INDUSTRIAL
AND
ENGINEERING CHEMISTRY
A N A L Y T I C A L E D IT1 0 N PUBLISHED
BY
THE
AMERICAN
CHEMICAL
SOCIETY
0
HARRISON
E.
HOWE,
EDITOR
Application of Infrared Spectroscopy to Industrial Research NOHIIIAN WHIGHT, ‘The Dow- Chemical (:onipany, >lidland, \rich.
T
4.45p, and those with the ( 2 4group have a band in the interval 5.6 to 5 . 9 , ~ .These cases and similar ones have long been known; considerable literature of such subgroup bands or “linkage” bands has been built up in recent years (1,8,12). A great many applications of infrared spectroscopy can be made in the field of industrial organic chemistry, both as ft tool for research and for actual control of plant processes. A list of such applications would include: (1) identification of organic compounds, (2) detection and identification of small amounts of impurities in organic compounds, (3) accurate quantitative determination of such impurities, (4)study of reaction mechanisms and speeds, and detection of intermediates, ( 5 ) study of isomerism and tautomerism, (6) study of association and compound formation, ( 7 ) study of polymerization and copolymerization in the field of plastics, (8) determination of geometrical structures, moments of inertia, and bond lengths, (9) determination of force constants and dissociation constants, (10) calculation of specific heats and other thermodynamic constants, and (11) study of crystal structure through use of polarized radiation. There are no doubt other applications. In view of the work of Coblentz ( 2 ) and others before 1905, pointing directly to many of the above applications, the question arises why these have not long ago been put into industrial practice. The answer lies almost entirely in the severe experimental difficulties encountered by early workers. From the standpoint of optics there was no great obstacle, but the extremely high sensitivity of the radiation-detecting devices to all sorts of external disturbances has been overcome only in recent years. This explains the scarcity of reports on efforts to utilize infrared spectroscopy in industry, particularly in the analytical field. Tn o instances of analysis by means of infrared which are no doubt adaptable to industrial use include a method developed by hlcAlister (9) for the accurate and rapid analysii of carbon dioxide in air, and an ingenious method devised by l’fund (11) for the determination of simple gases such as carbon dioxide, carbon monoxide, and methane without the use of a spectrometer. There are no reports, however, of the industiial use of infrared spectroscopy for quantitative analysis of more complex organic compounds. It is therefore the purpose of this paper to describe in some detail the methods of qualitative and quantitative analysis by means of infrared spectroscopy which are being employed on a routine basis in an in(lust 1ial 1a h r a t o ~ y .
HE range of the infrared spectrum discussed in this paper extends from about 2.51.1 (25,000 A.) to approximately
15p. Since photographic plates are sensitive to wave lengths no longer than 1 . 3 the ~ ~ spectrum here mentioned is detected and measured through the heating effect of the radiation, in the present case by a thermopile and galvanometer. It has long been recognized, through such early work as that of Coblentz (2) in 1905 on the infrared spectra of a large number of compounds, that the selective absorption (or emission) of infrared radiation arises in the mutual vibrations of the atoms constituting the molecules. A molecule does not absorb radiation of all wave lengths but selects only a fennarrow wave-length intervals which are known as absorption bands. The resulting absorption pattern is characteristir of the molecule. Theoretical treatment of the vibrations of molecules ant1 the correlation with infrared spectra has been sumniarizetl for the simpler molecules by Dennison ( 3 ) . He pointed out that the vibration frequencies within a molecule are determined by the masses of the atoms, the strength of the forces which bind them, and the geometrical structure of the molecule. I n the case of organic compounds there is only slight dependence on the state of aggregation of the molecules, and the factors mentioned lead to vibration frequencies corresponding to wave lengths lying for the greater part in the spectral range 2.5 to 1 5 , ~ . Inorganic compounds do not present as favorable a fielcl for infrared spectroscopy as do the organic. Chief of the disadvantages encountered with inorganic compounds is the fact that water, the commonest solvent, is nearly opaque t o infrared naves longer than 1 . 5 ~ . A second disadvantage is the great width of absorption (or reflection) bands of inorganic compounds. The field of organic chemistry, on the other hand, lends itself particularly well to the methods of infrarecl spectroscopy, and applications in this field only are discussed in the present paper. I n addition to the general fact that the infrared spectrum of an organic molecule is characteristic of that molecule, it is well known that certain groupings or subgroups of atoms within molecules behave more or less independently of the rest of the molecule and give rise to characteristic absorption bands. For example, the 0-H group gives rise t o a bantl in the vicinity of 2 . 7 5 ~(in unassociated molecules), irrespective of the type of molecule containing this hydroxyl group. Compounds with the C-S group posses:‘: a band at about I
Vol. 13, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
2
Apparatus The spectrograph is an automatically recording instrument employing a 60" prism of rock salt in a Littrow mounting. It was designed to meet the requirements of producing and rapidly recording an infrared spectrum of the proper quality and yet retaining the simplicity and dependability vital to industrial application. I n some respects it resembles the spectrograph of Strong and Randall ( I S ) which, however, uses the more complicated Wadsworth-Littrow mounting. The instrument was built to order by the shop of the Physics Department of the University of Michigan. A diagram of the optical system is shown in Figure 1.
a high sensitivity without sacrifice of time, and permits employment of more stable galvanometers. Automatic scanning of the spectrum is accomplished by slowly rotating G by a motor drive, causing the spectrum t o move slowly over the selector slit, H . Simultaneously the deflection produced in the final galvanometer is photographically recorded on the synchroniaed drum camera, L. A photograph of the instrument and room is shown in Figure 2. The radiation source and focusing mirror are enclosed in an airtight case, as is also the spectrometer proper. This allows removal of water vapor from the radiation path by drying agents placed within the cases. The source-box can be readily moved toward or away from the spectrometer to permit insertion of sample cells of different length. The room is lined with sheet metal t o shield against the electromagnetic effects of the highpotential spark sources used in the spectrographic laboratory nearby. This instrument has been in constant service for nearly 3 years.
Several records of infrared spectra made with this instrument are shown in Figures 3, 5, and 6. These records (15 cm., 6 inches, wide by 50 em., 20 inches, long) are graphs of galvanometer deflection produced by the transmitted radiation ns. wave length as indicated in microns along the bottom edge. Zero deflection of the galvanometer is a t the bottom edge of the records and is marked by the dots at the beginning and end of each section of the recording. The vertical fiducial lines are photographed on the records a t intervals of 20 units of a revolution counter geared to the Littrow mirror drive. Infrared wave lengths are easily read from a calibration curve obtained with the use of known standard wave lengths. The manner in which a record is made is illustrated in Figure 3 by the spectrum obtained with no material other than air in the radiation path. Ox'INFEARED SPECTROGRAPH FIGURE1. DIAQRAM
g l d a r P I ~ ~ P I I I ,.A The B ~ ~ I ~ Tof V Pthe infrared r d i 3 t i m i s (12.5 cm., 5 inches long), mounted rn a n.nter-coolcd jacket. Tlw radmtmn is foruxrd I W 11.e concnve n.mor. I
