Benzene, Toluene, Ethylbenzene, 0-Xylene, m-Xylene, and p-Xylene Determination by Ultraviolet Spectrophotometry D. D. TUNNICLIFF, R. ROBERT BRATTAIN, AND L. R. ZU&IWALT1 Shell Development Company, Emerpille, Calif. An ultraviolet absorption method is presented for the determination of the CS, C7, and Ca aromatic hydrocarbons. The accuracy of the determination of each component is about 1% of the total aromatic content, except for toluene and ethylbenzene which are determined to about 2% when both are present in the same sample. Tests to determine the presence of interfering components are included in the method. A chemical treatment is described which is effective in removing many interfering unsaturates and sulfur compounds.
HE determination of the individual cs,c7,and c8aromatics by chemical is difficultbecause of the great similarity in the chemical properties of these compounds. Fortunately, their ultraviolet absorption spectra differ sufficiently to permit an accurate determination of each component. Ultraviolet absorption is particularly useful for the determination of aromatics, for they are often associated with saturated hydrocarbons Ryhich have no absorption in the spectral region used in the analysis and, consequently, do not interfere Unsaturates and sulfur compounds which are often present in low concentrations with aromatics usually can be removed by chemical treatment. Shaking the diluted sample with mercuric nitrate solution has been found effective for this purpose. The spectrophotometric method is also comparatively rapid and does not require a high degree of skill in routine operation. The number of individual aromatics that can be determined simultaneously in a mixture is limited to the number of suitable spectral positions available for the analysis. The spectral positions must be taken a t wave lengths that will result in a set of independent simultaneous linear equations relating the concentration of each component to the observed optical densities. As the equations approach dependency the errors in the analysis become very large. This condition is usually observed only when there is a marked similarity between the absorption spectra of two of thr components. However, even when this does not occur a condition of linear dependence will exist if the absorption spectrum of one component is very similar to the absorption spectrum of a mixture of two or more of the other components. This condition can be detected by making a preliminary calculation of t h r mpected errors as described in a later section. In the analysis of complex mixtures of aromatics suitable spew tral positions may not be available, so the mixture must first b(5 separated by distillation into fractions rontaining only a limited number of aromatics. Methods are given below for determination of the individual aromatics present in the following mixtureSE. substitut? the optical densities observed a t 2745, 2725, 2710, and 2545 A. in the inverse equations (Equation 3, n = 4) and calculate the concentration of each component. From these values calculate the peicentage of each aromatic present in the sample. BEXZENE,TOLVENE, ETHYLBENZENE, 0-XYLENE,?n-~YLEKL, AND ~ X Y L E X ESubstitute . the optical densities found a t 2745, 2725, 2710, 2685, 2590, and 2545 A. in the inverse equations (Equation 3, n = 6) and calculate the concentration of cach coiiiponent. From these values calculate the percentage of each aromatic present in the sample.
CORRECTION FOR BENZENE LOSTIN CHEMICAL TREATM I t was first reported by Powell and Rappoport (8) and later confiimed in this laboratory that part of this benzene in a dilutr. solution is lost when the solution is shaken with aqueous reagents The nature of this loss is unknown; experiments have shown that only a negligible amount of the loss can be accounted for bv solubility in the aqueous phase. Similar losses have not becxii observed For the other aromatics. A number of experiments have showii that the loss is dependent on the ratio of the volumes of the solution and the reagent. For equal volumes the loss mas found to 1 x 1 approximately 27, of the benzene content of the solution. ( ' t i n wquently, the benzene content of treated samples should t w i r i meased by 2% of the value found. Test for Interfering Absorption. Occasionall) , saniplv~c o u tain unknon-n compounds M hich absorb in the spcctral regioii uwd for the analvsis and consequently the analvsis gives incorrect results. Unless the samples are known to contain no interfcring compounds, they should ti( tested h v the t n o mc3tliods dcscribed belon . 1. De$rmination of the optical density of the diluted samplr at 2900 A. Because none of these aromatics has appreckble absorption a t this wave length, an optical density at 2900 A. which exceeds 0.003 shows the presence of interference. 2. Using the concentrations determined from the analytical procedure described aboveocalculate the optical density that should be observed a t 2470 A. if this optical density is due solely to the aromatics. This value is calculated by substituting in Equation 2 the calculated concentration of each of th? aromatics and the corresponding extinction coefficients for 2470 A. as determined during the calibration. The observed optical density should not exceed the calculated value by more than 0.010. As the interference usually increases at shorter wave lengths, this test may show the presence of interference when the first test does not
Thrs same tests can also I>(, applied in the determination of a single component. I n this case, the agreement bet,\veen the concentrations determined a t thr two wave lengths is an additional test. Because the interference usually increascxa a t the shorter wave length, the optical density determined at the longer of the two wave lengths will be more nearly correct and should be used for calculating the concentration if there is a significant difference between the two values which cannot hc roduccd by chemical treat men t. If any of the above tests indicates interfererice, the diluted sample is treated with mercuric nitrate as described above. If the interference persists, it may be possible to devise some other chemical treatment. An algebraic method of correcting for intrrfering absorption is presented in another paper (11).
DIscussIo\ The results obtained by the analysis ot a iiumbcr of synthetic samples are given in Tables 11, 111, and IV. The average error in these results agrees satisfactorily with the theoretical errors calciilated from the expressions
I n
__-
IVIICII~ = average t'i'ror i n concentrat inn oi component
k,,
= valu~.sof I: irom Kquarion
AD, =
1
3 ior component i and
wave lrngth j averagi2 crror i n optical dvnsity at wave length j
The theoretical average errors given in Tables 11, 111, and IV were calculated by assuniing a value of AD. oi 0.003 a t all nave lengths. Expericnce has shmvn this to bc a conservative value. These errors \vere ronvertrd t o pc'rccntagc by assuming a total aromatic contciit oi 0.25 grani per liter, nhic.ti i: in the rstigc of aromatic coiiteiit used i n t i i t s ilvrerniination. These theoretical av,.~'wgt.1,rrors have bwn iourid usciul in predicting the errors oi B ncw analysis bcforc thrs actual work of the (dibration and thc. analysis of synthetic ssmplrs is undertaken. ,\pproxiniate estinrtioii rorfiicicnts such as msy be obtained diwctly frorii absorption y w t ra art' usually sufficicntly accurate for t Iii.. purpow. \CK?lOW LEDGME\T
The authors wish to acknowledge the excellent work of M. Morse and R. Hatch in obtaining the necessary calibration data and in analyzing the synthetic samples listed in this paper. LITERATURE CITED
R. S., a n d C r a v a t h , A . M.,J . A p plied Phys., 14, 418 (1943). (2) B r o d e , W.R . , Patterson, J. W., B r o w n , J. B . , a n d F r a n k e l , J., IND. Ewo. C H E M . ,Ax.4~.ED.,16, 77 (1944). (3) C a r y , H. H., a n d B e c k m a n , -4.O., J . Optical SOC.A m . , 31, 682 (1941). (4) Clout, P. D., Trans. A m . Inst. Elec. Engrs., 60, 1235 (1941). (5) F u l t o n , S. C . , a n d Heigl, J. J., Instruments, 20, 35 ( J a n u a r y 1947). (6) Graff, M. M., O ' C o n n o r , R. I., a n d S k a u , E. L., IND.ENQ. &EM., AKAL. ED., 16, 556 (1944). (7) Meloche, C. C . , a n d F r e d r i c k , W. G., J . Am. Chem. SOC.,54, 3264 (1932). (8) Powell, J. S., a n d R a p p o p o r t , D. A . , S o u t h e r n California G a s Co., Los Angeles. Calif., p r i v a t e communication. (9) R a s m u s s e n , R. S., Tunnicliff, D. D., a n d B r a t t a i n , R. R., J . Chem. Phvs., 11, 432 (1943). (10) T u c k e r , E. B., S t a n d a r d Oil Co. ( I n d i a n a ) , W h i t i n g , I n d . , pri(1) B r a t t a i n , R . R . , R a s m u s s e n ,
v a t e communication.
(11) Tunnicliff, D. D., R a s m u s s e n , R . S., a n d M o r s e , M., A N A L . CHEW.,21, 895 (1949). (12) Weissler, A.,IND. EKG.C H E Y . ,ANAL.E D . . 17, 695 (1945). RECEIVED August 10, 1948.
