Su bmicrodetermination of Oxygen-1 8-Benzophenone by Infrared Spectrometry ISRAEL LAULICHT and SHRAGA PINCHAS The Weizmann Institute o f Science, Rehovoth, Israel
b The OI8-benzophenone content of 100 to 700-pg. samples of 1 to 61
atom % enriched benzophenone can b e determined with the mean accuracy of *0.5% (absolute) from the infrared absorption intensity of their CC14 solutions at 1667 and 1637 ern.-', the locations of the C=0l6 and C=O's stretching bands. No weighing of the sample is generally necessary since the O'* content is calculated on the basis of the respective absorbances. It seems that this method can b e used for the 0l8determination in submicro samples of water after reacting it with pure dichlorodiphenylmethane.
H E N DICHLORODIPHENYLMETHANE
is refluxed with water enriched with oxygen-18, the enrichment of the benzophenone formed by this reaction is nearly the same as that of the original water used for its formation (1). Although this reagent usually contains small amounts of free benzophenone, which slightly decrease the 018enrichment of the formed benzophenone, this reagent can no doubt be purified satisfactorily without too much difficulty. Hence, an easy method for the precise submicrodetermination of the 018-benzophenone content of enriched benzophenone would be of interest for the isotopic analysis of H20I8 in submicro quantities of water. Such a method could be expected to replace the one which introduces the labeled oxygen atom into a di-p-tolylurea molecule and which was found to give considerable errors for low enrichments of oxygen18 (a). This paper reports the description of such an infrared spectrometric method and shows that it can give results accurate to about +0.5% (absolute) for benzophenone samples of 0.1-0.7 mg. which contain 1 to 61 atom Toof 0 ' 8 . This method is based on the characteristic C=0I6 and C=0l8 stretching bands for the carbon tetrachloride solutions of the benzophenone molecules, a t 1667 and 1637 cm.-I, respectively. If the instrument used for the absorbance measurement of these solutions has good resolution, the absorptivity of each isotopic species at the key frequency of the other can be 1980
ANALYTICAL CHEMISTRY
neglected for samples of 40 to 60y0 enrichment since the half width of both bands is only about 10 em.-' (3). The values for the respective molar absorptivities are so high (S) (about 652 and 540 liters mole-' em.-' for the C=0l6 and C=0l8 bands) that in a 2-mm. cell, reasonable absorbances are obtained even with concentrations as low as 0.1-0.2 mg. per ml. of each modification. When the key frequency absorbances of isotopically homogeneous, 0.1-1.0 mg. per ml., benzophenone solutions are plotted against the corresponding concentration practically all the points fall on two straight lines that pass through the origin-Le., both modifications obey Beer's law. The 0l8content of very small (down to 40 atom yo) enriched benzophenone samples can therefore be determined, without weighing them, by measuring the absorbances of their solutions in carbon tetrachloride a t 1637 and 1667 cm.-l, reading the concentrations of labeled ( A ) and normal ( B ) benzophenone from these lines, and solving B). the equation: % 018 = .4/(A Since a 2-mm. cavity cell (Type B of the Barnes Engineering Co., Stamford, Conn.) can be filled by about 0.15 ml., satisfactory results can be obtained by this method from about 100-pg. samples without using beam condensers. Such samples should be transferred unweighed into the cavity cell and dissolved in situ in any amount of carbon tetrachloride which fills the cell. For samples of 10 to 40% enrichment, the absorbance a t 1637 cm.-l must first be corrected for the absorption of the normal benzophenone concentration (as read from the working line) a t this point where it has a molar absorptivity of approximately 20 liters mole-' ern.-' If the 0l8content of the sample is less than 10% the net absorbance a t 1637 em.-', for samples of about 100 pg. in a 2-mm. cell, will be too low (< 0.05) for accurate determination. If the amount of sample introduced into the cell is increased considerably, the absorbance at 1667 ern.-' will then be too high (>0.7) for a precise determination. This difficulty can be overcome by two methods. According to method I1 the sample is now weighed (and dissolved in a known volume of solvent) and the determination is based only on the
+
absorbance of the solution a t 1637 em.-' The following equation is then solved: =
C (Xa L
-
+ (100 - X)a,) 100
v
where -4 = absorbance of the sample solution in a 5-mm. cell; C = weight of sample (mg.); X = per cent of labeled compound in the sample; aL, u4' = absorptivities of the labeled and normal benzophenone a t 1637 em.-' respectively, in this cell; and Ti = the total volume of the solution (ml.) after a L and are first evaluated separately from measurements on isotopically pure solutions of known concentrations. Alternatively, according to method I11 which seems to give better results, the sample of 1 to 10% enrichment which contains 500 to 700 pg. is transferred, unweighed, into a 5-mm. cavity cell, and dissolved in situ by the amount of carbon tetrachloride needed to fill it (about 0.35 ml.), The absorbance of this solution a t 1637 em.-' is measured and 0.20 ml. of it is diluted with 1.0 ml. of solvent. The cell is now cleaned and filled with the diluted solution to measure its absorbance at 1667 em.-' From this absorbance, the absorbance of the original solution a t this frequency is found by multiplying it by 6 and the original normal benzophenone concentration calculated from this absorbance. The absorbance of the original solution a t 1637 cm.-' is then corrected for the absorbance of the normal benzophenone a t this point and the net absorbance used to determine the concentration of the labeled species in this solution from the working line. The 0 ' 8 content is finally calculated, as in the first met,hod, by solving the equation: % 0 1 8 = A / A B.
+
EXPERIMENTAL
Apparatus. The instrument used for these determinations was a PerkinElmer 12C spectrophotometer equipped with a CaF2 prism. I t s entrance slit was kept const'ant a t 0 . 2 mm. throughout this work. The signal-tonoise ratio was about 200. Reagents. The normal benzophenone was a pure commercial product. The OI~--labeled benzophenone contained 89 atom % 0 1 8 and was synthesized as already described ( 1 ) . Khen
the absorbance of solutions of this material was measured, its normal benzophenone content, WEIS always taken into account. Procedure. T h e absorbance of a benzophenone solution at' a cert,ain point was the difference bet'ween its absorbance and t h a t (of the pure solvent in the same cell, a t this frequency. Each of these absorbances was the mean value of t,wo measurements, all measurements being carried out during a very short period. T h e estimated mean experimental error in absorbance was about 1 0 . 0 0 5 . RESULTS
The results obtained by the three spectroscopic methods of analysis described above, on solutions of known 0 ' 8 content, are collected in Table I. For enrichments of S-61y0 0 1 8 , both methods I and I1 gave very satisfactory results while for 1 to 4% enrichments both methods I1 and 111 gave results which were only accurate to 11% (absolute). The general mean
Table I. Determination of O18-Benzophenone Content of Known Solutions
Known content,
%
61 55 44 19 8 3
1
0 5 8
4 5 1 4 1 0
Found, Method I I I I I1 I1 I11 I11
56
Dlff.
61 4 55 0 44 5
+O 3
20 3 a
-8 4 1 1
7 6 5 9
0 0 0 0
+o
+o
.5
3 + 1 1 +01 t o 9 0.4
pended somewhat on the total concentration when in excess of 1.5 mg. per ml. I t is hoped that by using 20-mm. cells, and keeping this concentration within the 1.5 ma. per ml. limit, considerably more accurate results will be obtained with enrichments as low as 0.5%. ACKNOWLEDGMENT
The authors thank D. Samuel for the sample of 018-benzophenone.
~
mean a After correcting for the absorption of the normal compound a t 1637 cm.-'
error of all these methods however was about +0.5%. The main reason for the bigger error in the determination of lower 0l8 contents &-a5 the interference of the now relatively high concentration of the normal species a t the key frequency of the labeled benzophenone, which de-
LITERATURE CITED
( 1 ) Halmann, SI., Pinchas, S., J . Chem. Soc. 1958,p. 1703. ( 2 ) Lapidot, A., Pinchas, S., Samuel, D., Proc. Chenz. Soc. 1962, p. 109. ( 3 ) Laulicht, I., Pinchas, S., Israel Journ. Chem. 1, 404 (1963).
RECEIVEDfor review March 3, 1964. Accepted May 21, 1964. A grant from the National Institutes of Health, Bethesda, Washington I>. C., supported this investigation. Presented in part before the XIXth IUPAC Congress, London, U. K., July 1963.
End-Group Analysis and Number-Average Molecular Weight Determination of Some Polyalkylene Glycols and Glycol Polyesters Using Nuclear M a gnetic Kesonance Spectroscopy THOMAS F. PAGE, Jr., and WARREN E. BRESLER Battelle Memorial Institute, 505 King Avenue, Columbus, Ohio 4320 7
b Using nuclear magnetic resonance spectroscopy (NMR), it has been found that methylene and methine groups attached to a hydroxyl group can b e differentiated from those attached to an ether oxygen for some polyalkylene glycols and glycol polyesters. In the specific case of glycols, this differentiation makes it possible to calculate number-average molecular weights without using the -OH resonance area and thus circumvents the problem of obtaining low molecular weight values because of the presence of a small amount of water. By being able to determine the -CHzOH content of a glycol polyester, it is possible to correct the area of the resonance due to the acid and hydroxyl protons for the hydroxyl content, and water i,F present, and thus obtain the acid en'd-group content by difference. Number-average molecular weights of the glycol esters can also b e determined. The technique involves taking advaintage of the
complex formed between pyridine and -OH groups to shift the -CHzOH resonance from the -CHz-Oresonance.
I
that if one can perform a quantitative analysis of the end-groups of a polymer, this information can be used to calculate a number-average molecular weight (NAMW) of the polymer ( I , 11). Xuclear magnetic resonance (NSIR) spectroscopy is especially suited to this kind of determination if the resonance of the end-group protons is chemically shifted from that of the internal group protons and if impurities do not interfere with the respective resonances. The S M R spectra of polyethylene glycols (PEG) fulfill the former of these qualifications but do not usually fulfill the latter because the mall impurity of water which is usually present in conimercial P E G samples contributes to the -OH resonance area. This interference T IS WELL KNOWN
results in the measurement of a high -OH content and thus the calculation of a correspondingly low NAMW. The purpose of this paper is to report a technique which has been developed so that end-group analyses and S X M W determinations can be carried out fcr hydrosyl terminated polymers such as glycols by using only data from the
CH~-, -cH,-,
-CH
resonances.
EXPERIMENTAL
Apparatus. A11 spectra were obtained using a Varian Associates Model HR-60 nuclear magnetic resonance spectrometer operating a t 60 Me. per second. Varian 1Iotlel V-3521 electronic integrator was used for integration; the capacitor charge was monitored with a Hewlett-Parkard Model 3440A digital voltmeter equipped with a Hewlett-Packard Model 3442.1 automatic ranging unit. Chemical shifts are reported in csycles per second don nfield froin the internal VOL. 36, NO. 10, SEPTEMBER 1964
1981
