LITERATURE CITED
(1) Campbell, G. G., Tacker, S 8.,AXAL. CHEX 24, 1090-2 (1952). (2) Das, 31. X., Palit, S. R., J. Indian Chem. SOC.31, 34-8 (1954). (3) Phillips Chemical Co., Method VIA-5, Adams Terminal Vinylpyridine Plant Laboratory Rlanual, 1952. (4) Seaman, IT.,Sorton, A. R., Sund-
berg, 0. E., IND. EXG.CHEJI., . ~ X A L . ED. 12, 403-5 (1940). (5) Seidell, A,, Linke, IT.F., “Solubilities of Inorganic and Organic Compounds,” Suppl. to 3rd ed., p. 659, Van Sostrand, New York, 1952. (6) Synthetic Rubber Division, Reconstruction Finance Corp., “Butadiene Laboratory Manual,” Method L. M. 2 . 1 . 9 . 3 (Sept. 25, 1951).
( 7 ) Ibid., Method L. M. 2 . 1 . 9 . 2 (Feb. 2, 1944).
(8)7Takahashi, T., Kimoto, K., Takano, I.,J . Chem. SOC.Japan, Ind. Chem. Sect. 56, 591-3 (1953).
RECEIYED for review February 15, 1958. accepted July 16, 1958. Division of -4nalytical Chemistry, 133rd Meeting, .4CS, San Francisco, Calif., April 1958.
Potassium Bromide Method of Infrared Sampling ROBERT G. MllKEY U. S. Geological Survey, Washington 25, D. C.
b In the preparation of potassium bromide pressed windows for use in the infrared analysis of solids, severe grinding of the potassium bromide powder may produce strong absorption bands that could interfere seriously with the spectra of the sample. These absorption bands appear to be due to some crystal alteration of the potassium bromide as a result of the grinding process. They were less apt to occur when the coarser powder, which had received a relatively gentle grinding, was used. Window blanks prepared from the coarser powders showed smaller adsorbed water peaks and generally higher over-all transmittance readings than windows pressed from the very fine powders.
M
has been derived from the pressed-window method of infrared sampling (3, 7 4 , in which a finely divided solid, a solution, or a n oil is mixed thoroughly with a finely ground alkali halide matrix (usually potassium bromide) , and pressed under high pressure into the form of a transparent circular disk or rectangular prism. The amount of sample in the pressed disk can be controlled for application to quantitative analysis; the spectra contain no interfering absorption peaks, which n-ould appear if the sample were analyzed in a mull, and the windows can be stored for an indefinite time. This method, however, must be used ith caution, because of possible ehemical and structural changes in the sample. Rfwarchers have reported that the preparation of pressed disk n indows has altered the spectra of samples. When benzoic acid was pressed in a potassium bromide. window, its spectra exhibited a general deterioration of the spectral peaks (4). When steroids were agitated vigorously with potassium bromide in a ball mill, the -harp absorption peaks obtained when the sample had been hand-ground in a mortar, became altered to rounded and indefinite absorption bands (6). IrUCH USE
reversible changes took place in certain carbohydrates as a result of the pressedwindow procedure of sampling, and particularly, p-D-glucose changed to a - ~ glucose monohydrate ( 2 ) . When the potassium bromide disk method mas used to obtain the spectrum of ehemisorbed ammonia, ion exchange took place between potassium and ammonia, so that the spectrum obtained was actually that of ammonium bromide, rather than of XH4+( 5 ) . Various polymorphic changes occurred in the structures of succinimide and napthalene acetamide when they were pressed into n indows (1). Csually, the changes in sample spectra appear to be due to the grinding action during the mixing of sample and matrix, preparatory to pressing. It is possible that vigorous or prolonged grinding of the sample and matrix can change the crystalline structure of the sample, producing new sets of discrete absorption bands; or, a material may be rendered amorphous, I\ ith resulting broadcning or elimination of absorption peaks. .4 parallel source of potential error lies in the effect of the grinding on the potassium bromide matrix. In the spectra of the potassium bromide pressed disk, a minor absorption band a t lyave lengths betvieen 9 and 10 microns is often considered to be due to impurities in the potassium bromide. While not generally considered an interference to sample spectra, this absorption may, in fact, be greatly augmented under certain condition. of grinding. increasing in intensity as the intensity of grinding is increased. Spectra of prepared potassium bromide blank disks indicate that this absorption could interfere seriously with the spectra of a sample. APPARATUS
Spectrophotometer, Perkin-Elmer Xodel21, with the following instrument settings: slit schedule 927, gain 5 , response 1, source amperes 0.3, speed 2. and suppression 2. Pellet die, made of hardened tool steel, with a 0.50-inch bore, and facili-
I
3 0 1 ’ ” ’ ’ 1 ’1
*
9
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1
1
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IO II WAVE LENGTH MlCRONS
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Figure 1. Absorption band shown b y potassium bromide window blank 1. 2.
KBr ground gently in mullite mortar KBr ground strongly in mullite mortar and sieved through 400-mesh screen 3. KBr in Bakelite capsule pulverized b y mechanical vibrator 4, 5. KBr in steel capsule pulverized b y mechanical vibrator
ties for air-evacuation of pressing chamber (Hilger and Watts, Ltd., London). Vacuum pump, 2-stageJ Welch Duoseal, Model 1400, providing a vacuum of 0.1 micron or better. Grinding vessels, mullite mortar and pestle, and mechanical dental amalgamator (Crescent Manufacturing Co., Chicago, Ill.), equipped with Bakelite capsule and pestle, and stainless steel capsule and pestle. Hydraulic press, Carver hydraulic laboratory press, 10-ton capacity (Fred 8.Carver, Inc., Summit, K. J.). PREPARATION
OF
DISKS
One hundred milligrams of reagent grade potassium bromide was ground, by hand with mortar and pestle, or alternatively ground mechanically in the dental amalgamator, then pressed under vacuum a t barely compacting pressure for 1 minute, followed by a niaximum of 20,000 p s i . for 2 minutes to form a disk with a diameter of 0.50 inch and a thickness of about 0.03 inch. RESULTS AND DISCUSSION
Figure 1 presents the spectra of several potassium bromide disks, prepared VOL. 30, NO. 12, DECEMBER 1958
1931
by this procedure, except that the potassium bromide of each was subjected to different degrees of grinding action. The same peaks, a t 8.7, 9.05, 9.5, 9.8, and 10.05 microns, with steadily increasing intensity, are seen in curves 1 t o 5, corresponding to potassium bromide subjected to increasingly severe grinding action. This sequence of absorbance bands appeared in the spectra of many windows prepared over a period of time (although the relative intensities of the peaks mere sometimes different from those in Figure 1). It seems unlikely that impurities picked up during the grinding produced the absorption peaks, because grinding vessels of different materials m r e used-mullite for curves 1 and 2, Bakelite for curve 3, and stainless steel for curves 4 and 5. Rather, some degree of alteration in crystal structure of the matrix material is suggested. Crystal alteration that did occur, hoivever, mas not discernible in the x-ray diffraction pattern of the window, which produced the usual pattern for potassium bromide except for an occasional strengthening or diminution in reflection such as would occur if some preferred orientation had taken place during the pressing process. In curve 3, which represents mechanical grinding of the potassium bromide in Bakelite, a drop from the base line of more than 10% transmittance units occurred, and in curve 5, which represents mechanical grinding of the potassium bromide in steel, a drop from the base line of close to 30% transmittance units occurred. Accordingly, the possible occurrence of interfering absorbances of this magnitude must be kept in mind when vigorous mechanical grinding is used in the preparation of windows. I n practice, absorbance bands due to the matrix varied from disk t o disk. The bands were more apt to be noticeable in disks that were pressed from finely ground ( 200-mesh; 27.2% were 200 > 300-mesh; and 43.570 JTere < 300-mesh. For comparison with a window in which light scattering is a t a minimum, curve 1 shorn the spectra of an optical crystal blank from grown-crystal potassium bromide, 5 mm. in thickness and having 2n optically polished face. Curve 4 s h o w the transmittance of a window pressed from
