(7) Jacobs, M. B., "Analytical Chemlstry of Industrial Poisons, Hazards, Solvents," 2nd ed., p. 444, Interscience, New York, 1949. (8) Kusnetz, H. L., Saltzman, B. E., LaNier, M. A., A m . Ind. Hyg. Assoc. J . 21, 361-73 (1960). (9) Lubatti, 0. F., J . SOC.Chen. Znd. 63, 133-9 (1944). (10) Matusmk, M. P., IND. ENQ.CHEM., ANAL.ED. 6, 457-9 (1934). (11) Naval Research Laboratory, Rept. . 898 (January 1959).
(12) Post, M. A,, Moore, W. A., ANAL.
CHEM.31, 1872-4 (1959). (13) Saltrman. B. E., Am. Znd. Hva. . Quart. 16, 121-2 (1955). (14) Saltrman, B. E., ANAL. CHEM.26, -194%55 - - - -- ( 19.54). (15) Saltzman, B; E., Gilbert, N., A m . Ind. Hygiene Asaoc. J . 20,379-86 (1959). (16) Saltzman. B. E., Gilbert,, N.,, ANAL. . CHEM.31, 1'914-20' (1959). (17) Shepherd, M., Ibid., 19, 77-81 (1947) (18) Thomas, M. D., MacLeod, J. A., I _
\ - - - -
I
Robbins, R. C., Goettelman, R. C., Eldridge, R. W., Rogers, L. H., Zbid.,
28, 1810-16 (1956). (19) Verhoek, F. H., Daniels, F., J. A m . Chem. SOC.53, 1250-63 (1931). (20) Webster, S., H., Fairhall, L. T., J . Ind. Hyg. Tozzcol. 27, 183-92 (1945).
RECEIVED for review January 26, 1961. Accepted February 27, 1961. Division of Water and Wrtste Chemistry, 138th Meee ing, ACS, New York, N. Y., September 1960. Partial financial support given by the Bureau of Ships, Department of the Navy, under contract letter S23(649B) Ser 649B-333, February 25, 1959.
Determining Compositions of Labeled EthylenePropylene Copolymers R. L. STOFFER Research and Development Department, American Oil Co., Whiting,
Id.
W. E. SMITH Research Department, Amoco Chemicals Corp., Whiting,
b The compositions of labeled ethylene-propylene copolymer standards can be determined b y a method involving liquid-scintillationcounting. The copolymers are dissolved by heating in a liquid scintillator, cooled, and counted as gels or suspensions. Counting efficiencies of 70% have been obtained on such samples containing 0.5 gram of copolymer. The method should also be applicable to other labeled copolymers.
R
analytical methods for determining the compositions of ethylene-propylene copolymers, such aa infrared (3, 6) and mass (1) spectrometry, must be calibrated with standards. The best primary standards are ethylene-propylene copolymers prepared from feeds containing either and anaeth~1ene-C~'or pr0py1ene-C~~ lyzed by a radiochemical method. I n the only radiochemical method that haa been available (b), thin copolymer disks are counted with a thin-window GeigerMuller tube (X), and the observed specific activities are related to that of ethylene-cl' included in the feed. The precision of the method is good; however, with tubes of the usual l-inch size, the counting efficiency is less than 1% and the effective sample size is at most 0.16 gram. Liquid-scintillation techniques can count carbon-14 in certain types of samples as large as 10 grams with efficiencies of over 50% (4). Hence, if ethylene-propylene copolymers labeled with carbon-14 could be put into a suitable counting form, their compositions should be determinable with OUTINE
11 12
0
ANALYTICAL CHEMISTRY
Id.
the same precision as with the GeigerMuller tube, with a marked reduction in either the amount of carbon-14 or the counting time required. Because solid copolymers of ethylene and propylene are insoluble in practically all solvents a t room temperature, a liquidscintillation counting method for insoluble substances was sought. Samples were ground and then suspended @ a liquid scintillator containing Thixin, a gelling agent that aids in forming stable suspensions (6). However, this method lacked adequate precision because of variations in self-absorption of the radiation by polymer particles of nonuniform size. The partially amorphous copolymers could not be reduced to fine particles of uniform size. To reduce the effects of self-absorption, a liquid-scintillation counting method has now been developed in which gels are formed from the copolymers themselves. These gels are, in essence, intimate mixtures of liquid scintillator and polymer. The method depends upon the ability of aromatic solvents suitable for use in liquid scintillators to dissolve the copolymersand the homopolymers needed for calibration-at temperatures slightly above 100° C. Upon cooling, such solutions usually form gels that remain stable for at least an hour. METHOD
A Model 314 Tricsrb liquid-scintillation spectrometer is suitable for all counting. Fivedram glass vials are convenient for sample preparation and counting. A 0.5-gram sample of dried polymer is weighed directly into a &dram count-
ing vial. Then 15 ml. of a liquid scintillator, which consists of researchgrade o-xylene containing 7.5 grams per liter of scintillation-grade diphenyloxazole, is added. To dissolve the sample, the vial is heated between 115' and 125' C. for 15 minutes; it is then cooled to room temperature, capped, wiped clean, shaken well, held a t the temperature of the counting chamber for a t least 30 minutes, and shaken again just before counting. The samples are counted with the lower and upper discriminators of the two-channel pulse-height analyzer and the high voltage applied to the multiplier phototubes set to minimize statistical counting errors. [On our instrument, settings of 10, 100, and 1060 volts, respectively, were found best; they gave background counts on blanks prepared from unlabeled copolymers of 30 to 60 counts Der minute (c.D.m.) . . at - 15' C.] This counting method can also be applied to 0.1-gram polymer samples. In this case, a suspension of polymer in the liquid scintillator is obtained as the counting sample, instead of a gel. Sample activities are converted into per cent labeled monomer by the formula : %labeledmonomer
=
A X 100 CF X W ~
where A is the sample activity in c.p.m., CF is a conversion factor (specific activity of labeled monomer in c.p.m. per mg.), and W is the sample weight in mg. DISCUSSION
For use in evaluating the method, p~lypropylene-C*~,polyethylene, and
Table 1. Validity of a Single Conversion Factor
NomiMixture Conversion nal CoplPosition, Mg, Factor, C.p.M./M Samplc Polyprop I- poly- Polypropyf Wt., ene-C" Mg. ene-&( ethylene 500
lW
45.7 45.1 10.4 10.8 100 5 101 2 100
453.2 447.8 490.6 495.1
580 569 562 553 .., 560 557 ... 564 .., 562 Avo 561 & 2
...
z5
109.2a 4
Liquid.
14 copolymers were prepared in our laboratory from high-purity ethylene and propylene. All labeled polymers were prepared from the same propylene feed tagged with pr0py1ene-l-C~~. The validity of the method rests upon the reasonable assumption that conversion factors obtained from mixtures of polypropylene and polyethyleneeither one labdcd-can be used with copolymers of thcm. Our conversion factors were the propylene-CI4 specific activities in such mixtures; they were determincd by using the new method to obtain the activities of mixtures of the polypropylene-CI4 and the polyethylene, and dividing thr activities by the respective known wcights of the polypro-
p ~ 1 e n e - Cin ~ ~these mixtures. These conversion factors are shown in Table I, along with values for the polypropylene-C" alone. The two liquid samples were prepared like the solid mixtures, except that no heating was necessary. Inasmuch as the conversion factors did not vary, within experimental error, with sample size, composition, or physical form, an average factor proved valid over the composition range studied. Because their propylene-C'd specific activities were the same, all samplea had the same absolute counting efficiency. By comparison with a benzoie-C*4 acid standard, this efficiency was found to be 70%. The propylene contents of the 14 different copolymers were determined in duplicate and are shown in Table 11. Counting times of 2 to 4 minutes and 6 to 12 minutes were used with the 0.5- and 0.1-gram samples, respectively. The differences in the duplicate determinations are small, and most of them can be attributed to statistical counting errors. CONCLUSION
The new method gives the precision of the previous method but saves either carbon-I4 or counting time because larger samples can be counted with much higher efficiency. It can be applied to other copolymers prepared from monomers, one of which is labeled with a
Table It.
Precision of Copolymer Analyses
( % propylene) 0.5-G. Samples 0.14.Samples 1.99,' 1.99 1.89. 1.86 2.05; 2.09 2.0s; 2.07 3.23,3.24 3.11, 3.09 3.42,3.46 3.13, 3.10 4.48,4.31 4.33, 4.32 8 . 7 3 . 8.74 7.89.8.26 10.48;10.62 9.42; 9.41
suitable beta emitter. With distinctive labeling, more than one component could be so determined. For polymers that will not dissolve in o-xylene at temperatures below the boiling point, suitable scintillators could be prepared from higher-boiling .solvents, such as mesitylene or triethylbenzene. LITERATURE CITED
(1) Bua, iF., Manaresi, P., ANAL. CHEW 31,2022 (1959). (2) Danusso, F.,Pajaro, G., Sianesi, D., J . Polymer Sci. 22, 179 (1956). (3) Natta, G.,Mazxanti, G . , Valvaasori, A., Pajaro, G., Chdm. e ind. 39, 733 (1957). (4) Wagner, C. D., Guinn, V. P., 1vucleonics 13, No. 10,56 (1955). (* -5, ) Wei. P. E.. ANAL. CHEM. 33. 216 (1961j. (6) White, C. G.,Helf, S., Nucleonics 14,No. IO, 46 (1956). RECEIVED for review January 9, 1961. Accepted May 8,1961.
Absorptivities for the Infrared Determination of Trace Amounts of Ozone PHILIP L. HANST,' EDGAR R. STEPHENSI2 WILLIAM E. SCOTT,2 and ROBERT C. DOERR laborafories for Research and Development, Franklin Insfitufe, Philadelphia, Pa.
b The infrared absorptivity of gaseous ozone was determined for use in the analysis of polluted air. Absorption ce1l.s of various lengths were charged with ozone, which was measured by two physical methods. The absorptivity determined at 9.48 microns in the infrared is 3.74 X lo-,* p.p.m.-' meter-' for 1 atm. total pressure over a wide range of concentration and path length. Results obtained by the long-path infrared instrument agreed with those found with an ultraviolet ozone photometer when synthetic ozone was determined in air at parts per million concentrations.
* Present address, Avco Research and
Advanced Development Division, Wilmington, Mass. * Present address, Scott Research Laboratories, Inc., Perkasie, Pa. ~
T
HE measurement of ozone concentrations of less than 1 p.p.m. in air became important in studies of chemical reactions of air pollutants, when this substance was discovered in polluted air. Long-path infrared spectrophotometry provides the most specific method available for this measurement, although it is neither as sensitive nor as simple as the chemical methods. The instrument used for this work haa been described ( 4 ) . When the long-path infrared technique was first applied to air pollution analysis, i t was necessary to determine the absorptivity of ozone. Since there was some doubt as to the reliability of the usual potassium iodide methods (3) when applied to these very dilute mixtures, two methods based on the physical measurement of known quantities of ozone were adapted. The
first of these methods waa based on the one described by Birdsall, Jenkins, and Spadinger (1) and used the change in pressure and volume produced by the ozonization of a known volume of oxygen, In the second method, liquid ozone was vaporized into a known volume and its pressure measured. The first of these methods was used to determine the absorptivity of o9ione at concentrations in the parts per million range (in air), while the second waa used in the range of millimeters of Hg pressure. The absorptivity was virtually the same in the two c w s . MEASUREMENTS AT
P.P.M. CONCENTRATIONS
An apparatus (Figure l), patterned after one described previously (I), was used to prepare ozone samples of known volume. VOL. 33, NO. 8, JULY 1961
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