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V O L U M E 28, NO. 8, A U G U S T 1 9 5 6 linearity between sensitivity and molecular weight the sensitivities derived on the basis of the assumption were applied to cuts from the distillation falling among the CS,CS, and C12 concentrates. The analyses of these various cuts gave results totaling between 90 and 110% olefin, thus indicating not greater than a 10% error in the sensitivities. Actually, because the distillation would be expected to effect some separation of the various isomers, and average coefficients are applied, these sensitivities when applied to a propylene polymer sample of m-ide boiling range should give good results. Application to High Boiling Materials. It v a s expected that the low voltage technique would have a particularly useful appliration to the quantitative determination of the compound types present in fractions obtained from the percolation of heating oil :ind gae oil fractions over alumina gel or similar absorbent. Preliminary investigations of this possibility have been successful,
and i t is anticipated that this techniquc will prove useful iii the eventual unraveling of the composition of high boiling aromatics. ACKYOWLEDGMENT
The authors gratefully acknowledge the contributions oi Burl
L. Clark, who performed the greater portion of the e.;pciiniental work. Joe Dzilsky joined the project in its later stages .ind his effortg, too, are greatly appreciated. LITERATURE CITED
(1) Field. F. H., Franklin, J. L., ,J. Chem. Phya. 22, IS05 \ l ! ) S ) .
( 2 ) Franklin, J. L.. Ihid.. 22, 1304 (19.54). (3) Honig, R. E., Ibid., 16, 105 (1948).
(4) Stevenson, D. P., TVagner, C. D., J . A m . Chem. Soc. 72, 5tj1'7 (1950). ( 5 ) Taylor, D. D., U. S. Patent 2,373,151(1900). RECEIVED for review February 10,1956.
Accepted M a l - 3. 1956
Quantitative Infrared Absorption Spectroscopy in Water Solution W. J. POlTS, JR., Dow
and NORMAN WRIGHT
Chemical Co., Midland, Mich.
Quantitative infrared absorption spectroscopy can be carried out in water solution by using a very thin absorption cell with barium fluoride windows. Useful transmittance in the region from 6.5 to 10 microns is obtained on a double-beam spectrometer by insertion of a transmittance screen in the referencebeam; energy is recovered by widening the spectrometer slits by a suitable amount. The method is applicable to many cases where organic materials are soluble in water.
T
HAT water can be used as a solvent for infrared absorption spectroscopy was shown as early as 1905 by Coblentz ( 3 ) . Nore recently Gore, Barnes, and Petersen ( 3 ) and Blout and Lenormant (1) have shown that water, used in conjunction with heavy mater, can have considerable use in this respect, enabling one to obtain an infrared absorption spectrum throughout almost the entire rock salt region. Their results were of a qualitative nature only, however. Plyler and Acquista ( 4 ) have given quantitative absorption spectra of pure water, and have shown that there is a region from ~ 6 . to 5 =10 microns where there is still enough infrared transmittance in reasonable path lengths of water to suggest its use as a solvent for quantitative analytical purposes. The advent of barium fluoride as an optical material has made possible the construction of a permanent absorption cell. Barium fluoride seems ideally suited for this use, as it is commercially :tvailable, hard, easily polished, and essentially insoluble in uater. With such a cell, a a t e r solutions can be used in much the same way, and m-ith the same accuracy, as carbon tetrachloride or carbon disulfide solutions are used for quantitative absorption spectroscopy a t present.
an amalgam with the brass, which sticks to the fluoridc plate surface. The barium fluoride plates were obtained already cut, ground, and polished from the Perkin-Elmer Corp., Sorwalk, Conn. The cell so constructed has a path length of 0.027 inin.: this distance was determined in the usual way by a fiinge pattern ( s h o m in Figure 2, a ) of the empty cell. The depth of the fringes and their general regularity indicate that, even w t h this short path length, a cell can be made with nearly perfectly pal allel faces if care is used. ,411 spectra were obtained on a double-beam infr arcd spectrometer equip ed with a rock salt prism. The instrument was designed a n z b u i l t in this laboratory; a publication desciibing its construction and features is in preparation. Figure 2, b, is the absor tion spectrum of pure n-ater obtained in the cell just described. 8omparison with Figure 2, n, shoa s that water ceases to transmit a useful amount of radiation at somewhat shorter wave length than the barium fluoride rntoff point; hence, barium fluoride is by no means the limiting factor in the use of water solutions.
APPARATUS AND TECHNIQUES
The absorption cell (Figure 1) is constructed in much the same manner as the conventional rock salt cells. The spacer between the barium fluoride plates is made from 0.001-inch shim brass, and is sealed to the plates by coating the brass n i t h mercury to form
Figure 1.
Barium fluoride absorption cell for use w-ith water solutions
A N A L Y T I C A L CHEMISTRY
1256 I n order t o obtain a more useful IO in the region from 6.5 to 10 microns, the spectrometer is stopped a t 6.1 microns (absorption maximum of the water deformation vibration), and a transmittance screen is inserted in the reference beam. The absorption under these conditions is shown in Figure 2, c. This now results in an ZO near the region of 1 0 0 ~ otransmittance (on the chart paper), a situation desirable in accurate photometry and convenient for qualitative study of spectra. A useful ZO might be obtained by placing a second absorption
cell filled with water in the reference beam of the spectrometer. However, this would tend to hide the fact that the spectrometer is receiving no energy and is, therefore, useless in the regions of strong water absorption (near c 6 . 1 microns and above clO.5 microns). Also, this would require the time and expense of a second cell, when a simple screen does just as well. Hence, this technique was not used. The energy lost by water absorption and transmittance screen is recovered by widening the monochromator entrance and esit
FREQUENCY, WAVE NUMBERS
20,
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:
Figure 2.
Absorption spectra
a. Fringe pattern of empty cell, from which is obtained optical path length of 0.027 mm.
b . P u r e water
c. Pure water, with screen placed in reference beam from 6.1 t o 11 microns
V O L U M E 28, NO. 8, A U G U S T 1 9 5 6 slits by a factor
d:,
here 7’is
1257
Yc light transmittance of water
in the region from 6.5 to 10 microns. This gives the same signalto-noise ratio as if neither water nor screen were in the spectrometer beams. Of course, this Tyidening of the spectrometer slits is done a t the cost of loss of resolution, but in all cases encountered thus far this loss has not been serious. -1somewhat greaterdeparture from Beer’s law might be encountered than in the case of normal
slit openings, but corrections for Beer’s law deviations can be made just as is usually done when effective slit widths are greater than natural band half-widths. APPLICATIONS
Metallic Salts of Organic Acids. solids insoluble in the usual infrared-transmitting solvents are often difficult to analyze accurately with infrared methods. Metal salts of organic acids
FREQUENCY, W A V E NUMBERS 3000
2500
1500
2000
00
01
02
03 04
05
I O 20
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01 Y
02
m-
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03
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0.4
m
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10 20
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Ol 02
03 04
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I.o 20 25
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Figure 3.
Absorption spectra of 10% aqueous solutions a . Sodium 2-chloropropionate b. Sodium 2,2-dichloropropionate c. Sodium 2,2,3-trichloropropionate
9
10
ANALYTICAL CHEMISTRY
1258 are often determined by conversion to the acid and extraction with a suitable solvent such as carbon disulfide. The use of water solutions now gives a simpler, more direct, and generally more accurate method. Such a problem which has been successfully solved in this laboratory with the water solution technique is the simultaneous determination of the sodium salts of 2-chloro-, 2,2-dichloro-, and 2,3,3-trichloropropionicacids. The spectra of 10'70 solutions in
water of these three materials are shown in Figure 3. Examination of these spectra shows that they are sufficiently different to be used as the basis for the determination of these three salts in the presence of each other. Applications in this laboratory have given results generally reproducible to within 2% of the amount present for the main constituent. Glycols. Liquids insoluble in the usual infrared solvents are also often difficult to analyze directly. The various glycols are
FREQUENCY, W A V E NUMBERS
W A V E LENGTH, M I C R O N S
Figure 4.
Absorption spectra of aqueous solutions
a . Ethylene $lycol, 10%
b. Diethylene glycol 10% Ethylene glycol Bhd diethylene glycol. 3% each
C.
V O L U M E 28, NO. 8, A U G U S T 1 9 5 6
1259
a
8m
