantitatively recovering the adsorbate of interest in an unchanged condition can be worked out. For many adsorbates, displacement by a more strongly adsorbed gas such as water vapor may be found preferable to desorption at an elevated temperature. LITERATURE CITED
(1) Bartell, F. E., and Almy, E. G., J . Phys. Chem., 36, 475 (1932). (2) Holmes, H. N., and Elder, A. L., Ibid., 35, 82 (1931). (3) Kalberer, W.,and Schuster, C., Z . physik. Chem., A141, 274 (1929).
Lewis, TV. K., Gilliland, E. R., Chertow, B., and Bareis, D., J . Am. Chem. SOC.,72, 1160 (1950). (5) Lewis, W. K., Gilliland, E. R., Chertow, B., and Cadogan, W.P., Ind. Eng. Chem., 42, 1326 (1950). (6) Lewis, W.K., Gilliland, E. R., Chertow, B., and Milliken, W.. J . Am. Chem. soc., 72, 1157 (1950). (7) Patrick, W.A., Frazer, J. C. W.,and Rush, R. I., J . Phys. Chem., 31, 1511 (1927). ( 8 ) Reyerson, L. H., and Swearingen, L. E., Ibid., 31, 88 (1927). (9) Rohrbaugh, P. W., Plant Physiol., 18, 79 (1943). (10) Steacie, E. W. R., and Stovel, H. V., J. Chem. Phys., 2, 581 (4)
(1934). (11) Stitt, F., Tjensvold, A. H., and Tomimatsu, Y . , ANAL.C R m i . , 23, 1138 (1951). (12) Stitt, F., and Tomimatsu, Y., I b i d . , 23, 1098 (1951). (13) Young, R. E., Pratt, H. K., and Biale, J. B., Ibid., 24, 551 (1952). (14) Zimmerman, P. W., Cold Spring Harbor Symposia Quant. Biol.. 10, 152 (1942). RECEIVEDfor review August 18, 1952. Accepted September 19. 1932. Mention of a product does not imply t h a t i t is endorsed or recommended by the Department of rlgriculture over others of a similar nature not mentioned.
Ultraviolet Absorption Determination of Cp and Cl0 Aromatics MATTHEW S. NORRIS AND NORMAN D. COGGESHALL Gulf Research & Dezelopment Co., Pittsburgh, Pa. ETHODS of analyzing gasoline and other hydrocarbon fractions for the lower molecular m-eight aromatics are well known ( 2 , 4). For example, a gasoline may be accurately analyzed for benzene toluene, and the individual xylene isomers by precision distillation f o l l o ~ed by multicomponent analyses of the cuts. To extend determination of the individual aromatics t o the higher niolecular weight members becomes increasingly difficult because of the proximities of their boiling points and the close similaiity of the spectra. Fortunately, many cases do not require a detailed knowledge of the Concentrations of the individual aromatics but require a total aromatic concentration or concentrations by classes. The method presented in this paper is an extension of previous methods and is a method for the analysis of hydrocarbon cuts boiling between 150" and 180" C. for different classes of Cs and Clo aromatics. These results may then be combined to yield the total concentration of aromatics. This method is based on the close similarities in absorption characteristics of monosubstituted aromatics as a class, ortho- and meta-substituted aromatics as a class, and para-substituted aromatics as a class. EXPERIMENTAL
The instrument used was the Beckman quartz spectrophotometer, equipped for ultraviolet absorption measurements, as manufactured by the National Technical Laboratories, South Pasadena, Calif. The solvent u s d was spectro grade iso-octane (2.2,4-tri-
methylpentane) obtained from the Special Products Division, Phillips Petroleum Co., Bartlesville, Okla. The aromatics used for calibration were ilmerican Petroleum Institute standard samples obtained from the Department of Chemistry, Carnegie Institute of Technology, Pittsburgh, Pa. Glassware was cleaned in all cases by washing with Drene solution, followed by oven drying. Samples containing olefinic materials were treated with an alkaline permanganate treatment to remove possible interference. This treatment is widely used in the treatment of samples prior to ultraviolet analyses for CSand lighter aromatics. The solution of sampleis agitatedin iso-octane with a strong aqueous solution of potassium permanganate and potassium hydroxide for a period approximately 15 minutes a t room temperature. SPECTRAL BEHAVIOR
The absorption of radiation in the ultraviolet range by mononuclear aromatics is due to the electronic system of the benzene ring. This electronic system is affected by variations in direct substitution on the ring-ie., ortho versus para, and conjugation with olefinic or other active groups. However, variations in hydrocarbon substitution of carbons not directly attached to the ring produce only minor changes. For example, the spectra of n-propylbenzene and of isobutylbenzene are very similar in shape and intensities, as shown in Figure 1. An examination of the physical constants of the hydrocarbons compiled by the hmerican Petroleum Institute Research Project No. 44 ( 1 ) showed that 14 of the possible Cg and Cl0 aromatics possess boiling points in the 150' to 180" C. range. These have
ANALYTICAL CHEMISTRY
184 Table I.
Cg and CIOAromatics Classified According to Spectral and Structural Characteristics
Mono-Substituted Ortho- and Meta-Substituted 11-Propylbenzene Isoprop, lbenzene Isobutrlhenzene see-Butylbenzene tert-Butylbenzene
o-Erhyltoluene m-Et!iyltoluenc l-llerhy!-3-i;oprop).lbenzene 1,3-Diethylbenzene 1,2,3-Trimethylbenzene 1,3,5-Trimethylbenzene 1-Methyl-2-isopropylben. zene
Para-Substituted l - l I e r h s I - ~ - i s o C r o p1br.n) zene p-Ethvltoluene 1.2.4-Trimethvlbeneene
There are two compounds-l,2,3-trimethylbenzene and 1,3,5trimethylbenzene-that are markedly dissimilar from the others. They do, however, fit the ortho and meta classification better than the mono- or para-substituted classes and are included in the former c*lass for the sake of simplicity. The absorptivity 0.55I
0 50
0 45
tl I-METHYL-2- ISOPROPYLBENZENE
A I -METHYL-3- ISOPROPYLBENZENE
- I,2,3-TRIMETHYLEENZENE 0 40
0.20
0 0
>.
a Q
0 35
0.15
I-
x
ORTHO-ETHYLTOLUENE META-ETHYLTOLUENE I,3,5-TRIMETHYLBENZENE 43 DIETHYLBENZENE
-
>.
I-
2
a10
030
I-
2
n
K
0
025
0.05
0
230
a
240
250
WAVE
260
LENGTH I N
0 20 270
280
rnp
Figure 1. Ultraviolet Absorption Spectra for Monosubstituted Benzenes, Boiling Range 150' to 180" C. X
- -A00
tert-Butylbenzene lsopropylbenzene n-Propylbenzene Isobutylbenzene sec-Butylbenzene
0.15
010
been classified in Table I according to their spectral and structural characteristics. I n addition, 1,3-diethylbenzene, boiling 3 point 181.1" C., has been included. WAVE LENGTH IN m p In Figure 1 are shown the spectra of the mono-substituted Figure 2. Ultraviolet Absorption Spectra for itromatics. These are plotted as absorptivity calculated with the Ortho- and Meta-Substituted Benzenes, Boiling cv)iicentration expressed in moles per ml. The data are repreRange 150' to 180" C. wntative of absorption per molecule throughout the clitw. These mnteri:ile not only posTable 11. Molar Absorptivities of C9 and ClO Substituted Benzenes, 150' to 180" C. sess very siiuilar spectra in Boiling Range Av., terms of shapes and positions ?dolar Abdorptivities of Sitbstituted Benzenes, Moles/Ltter Average P a r t s / l 0 0 0 of maxima but also in terms Wave Length w Class A of equivalent intensities. Of IsotertSedthese compounds, tert-butyln-Propyl propyl Butyl Isobutyl Butyl benzene is a bit distinct from 259 205 198 192 1.27 173 189 194 272 27.6 18.6 13.1 25.0 20.3 0.135 16.9 the others but is still similar 274 12.1 8.31 5.63 8.70 0.0Xl 10.0 7.50 276 4.74 2.87 enough to be included in this 2.90 0 019 1.25 3.75 1.88 class. Maximum absorption Class B 1,3,5-Tri- m-Ethyl- o-Ethyl- 1,2,3-Tri- 1-Methyl-% 1,3-Di- 1-Methylfor this class occurs a t about methyl toluene toluene methyl isopropyl ethyl 2-isopropyl 259 mpI which is considerably 259 153 200 212 154 194 192 200 187 1.26 272 173 220 removed from the principal 184 86.4 1. I 9 166 177 210 200 274 133 133 80.2 40.0 64.4 89.7 0.610 85.4 91.3 maxima displayed for the other 276 80.1 43.9 26.4 0.250 23.1 36.4 22.1 28.0 31.1 classes of aromatics (Figures Class C p-Ethyl- 1-Methyl2 and 3). toluene 4-isopropyl The spectra of di- or trisub259 290 302 296 1.96 stituted aromatics which are 272 216 283 250 1.64 274 465 396 430 2.87 characterized by either or both 276 219 130 174 1.18 of ortho and meta substitution Class D 1,2,4-Tribut not para substitution are methyl shown in Figure 2. For sim259 260 1.86 plicity, in this paper these 272 350 2.50 274 329 2.35 are referred to as ortho- or 276 449 3 21 meta-substituted a r o m a t i c s .
185
V O L U M E 25, NO. 1, J A N U A R Y 1 9 5 3 Table 111. Analysis of Synthetic .iromatic Blends Concentrations, Volume %
Concentration, Rloles/Liter SamP le 1 2
3 4
Total aroD matics
Class A Blended 1.02 Found 0.96 Difference - 0 . 0 6 Blended 0.4'2 Found 0.34 Difference - 0 . 0 8 Blended 0.96 Found 0.83 Difference - 0 . 1 3 Blended 0.28 Found 0.28 Difference 0.0
B
C
-.
A
B
C
D 5,O
Total aromatics
1.06 0.35 0.36 0.880.480.38 -0.18 0 . 1 3 0.02
2.79 2.70 -0.09
15.0 14.4 -0.6
15.0 13.4 -1.6
5 0 7.1 2.1
5.4 0.4
40.0 40.3 0.3
0.92 0.21 0.74 0.790.320.78 -0.13 0.11 0.04
2.29 2.23 -0.06
6.0 5.0
12.9 12.0 -0.9
2.9 10.4 4.810.9 1.9 0.5
32.2 32.7
-1.0
2.08 0 . 7 8 0.65 1.900.940.69 -0.18 0.16 0.04
4.47 4.36 -0.11
15.1 12.1 -3.0
+1.3
0.72 0 . 0 8 0 . 7 4 0.630.150.77 -0.09 0 . 0 7 0.03
1.82 1.83 0.01
4.2 4.2 0.0
10.2 9.5 -0.7
SCHESlE O F AY4LYSIS
0.5 65.9
A4foul-coinponent matrix system using average values for the different classes M-as formulated. In Table I1 are given 1.2 10.4 26.0 2.210.8 26.7 the molar absorptivities (calculated 1.0 0.4 0.7 with concentrations expressed in moles per liter) for all of the 15 aromatics, the wave lengths chosen for each class, and the classification symbols. IVith a few exceptions, the absorptivities for a given class a t ii particular wave length do not deviate too severely from an average value. Those members which shox the largest deviations from the averages for their classes are the same ones that show distinct differences in spectra in Figures 1, 2, and 3. Of the fifteen aromatics examined, eight are Cg'sand seven are Clo's. This suggests that the deviations from average values of the absorptivities calculated with concentrations expressed iu volume fractions would be sindl criough to d o \ \ - satisfact,or>* analyses on a volume per cent btisis. This vas found to be the case. This is fortunate, as the volume concentrations of aromatics are frequently needed, and it is desirable to be able to calculate them directly. The average absorptivities for concentrations expressed in parts per thousand are also given in Table 11.
30.3 11.4 29.014.1 2.7
is calculated for concentrations expressed on a moles per milliliter basis. I n Figure 3 are shown the spectra of the three para-substituted aromatics boiling in the 150" to 180" C. range. Theseare generally similar. However, for purposes of the calculations, it is necessary to group the two very similar ones, p-ethyltoluene and 1methyl-4-isopropylbenzene, as one subclass and the 1,2,4-trimethylbenzene as another subclass. Therefore, there are four classes of aromatics, based on spectra: mono-substituted orthoand meta-substituted, and two subclasses of para-substituted members.
other classrs at all points and the orthoand meta-substituted compounds dominate the mono-substituted ones almost throughout the entire range. Howcver, a multicomponent analysis scheme \vas formulated and was found to gives satipfactory results.
9.1 9.5 0.4
64.7 -1.2
0.500
0.4%
0.400
0.350
*
t-
5
0.30C
I=
a (r
0
p a
0.250
w W
2 3 Y
0.200
0.150
230
240
250
260
270
280
2
WAVE LENGTH IN n+!
Figure 3. Ultraviolet Absorption Spectra fo: Para-Substituted Benzenes, Boiling Range 150 to 180' C. 0
-
IO
1,2,4-Trimethylbenzene
0 p-Ethyltoluene
1-Methyl-4-isopropylbenzene
The average absorptivities a t each wave length for each class wprr calculated and are plotted in Figure 4. These spectra would not indicate that a multicomponent analysis for the different rlasses would be feasible. The para compounds dominate the
WAVE
LENGTH IN
mp
Figure 4. Average Absorptivities of Four Classes of Substituted Benzenes 0 Class A A Class B X
Class C
C Class D
186
n
ANALYTICAL CHEMISTRY
..---
.
Table I\ Total Aromatic Concentrations Determined by Ultraviolet Absorption and Chromatographic Adsorption ConrentTntion Volume "0 Ultiaiiolet Chi oniatographic ab>orption adsorption 19.9 20.9 24. fi 23.0 43.0 45.7
Sample 1
2 3
A - UNTREATED 8 - TREATED ONCE C TREATED TWICE
0.900
0.80
-
RESC LTS
The first tePt of the accuracy of the scheme was made by analyzing synthetically prepared samples. These samples were blended t o include various amounts of the different individual aromatics used t o provide the calibiation data. Since both Cg's and Cl
