ANALYTICAL CHEMISTRY
1006 I
1.300
I
I
I
LEGEND
-
C6 FRACTION FROM UNTREATED SYNTHESIS GASOLINE
-
1.100
----%
FRACTION FROM SYNTHESIS GASOLINE
c
w
-
T6FATE-D
,900
Y U
0
--
,700
R\
c = c,I n
R“
R”
of compound types in complex hydrocarbon mixtures. Studies are currently in progress on the development of methods for determining various types of alkyl aromatics, based on the absorption of these compounds in the 10- to 15-micron region of the infrared, and on the determination of CHI groups, CHI groups, and Ch- and Ce-membered naphthene rings in complex mixtures. Results of these studies when completed will be presented in subsequent publications. ACKNOWLEDGMENTS
8 L
::
,
,+.;
9
I
The work discussed in this article was conducted in the laboratory of the, Humble Oil and Refining Company, to which the authors express thanks for permission to publish the data obtained. The contribu-
‘.
. e . ,
a . SO0
I
\
I)
\
\
,300
-
Figure 3.
Infrared Spectra of Ce Fractions from Untreated and Treated Synthesis Gasoline
Table IX.
Oxygenated Compound Type Analyses of Treated Fischer Synthesis Products
Treat No. Functional
0
1
2
3
4
5
0.54
0.07
0.01
0.00
0.02
0.02
%TU@ OH CHO
...
...
...
. ..
Nil
2.49 0.18 0.00 0.00 0.00 0.00 coo 1.76 1.73 1.47 1.31 1.11 1.04 co 2.29 0.69 0.30 0.20 0.18 0.08 N o absolute valuea obtained because of interference of acids i n determining aldehyde conoentration. Indications are that aldehydes are reduced to an immeasurably low concentration. COOH
application are given in Figure 3 and Table IX. I n such cases, the precision obtained with the method is more important than the absolute accuracy. It is hoped that the accuracy of the method can be improved through further study and especially by the acquisition of more calibration standards of high purity. There are other potential applications for the determination
(1) Avery, W.H.,J. Optical SOC.Am., 34,633 (1944). (2) Barnes, R. B.,Gore, R. C., Liddel, U., and Williams, V. Z.,“Infrared Spectroscopy,” New York, Reinhold Publishing Corp., 1944. (3) Barnes, R. B., Liddel, U., and Williams, V. Z., IND. ENO.CHEM., ANAL.ED.,15,83(1943). (4) Brattain, R. R., Calif. Oil World Petroleum Ind., 36 (2). 9 (1943). (6) Coggeshall, N. D., J. Am. C h m . Soe., 69,1620 (1947). (6) Hersberg, G.,“Infrared and Raman Spectra,” New York, D. Van Nostrand Co., 1945. ( 7 ) Johnston, R. W. B., Appleby, W. G., and Baker, M. O., paper CHEMICAL presented at Southwest Region Meeting, AMERICAN Dec. 13, 1947. SOCIETY, (8) Kent, J. W., and Beach, J. Y., ANAL. CHEM., 19, 290 (1947). (9) Lecomte, Jean, Compt. rend., 178,1530-2 (1924). (10) Ibid., 180, 1481-2 (1925). (11) IMd., 203, 1501-3 (1936). (12) Liston, M.D.,Quinn, C. E., Sargeant, W. E., and Scott, C . G.. Rea. 8&. I 7 l 8 t f U d 8 , 17,194 (1946). (13) Nielsen, J. R.,Oil Gas J., 40 (37),34 (1942). (14) Wright, N., IND.ENG.CHEM.,ANAL.ED.,13,l(1941).
RECEIVED March 25, 1948. Presented before the Division of Analytical and CH~MICAL SOCI~TT Micro Chemistry a t the 113th Meeting of the AMERICAN Chicago, Ill.
Isomeric Xylene and Ethylbenzene Mixtures Infrared Spectroscopic A nalys is WILBUR I. KAYE AND MARSHALL V. OTIS, Tennessee Eastman Corporation, Kingsport, Tenn.
T
H E xylene isomers and ethylbenzene have been extensively studied by infrared and Raman spectroscopists (1-4). Mixtures of these substances have been the subject of considerable research by chemists, who have attempted to develop efficient and economical separation procedures for these constituents which are normally present in commercial xylene (6). In many instances the results have been difficult to evaluate, because no accurate means of analysis was known. With the advent of infrared absorption spectroscopy, a method became available for conducting such analysis. The infrared absorption spectra of the xylene isomers and ethylbenzene in the liquid phase possess characteristic differences which facilitate accurate analysis of mixtures of these compounds
by infrared spectroscopic methods. The wave-length region chosen for the analysis is one in which certain bands characteristic of the phenyl ring structure manifest frequency shifts due to the addition of functional groups. The high intensity absorption bands at 12.54, 12.98, 13.44, and 14.30 microns are characteristic of p-, m-, and o-xylenes and ethylbenzene, respectively. INSTRUMENTAL
The spectral measurements were made with the null meter circuit incorporated in the Beckman Model IR-2 infrared spectrophotometer. Wave-length settings are reproducible to 0.001 micron by means of the turret stop feature of this instrument (6). The slit width settings are reproducible to 0.1% by means of a micrometer. The high sensitivity of the photoreceiver, stability
V O L U M E 20, N O . 11, N O V E M B E R 1 9 4 8
1007
A liquid-phase method is presented for the quantitative analysis of isomeric xylene and ethylbenzene mixtures in carbon disulfide solvent. A n accuracy of 1% is substantiated by the analysis of several synthetic mixtures prepared from high-purity National Bureau of Standards reference samples. The absorption coefficient abnormalities for the xylene isomers and ethylbenzene are reported for varying concentrations in carbon disulfide solvent.
100
1
I
1
I
concentrations are known in order to maintain the optical density of the major constituent in the neighborhood of 0.5. High-purity xylene isomers and ethylbenzene samples were secured from the Sational Bureau of Standards and used for the determination of absorption coefficients of these compounds a t the four selected wave lengths used in this analysis. The purity of these samples is reported by the National Bureau of Standards as follows:
I /
El 14.3%
14 4 4 Y
33 3 9 .
2 3 . 0 ~
I-
50
The sample mixture wax introduced into the absorption m1l with the aid of a hypodermic syringe. Spectral measurements were made a t the following instrumental settings for each of the pure compounds:
U
M E T A - XYLENE 33.3 9. 13 A4*
-
40
12 0 % ~
a
Impurities, Mole % 0.010 * 0.007 0.06 0.04 0.06 * 0.03 0.20 * 0.07
Compound o-Xylene m-Xylene p -X y1ene Et hylbensene
Y E T A - XYL
ORTHO- XYLENE
Y 0
30
I
20.6 %
I
12.40
I I I 12.80 13.20 13.60
I
14.00
Compound p-Xylene m-Xylene o-Xylene Ethylbenzene
I I I 14.40 14.80
Wave Length, Microns 12.54 12.98 13.44 14.30
Slit Width, Mm. 0.600 0.690 0.830 1.340
W A V E L E N G T H I N MICRONS
Figure 1.
Infrared Absorption Spectrum of Synthetic Mixture
of the amplifier, and the linear characteristics of the electrical measuring circuit permit optical density measurements within 0.005 at 0.400 or 0.002 a t 0.050 of a density unit in the wavelength region selected for this analysis with the rock salt optics of this instrument. The absorption cell used for the analysis was the standard Beckman model provided with a 0.1-mm. amalgamated lead spacer. ANALYTICAL PROCEDURE
The high intensity absorption bands selected for quantitative measurements are reduced by a dilution of 1 to 20 parts by volume n ith carbon disulfide solvent. The instrumental resolution of the characteristic absorption bands may be seen in Figure 1, which is a replot of the original automatically traced graph of the infrared absorption by a typical mixture. Care must be exercised in the determination of ethylbenzene in the presence of mxylene, inasmuch as characteristic absorption bands of these two compounds are relatively close together in the spectral region selected for analysis. The dilution procedure may be varied when the approximate
Table I.
Io = kicil D = log 7
+ ha1 4- . . . .
in which 1 is neglected, since the same cell (0.1 mm. thick) was used in every case. The absorption due to the solvent and rock salt plates was carefully determined and checked frequently. It varied from 0.051 at 12.5411 to 0.092 a t 14.30~. The values of the absorption coefficients at various concentrations are recorded in Table I. There are significant deviations from Beer’s law at low concentrations which require that a series of approximations be used in the mathematical solution of the linear equations representing Beer’s law a t the characteristic wave lengths. In each successive approximation the absorption coefficients are used which are representative of the concentration found in the preceding approximations. The absorption coefficients used in calculation procedures are selected from Table I after a f i s t rough approximation of the
Absorption Coefficients Versus Concentration of N.B.S. Standard Samples of Isomeric Xylene and Ethylbenzene in Carbon Disulfide Solvent in 0.1-Mm. Cell
Concn., MI./Ml. CSn 0.0476 0.0238 0.0119 0.0060 0.0030 0.0015 0
The absorption coefficients for each of the pure compounds a t each of the four wave lengths were calculated from the known concentrations in carbon disulfide and the optical density measurements according to the familiar Beer’s formula:
Ethylbenzene.
Para 19.51 21.17 22.10 22.50 22.60 24.90
12.54 Microns Meta Ortho 0.23 0.15 0.29 0.17 0.42 0.17 0.66 0.33 1 .oo 0.66 1.33 1.33
E.B.0 0.90 0.97 1.09 1.16 1.33 2.00
Para 0.29 0.42 0.67 1 .oo 1.33 2.00
12.98 Microns M e t i Ortho 15.97 0.42 19.83 0.50 20.92 0.68 21.66 1.00 1.66 22.33 24.66 2.66
E.B. 1.76 1.89 2.01 2.33 3.00 4.00
Para 0.15 0.17 0.25 0.50 1 .oo 2.00
13.44 Microns Meta Ortho 0.23 25.65 0.29 31.09 0.34 33.36 0.50 35.50 0.66 36.33 1.33 38.00
E.B. 4.62 4.78 4.96 5.50 5.66 6.00
Para 0.11 0.13 0.24 0.50 1.00 2.00
14.30 Microns Meta Ortho 1.11 0.15 1.21 0.17 1.18 0.17 1.50 0.33 1.66 0.66 1.33 2.00
E.B. 11.24
11.84 12.10 12.86 13.00 13.30
ANALYTICAL CHEMISTRY
1008 _ _.
Table 11. Sample So. 1
p-Xylene Known Exptl. Error
. -
Analytical Results of Synthetic Mixtures m-Xylene Known Exptl. Error
o-Xylene Known Exptl. Error
Ethylbenzene Known Exptl. Error-
25.0
23.7
-1.3
25.0
25.4
+0.4
25.0
24.6
-0.4
25.0
25.2
+0.2
2 3 4 5 6 7 8 9 10 11 12 13
40.0 55.0 20.0 45.0 47.5 60.0 35.0 25.0 23.8 25.0 25.0 13.5
39.2 53.4 19.2 44.2 46.1 58.8 33.6 24.8 22.4 25.4 26.8 12.9
-0.8 -1.6 -0.8 -0.8 -1.4 -1.2 -1.4 -0.2 -1.4 +0.4 +1.8 -0.6
20.0 15.0 30.0 25.0 23.8 25.0 25.0 60.0 33.3 40.0 5.0 0.0
19.2 13.4 28.0 23.6 24.2 25.2 23.0 59.2 33.9 40.8 5.6 0.6
25.0 25.0 25.0 15.0 19.0 10.0 5.0 0.0 28.6 3.5.0 40.0
15.0 5.0 25.0 15.0 9.5 5.0 3.5,O 15.0 14.3
14.4 4.6 24.8 14.8 9.5 4.4 34.2 l5,4 16.1 0.0 31.2 30.2
-- 00 .. 64 -0.2 -0.2
0.0 -0.6 -0.8 +0.4 +1.8 0.0 +1.2 11.0
151.. 01
151 .. 21 0.2
0.0 4-0.2 f0.2
150 .. 50 15.0
140.. 90 14.4
05 a5..o0
24.4 24.8 27.6 16.4 18.8 10.0 4.0 1.4 29.6 36.6 41.6 51.6 5 55 6 .. 3 8
-0.6 -0.2 +2.6 +1.4 -0.2
14 15 16
-0.8 -1.6 -2.0 -1.4 +0.4 fO.2 -2.0 -0.8 f0.6 f0.8 f0.6 f0.6 - 00. 6 .0
-0.6
60.0
60.8
2 63 .. 94 2 24.2
-- 10 .. 68 -0.8
0.0
;:.O
0.0
-1.0 +1.4 f l . 0 . +1.6 f1.6 -1.4 0 .. 28 f- 1 +0.8
0.0 30.0 29.2 2 27 5 .. 7 0 25.0
_concentrations of the compounds in an unknown mixture is made with the following absorption coefficients: Wave Length, Microns 12.54 12.98 13.44 14.30
@-Xylene 21.0 0.4 0.2 0.2
m-Xylene
+Xylene
Ethylbenzene
0.3 18.0
0.2 0.5 31.0 0.2
1.0 2.0 5.0 12.0
0.3
1.2
Not more than two approximations are usually necessary to obtain the concentrations of the xylene isomers and ethylbenzene within an accuracy of 1% of the total concentration of the constituents. The use of a spectrocomputer facilitates rapid calculations.
EXPERIMENTAL RESULTS
The analysis of sixteen synthetic mixtures which were prepared from the National Bureau of Standards’ reference samples gave experimental results with a standard deviation of 1.05% for the 64 determinations reported in Table 11. The total concent’rationof 1 ml. of the mixture to 20 ml. of carbon disulfide solvent was held constant as the concentrations of the constituents were varied in this series.
LITERATURE CITED
(1) Barnes, James, and Fulweiler, W. H., J.;lm. Chem. ,SOC.,49, 2034 (1927). 12) Dadieu and Kohlrausch, Monatsh., 52, 220 (1929). (3) Ellis, J. W., Phgs. Rev., 27, 298 (1926). (4) Fenske, M . R., Braun, R. V., Quiggle, D., McCormick, R. H., and Rank, D. H., ANAL.CHEM.,19, 700 (1947). (5) National Technical Laboratories, South Pasadena, Calif., Beckman Bull. 153. (6) Oronite Chemical Co., San Francisco, Calif., “Xylene Technical Review,” 1947.
.-,
RECEIVED dpril 14, 1948.
Instrument for Automatic Continuous Titration PHTLIP A. SHAFFER, JR.’, ANTHONY BRIGLIO, JR., AND JOHN A. BROCKMAN, JR. California Institute of Technology, Pasadena, Calif.
There is described an automatic continuous titrating instrument, originally developed for the determination of mustard gas in air. The unknown gas sample is continuously aspirated through a titration cell in which it is absorbed in solution. Titration is effected by electrolytic generation of the titrating agent in the cell; the electrolysis is so con-
D
URIXG the war the need arose for instruments capable of
measuring and recording, automatically and continuously if possible, the concentrations of toxic gases obtaining in chemical warfare. Because of the importance of such instruments in field trials, the development of the instrument herein described was undertaken. 4 complete description of the instrument is given in a wartime report ( I ) ; therefore sufficient information is not given here t o enable the construction of an instrument. The aim of this paper is to describe the novel aspects of the instrument and to discuss the principles of its operation, to the end that these principles may be applied further to chemical problems. In earlier types of titrating instruments ( I S ) the titration takes place a t a constant rate for a measured period following the absorption of the sample from the air. In contrast to this sequential operation, the device described here achieves continuous titration by the maintenance of an end-point condition by introducing the titrating agent continuously a t a rate which closely approximates, stoichiometrically, the rate of absorption of the unknown gas sample. The general relations between the several elements in the con1 Present address, E. S. Naval Ordnance Test Station, Pasadena, Calif.
trolled that a ver3 small excess of the titrating agent is maintained in the cell. The control is achieved by means of negative feedback, characteristic of servomechanism applications; the feedback loop involves chemical reaction. The dynamic behavior , is analyzed with the aid of equivalent electrical circuits.
tinuous titrating system are shown schematically in Figure 1. The diagram makes evident the feedback loop (6), by means of which the rate of addition of titrating agent is made to follow very closely the rate of absorption of the sample. The direct current amplifier of Figure 1 may be replaced by other equivalent devices; the authors’ first design made use of a relay-controlled reagent pump. Upon surveying the known quantitative methods for the determination of mustard gas, the authors decided that the bromine titration method utilizing a potentiometric end point described by Northrop ( 7 ) was perhaps the best suited for use in an automatic instrument. While work on a mechanical instrument making use of a titrating reagent pump ( 2 ) was under way, it occurred to them that the titrating agent might be generated electrolytically (12). Automatic equilibration of the rates of introduction of the two reactants into the reaction cell could be achieved then by using an electronic power amplifier driven by the end-point potential and delivering current to the generating electrodes. Because of the many important advantages which this new basis of operation holds over the mechanical one, attention xas turned from the titrating reagent pump to the electrolytic type of instrument. Detailed results of the developmental work have been reported ( 8 ) .
