ANALYTICAL CHEMISTRY
1530
Use of the acetone-water medium has the advantages of greater sensitivity and reproducibility over use of the aqueous medium. ACKNOWLEDGMENT
The author is indebted to Cyrus Feldman for his translation of the Russian papers by Kuznetsov and Nikolaev and Sorokina, and to Helen P. Raaen for her careful editing of the manuscript. LITERATURE CITED
(1) Crouthamel, C. E., Johnson, C. E., ANAL.CHEM. 24, 1780 (1952). (2) Furman, N. F., ed., “Scott’s Standard Methods of Chemical
Analysis,” vol. I, 5th ed., p. 888, Van Nostrand, New York, 1939.
(3) Horton, A. D., ANAL.CHEM.25, 1331 (1953). (4) Horton, A. D., Thomason, P. F., Miller, F. J., Ibid., 24, 548 (1952).
(5) Kuznetsov, V. I., Zhur. Anal. Khim. 3, 295 (1948). (6) Nikolaev, A. V., Sorokina, A. A., Doklady Alzad. Nauk S.S.S.R. 77, 427 (1951). (7) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 414, Interscience, Kew York, 1950. (8) Thomason, P. F., Perry, M.A . , Byerly, W. M., ANAL.CHEM.21, 1239 (1949). RECEIVED for review February 2, 1956. Accepted M a y 26. 1956.
Ultraviolet Determination of Phenolic Antioxidants in Rubber COE W. WADELIN Research Division, Goodyear Tire and Rubber Co., Akron 16, O h i o
The determination of phenolic antioxidants in rubber extracts by direct ultraviolet measurement is subject to interference from other extractable materials. However, in extracts of both hot and cold GR-S, the only substances which undergo a change in ultraviolet spectrum as a function of basicity are phenolic antioxidants. By choosing a wave length where the antioxidant has a low absorptivity in neutral solution and a higher absorptivity in 0.1N potassium hydroxide solution and measuring the increase in absorbance, the amount of antioxidant can be accurately determined. The method has been applied to Wingstay S (alkylated phenol), 2,6-di- tert-butyl-4-methylphenol, and methylenebis-2,2’-(6-tert-butyl-4-methylphenol).
APPARATUS AND REAGENTS
Absorbance measurements were made with a Cary Model 11 spectrophotometer and a Beckman Model DU spectrophotometer. Measurements were made in matched pairs of 1-em. quartz cells, using solvent blanks in the reference cell. Absorption intensities are plotted in terms of absorptivity, a, liter gram-’ centimeter -l(2).
1
T H E determination of phenolic antioxidants in synthetic rubber by ultraviolet absorption is subject to interferences from other materials present in the samples. This interference is more serious than that encountered in measuring amine antioxidants because the absorption maxima of the phenols occur a t shorter wave lengths than the maxima of the amines, and the absorptivities of the phenols are about one tenth as great as those of the amines. Ultraviolet measurements have been made on extracts of the rubber samples (6)or by complete solution methods (S). I n either case some average factor must be used to correct for the background absorption. The background for the complete solution method varies greatly with different samples, and accurate determinations cannot be made by using an average correction factor (4). Thr background absorbance correction for Wingstay S (alkylated phenol, Goodyear Tire and Rubber Co.) analysis by the extraction method has been found to range from 0.065 to 0.124. This is sufficient variation to cause about 15% error in the analysis. Coggeshall and Glessner have found that the ultraviolet absorption peaks of various phenols in ethyl alwhol solution are shifted about 20 mp toward longer wave lengths when the solutions are made basic and that the absorptivities a t the absorption peaks are increased (1). This shift is in the proper direction to minimize the interference shown in Figure 1. Therefore, this technique was applied to the determination of Wingstay S in Plioflex 1006 (copolymer of butadiene and styrene made at 122’ F.) and Plioflex 1502 (copolymrr of butadiene and stjrene made at 41’ F.).
A. B. C.
D
300 WAVE LENGTH, m g
340
220
260
Figure 1.
Ultraviolet absorption spectra
380
Wingstay S in absolute ethyl alcohol Wingstay S in 0.1N potassium hydroxide in absolute ethyl alcohol Plioflex 1502, antioxidant-free, absolute ethyl alcohol extract Plioflex 1006, antioxidant-free, absolute ethyl alcohol extract
A liM solution of potassium hydroxide was prepared by grinding 22.4 grams of the reagent with 50-ml. portions of absolute ethyl alcohol until it had dissolved and then diluting to 400 ml. with absolute ethyl alcohol. The solution was allowed to stand overnight for potassium carbonate to settle. When the solution began to turn yellow, it was discarded. The solution should be between 0.9 and 1.1N. Commercial grade antioxidants were used without purification. PROCEDURE
Sheet the sample to a thickness of 0.020 inch and cut into strips 1 x 5 cm. Weigh a &gram sample and add i t to 100 ml. of absolute ethyl alcohol, one strip at a time to prevent sticking together. Reflux for 60 minutes, pour off the solvent, add 100
1531
V O L U M E 28, NO. 10, O C T O B E R 1 9 5 6 ml. of fresh absolute ethyl alcohol, and reflux 60 minutes more. Pmir this off and rinse the sample with three 10-ml. portions of absolute ethyl alcohol. Combine all extracts and washings in a 250-ml. volumetric flask. Cool to room temperature and dilute to the mark with absolute ethyl alcohol.
Table I.
Absorptivities of Various Lots of Wingstay S Absorptivity Obsd. Av.
Lot 26-16 52-21 PiBS standard sample 1931-152
14.0, 1 4 . 2 1 4 . 6 ,1 4 . 5 1 4 , 6 ,1 4 . 8 l5,l,l5,l
Over-all av.
14.1 14 6 14.7 15.1
sorptivities are 8.4 a t 279 mp in neutral solution and 14.8 a t 301 mp in base. Increasing the base strength to 1N did not cause any further change in the m v e length of maximum absorption or in the absorptivity. It was also observed that the absorptivity of the antioxidant in neutral solution is 0.2 a t 301 mp, giving an increase in absorptivity of 14.6 a t this wave length. Beer's Ian- is followed from 0.005 to 0.050 gram of iT7ingstayS per liter. Variation in Lots. The absorptivities of four lots of Wingstay S were determined by the method described above. The results are shown in Table I. On the basis of the lot-to-lot variation observed, it is recommended that the absorptivity be determined on the same lot of antioxidant as that used in the samples. If this lot is not available, probably no gross error will be made by using an absorptivity of 14.6.
14.6
RESULTS
Transfer 5-ml. aliquots of the extract to two 50-ml. volumetric flasks. Add 5 ml. of I N alcoholic potassium hydroxide to one of the flasks. Dilute both solutions to the mark with absolute alcohol. Prepare a blank solution by diluting 5 ml. of the 1-1' alcoholic potassium hydroxide to 50 ml. with absolute alcohol. Measure the absorbances of the extract solutions at 301 mp in 1-cm. cells, using absolute ethyl alcohol in the reference cell for the neutral solution and the alcoholic potassium hydroxide blank solution in the reference cell for the basic solution. For calibration prepare a solution of 0.300 gram of Wingstay S per liter of absolute ethyl alcohol, preferably from the same lot of Wingstay S as that used in the samples. Transfer 5-ml. aliquots of this solution to two 50-ml. volumetric flasks. Treat and measure in the same manner as with the sample extracts. From the measurements on the standard:
Determinations on Standard Samples. Four known polymers were prepared and analyzed for Wingstay S by the recommended procedure. The polymers were coagulated by the salt-acid method from antioxidant-free latex. Weighed amounts of Kingstay S, lot 26-16, were milled into weighed amounts of polymer. The results are shomn in Table 11.
Table 11. Determifiations on Known Samples
5% WingRtay S Sample
Added
Found
Plioflex
1006
None
Plioflex
1006
1.21
1 . 2 3 , 1 . 2 0 ,1 . 2 5
ave.
1.23
Plioflex
1502
Sone
0.01,0.00,0.02
ave.
0.01
Plioflex
1502
1.24
1.22, 1.23,1.25
ave.
1.23
-0.04,0.02,
-0.05
ave.
-0.02
Standard deviation 0 . 0 2 5 %
From the measurements on the sample:
70Wingstay where absorptivity, liter gram-' centimeter -1 absorbance in basic solution absorbance in neutral solution concentration in grams/liter cell length, centimeters sample weight in grams volume of combined extracts, milliliters volume of aliquot, milliliters volume to which aliquot is diluted
Oil-Extended Plioflex. When the method was tried on synthetic polymers extended with Circosol 2XH oil, the absorbances were too high to permit application of this method. Other Antioxidants. The method has been successfully applied t o Ionol (2,6-di-iert-butyl-4-methylphenol) and Antioxidant 2246 [methylenebis-2,2'-(6-tert-butyl-4-methylphenol)] . ACKNOWLEDGMENT
The author wishes to thank the Goodyear Tire and Rubber Co. for permission to publish this paper.
DISCUSSION LITERATURE CITED
Samples of Plioflex 1006 and Plioflex 1502 containing all of the manufacturing materials except Wingstay S were extracted and measured by the above procedure. The curves, which are shown in Figure 1, are the same in neutral and basic solutions. They are shown a t the relative intensities they have when the polymers contain 1.25% Wingstay S. Because Wingstay S is the only constituent in the absolute ethyl alcohol extract whose spectrum changes as a function of basicity, the increase in absorbance a t 301 mp can be used to determine it quantitatively. The fact that the background varies from one sample to another causes no difficulty, because it is actually measured and corrected for on each individual sample. Effect of Base Strength. The absorption curves of Wingstay S in absolute ethyl alcohol and in 0.1N potassium hydroxide in abRolute ethyl alcohol are shown in Figure 1. The peak ab-
(1) Coggeshall, N. D., Glessner, A. S., J . Am. Chem. SOC.71, 3150 (1949).
Hughes, H. K., others, ANAL.CHEM.24, 1349 (1952). (3) Linnig, F. J., Peterson, J. M., Edwards, D. M., Acherman,
(2)
W. L.. Ibid ,25, 1511 (1953).
Prem, D. C., Duke, J. T., Larchar, T. B., Sr., private communication to Federal Facilities Corp., Office of Synthetic Rubber. (5) Reconstruction Finance Corp., Office of Synthetic Rubber, "Specifications for Government Synthetic Rubbers,'' revised ed., Oct. 1, 1952. (4)
RECEIVED for review February 18, 1956. Accepted June 13, 1956. Pittaburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 2, 1966. Contribution 218 from the.Research Division, Goodyear Tire and Rubber Co. Part of this work was sponsored by the Office of Synthetic Rubber, Federal Facilities Corp., in connection with the government synthetic rubber program.
