1382
ANALYTICAL CHEMISTRY
amount of substituted thiophenes would be calculated as approximately 0.6% alkyl benzenes. SIGNIFICANCE O F ULTRAVIOLET METHOD
This analytical procedure was developed from a combination of the need for a test confirming ASTM D 875-16T results and a study of the absorption intensit,y of alkyl benzenes in the short wave-length region. Examination of many spectra was made possible by the use of automatic rec-oi,dinp u1trnviolr.t instrumeritation. The ultraviolet test proves that thc ALYl'.\;Iprocedure, a t least in the absence of high broinine number, is very reliable for straight-run petroleum stocks in the range of 1 to 95% alkylbenzene content. As a supplementary test for aromatics i t has proved its merit in cases of disagreement between the POSA ( 2 ) and ASTM methods. The aromaticity of crude fractions boiling between 250" and 395' F. may be estimated in an average elapsed time of 10 to 20 minutes with an average reproducibility of analysis of i O . 5 weight % alkyl benzenes. Calibration data 6 months old are still in use. The test can be utilized as an aid to existing standard procedures. Its main advantage is the facility of rapid determinatioii of total alkyl benzene content in crude fractions boiling between 300" and 395" F. A need for ultraviolet equipment for use in the region between 183 aud 210 m p is indicated. Absorption data within this interval could be of considerable value in the evnluntion of petroleurn stocks. LIMITATIONS OF THE METHOD
A knowledge of the sample absorption characteristics a t as many wave lengths as possible is an asset to any spectrometric. analysib. For routine analyses this becomes possible only with the aid of automatic recording equipment. Absorbance measurements a t 215 mp should be accompanied by additional measurenirnts a t several other wave lengths to cnsure the absence of inter-
fering components. If done by a manual procedure, the method becomes less effective. Measurements at 215 mp are very close to the lowest working limit of many ultraviolet instruments. This, together with criteria of cell matching and reproducibility of absorbance values obtained from fiteep curves, imposes restrictions in regard to the method of analysis. Reproducibility of analyses and calibration data, together with the fact that unmatched cells can be compensated on the Cary instrument, has justified application of the procedure. The alkyl benzene absorptivities tabulated in Tables I and I1 w e significant for the instrument used for development, of calibration data. In view of the probable variations between instruments, some of the values should be rechecked and a correction i:ictor applied if necessary. .411 data have been developed from straight-run stocks. Preliminary results on cracked products indicate a substantial discrepancy in aromatic content as determined by the ASTM and ultraviolet methods. Further investigations t o determine the applicability of the ultraviolet method to surh petroleum fractions are in progress. ACKNOWLEDGMENT
The author is indebted to many members of the department for their cooperation and assistance in the development of analytical data. Appreciation t o Sinclair Research Laboratories, Inc., is vxpressed for permission to publish this paper. LITERATURE CITED
(1) Am. Soc. Testing Materials, B.S.T.M. Standards on Petroleum Products and Lubricants. pp. 323-30, D 575-461T, Xovember
1950. (2) Grosse, A. V., and Wackher, R. C., IND.EXG.CHEM.,Ah-ar.. ED.,
11, 614~-24(1939). ( 3 ) )fellon, hl. G., "Analytical Absorption Spectroscopy," p. 93, Yew T o r k , John JViley 8: Sons, 1950. (4) Platt, J. R., and Klevens, H. B., Cheni. Revs.. 41, 301 -10 (1947).
(5) Tunnicliff, D. D.. Brattain, R. R., and Zumwalt. L. R., (!HEM.. 21, 590-94 (1949).
.\K.
