Mull and Solvent Media for Infrared Use JESSE S. ARD Eastern Regional Research Laboratory, Philadelphia 18, P a . N WORK on antibiotics, alkaloids, and plant regulators, the
1 samples usually have been solids, and the methods most fre-
quently used to obtain their infrared spectra have involved a medium. This led to frequent concern over what bands may have been obscured by the intense absorptions of the medium. In an earlier presentation and abstract ( I ) , media were discussed that would relieve this situation by permitting complete supplementations. Attention was called to the special advantages of a inull system whereby hexachlorobutadiene supplements paraffin oil, and a solution system whereby tetrachloroethylene supplements carbon disulfide. Information concerning the use of these systems is recorded here. HEXACHLQROBUTADIENE AS A MULL MEDIUM
As shown by the dotted lines of Figure 1, paraffin oil of medicinal grade, as used for most mulls, gives intense obscuring absorptions near 3.4, 6.85, and 7.26 micrbns. I n addition, there are less serious interferences near 2.4, 3.8, and 13.8 microns that often can be ignored but occasionally are disturbing. As shown by the solid line, the transmittance of hexachlorobutadiene exceeds that necessary for supplementation in each of these regions. The thickness in each case is 0.10 mm., which exaggerates the media effects by being roughly three times the thickness of the average mull preparation. I t is evident that paraffin and hexachlorobutadiene supplements together can give complete supplementation a t mull thicknesses without any interferences of a significant magnitude.
I 10
2i"
Figure 1.
12
Supplementing Transmittances of Paraffin Hexachlorobutadiene
A . Paraffin oil (heavy liquid petrolatum, U.S.P.), 0.10 mm. B . Liquid hexachlorobutadiene, 0.10 m m .
Perfluorokerosine (4, 11) has also been used to obtain spectra that are obscured by paraffin oils, and tests have shown that its useful region extends from below 2 to an absorption edge a t about 7.20 microns when the thickness is 0.03 mm. Although the supplementing capacity is adequate for many special purposes, the utility of perfluorokerosine seemed inefficient or doubtful in the 7.3- and 13.8-micron regions, and therefore it was considered to be less complete than that of hexachlorobutadiene. The reduction of light scattering by perfluorokerosine was also believed to be less efficient because compatibilities resulting in partial solution or gelation of the sample seem less likely to occur, and the very low refractive index (nY found, 1.3311) probably causes more abrupt changes of refraction a t particle interfaces. The cost and availability of perfluorokerosine are also somewhat restrictive. It was concluded, therefore, that hexachlorobutadiene can advantageously replace perfluorokerosine for most mull purposes. As widespread usage of hexachlorobutadiene has already resulted from the earlier announcement ( I ) , it can now be recommended without reservation as a product whose general suitability has been proved by service. I t is an oily liquid having a boiling point of 215" a t 760 mm., and a refractive index (%Lo)of 1.5542 (6). Although the stability may not be as great as that of perfluorokerosine, it far exceeds the requirements. Fruhwirth (6)
reported that it behaves like a fully saturated compound; he could not polymerize it a t 100 atmospheres' pressure, nor would it react with chlorine in the sunlight, with diene reagents like maleic anhydride, or with acids and bases. Other evidence of its unusual stability was reported by McBee and Hatton (IO). The reagent used here was a commercially available product, of nkS 1.5524, with the boiling point listed as 210-212". This product did not need further purification and has shown no evidence of change in 3 years. Because solubility and other properties specific to the sample are not limiting factors by the mull technique, the single supplementing combination of hexachlorobutadiene and paraffin oil adequately covers the entire general need for media in the mull field. Without further concern over media selections, these may be used to obtain qualitatively useful spectra of nearly every type of dry powderable sample. Highly bonded substances may give spectra that are not as sharp as desired, but this is not due to any fault of the media. Since the intensity of the CH bands as revealed by hexachlorobutadiene often is of understandable significance as a quantitative standard, having these and one additional band that is common to both supplements gives a basis for evaluating all intensities relative to each other, despite loses due to scattering. Although the accuracy is low, such evaluations are useful for estimating if a band is intense enough to indicate a specific group of the principal ingredient, as distinguished from minor bands that may arise from contamination or from secondary effects of fundamental vibrations. When there is an unusual need to conserve the sample, the hexachlorobutadiene supplement can be done after the paraffin oil supplement on the same sample after the paraffin oil has been carefully flooded off with a volatile rinsing liquid like hexane. I4 Bonding patterns may be on a simpler basis in Oil and hexachlorobutadiene than in a paraffin oil medium, although such differences are of rare occurrence and have not been observed outside the 3-micron region. The most logical explanation seems to be that the hydrogen-containing oil has mild bonding properties, and the imbibing of this oil by the sample may force a change in weakly established bonding equilibria to give additional complexities. Although not observed, it seems reasonable to expect that imbibing an oil could influence the spectra of hydrates by displacing water in some instances.
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TETRACHLOROETHYLENE AS AN INFRARED SOLVENT
As reported earlier ( I ) , and evident from published spectra (3,8, If ), tetrachloroethylene can fill a critical need as an infrared solvent in the region from below 2 to 7.3 microns. It is possible to supplement completely the favorable region of carbon disulfide from 7.3 to beyond 15 microns. It seems evident, however, that the use of tetrachloroethylene has been hindered by troubles with impurities, and that there is a need to understand and take special precautions against these. On exposure to light, air, and moisture, the unpreserved material becomes oxidized, resulting in contamination with triehloroacetyl chloride, phosgene, trichloroacetic acid (5, ?), and presumably hydrochloric acid. Ordinarily, the access of air in storage is not sufficiently prevented by glass stoppers. After contamination, a simple distillation does not adequately remove trichloroacetyl chloride, as the boiling range is similar. Trichloroacetyl chloride may be detected both by its odor and by a prominent band a t 5.56 microns having a subsidiary peak a t 5.72. 1743
ANALYTICAL CHEMISTRY
1744 Commercially available tetrachloroethylene usually already contains a preservative, often about 0.5 to 1% alcohol (f2),although advantages have been reported for pyrroles (Q), and for thymol and other compounds ( 2 ) . The purity has been satisfactory for infrared purposes when small portions were distilled as needed from a stock preserved with alcohol in brown bottles, and the first portion distilled was discarded. It seems advisable also to discard the excess remaining after use instead of attempting to preserve it. The need for such attention evidently will greatly restrict the use of tetrachloroethylene until more convenient means of maintaining its purity have been established, but for many purposes the advantages of complete supplementation warrant the extra effort. LITERATURE CITED
(1) Ard, J..S., Second Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1951; . 4 ~ . 4 ~CHEW, . 23, 680
(1951) (-4bstract).
(2) Bailey, K. C., J . C h e w Soc., 1939,767. ( 3 ) Bernstein, H. J., J . Chem. Phys., 18, 478 (1950).
(4) Blout, E. R., and Nellors, R. C., Science, 110, 137 (1949). (5) Dickinson, R. G., and Leermakers, J. A., J . Am. Chem. Soc., 54, 3852 (1932). (6) Fruhwirth, O., Ber., 74,1700 (1941). (7) Huntress, E. H., “Preparation, Properties, Chemical Behavior, and Identification of Organic Chlorine Compounds,” pp. 692-700, New York, John Wiley & Sons, 1948. (8) Jones, R. N., and Lauson, R., “Selection of Solvents for Infrared Spectrometry,” Bull. 3, Sational Research Council, Ottawa, Ontario, Canada, 1953. (9) Klabunde, W., U. S. Patent 2,492,048 (Dec. 20, 1949). (10) McBee, E. T., and Hatton, R. E., I n d . E n g . Chem., 41, 809 (1949). (11) Pristera, F., A p p l . Spectroscopy, 6, S o . 3, 29 (1952) (12) “United States Pharmacopeia,” 14th Revision, pp. 614-15, Easton, Pa., Alack Publishing Co., 1950. RECEIVED for review .4pril25, 1953. Accepted J u l y 29, 1953. Presented in part before the Pittsburgh Conference on Analytical Chemistry and .4pplied Spectroscopy, March 5 to 7, 1981, and before the Chemical Society of Washington, D. C., M a y 10, 1951.
Ultraviolet Spectrophotometric Determination of Antimony as lodoantimonous Acid ANITA ELKIND, K. H. GAYER, AND D. F. BOLTZ V a y n e University, Detroit, Mich. study of the solubility of antimonous oxide it became necItimonous essary to determine accurately very low concentrations of anions. A preliminary study of several colorimetric N A
methods indicated that the iodoantimonous acid method offered advantages in respect to accuracy, precision, and sensithit! . This method is based on the formation of yellow iodoantimonous acid (HSbI4) when trivalent antimony in sulfuric acid solution is treated with an excess of potassium iodide solution (I). McChesney (3) studied this colorimetric method in reference to the effect of iodide concentration, acidity, and certain diverse ions and applied the method to the determination of antimony in biological materials. He made photometric measurements in the visible region a t 425 mp. Holler (2) used the iodoantimonous acid method to determine antimony in copper-base alloys using photometric measurements in the visible region, Nikitina (4)also used the iodoantimonous acid method in determining antimony in copper and tin alloys. The purpose of this investigation was to determine the ultraviolet absorption spectrum of iodoantimonous acid and to find whether it could be used as the basis for a more sensitive spectrophotometric method for the determination of very low concentrations of antimonous ions.
Experimental Procedure. The procedure given below was followed in determining the effect of various solution variables.
A definite amount of the standard antimony solution was transferred to a 50-ml. volumetric flask. The potassium iodide reagent solution was added, followed by the addition of the sulfuric acid reagent. I n the case of the diverse ion study, the diverse ions were added before the potassium iodide. The final volume
GENERAL EXPERIMENTAL WORK
Apparatus. A Beckman Model DU Bpectrophotometer with 1.00-cm. silica cells was used for all absorbancy measurements. A hydrogen discharge lamp was used for measurements in the 220to 400-mu region and a tungsten filament lamp was used for the 400- to 500-mp region. Solutions. A standard antimony solution was prepared by dissolving 0.2743 gram of KSbC4H,0?.’/2H,O in redistilled water, adding 160 ml. of concentrated sulfuric acid, and diluting to 1 liter with water. Each milliliter of this solution contains 0.100 mg. of antimony per ml. This concentration was checked spectrophotometrically using a standard solution prepared from pure antimony metal. Sulfuric acid reagent solutions were prepared by adding concentrated acid to distilled water and diluting to 1 liter. A potassium iodide solution was prepared by dissolving 140 grams of reagent grade potassium iodide and 10 grams of crystalline ascorbic acid in redistilled water and diluting to 1 liter. The purpose of the ascorbic acid is to reduce any trace of iodine whirh may be liberated by traces of oxidants, or by actinic rays.
Figure 1. .4bsorption Spectra for Iodoan timonous Acid 1. 2. 3. 4. 5.
3 p.p.m. 2 p.p.m. 1 p.p.m. 24 p.p.m. 12 p.p.m.
antimony antimony antimony antimony antimony
