greater the accuracy. The solubility is such, however, that the maximum concentration attainable at room temperature was 40 mg. per 100 ml. of solution. The results obtained with synthetic mixtures containing known amounts of R D X and HR4X are shown in Table I. The grand average of the results obtained on six synthetics, each analyzed in triplicate, was 99.94y0 recovery with a standard deviation of 0.48%. The R D X and Hh4X used to prepare the synthetic mixtures were purified by recrystallizing small batches two times from acetone. The melting points were in agreement with the published values. Results obtained on plant manufactured samples are s h o m in Table 11. The method has
been very useful in controlling the quality of the manufactured products. LITERATURE CITED
(1) Bandelin, F. J., Pankratz, R. E., ANAL.CHEM.30, 1435 (1958). ( 2 ) Connor, R., “Final Report of the
Analysis of RDX/HRIX Rliatures,” O.S.R.D., California Institute of Technology Report. 1711 (July 1, 1943). (3) Davis, T. L., “Chemistry of Powder and Explosives,” p. 398, Wiley, Sew York, 1943. (4) Feigl, Fritz, “Qualitative hnalysis by Spot Test,” 2nd ed., p. 210, Nordemann Publ. Co., Sew York, 1939. (5) Laccetti, Mario, Semel, Stanley, Roth, Milton, Ax.4~.CHEY.31, 1049 (1959). ( G I Los Alamos Scientific Laboratorv. proposed purchase description for Typl X, HRIX, Feb. 11, 1938. ( 7 ) Malmberg, E. W., Trueblood, K. N., Kaugh, T. D., ANAL. CHEM.25, 901 (1953). \
,
(8) Manno, R. P., Matsuguma, H. J., “Quantitative Analysis of HMX/RDX hIixtures of Infrared Spectrophotometry,” Picatinny iirsenal Tech. Rept. 2554 (Confidential) (November 1958). (9) Office of Scientific Research and Development, California Institute of Technology, Rept. 5943 (Nov. 13, 1943). (10) Sirotkin, G. D., Starostin, V. V., J . A p p l . Chem. U.S.S.R. 27,1081 (1954) (English trans. by Consultants Bureau Inc., New York). (11) Swann. & H.. I. ildams. \I. L.. . 4 x a ~ .
Determination of RDI fense Corp., Kingsport, Tenn., July 6, 1953. RECEIVED for review September 26, 1958. Accepted January 19, 1959. Presented in part, Division of Analytical Chemistry, hleeting-in-~liniature, North Jersey Section, ACS, January 1958.
Infrared Analysis of Isomeric Dicyanobenzene Mixtures NINA HADDEN and W. F. HAMNER Monsanto Chemical Co.,Texas City, rex.
,An infrared analysis for the distribution of isomers in mixtures of the dicyanobenzenes has been developed. Because no suitable solvent could b e found for these solids, the potassium bromide disk technique was used. A normalization procedure eliminated the necessity for accurate control of sample size. The analytical wave lengths selected were 12.90, 12.35, and 11.80 microns for 0 - , m-, and p-dicyanobenzene, respectively.
T
pressed disk technique (11-18) has become widely used in the preparation of solid samples for qualitative infrared analysis. It produces a uniform distribution of very small sample particles in a transparent suspending medium, usually potassium bromide or chloride. Reproducible spectra free of interfering bands and with low scattering losses are obtained. These advantages make the pressed disk technique particularly adaptable to the quantitative analysis of insoluble materials. Kirkland (9) has discussed general procedures for the preparation of samples for quantitative analysis using the disk technique. Bonhomme (Z), Duyckaerts (6, I O ) , and Jones (8) have reported on the relationship between particle size and absorbance of the material incorporated in the disk. Others (3, 4, 7 ) have discussed the use of the disk technique for specific analyses. This paper describes the application of the pressed disk technique to a rapid determination HE
1052
ANALYTICAL CHEMISTRY
of isomer distribution in niiutures of 0-, m-,and p-dicyanobenzene. Precise measurement of sample size was eliminated through normalization of the rpsults. EQUIPMENT
An evacuable die similar to the one described by Ford (6) was used to prepare disks 19 mm. in diameter. It v a s made of hardened tool steel with the die casing in one piece. S o difficulty was encountered in removing the disks from the die nithout breakage. A C a r w r laboratory press of 10-ton capacity wis used for pressing the disks. A Perkin-Elmer Model 21 spectrophotometer equipped with sodium chloride optics was employed to obtain the spectra. Slit widths for quantitative measurements were 0.285 mm. a t 11.5 microns and 0.490 mm. a t 13.5 microns with a scanning speed of 1 micron per minute. MATERIALS
Potassium bromide. Pondered optical grade material was used (Harshan Chemical Co.). The salt ground to pass a 200-mesh screen and dried for several days at 125’ C. was also satisfactory (Baker Analytical reagent, potassium bromide). o-Dicyanobenzene was prepared by converting phthalic anhydride to phthalic diamide, which was dehydrated to the dicyanobenzene (1). m-Dicyanobenzene n-as prepared from m-phenylenediamine. p-Dicyanobenzene. The compound was recrystallized from ethyl alcohol (Eastman Kodak, White Label).
EXPERIMENTAL PROCEDURE
Disks of each pure isomer were prepared by combining 0.005 gram of the isomer with 1.6 grams of potassium bromide. This mixture was ground for 5 minutes in a mechanical vibratorgrinder containing small steel balls. This was the time required for the particular system to give a constant and maximum absorbance for the pure compounds a t their analytical m-ave lengths. The grinding conditions vary with size of sample, type of grinder and matrix used and must be determined for a given system. For example, a smaller size sample in a Rig-L-Bug grinder requirw only a few seconds. A 0.400-gram aliquot was transferred to the die, which was assembled and placed in the press. After evacuation for 1 minute, the full force of 10 tons was applied to the plunger for 5 minutes to form the disks. Absorptivities of each isomer a t the analytical wave lengths were determined from thesc disks and used in setting up the calibretion equations. Synthetic mixtures for calibration checks were prepared by mixing knon n amounts of each monomer and grinding, or by dissolving known amounts in acetone and alloving the solution to eraporate to dryness. For analysis, 0.005 gram of a sample was added to 0.800 gram of potassium bromide and mixed for 5 minutes in th(A vibrator; a 0.400-gram portion of this was taken for the disk. After the 0 and looyo transmittance levels had been checked, disks were scanned from 11 to 14 microns v-ith a pure potassium bromide disk in the reference beam.
DISCUSSION AND RESULTS
The spectra of the dicyanobenzenes are shown in Figure 1. The absorption bands a t 12.90, 12.35, and 11.80 microns were chosen as analytical wave lengths for 0-, m-, and p-dicyanobenzene, respectively. The intensity of the energy through the disks was measured at a reference wave length of 12.10 microns characterized by low absorbance for all cornponents. A base line drawn from 11..5 to 13.5 microns may be used, instead of a reference ITave length; it leads to better accuracy for samples con-
taining small amounts of o-dicyanobenzene. The accuracy of the method depends largely upon the accuracy of the absorptivities, for the pure compounds, and thus great care must be taken in the preparation of the disks of the individual isomers for calibration. The ratio of the weight of isomer to potassium bromide and the total weight of the mixture used for calibration should be kept constant. The absorbances a t the analytical wave lengths of these disks should be corrected for variations in thickness. Because the concentration of the pure
Table
I
Dicyaiiobenzrne
Iinoi\ n
0-
0 54 2 45 8
in -
7'-
I.
Results on Synthetics
I1 Found 0 5ti a 43 2
isomer in the potassium bromide is constant for all three isomers, the absorbance is proportional to the thickness of the disk. The thickness is not identical for each of the standards, because of variations in pressing. The average thickness is determined by measuring over the face of the disk with a micrometer. Then the absorbance of each of the disks is adjusted to a constant thickness. I n our particular case the adjusted thickness was 0.021 inch. The corrected absorbances may be used directly in the calibration equations to determine the relative concentrations
I11 I
