(13) E. C. Horning and W. J. A. VandenHeuvel. Adv. Chromatogr., 1, 161 (1965). (14) E. C. Horning, "Gas Phase Chromatography of Steroids", K. E. Eik-Nes and E. C. Horning, Ed., Springer-Verlag,New York. 1968, p 1. (15) A. Vermeuien. Clin. Chim. Acta, 34, 223 (1971). (16) A. Hiscoe, D. W. Mathieson, and R. H. Perrett, J. Chromatogr., 81, 144 (1973). (17) A. Kuksis, Fette, Seifen, Anstrichm., 75, 420 (1973). (18) J. D. Baty and A. P. Wade, Anal. Biochem., 57, 27 (1974). (19) E. Bailey, M. Fenoughty and J. R. Chapman, J. Chromatogr., 96, 33
(23) D. Kritchevsky, "Comprehensive Biochemistry", M. Florkin and E. H. Stotz, Ed., Vol. 10, Eisevier, New York, 1963, p 1. (24) L. F. Fieser and M. Fieser. "Steroids", Reinhold. New York, 1959, Chap. 1. (25) J. R. Chapman and E. Bailey, Anal. Chem., 45, 1636 (1973). (26) E. M. Chambaz, G. Defaye, and C. Madani, Anal. Chem.. 45, 1090 (1973). (27) G. M. Janini. G. M. Muschik, and W. L. Zielinski, Jr., personal mrnmunication, 1975.
(1 974).
(20) D. N. Kirk and P. M. Shaw, J. Chem. SOC.C, 1971, 3979. (21) G. M. Janini, K. Johnston, and W. L. Zielinski, Jr.. Anal. Chem., 47, 670 (1975). (22) IUPAC Commission on the Nomenclature of Organic Chemistry and IUPAC-IUB Commission of Biochemical Nomenclature, J. Steroid Bio-
chem., 1, 143 (1970).
RECEIVEDfor review September 29, 1975. Accepted January 5, 1976. This study was sponsored by the National Cancer Institute under Contract No. N01-CO-25423 with' Litton Bionetics, Inc.
Determination of Trace Amounts of Diethylene Glycol in Nitroglycerin by Infrared Spectrometry A. S. Tompa Research and Technology Department, Naval Surface Weapons Center, White Oak Laboratory, Silver Spring, Md. 209 10
Diethylene glycol concentrations of 0.01 to 0.20% in nitroglycerin were determined to within 0.005 YO by infrared spectrometry using the 1122 cm-' C-0 absorption band.
Nitroglycerin (NG) is a powerful and very shock-sensitive explosive plasticizer which is used with nitrocellulose in double-base propellants. Accordingly, the shipment of NG is required by Interstate Commerce Commission regulations to be desensitized by the addition of 25% diethylene glycol (DEG). However, before NG can be used in propellant formulations, the DEG must be removed because it has deleterious effects on propellant properties. NG is therefore water-washed three times in order t o lower the DEG concentration to below 0.02%. The determination of DEG in water-washed NG has been attempted by dichromate oxidation ( 1 ) and near-infrared spectrometry (2). The oxidation method is time consuming (requires several extractions and a two-hour digestion period) and is nonselective as any organic impurities present in the extract are also oxidized. The near-infrared procedure is a pass or fail method based on a weak shoulder band a t 2.92 Mm. Any residual water must be corrected for by a Karl Fischer titration. The correction factor is usually of a greater magnitude than the DEG concentration and, hence a serious source of error. Therefore, alternate methods of analysis which are quick, safe, and accurate are highly desirable. Two promising methods ( 3 ) developed in this laboratory are gas chromatography and infrared spectrometry. The gas chromatography method is based on Trowell's ( 4 ) method for analysis of aged double-base propellants.
EXPERIMENTAL Apparatus. The infrared spectra were recorded on a PerkinElmer Model 521 spectrophotometer. The spectra were obtained from 1170 to 1090 cm-' using a 2X slit program and a scanning speed of 30 cm-'/min. The mechanical slit opening was 541 Fm a t 1122 cm-'. The infrared cells had pathlenghs of 1.0 and 0.10 mm. Reagents. Reagent grade DEG and chloroform were used. T h e water-washed NG was at least 99.7% pure and was either a clear solution or slightly cloudy because of the presence of trace amounts of water.
Procedure. Three procedures were investigated. The first method used neat NG as the solvent. The second was a modification of the first by using the method of additions. The third used a solution of NG in chloroform. In the first method, 0.50 to 15.0 mg of DEG was added directly to 1 to 4.000 g of NG to give 0.01 to 1.20% solutions of DEG in NG. T h e vials containing the solutions were gently swirled to ensure thorough mixing. The solutions were then run in a 0.1-mm cell with neat NG in a 0.1-mm reference cell. The cell thicknesses should be sufficiently matched so that neat NG in both the sample and reference cells would give a straight line in the 1170 t o 1090 cm-' region. If the sample contains 0.01% or less of DEG in NG, then 5X scale expansion should be used t o increase the precision of the measurement. The second method involves adding a known amount of DEG to the sample and running the original and spiked samples in the same infrared cell. Generally, the spiked samples would contain 0.01 t o 0.05% DEG in NG. In the third method, solutions of 0.001 to 0.02% DEG in chloroform containing 8.00 to 10.00% NG were prepared. Two to 10 mg of DEG were added to the 10-ml volumetric flasks and filled to the mark with chloroform. Aliquots of 1 t o 2 ml of these solutions were added to 10-ml volumetric flasks with plastic stoppers. Chloroform was added to approximately half-fill the flask, the flask weighed, and then 0.8 to 1.000 g of NG was added. The flask was reweighed and then diluted to the mark with chloroform. A standard reference solution of 10.00% NG in chloroform was prepared. Aliquots of this solution were diluted with chloroform t o match the NG concentration in the sample solution. The chloroform solutions are run in 1.0-mm cells. Chloroform in a 1.0-mm cell has a strong absorption band in the 1100 to 1000 cm-' region. It was observed that the absorption band a t 1122 cm-' was more intense and symmetrical when the mechanical slit width was doubled. The 10 baseline was taken as the horizontal portion of the spectrum between 1170 to 1140 cm-'. I t was found using chloroform solutions in order to get good quality spectra suitable for quantitative analysis, the product of the NG concentration and cell thickness in the reference cell must be a t least 95% of the product of the NG concentration and cell thickness in the sample cell. On some occasions, the nitroglycerin sample will contain 0.1% of 2-nitrodiphenylamine which acts as a stabilizer. I t imparts a characteristic red color to the sample. The stabilizer has a sharp band a t 1150 cm-' of medium intensity in a 0.1-mm cell a t the 0.1% level. It does not interfere with the DEG determination; however, better precision is obtained if the method of addition is used.
RESULTS AND DISCUSSION The infrared spectra of DEG and NG in the 4000 to 400 cm-' region showed that the 1122 cm-l absorption band of ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
911
chloroform solution method is that a person does not have to handle neat NG. The disadvantages are twofold; namely, it is more time-consuming and not as accurate. Its accuracy is estimated to be f0.01% DEG in NG. This accuracy is obtained if the product of the NG concentration and pathlength in the reference cell is within 95% of the product of the NG concentration and pathlength in the sample cell. If the product is much greater, then lower values of DEG would be found, and, conversely, if it is much less, then higher values of DEG would be found. Using neat NG, the accuracy is estimated to be f0.005% DEG in NG. The method of additions is preferred because it eliminates any changes in the pathlength of the cell and any changes in the instrument performance since the calibration data were obtained. The following calculation applies to data obtained using the method of additions
Frequency (cm-’)
Figure 1. Infrared spectra of DEG and NG ( A ) Films of DEG and NG. (B) (1) 0.044%; (2) 0.17%; (3) 0.42%; (4) 1.19% DEG in NG vs. NG in 0.1-mm cells. (C)(1) 0.0016%; (2) 0.0050%; (3) 0.0063%; (4) 0.0070%; (5)0.0098%; (6) 0.016% DEG in CHCI:, containing 8 to 10% NG vs. 8 to 10% NG in CHC13 in 1.0-rnm cells
DEG due to the C-0 stretching mode of the COH group may be used for quantitative analysis. It has the advantage over the OH stretching band a t 3600 cm-l in that trace amounts of water do not interfere in its determination. Figure 1A shows the infrared spectra in the 1200 to 950 cm-l region of films of DEG and NG. In order to avoid the interference of the weak shoulder band in NG in the 1130 to 1120 cm-I region in the determination of DEG, NG must be added to the reference cell to provide compensation. I t is seen in Figures 1B and 1C that a more symmetrical band shape is obtained in neat NG and that the detection limit could easily be extended to 0.005% DEG in NG using 5X scale expansion. Straight line calibration plots passing through the origin were obtained from the curves shown in Figures 1B and 1C. From the slope of these plots, the following relationships were obtained: % DEG =
0.385 Allzz/mm
% DEG = 0.20 Allzz/rnrn
(1) (2)
where AllZz/rnrn is the absorbance of the 1122 cm-’ band per mm of cell thickness in neat NG and in CHC13 containing 8 to 10% NG, respectively. The main advantage of the
%DEG=-
AzCi Ai -Az
(3)
where A1 is the absorbance a t 1122 cm-l of the spiked sample, A2 is the absorbance a t 1122 cm-l of the unknown sample, and C1 is the % DEG added to the unknown sample. These methods with emphasis on neat NG have been applied successfully to the analysis of several production lots of NG. ACKNOWLEDGMENT The author thanks Wilbur W. DeAtley for helpful discussion. LITERATURE CITED (1) C. L. Whitrnan, G. W. Roecker. and C. F. McNerney. Anal. Chem., 33, 781 (1961). (2) W. H. Jones, Indian Head, Tech. Rep. 279, Naval Ordnance Station, Indian Head, Md.. Dec. 1968. (3) A. S. Tornpa and W. W. DeAtley. Indian Head, Tech. Rep. 358, Naval Ordnance Station, Indian Head, Md. (4) J. M. Trowell. Anal. Chem., 42, 1440 (1970).
RECEIVEDfor review January 13, 1975. Accepted December 10, 1975. The opinions or assertions made in this paper are those of the author and are not to be construed as official or reflecting the views of the Department of the Navy or the Naval Service a t large.
I CORRESPONDENCE Effect of Reagent Impurity on Chromotropic Acid Method for Determination of Chloride Sir: In a previous publication, we described our new automated colorimetric method for the determination of chloride using chromotropic acid ( I ) . This method has been used to analyze various natural waters. With continued use of the method, our original stock of monosodium salt of chromotropic acid, obtained from the Eastman Chemical Company, Catalogue No. P-230, was depleted and, upon reordering from the same company, it was found that the monosodium salt had been replaced with disodium salt purchased under the same catalogue number. The new reagent had completely different characteristics and did not give the same results as the earlier stock. Upon detailed in912
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
vestigation, it was found that the majority of the stocks obtained from various companies contained varying amounts of chloride as impurity and resulted in varying high backgrounds and different results. In the present communication, the effect of chloride impurity in chromotropic acid reagents from various manufacturers is discussed. Some modifications are also described to obtain reproducible and accurate results, using these reagents. EXPERIMENTAL Apparatus. Standard Technicon AutoAnalyzer modules a n d
