stoichiometry. Consistent application is all that is necessary to both the standard samples and the unknowns.
Table 11.
Analysis of National Bureau of Standards Samples
Certified
Sample
Alloy 106.4" Cr, 510,A1 steel 162A Cupro-Si 164A Al-bronze 349 waspaloy 416A Kitralloy G 1189 Nimonic BOA 1191 Waspaloy 1192 W aspaloy Acid-soluble aluminum only. NO.
RESULTS
The results of the analyses of eight Xational Bureau of Standard samples are listed in Table 11. The reported values represent an average of five determinations for each standard. The average standard deviation is 0.010 within the concentration range investigated. The method is a t present used in our laboratory for the control determination of aluminum in cobaltand nickel-base alloys. ACKNOWLEDGMENT
4
the procedure. The permission of Sierra Metals Corp., Division of Martin Marietta Corp., to publish this paper is gratefully acknowledqed.
A V . (5 Detns.), yo 1 .os 0.492 9.61 1.24 1.08 1.20 1.55 1 .OS
Value,
%
1.os
0.50 9.59 1.23
1 .os
1.21 1.55 1.07
Std. Dev. 0,0023
0.0107 0.0283
0.0105 0.0016 0.015 0.0016 0.0112
F., Brecher, c., Arch. '14 (1932). (4) W-atts, H. L., ANAL. CHEM.30, 967 (1~~~8). (5) Watts, 13. L., Ctley, D. W., Ibid., 2 8 , 1731 (1956). (3) Viebock, 2709
LITERATURE CITED
The author is grateful to W. A. Fahlbusch and H. L. Watts for helpful criticism and assistance in evaluating
(1) Hale, M. S . ,IND.ENG.CHEU.,ANAL. ED. 18, 568 (1946). (2) Snyder, L. J., Ibid., 17, 37 (1945).
GEORGEA. BORUN Sierra Metals Corp. Wheeling, 111.
Determination of Nitrogen in Nitrocellulose by Infra red Spectrometry SIR: Nitrogen in nitrocellulose is usually determined by the du Pont nitrometer method (9) or by titration (10,If). The nitrometer method is accurate but the working time required per sample is considerable, and the expense and hazard of using the large amount of mercury are objectionable. The titration methods are less timeconsuming than the nitrometer method, but they are not quite as accurate and preparation and standardization of reagents are required. One of the most satisfactory of the titration methods is that of Pierson and Julian (If). The only investigator to report on the determination of nitrogen in nitrocellulose by infrared spectrometry was Kuhn (8) who worked with nitrocellulose films obtained by evaporating ethyl acetate solutions of nitrocellulose. He found that nitrogen could not be determined by measuring the absorbance a t the nitrate band a t 6.0 microns because the fdm showed almost complete absorption at that wavelength. He suggested that the determination could be performed by measuring the ratio of the hydroxyl band a t 3 microns to the carbon-hydrogen band a t 3.5 microns. Rosenberger and Shoemaker (IS) determined nitrocellulose in mixtures of cellulose resins by dissolving in acetone and measuring the absorbance a t the nitrate band a t 11.9 microns. This laboratory developed an infrared method for the determination of nitrogen in nitrocellulose by dissolution in
tetrahydrofuran and measurement of the absorbance a t the nitrate band at 6.0 microns. PROCEDURE
Rinse a 125-ml. Erlenmeyer flask (with a ground glass stopper) with acetone, dry a t 130' C., allow to cool for a half hour or more, and weigh to the nearest milligram. Place 0.31 to 0.32 gram of the sample, previously dried at 65' C. a t 2 to 5 cm. pressure for 4 hours, into an aluminum scoop-type balance pan, and weigh to the nearest 0.1 mg. Pour the sample into the flask, tap the pan, weigh again to the nearest 0.1 mg., and calculate the weight of the sample by difference. Add 45 ml. of high purity tetrahydrofuran using a 50-ml. tall type graduate; do not add the solvent around the sides of the flask as this will cause volatilization losses. Cover and allow to stand overnight in the room (constant temperature) containing the infrared instrument. Swirl the flask and weigh to the nearest milligram. Using a 1-cc. syringe pipet, rinse a 0.2-mm. cell once with tetrahydrofuran and three times with the solution of the sample, then fill it with the solution of the sample. Run the infrared spectrum from 5.80 to 6.05 microns, using the following settings on the Perkin-Elmer Model 21 spectrophotometer: resolution, 941; speed, 1; gain, 5; response, 2; suppression, 0; approximately 0.5 micron per minute. Calculate log ( I A / ~(Figure ) l ) , and convert this reading to milligrams of nitrogen per gram of solution by con-
sulting the calibration curve. Calculate the per cent nitrogen as follows: mg. of N per gram of solution x grams of solution %N in NC = grams of NC x 10
For the preparation of the calibration curve carry three or more samples of nitrocellulose of known nitrogen content through the procedure, calculate the nitrogen concentration (milligrams of nitrogen per gram of solution), and plot log ( I A / I ) against concentration, using regular graph paper. RESULTS AND DISCUSSION
The most feasible method for determining nitrogen in nitrocellulose by infrared spectrometry was to dissolve the sample in a solvent and to measure the absorbance a t one of the three strong bands due to the nitrate group. These bands occur a t 6.0, 7.8, and 11.9 microns (7,8, l a ) . For the proposed procedure the solvent had to show small absorbance a t the nitrate band and also dissolve nitrocellulose of high and low nitrogen content. Of the solvents that have been used for dissolving nitrocellulose [esters, acetone, methyl ethyl ketone, cyclohexanone, dioxane, methanol, nitrobenzene, nitroethane, tetrahydrofuran (3, 4), propylene oxide, pyridine, and a mixture of ethyl alcohol and ether] only acetone and tetrahydrofuran merited consideration for the problem a t hand. Some work was done on the use of VOL. 34, NO. 9, AUGUST 1962
1167
tetrahydrofuran or to dry it, Distilled tetrahydrofuran gave a spectrogram that did not differ significantly from that obtained with the undistilled solvent. High purity tetrahydrofuran contains 0.02% water (6). Satisfactory results could only be obtained by allowing the sample to dissolve in the tetrahydrofuran by standing overnight. The results obtained when the samples were dissolved by stirring with a magnetic stirrer \\ere erratic, probably because stirring causes depolymerization of nitrocellulose ( I ) .
80
60
40
20
Figure 1. Spectrum of nitrocellulose in tetrahydrofuran from 5.80 to 6.05 microns
acetone a t 11.9 microns but the results were only fair. The transmittance of acetone at 11.9 microns using a 0.2-mm. cell was about 74%. The spectrum of acetone indicated that that solvent could not be used at 6.0 or 7.8 microns. The only satisfactory solvent was tetrahydrofuran. The infrared spectrum of tetrahydrofuran using a 0.03mm. cell has been reported (I, 14); the spectrum obtained in this laboratory using a 0.2-mm. cell is shown in Figure 2. The transmittance of tetrahydrofuran at 6.0 microns was about 87% so the use of the nitrate band at this wavelength was feasible. It was not necessary to distill the
The concentration was controlled at approximately 0.78 gram of nitrocellulose per 100 grams of solution to obtain the best accuracy and maximum change in absorbance per milligram of nitrogen. This concentration gave a transmittance (nitrocellulose plus solvent) of about 18 t o 23%. Control of the temperature of the solution was important. This factor was controlled in this laboratory by allowing the solution to take place in the air conditioned room (20" C.) that housed the infrared instrument. A constant temperature bath probabIy could also be used. I n laboratories not possessing an air conditioned room for the infrared instrument or a constant temperature bath, a new curve could be drawn up with each batch of samples. As the working time per sample is very sinall, this would not be too objectionable. The method of calculation shown in Figure 1 amounted in effect to subtracting the absorbance at the top of the band from the absorbance a t the bottom of the band. The use of the transmittance scale is preferable to the absorbance scale because the former scale can be read more accurately than the absorbance scale in the region of 20% transmittance. A straight line calibration curve was obtained on carrying standard samples of nitrocellulose containing approxi-
Table I.
Results for Nitrogen in Nitrocellulose
Xi trogen Content by
Source Wood pulp
Wood pulp
Cotton linters
Cotton linters
Cotton linters
Nitrometer (%)
Nitrogen Content by Infrared
(7%)
12.28
12.28 12 17 12.24 12 29 12 31 Av . 12.26 Std. dev. 0 055
13.19
13.12 13.23 13.29 13 08 13.09 Av . 13.16 Std. dev. 0 . 0 9 3
3,27
13.32 13.27 13.15 13.06 13.28 AV. 13.22 Std. dev. 0 . 1 0 8 2.24 12.15 12,29 12.12 12.30 12,27 12.23 Av. Std. dev. 0,085 12.64 12.63 12.91 12.70 12.62 12.61 Av . 12.69 Std. dev. 0 . 1 2 6
mately 12 to 13.6% nitrogen through the recommended procedure. This range of nitrogen is the amount of nitrogen found in nitrocellulose used in ammunition and explosives. For 10.5 to 12% nitrogen, a different calibration curve might have to be prepared. This was not investigated. Nitrocellulose containing less than 10.5% nitrogen is of limited commercial importance be-
FREQUENCY (CM')
2
1
3
4
5
Figure 2.
1168
0
ANALYTICAL CHEMISTRY
6
7
8 9 WAVELENGTH (MICRONS)
IO
11
Infrared spectrum of tetrahydrofuran (0.2-mm. cell)
12
13
14
15
cause it is tiifficultly soluble in organic solvents (6). The results obtained by tlic proposed method for five samples of nitrocellulose that had been analyzed accurately by the nitrometer method by Picatinny Arsenal are shon-n in Table I. The results compared 1x11 with the results by the nitrometer method. The precision for the proposed method (average about i0.09yo) is satisfactor>- but is not as good as that obtainable by the nitrometer method in the hands of an experienced operator. The source of the nitrocellulose did not affect the results obtained by the infrared method. ACKNOWLEDGMENT
The authors are indebted to Kilmer White for his assistance and Samuel Sitelman for his suggestions. Also, they
thank Milton Roth and Hyman Jadon-ita of Picatinny Arsenal for furnishing accurate standards needed for this project. LITERATURE CITED
(1) Allen, P. W., “Techniques of Polymer Characterization,” pp. 11, 12, 15,
Butterworths, London, 1959.
( 2 ) Barrow, G. hi., Searles, S., J . Am. Chem. SOC.75, 1173 (1953). (3) Doolittle, A. K. “Technology of
Solvents and Plaiticizers,” p. 527, Wiley, New York, 1954. (4) du Pont de Semours, E. I., and Co., LLPropertiesand Uses of Tetrahydrofuran.” Wilmineton. Del.. 1960. (5) Fisher Scientih Co., “Fisher Chemical Index,” p. 280, New York, 1959. (6) Hercules Powder Co., “Nitrocellulose, Prouerties and Uses.” . _UP. - 1,. 2,. Wilminaton; Del., 1955. ( 7 ) Kagarise, R. E., Weinberger, L. A “Infrared Soectra of Plastics a d Resins,” 1L’a;al Res. Lab. Rept. 4369,
Xay 1954 (U. S. Dept. of Comrnerce ltept. No. 111438). (8) Kuhn, L. P., ANAL.CHEM.2 2 , 276 (1950). (9) Military Standard, Propellants: Sampling, Inspection, and Testing, MILSTD-286, p. 56, June 1956. (10) Mitchell, J., Kolthoff, I. AT., Proskauer, E. S Weissberger, A., “Organic Analysis,” $01. 2, p. 102, Interscience, New York, 1954. (11) Pierson, R. H., Julian, E. C., ANAL. CHEM.31,589 (1959). (12) Pristera, F., Halik, hl., Castelli,. A., Fredericks, W., ”Analysis of Explosives by Infrared Spectroscopy,” Picatinny Arsenal Tech. Rept. 2254, May 1956. (13) Rosenberger, H. M., Shoemaker, C. J., ASAI,. CHEM.31,1313 (1959). (14) Shreve, 0. D., Heethcr, M. R., Knight, H. B., Swern, D., Ibid., 23, 277 (1951). HARRYLEVITSKY GEORGE NOR-WITZ Pitman-Dunn Laboratories Frankford Arsenal Philadelphia 37, Pa.
Conductometric Determination of Sulfate by the Nonaqueous Barium Acetate Method SIR: A method for the determination of sulfate has been reported in which the sulfate is precipitated in an acetic acid medium by the addition of an excess of barium acetate [Goldstein, G., Menis, O., Manning, D. L., ANAL. CHEM. 33, 266 (196l)l. The excess barium acetate is then determined potentiometrically by titration with perchloric acid. Conductometric titration of the excess barium acetate has since been found to be satisfactory and to offer a n advantage over the potentiometric method in that water does not have to be excluded completely from the sample. A Leeds and Sorth-
0
L
1
2
3
4
5
6
V O L U M E OF 0 . 0 3 N HCI04, ml.
Figure 1 . Effect of water and acetic anhydride on conductometric titration with perchloric acid of barium acetate in acetic acid Conditionr;
Total volume of solution titrated, 50
ml.
Ba(CHaC0O)t present, 0.1 5 meq.
rup conductance bridge (Catalog Xo. 4866) and two platinum electrodes (2 X 2 cm. and fixed 1 cm. apart) were used in all titrations. Effects of Water and Acetic Anhydride. The results of t h e conductometric titrations of barium acet a t e with perchloric acid are shown in Figure 1. The titrations were conducted in anhydrous acetic acid and also in acetic acid solutions t h a t contained 2 vol. yo water or 10 vol. % acetic anhydride. The reaction before t h e end point is t h e neutralization of the barium acetate ion with the perchloric acid. If i t can be assumed that the degrees of dissociation of barium acetate and of barium perchlorate in anhydrous acetic acid are approximately the same, then the small initial increase in conductance is probably due to the greater ionic conductance of the perchlorate ion. A leveling off of conductance with increasing concentration is typical of weak electrolytes. Conductances are greater in acetic acid solutions that contain 2% water or 10% acetic anhydride than in anhydrous acetic acid, because of the increase in the dielectric constant. The slope of the titration curve in the region beyond the end point is greater in the presence of 2% water than when water is absent. This small volume per cent of water is therefore beneficial in the conductometric titration b u t is detrimental in the potentiometric method. However, when the concentration of water is as high as 10 vol. yo, the initial conductance is very high, and the end
point of the titration is not clcnrly defined. Effect of Barium Acetate. T h e titration curves of solutions t h a t contained 0.07 meq. of sulfate a n d various amounts of excess barium acetate are shown in Figure 2. In general, a rather wide range i n concentration of excess barium acetate can b e tolerated without effect on t h e shape of t h e conductometric titration curves. For optimum conditions, however, i t is desirable to adjust the
c , ; 0
]
I
,
_A_--
1 2 3 4 5 VOLUME O F 0 . 0 3 N HCIO,,
6 rnl
Figure 2. Effect of excess barium acetate on conductometric titration of SUIfate with perchloric acid Conditions: Total volume of solution titrated, 50 ml. Volume of 0.05N Ba(CH3COO)t used, as indicated Sulfate present, 0.07 meq. VOL. 3 4 , NO. 9, AUGUST 1962
1169
