made on different sample distributions by swirling the same sample before each peak measurement and allon-ing the 1)articles to settle for 1 minute. It' is evident, from Table VI, that little error is incurred from this souce. Finally. the standard deviation of the ]leak height was determined from a set of 15 different extractions. The standard (leviation of 1.10% s h o w that the major source of error in the method is that due to counting errors. In most cases 0.7 to 3.0 X lo5 counts were recorded over 60 second. and it is possihle t'hat the rc~producibilityrimy be improved even further by counting for longer periods. Only counting and extractionerrorsare incwred when extracting with a liquid rwin axid the experimental standard (Irriationi for these are also given in T:iMe V I , The counting error is higher :is a recult of the less favorable peak-tol~ackgrounrlratio (1.38 a s compared to 1% for -1C-lX). It, does appear, l i o \ v c ~ ~ was. if this extraction technique is t h r 1r.s reproducihle of the two varixnti. S o m ~sample deterni:iiat'ioiis by both techniyur. nre givm in T:iblc: T'II. r . I lw a c c ~ a c yappear.; to be w l l within thc. limit- predicted by the rrproduciIiility>namcly 3 to 5%. Sensitivity. Using molybdenum railintion. the lowest concentration of uraniiini t h a t i t is pract'icable to :tnalyzc~ I ty the solid resin t'echnique is estimntcd at 0.1 p,p.ni. The low ~,euk-to-bacliground ratio (1.05) ivoiild then givp rise t o a counting ( w o r of about 6% for 60-second (minting intervals. Iiicreascd enrich-
Table VI.
a
Main Sources of Error in Uranium Determination
Conditions Solid resin, stationary Solid resin, redistributed Solid resin, different extrartions Liquid resin, single sample Liquid resin, different extractions Computed statictical counting crror. Table VII.
Standard Deviation, yc 0.63, (0 7 1 p 0.78 10 1 . 10 0.8 83, 3 , (1.19)"
2.95
Population 15 30 15 10 10
Sample Uranium Determinations in Barren Solutions P.P.11.
u,
A(:-lS Resin .-\dded Found 1 70 1 68 3 76 3 60 4 35 4 35
L.4-2 Solution Added Found 2 30 2 44 3 1-0 3 50
ment ratios and longcr counting tinies should, however, improve the> sensitivity. ACKNOWLEDGMENT
K e are indebted t o F. K. E. Strelow of the National Chemical Rewarcli Laboratory, Pretoria, for valuable advice on ion exchange reqini, as n ~ l l as for a numbpr of chemical a n n l ~kc-. LITERATURE CITED
(1) Clegg, J. IT., Foley, D. D., "Uranium
Ore Processing," chap. 3-11, XddisonWesley Pub. Co., Reading, IIass., 1956. ( 2 ) Flikkenia, 1). S., Larsen. R. P., Schablaske, R. V., U. 8. Atomic Energy Comm. ANL-5641,Sovember 1956.
LA-2 Solution with Internal Standard (CsHjBr) A4dded F o u n d 2 :30 2 30 3 40 3 30
(3) Grubb, K.T., Zemany, P. I)., Sat,cre 176,221 (1955). (4) Kehl, L. IT., Russel, R . G., .4sa~. CHEX. 28, 1350 (1956). ( 5 ) Pish. G.* Huffman. A . A.. Zbid.. 27. 18$5 11955). (6) Smithsdn, d L., Eager, R. I,., \'an Cleave, A. B., Can. J . Cheni. 37, 20 (1959). ( 7 ) van Siekerk, J. X., de Wet, J. F., S n t i r r e 186, 4722 (1960). (8) van Siekerk, J. N., Wybenga, F. T., d p p l . Spectroscopy 14, 56 (1960). (9) \Tilpon, H. ll., Wheeler, G. V., Ibid., 11, 128 (1'357). (10) Zemany, P. I)., Pfeiffer, H. C., Liebhafsky, H. h..As.41,. (?mar. 31. 1776 (1959). (11) Zemany, P. I)., Welbon, IY. B., Gaineq, G. L., Ibid., 30, 299 (1958). RECEITEI)for review June Accepted October 19, 1960.
13, 1060.
Determination of Ethylene-Propylene Copolymer Composition by Infrared Analysis PETER E. WE1 Chemicals Research Division, Esso Research & Engineering Co., linden,
b Previous analyses of the composit i m of ethylene-propylene copolymers have been based on radiochemical am'ly5is of copolymers prepared with radioactive ethylene, and infrared analpis of copolymer solutions. Agreement :has been excellent, but both methods are time-consuming and applicable primarily only as research tools. A simple, rapid method for determining the composition of ethylene-propylene copolymers i s reported here. It i s carried out with easily prepared copolymer films, rather than with solutions. Measurements are made of the absorbances at 8.7 and 13.9 microns. The
N. 1.
logarithm of the ratio of the two absorbances i s a linear function of the propylene content of the copolymer, and the composition i s quickly determined from the correlation line. Complete analysis, including film preparation, requires only 10 to 15 minutes and i s independent of film thickness between rather wide limits. Its precision i s comparable to that obtained by solution analysis.
T
have been used for determining ethylenepropylene copolymer compositions by Natta and coworkers ( 3 ) . One is a W O DIFFEREKT YETHODS
radiochemical method, applied to copolymers synthesized from radioactive ethylene; the other is an infrared method which measures the methyl group absorption a t 7.25 microns in carbon tetrachloride solutions. They have found that the radiochemical analysis is the more accurate and reproducible; however, the results obtained b y infrared analysis agreed closely with those from radiochemical analysis and are quite satisfactory. By adopting the infrared method, monomer reactivity ratios of ethylene and propylene in good agreement with those reported by Katta were obtained VOL. 33, NO. 2, FEBRUARY 1961
215
WAVENUMBESS :N C M '
10
8 6
1400
1500
1100
1200
I100
900
COG
-
I00
4
-
-
".
2
-2
-
80
I
I---
-
--
700
800 I
1
b25
ABSORBANCE RATIO
MOLE % C3 SOLUTION bNALYSlS
- --
0423
37 I
I
I ~
I
4
0
'
F 0.8
b0
L
206
2 5
e 0.4
40
Y
4
0
m $02 m
0: Y
0.1
0.08 0.06
1 1 fi
0.04 o,02 0.01
~
~
~
,I
0
IO 20 30 40 50 60 70 PROPYLENE IN COPOLYMER, MOLE %
Figure 1. Correlation of absorbance ratio from film with propylene content from solution measurement
,
I
here. However, dissolving the copolymer is a slow process which usually takes 1 to 2 hours, depending on the molecular \veight and composition. Also, higher rnolecalar weight or higher ethylene content generally reduces copolymer solubility. Bua and hfanaresi (2) have recently reported an analytical method based on the mass spectrography of copolymer pyrolyzates. These methods are all difficult or time consuming and are primarily useful only for research purposes. A simple, rapid method for determining the composition of ethylene-propylene copolymers is reported here. It consists of infrared analysis of solid copolymer films. with measurement of the absorption intensities a t 13.9 and 8.T microns. The ratio of the two intensities is directly proportional to the ratio of c>thyleneand propylene units in the copolyiiier, and is independent of the film thickness. This procedure requires only 10 to 15 minutes for each analysis, including the preparation of film. The ethylene-propylene copolymers used in this work were laboratory products prepared with Ziegler-type ratalysts. PROCEDURE
216
ANALYTICAL CHEMISTRY
-~
I
0 b
_ _
-
7
_cc)
8
9
Figure 2.
--
-
IO V r ! E.rvC-d
The copolymer film is prepared by squeezing out about 40 to 50 mg. of sample between aluminum foil in a hydraulic press. I n the present work a Pasadena Hydraulics, Inc., press was used a t a pressure of about 30 tons per sq. foot for 30 seconds a t 400' F. The infrared spectrum of the film is obtained with a suitable spectrophotometer and a WaCl prism. The peak intensities at 8.7 and 13.9 microns are measured, using a glass scan at the former and a LiF scan a t the latter wave length as zero transmission lines, and parallel lines drawn from minima .at about 8.15 microns and 12.9 to 13.0
'
I1
12
-
. _ = E
13
14
-
lb
IN MICPONS
Infrared spectra for ethylene-propylene copolymers
microns as lOOyo transmission lines. The ratio of the absorbances thus measured, A(8.7 ~ ) / A ( 1 3 . 9PI, is entered into Figure and the corresponding value of the copolymer propylene content is read. DISCUSSION
Figure 1 was constructed by correlating the film absorbance ratios of a range of samples with the corresponding values obtained from solution analysis
(3). The straight line was fitted by least mean squares. Since the infrared analyses of solution and of film are both subject to error, the precisions of the two methods were tested a t three different levels of copolymer composition. Detailed data are given in Table I. The values of copolymer propylene content listed under film analysis were read from the correlation line in Figure 1. The absolute standard deviations in Table I show that t h e
Table 1.
Replicability
of Analyses for Ethylene-Propylene Copolymers a t Three Different Composiiion levels
Film, Sample 1 Foln.. ThickMol. % ness, A8.,/ Mol. To mils A13.8 CI CI 33.7 4 4 0 395 33.5 4 1 0 392 33.2 33 7 2 5 0 334 31.0 35.4 33.7 2.7 0 389 33.9 4 4 0 424 34.2 34 6 31.2 2.3 0.340 33.1 ... 2.3 0.337 31.0 1.9 0,319 ... 30.2 1.8 0.338 31.0 3.2 0.360 32.4 .. 2.3 0.345 31.5 3 8 0.383 33.0 32.3 34.0
...
;IV.
Xhs. std. dev.
0.784
1.405
precisions of the two methods are practically the same, and t h a t the film thickness has no significant effect on precision within the thickness range examined. Polypropylene gives rise to many infrared absorption peaks, and of these the symmetrical C-H bending vibration of the methyl group a t 7.25 microns is much too intense for use in film analysis. Many of the other peaks characterize polymer crystallinity and are likewise useless for the present purpose. On the other hand, the propyl skrletal vibration a t 8.7 microns has moderate intensity and is not due to crystallinity ( I ) . It also gives good mc3asurement and was therefore selected for the measurement of copolymer propylene content. Figure 2 shows the infrared spectra for four different rthylene-propylene copolymers, correFponding to different compositions and conditions of synthesis. Polyethylene
Soh, Mol. yo Ca 48 0 48 0 48 4 48 2 47 9
... . .
... 48.1
Film. Samule 2 Thickness, A8.,/ ?viol. 7 0 mils AI3.9 CB 3 9 0 895 45 8 4 4 0 872 45 4 4 5 0 852 45 1 6 0 0 897 45 8 46 0 897 45 8 4 9 0 898 45 8 6 2 0 901 46 0 6 8 0 854 45 2 4 9 0 895 45 8 5 0 0 857 45 2 4 4 0 871 45 3 6 8 0 902 46 0 45 6
0.200
0.333
has very few characteristic infrared absorption peaks. The most suitable peak occurs a t 13.9 microns; this represents the block methylene rocking vibration, for blocks containing a sequence of three or more methylene units. This sequence length decreases as the copolymer propylene content increases, and the intensity of the 13.9-micron peak falls off exponentially. This explains why the linear relationship shown in Figure 1 requires a semilogarithmic plot. The least mean squares line of Figure 1 is given b y the equation log,, AI,.^) = 0.0286 (C,) - 1.3730 where (CJ is the mole per cent of propylene units in the copolymer. The analyses from this method represent, of course, the average compositions of the various copolymer species present in the sample. If one is dealing m t h copolymer fractions. or with physical mivtures of polyethvlene and polypropylene, the
Soh, Mol. yo C3 58.1 54.8 56.5 57.6 57.1 57.1
...
Thickness, mils 3 4 2 6 2 1 2 4 4 4 3 5 4 3 2 9 3 2 3 2 2 9 4 2
Film, Sample 3__AS.,/ A13.9 2 423 2 410 2 230 2 170
2 420 2 320 2 480 2 200 2.120 2.400 2 390 2.220
Fj6.9 1.045
Mol. yo Ci 60.8 60.7 59 5
.w
1
A0 8 80 2 61 1
59 ,58 80 60 59 60
4 8
6 5
6 1
0.703
values of the numerical parameters given in our correlation equation may require modification. ACKNOWLEDGMFNT
I express m y indebtedness to John Rehner, Jr., for his personal inspiration and many fruitful suggestions. I a m also very grateful to Herbert F. Strohmayer for kindly supplying the copolymers. LITERATURE CITED
(1) 4be, K., Yanagisawa, K., J . Polymer Sci. 36, 539 (1959).
12) Bua. E.. Manaresi. P.. ANAL. CHEM. 31.2022 ri959i. (3) Natta, ~ G .Mazzanti, , G., Valvassori, A., Pajaro, G., Chim. e ind. (Milan) 39, 733 (1957). I
,
RECEIVEDfor review May 11, 1960. hccepted October 31, 1960. Presented before the Division of Polymer Chemistry, 137th National Meeting, ACS, Cleveland, Ohio, April 9, 1960.
Rapid Fluorine Analysis by Wide-Line Nuclear Magnetic Resonance HERBERT RUBIN Schlumberger Well Surveying Corp., Ridgefield lnsfrumenf Group, Ridgefield, Conn.
ROBERT E. SWARBRICK Analytical Research Division, Esso Research and Engineering, linden, N. J.
b The salient aspecis of the instrumentation and method for the determination of fluorine in fluorocarbon liquids by wide-line NMR are presented. By means of an integrator, fluorine determinaiions may b e quickly made on single compounds and mixtures. The influence of dilution and chemical shifts in mixtures was studied.
With measuring times of 1 minute, the fluorine content of 20-ml. samples was measured with a relatibe error of about 1%. The limit of detection in peak measurements was about 12 mg. of F. Samples of 0.2 ml. were measured with a relative error of about 2% and have a deiection limit of 3 mg. of F.
CONSIDERABLE WORK has been done with nuclear magnetic resonance in elucidating molecular structure and motion. this method has been little used in quantitative analysis. The specificity of NMR for a particular isotope gives i t a strong position in the field of elemental analysis. I n wide-line measurements, fine spectral HILE
VOL. 33, NO. 2, FEBRUARY 1961
217
