integrated Absorption Standardization for Infrared Determination of Ester Content in Oxidized Polyethylene JOHN N. LOMONTE W.
R. Grace
& Co., Washington Research Cenfer, Clarksville, Md.
b The integrated absorption intensity of the 1175-cm.-' C-0 stretching vibration band of ester groups was measured for six esters selected as model compounds. These integrated intensities were compared to those obtained for a group of oxidized polyethylene samples selected as typical of the product. The ester content of the oxidized polyethylene was calculated from this comparison. An absorptivity constant was calculated from these selected samples for the routine analysis of ester content in this material. A method i s also described for the measurement of optical film thickness of the oxidized polyethylene samples as the waxy nature of this material precludes accurate micrometer thickness measurements.
drop out of the calculations, it need not be known absolutely. A number which is proportional to it, such as calculated here, will be sufficient. The infrared spectrum of oxidized polyethylene as shown in Figure 1 is characterized by a broad absorption band in the 1700- to 1750-cm.-l region due to the C=O stretching vibrations of the various types of carbonyls produced in the oxidation. Rugg (6) reported the following frequencies for the carbonyl stretching modes of the various types of compounds listed:
T
These various types of carbonyl vibration bands cannot be sufficiently resolved by the spectrometer to allow their use for specific analytical determinations. However, it is possible to obtain an analysis for ester content in this system due to a C-0 stretching vibration of ester groups located a t 1175-cm.-l (1, 8 ) . In the oxidized polyethylene system, there are no interferences in this band due to either the polyethylene backbone or the other oxidation products. Because of the inherently waxy nature of oxidized polyethylene and the necessity for the use of thin films in the infrared analysis, measurements of film thickness by conventional methods are
integrated absorption intensity for functional group standardizations for the infrared analysis of polymers has been reported previously (4, 6). In these cases, the band being examined was symmetrical and the band shape could be represented by a Lorentz function, so that the integrated intensity could be calculated from the peak absorbance and the band width a t one-half peak absorbance. Such is not the case with the 1175-cm.-' C--0 stretching vibration band of esters. This band is not only asymmetrical about the band center but in some cases there are two bands in the integration interval. The integrated absorption intensities for these asymmetric bands are measured by a method developed in this laboratory ( 7 ) . If the instrument being used for the investigation of an asymmetric band is fitted with a logarithmic potentiomGter to give spectra which are linear in absorbance, the integrated area can simply be measured with a planimeter However, if such is not the case, the integrated area cannot be measured absolutely but a number which is proportional t o it can be calculated. This calculation consists of measuring the area with a planimeter and dividing this area by the intensity of the transmitted light a t the band center, lo. As the integrated intensity is used only for standardization and will subsequently HE USE OF
192
ANALYTICAL CHEMISTRY
Frequency, ern.-' 1713 1722 1733 1743
Carbonyl type Acids Ketones Aldehydes Estere
not sufficiently accurate. It was necessary to find an absorption band in the oxidized polyethylene spectrum which could be related to film thickness. This method would have the advantage of determining the optical thickness by spectrometric means a t the same time the analysis was being performed. Such a band was found a t 4350 cm.-' which is probably a combination band of the 2926-cm.-l CH stretching frequency and the 1465-cm.-l CH deformation frequency. As this band is a function only of the carbon and hydrogen present in the polymer, the small amount of oxygen present cannot seriously affect the accuracy of this method of film thickness measurement. EXPERIMENTAL
All scans were made on a Beckman IR4 spectrophotometer with sodium chloride optics and a double prism monochromator using the following instrumental conditions : Operating mode slits Filter period Scan speed Scan length
Double beam 1.5 X standard 2 seconds SO cm.-1 per minute 650 to 5000 crn.-l
The esters selected as model compounds for this standardization were scanned in carbon disulfide solution in a 0.0026-cm. sodium chloride cell which had been calibrated by interference fringes. The polymer samples were scanned as self-supporting films which had been molded a t 350' F. The films were pressed between sheets of 0.002-
100- 1 I
O 5000
L
4400
3600
Figure 1.
d \ i 2800
, w , 2000 1800 1600 Frequency, cm. -1
I!
,
1400
1203
1009
Infrared spectrum of a typical oxidized polyethylene
800
I 50
gauge Mylar on a Pasvdena Hydraulics press with heated plat3ns. The sample is removed from the press and allowed to cool. The Mylar sheets are separated and the sample is removed from the sheet it clings to by rolling the edge of the sheet. The rigidity of the sample causes it slowly to pull itself free of the Mylar. The band a t 4350 cm -1 selected as the internal standard band for film thickness was investigated on a series of films pressed from a sztmple of the base resin polyethylene from which the oxidized polymer was made. The thicknesses of the films wai3 measured accurately with a micrometer to be compared to the absorbance a t 4350 cm.-' measured for each film. Solutions were macle of the esters selected as model compounds for this standardization, and the ester content of each solution in gre,ms per liter was calculated from the sample weight used and the molecular meght of the ester group taken to be 44. The area of the 1175-crn.-l band was measured with a planimeter between the frequencies of 1140 and 1240 em.-' The peak absorbance and the intensity of the transmitted light a t the band center, lo, were recorded. Three samples of oxidized polyethylene were scanned in duplicate according to the given conditions. The absorbances were measured a t 1175 and 4350 cm.-l The areas of the 1175-cm.-l bands were measured by planimeter and the value of lo was recorded for each band. RESULTS AND DI!ICUSSION
In Table I are shown data from the investigation of the m3del compounds. The integrated intensity, A , was calcula t ed by : A = l/bc X areall,,
where
b = cell thickness, 0.0026 cm. c = concentration of ester groups,
grams/%ter .
area = band area by phimeter, sq. cm.
Ah absorptivity constant, a, was also calculated by: a = absorbance at I175 cm.-'/bc
The extremely good agreement among the integrated absorption values for these esters is to be noted. Also to be noted is the variation in the values of the absorptivity consi,ants, especially that of the n-amyl butyrate. This is caused by the calculation of the absorptivity constant not taking the band width into account. The n-amyl butryate band is deeper than the others but its narrow band width causes its area to be the same as the other model compounds. X set of three methll esters of longchain fatty acids was E,ISO investigated as to their use as model compounds. Although these methyl esters agreed among themselves, they did not agree
Table I.
Compound Ethyl octanoate Ethyl myristate n-Amyl butryate Ethyl nonanoate +Butyl stearate Lauryl palmitate
Integrated Absorption Data for Model Compounds AbsorpEster Integrated content, Absorbance tivity Area of absorbance at 1175 liter/ band, li ter-cm,/ grams/
liter
cm.-'
22.3 14,7 24.2 20.5 11 . O 13.2
0.365 0.208 0.730 0.286 0.165 0.185
gram-cm. 6.29 5.44 11.60 5.37 5.77 5.39 Av. 6 . 6 4
Table II. Calibration of 4350-Cm.-' Band for Film Thickness Measurement
Absorbance at 4350 cm.-' 0.056 0.196
n .am
0.614 0.710
Film Absorbance/ film thirkness, cm. thickness 0.0020
28.0 28.0 0.0128 30.2 0.0210 29.2 0.0250 28.4 Av. 2 8 . 7 0 . 0070
Film thickness, cm. = absorbance at 4350 cm.-l/28.7
This method of film thickness measurement has the advantage of being able to use uneven films as long as there are no air bubbles in the sample as the spectrometer records the internal standard and ester bands under the same conditions. In Table I11 are shown data obtained from the scans of the oxidized polyethylene scans. The film thicknesses were calculated from the internal standard band and the ester content values were calculated from: c = 1/Ab X area/Ir,
thickness, cm.
760-1 760-2 761-1 761-2 787-1 787-2
0.00254 0.00289 0.00362 0.00226 0.00275 0.00240
388 374 373 381 378 382 Av. 379
where A is the average value of the integrated absorption intensity from the model compounds and the other terms have the same definitions as before. As it is desirable to report analytical results in weight per cent, the value of ester content in grams per liter can be converted by:
From the now standardized samples of oxidized polyethylene, a weight per cent absorptivity constant, a, is calculated from the ester content and the peak absorbance by: a = absorbance at 1175 cm.-'/bc where c is now ester content in weight per cent. From this value of a also listed in Table III it is now possible to calculate the ester content of oxidized polyethylene from the peak absorbance at 1175 em.-* CONCLUSIONS
This method presented for the integrated intensity standardization for ester content is more evidence that this method of standardization, especially for functional groups in polymers, is a very useful concept. Because it is the only valid method for the comparison of band intensities of a solid and its solution and because it also eliminates the necessity for standardization by synthetic mixtures, it is probably the easiest and most accurate method for standardization. In comparison with the wet analytical
Data from Oxidized Polyethylene Samples
Film
Sample No.
gram
ZO 0.71 0.75 0.74 0.71 0.77 0.77
Wt. % ester = c/1000d X 100 = c/lOd where c = ester content, grams/liter d = density of sample, grams/ml.
with the integrated intensities from the other model esters. This is because the infrared spectrum of first members of homologous series are usually not typical of the rest of the series. The data from the investigation of the 4350-cm.-' internal standard band are shown in Table 11. As can be seen from this data, the absorbance is a linear function of film thickness so that:
Table 111.
sq. cm. 16.2 10.7 17.4 14.4 8.3 10.1
Area,
sq. cm. 5.6 6.1 6.1 4.1 5.0 4.5
I, 0.77 0.76 0.74 0.80 0.78 0.80
Ester concentration Grams/ liter Wt. % 7.56 7.34 6.01 5.99 6.15 6.19
0.80 0.77 0.63 0.63 0.65 0.65
Absorbante at
1175
cm.-l
Absorptivity, l / w t . %cm.
0.075 0.088 0.094 0.059 0.071 0.061
36.9 39.5 41.3 41.4 39.7 39.1 Av. 39.7
VOL. 36, NO. 1, JANUARY 1964
193
method for the determination of ester content by quantitative saponification, the values by this procedure are somewhat low. However, it has been shown by work done in this laboratory (2), ‘hat because this saponification must be done at temperatures to keep the Polymer is solution, there are side reactions which cause the saponification numbers to be too high. A series of five samples of oxidized polyethylene ranging in oxygen content from 0.73 to 3.09% was analyzed in quadruplicate by this procedure with
ester content values ranging from 0.05 to 0.80%. The standard deviation of these determinations was 0.035.
(2) Clancy, D. J., W. R. Grace & Co.,
LITERATURE CITED
Clarksville, Md., unpublished data, 1963. (3) Grafmueller, F., Husemann, E., Makromol. Chem. 40, 173 (1960). (4) Lomonte, J. N., ANAL. CHEM. 34, 129 119621. (5) Ri&a&n, W. s., Sacher, A., J . Polymer Sci. 10,353 (1953). (6) Rugg, F. M., Smith, J. J., Bacon, R. C., Ibid., 13, 535 (1954). ( 7 ) Woodbrev, J. C.. Peters. L. L.. W. R. Grace & Eo.. Ciarksvide. Md.. unpublished data, 1961.
(1) Bellamy, L. J., “The Infrared Spectra of Complex Molecules,” p. 179, Wiley, New York, 1958.
RECEIVED for review July 5, 1953. Accepted October 4, 1963.
ACKNOWLEDGMENT
The author thanks J. D. XIoyer of this laboratory for the preparation of the lauryl palmitate used in the standardization.
Determination of Organic Peroxides by Iodine Liberation Procedures R. D. MAlR and ALDA J. GRAUPNER Research Cenfer, Hercules Powder Co., Wilmington, Del, b Peroxides from the most active to the most stable can b e quantitatively determined by means of a coherent group of three methods. Method I (refluxing Nal-isopropyl alcohol) is recommended for all easily reduced peroxides. Method I 1 (refluxing Nalacetic acid-6% HzO) is primarily an assay procedure for diaralkyl peroxides, such a s dicumyl peroxide. Method 111 (refluxing Nal-acetic acidHCI) will determine the most stable di-tert-alkyl peroxides and quantitatively reduce many nonperoxidic compounds. Several interim procedures are discussed. The importance of excess iodide and the key role of H 2 0 content in the regulation of reducing power are stressed. The scope of the methods is demonstrated by their application to the determination of many pure compounds. Relative standard deviations were 0.23% for Method I, 0.30% for Method II, and 0.32% for Method 111.
A
today are called upon to determine organic peroxides in many and diverse analytical situations. For instance, commercial production of peroxides as industrial chemicals requires assays of purity. Process control needs rapid methods which can determine not only pure peroxides but intermediate and low concentrations as well, often in the presence of byproducts which may interfere. Trace methods are needed in many situations, such as the detection of peroxidation in solvents and polymers or incipient rancidity in food fats and oils. The system under analysis may contain highly reactive, moderately inactive, NALYSTS
194
ANALYTICAL CHEMISTRY
or very stable peroxides, and mixtures of peroxides with similar or dissimilar characteristics. It is not surprising, therefore, that no single method or technique can fill the needs of every peroxide analyst. On the contrary, analytical chemists have been called upon to devise a multitude of peroxide methods, and, as a result, there is a large body of literature on the subject. Ready access to this literature is provided through a recent review by Martin (19) and books by Davies (6) and Hawkins (11). Instrumental techniques include infrared (19, 29) and near-infrared (9, 32) absorption, gas chromatography (5), and polarography (2, 17, 24, 29, 31, 35). There are a number of colorimetric methods (7, 12, 16, 23, 27, 33, 59) for trace concentrations. Volumetric methods have been based on a variety of reagents, including stannous chloride ( I ) , arsenious oxide (28), and titanous chloride (14). More frequently used than any of these methods, however, and more generally useful, are the so-called iodine liberation methods, a family of volumetric procedures based on reduction of the peroxide bond by iodide. Nearly all the peroxides of early analytical interest could be reduced in this way and would liberate iodine quantitatively or nearly so even under conditions far from ideal. So the methods were useful and became popular, although apparently not always well understood. The classic work in this field is that of Wagner, Smith, and Peters (57), who expanded on a discovery by Kokatnur and Jelling (16), that intereference caused by air oxidation of the iodide reagent can be completely suppressed
simply by using an alcohoI as solvent. A similar beneficial effect of acetic anhydride has been cited by Nozaki (22). The procedure of Wagner, Smith, and Peters, which is based on sodium iodide and isopropyl alcohol, is simple and highly satisfactory for all easily reduced peroxides. Yet its good points seem not to have been fully appreciated by a number of workers who more recently have published procedures still based on acetic acid or chloroformacetic acid solvents (13, %), and it has been completely overlooked by others (8, 12, 26, 34). Perhaps its accuracy and precision have been questioned, since relevant data were meager. The recent procedure of Wibaut and coworkers (58) (acetic acid solvent) has the highest published accuracy and precision of any peroxide method (19). However, our experience with the sodium iodide-isopropyl alcohol method, discussed here, has shown that its precision and accuracy are fully equivalent to those of the newer procedure. Wagner, Smith, and Peters also pointed out the twin benefits to be derived in iodine liberation methods from the use of a liberal excess of iodide. This strategy keeps the equilibrium, Iz I- ~ r I3-, ? far to the right. As a result, since the triiodide ion is not volatile, loss of liberated iodine either from boiling the reaction mixture or from purging the reaction flask with a stream of inert gas is prevented. In addition, since triiodide ion also will not add to olefinic double bonds, errors due to absorption of liberated iodine by unsaturated components of the sample, a cause of much concern
+
