Refined Calculation for Determination of Nitrogen in Nitrocellulose by Infrared Spectrometry
SIR: The results obtained by Levitsky and Xorwitz ( I ) for the nitrogen content of nitrocellulose determined by their infrared procedure show that the method does not compare with good nitrometer operation. However, our investigations show that by use of a more refined calculation the infrared method can approach the nitrometer in accuracy and reproducibility. The over-simplified calculation given by Levitsky and Sorrvitz introduces a systematic error because the slope of the plot of peak height absorbance against concentration of nitrogen in solution depends on whether the change in concentration is brought about by (a) a change in concentration of nitrocellulose of given nitrogen content or (b) a change in the nitrogen content of nitrocellulose at a fixed concentration. To obtain reliable results the two effects must be considered separately. These effects are illustrated in Figure 1, on which are plotted the absorbances (peak height) of solutions of two nitrocelluloses d (12.20y0 Sp)and B(13.l6yO N2) made up from sample lveights a t the limits of the range allowed in the method (0.30 to 0.31 gram), each dissolved in 50 ml. of tetrahydrofuran. The four points are far from colinear and a standard line cannot be drawn. The and BO.30-BO.31 can be lines d0.30-&.31 shown to pass through the origin and are expressions of Beer’s law for the two nitrocelluloses. Lines 21o.~o-Bo.~oand &.31-BO.31 relate the absorbances and nitrogen contents of the solutions only when the concentration of nitrocellulose in solution is fixed a t the appropriate level. The magnitude of the errors that may arke can be found by considering the apparent nitrogen content of nitrocellulose A if the sample weight taken is 0.31 gram while the standard line was prepared using 0.30-gram portions (do.30-Bo.30). The absorbance of the test solution will therefore be z (Figure 1) which, from the standard line, gives a corresponding reading of y mg. of N2/ gram of solution. This represents an apparent nitrogen content of 11.91%, a n error of 0.29Y0 absolute (or 2.5% relative). I n practice the error could be even larger than this as the amount of solvent taken is controlled only approximately (by measuring cylinder) and the concentration of the solutions formed may therefore vary more widely than the range of permitted sample weights suggests. 522
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ANALYTICAL CHEMISTRY
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0.54 Y
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Figure 1 . Plots of absorbance vs. nitrogen concentration for solutions containing 0.30 and 0.3 1 grams of two nitrocellulose samples (1 2.20 and 13.15% N2)
The reason for the difference in effect of nitrocellulose concentration and nitrogen content of the nitrocellulose probably lies in the use of peak heights to measure absorbance, as the shape of the nitrate band changes with variation in the nitrogen content of the nitrocellulose. However, measurement of peak area is inconvenient. Close control of the concentration of the nitrocellulose solution would eliminate the error, but would not be practicable. An improved method of calculation of the results obtained by the existing procedure is therefore proposed. As nitrocellulose solutions under the conditions of test obey Beer’s law-i.e., absorbance is proportional to concentration-all observed absorbances are first corrected to a selected nominal concentration. The corrected absorbance of a test solution is compared with a standard curve prepared from the corrected absorbances of the range of standard nitrocellulose solutions. The calculation is as follows. If concentration of nitrocellulose = C, gram/gram of solution and absorbance (peak height) = A I , then -4, = A I X CJC, where A , = calculated ab-
sorbance of solution of the same nitrocellulose at concentration C, and C, = selected nominal concentration (conveniently 0.008 gram of nitrocellulose per gram of solution). Preparation of Standard Curve. T h e values of A , for nitrocelluloses of different nitrogen contents are plotted directly against t h e nitrogen contents of the nitrocellulose. Determination of Nitrogen Content of Nitrocellulose Sample. T h e nitrogen content of the sample is read from t h e standard curve, corresponding t o t h e corrected absorbance A , calculated as above. A standard deviation of 0.03% is obtainable using this improved method. This compares favorably with that obtained by experienced operators using the nitrometer method. LITERATURE CITED
(1) Levitsky, H., Norwitz, G., AUL. CHEM.34, 1167 (1962). ALBERT CLARKSOX C. hI. ROBERTSOX
PII.G.O.I.,
Chemical Inspectorate,
Woolwich, London, S.E. 18
