Ultraviolet Spectrophotometric Determination of Styrene in Ethylbenzene NINA ILADDEN
ANI
JOHN A. PERRY’, Monsanto Chemical Co., Texas City, Tex.
n
E T E R M I S A T I O S of styrene in ethylbenzene is usually performed by bromination titrations using various combinations of solvents and reagents or titrations based on the reaction of vinyl groups with mercuric acetate ( 2 , 3 ) . Such titrations are time-consuming and show excessive relative error for concentrations below 0.1%. The mass spectrometer has been used in this laborat’ory. but it also shows high relative error below 0.1%. Therefore, this investigation was undertaken to obtain a more rapid and sensitive mcxthod of analysis for small amounts of styrene in ethylbenzene. Styrene displays strong general absorbance in the region 2900 to 3100 A. while tho absorbance of ethylbenzene is comparatively weak; the analysis n a s set u p in this region t o cover the range 0 to 10% styrene. Small amounts of benzene and toluene in the ethylbenzene-styrene mixtures do not interfere.
Table 11. D e t e r m i n a t i o n of S t y r e n e i n E t h y l b e n z e n e Samples Sample Yo Found Known 1
0 0 0 0 0 0 0 0
0036 0078 010 045 134 243 455 2 53 6 50
Sample 1 2 3 4
The Cary recording spectrophotometer, Model 11, with a mercury arc source and matched cells with quartz windows, was used for all measurements. Styrene-free ethylbenzene (99.9%) and styrene (99.774, both from Monsanto Chemical Co., were used to make u p the necessary synthetic solutions. Purities were determined by freezing point.
Known, % 0.60 0.55 0.40 0.20
The difference in :Lbsorbance bctiwen styrene and ethylbcnzene becomes larger a t lower wave lengths and higher sensitivities are obtainable. In order t o attain the highest possible precision and e was used only as long as the calisensitivity, n given ~ a v length ~ linear. Instrumental data for the bration curve w : essentially v:ii-ious concentration rangcis with thoir associated uncertainties iim prewnted in T:thle I.
iinalj tical Data for Ultraviolet D e t e r m i n a t i o n of S t y r e n e in Ethylbenzene
Styrene, ‘/c 0-0 00: 0 005-0 015 0 015-0 20 0 20-0 80 0 80-5 0 5 0-10 0
(1)
=to ~0
fO rt0 1 0 1 0
0002 002 00.5 009 02 05
Found, % Ultraviolet ( 0 = 0 009) 0.607 1 0 . 0 0 9 0.558 1 0 . 0 0 9 0.400 1 0 . 0 0 9 0.192 1 0 009
RESULTS
The analyt,ical systems are based on ethylhenzene as a reference for measurement of the incident light. Calibration curves of styrene concentration versus absorbancy were prepared at 2916, 2921, 2967, 3023, and 3067 A. using synthetic samples made up volumet~rically. Kave lengths were selected to eliminate dilution of the sample and to give the highest possible accuracy for a given concentration range. -4s these wave lengths were on the side of a n absorbance peak, reproducibilit’y of wave length was ensured by use of mercury lines, two of which, 2967 and 3023, \yere used in the analysis. The other wave-length settings were referred t o these as a waw-length check. R70rking on the side of an absorbance peak is suhject to considerable error unless thew i,$ some guarantee that the wave-length setting is reproducihle. The mercury linw furnish this guarantee and experimmtal rcsult. hear this out.
Instrurnental Data Spectral Cell slit A, length, width, A. mm. A. 2916 50 1.38 2921 25 0.92 2967 25 0.24 3023 50 0.26 3067 50 0.84 3067 50 0.84
Titration 0 01) 0 . 6 1 1 0 01 0 . 5 4 1 0 01 0 . 4 0 1 0.01 0 20 1 0 . 0 1 (u =
st,runient conditions are selected for the relevant styitlie coiicentration range; absorbancy is measured :tnd perccsntagr styrene is r m d from t,he calibration curve.
PROCEDURE
Standard Deviation
0 0005 0 003 n 008 0 011 0 040 0 130 0 250 0 450 2 50 G 50
=t 0 0002 10 002 1 0 002 1 0 005 1 0 005 1 0 009 1 0 009 1 0 02 & 0 05
Table 111. Comparison of Lltraviolet and 13roniinntion Titration hlethods
EXPERIMEBTAL
T a b l e I.
0004 1 0 0002
% Concn. Benzene 6 31 Q
8
2 4
Toluene 15
73 100 14 5 8
To check thc method, synthetic saniples were :~11:11yzcdfor each concentration range; results are shon-11 in Tablc 11. Thc uncertainties associated with the several concentration r:~ngesarc expressions of both precision and accuracy. Approsimntc~ly1yo relative error in these determinations of styrene can 1 1 attributed ~ to thc probable error in measurement of ahsorb:rncy. IIo\wver, thc uncertainty of the determinations in thc lo\rcr prcentage ranges is higher than that expected from errors in absorbancy nicaeurements. This is most evident in the two lowwt concc.ntration ranges, where sample handling and voluinet,ric ~ r r o r s probably responsible. Small amounts of txnzcne and tolucnc can be pi samples to be analyzed, as their absorbancy in the region 2900 to 3100 -4.is similar to that of ethylbenzene. The nppi~osimnte amounts of benzene and toluene which can be tolerated in the samples before the styrene results will be outside the sigma limits ( 1 ) are given in Table I . These allowable amounts of bcnzcncb and toluene are a function of the benzeiie-ethylbenzenc, and toluene-ethylhenzene absorbancy differcnces and of the iigma for :i given range a t a given wave length; the figures quoted :IN’v:tlitl but approximate. In order to compare the accuracy and precision of tlit! u1tr;iviolet and the bromination titration methods in th(, r:~ngv0.2 to O B % , styrene samples were analyzed by both n i , ~ : i i i ~ . 8ever:iI samples of known composition were titrated until duplicato results for each were found to agree within 0.01% or I c w . Chlculations showed that titration results could be obtained 1i:iving ~i sigma of O.Ol%, as cited in Table 111. Thus, belo\\ 1% the precision and accuracy of the ultraviolet method are greiLtCr th:m those obtainable by titration. Moreover, the ultiuviol(~tis thtr faster method; all operations, including preliniin:iiy insti~uinc~nt adjustments, can be completed in 5 minutes.
SAMPLE ANALYSIS
LITERATURE CITED
(1) Brownlee, K. A , “ I n d u s t h l Experimentation,” P. 58, UI.~CI~JYII. N. Y., Chemical Publishing Co., 1947. (*) hfarquardt, R. p., L ~ E, ~s,, ~ Ayar,, , cEIEII,, 20, 05,-:J (1948). (3) Martin, R. IT., Ibid., 21, 921-2 (1049).
Akfter sp,ctrophotometer is bal;lIlced \\i\ith &hylbenzene in both the sample and reference cells, the sample cell is first rinsed :ind then filled n.ith the dri6.d s:imple. The wave length and inPresent address, College of Chemistry and Physics, Louisiana State Cniversity, Baton Rouge. La.
RECEIVED August 24, 1950.
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