Rapid Determination of the Nitrogen Content of Cellulose Nitrate and Other Nitrate Esters by Means of a Modified Devarda Method J. G. M. M. Srneenk’ Technological Laboratory, the Netherlands Organization for Applied Scientific Research TNO, Rijswijk (2. H. ) , The Netherlands
The nitrogen content of cellulose nitrate (CN) is commonly determined by the Lunge, the Schlosing, or the Devarda method. However, these methods are slow and very sensitive to environmental conditions (atmospheric pressure, temperature, solubility, and vapor pressure factors). In practice this means that exact and reproducible results can only be produced by a specialized, skilled analyst. At the Technological Laboratory of the Netherlands Organization for Applied Scientific Research, a rapid method of determining the nitrogen content of cellulose nitrate was needed. This method should be at least as accurate as the Schliising method, and less dependent on the specialization and skill of the analyst. These requirements were met by a modified Devarda method. In its classical way of operation (1-3) the Devarda method is rather time-consuming. Some time ago Potrafka et al. described an apparatus in which Devarda analyses of inorganic nitrate could be completed in a few minutes ( 4 ) . For the analysis of organic nitrates, this time gain is almost totally offset by the slow conversion of the nitrate ester into a n inorganic nitrate in alkaline medium, which must necessarily precede the Devarda reaction. For CN, this hydrolysis can take as much as 1.5 hr. In addition, this cqnversion is not always quantitative, as is the case with pentaerythrite tetranitrate (PETN). Vinsson et al. (5) showed that certain stable esters hydrolyze very rapidly in a medium containing dimethylsulfoxide (DMSO), water, and alkali. Combination of this rapid hydrolysis technique with the apparatus described by Potrafka made possible a rapid and accurate determination of the nitrogen content of CN. In addition, the method is also practicable for the analysis of other nitrate esters such as glyceryl trinitrate (GN) and PETN. EXPERIMENTAL Apparatus. The apparatus is similar to the apparatus described by Potrafka. Over the titration vessel, a suction pipe has been installed to remove vapors of DMSO. Reagents. The reagents used, except CN, GN, and PETN, were of analytical reagent quality and were used without further purification. All solutions were made using deionized water if not otherwise specified. Hydrogen peroxide was used as a 20% solution and sodium hydroxide as a 30% solution. The indicator solution was prepared by mixing 1 part of Methyl Red solution (0.1%) and 9 parts of Bromocresol Green (0.1%). Procedure. Samples containing GN or PETN are dried under vacuum at 30 “C for 2 hr and stored in a vacuum desiccator. After quickly weighing approximately 100 mg of CN or 80 mg of PETN in the reaction vessel, the sample is “wetted” with a few drops of ethanol. It is important that the sample does not contain coarse particles as this will result in low recoveries. Present address, Laboratorium Drinkwaterleiding Amsterdam, 73, Leidse Vaartweg, Heemstede, The Netherlands. (1) P. Howard, Chem. lnd. (London).1963, 1031. ( 2 ) M. M. Koehler et a / . ,Ann. Chim. Anal. Appi. lnd., 1 8 , 4 5 (1913). 1 8 , 45 (1913). (3) Y. Lacroixef a / . , MBm. Poudres, 39, 459-68 (1957). (4) K. A . Potrafkaetal., Anal. Chim. Acta, 31, 126-38 (1964). (5) J. A. Vinssonefal., Talanta, 13, 1673-77 (1966).
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9
Samples of approximately 80 mg of GN are dissolved in 3 ml of methylene chloride. Five millimeters of DMSO are now added to the sample, which dissolves rapidly. Very carefully, 1 ml of 20% hydrogen peroxide is added; then the solution starts to generate heat. After adding 5 drops of 30% sodium hydroxide, the hydrolysis proceeds rapidly. As a result of the reaction with DMSO, the hydrogen peroxide is readily consumed, and therefore a t regular intervals a few drops of hydrogen peroxide are carefully added to the solution. In this way, approximately 2 ml of 20% hydrogen peroxide are added. Thus a clear solution is obtained from which ethanol and methylene chloride (if present) have evaporated. As heat is generated, the solution starts boiling and, as a consequence, the last traces of hydrogen peroxide are removed. The solution is cooled to room temperature and approximately 1ml of 30%sodium hydroxide is added. Five drops of silicon oil and 1 gram of Devarda alloy are added and the reaction vessel is inserted in the apparatus. The apparatus is now ready for the titration of the distilling ammonia. In the same way, a blank is carried out. The normality of the titrant (approximately 0.05N H2SOr) can be determined by repeating this procedure with pure, dried, potassium nitrate.
RESULTS AND DISCUSSION Cellulose Nitrate. The quality of this procedure was checked by means of three samples of CN, whose nitrogen content was also determined by the Schlosing method (6), modified by Schulze and Tiemann (7) and by the Dumas method. PETN was used as a test compound. The results of the nitrogen determinations in the CN samples and the PETN sample by the Schlosing method and the Dumas method are presented in Table 111. The results of the Devarda analyses are listed in Table I. In general, the conversion of the nitrate ester into inorganic nitrate is a critical step in the analysis. It may be the cause of too low a nitrogen content. For instance, during an acid hydrolysis, evolution of nitric oxide is possible and, during an alkaline hydrolysis, reduction of the nitrate ester to nitric oxide, nitrogen, and ammonia should be prevented. According to Howard (1) this reduction can be prevented by adding hydrogen peroxide during alkaline hydrolysis. It is clear from Table I that the results of the Devarda analysis are somewhat lower than the results of the Schlosing method. This is not specific for the application of DMSO in the hydrolysis medium. The classical Devarda method, in which aqueous alkali and hydrogen peroxide are used, shows results which do not significantly differ from the results, found with the application of DMSO, if the same CN samples are analyzed. This may be seen from Table II. The occurrence of a similar systematic error was confirmed by the results of the analysis of PETN. For this compound, the modified Devarda method gave a nitrogen content of 17.60%, whereas the Dumas analysis of this (6) M. Schlosing. Justus Liebigs Ann. Chem., 40 ( 3 ) , 479 (1853). (7) M. Tiemann, Ber. Deut. Chem. Ges., 6, 1034 (1873).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
Table I. Results of Nitrogen Determinations in 3 Cellulose Nitrate Samples by the Modified Devarda Method, Using DMSO in the Conversion Reaction Nitrate ester
Mean Absolute stnd dev Schlosing method
Table 111. Results of Nitrogen Determinations in Samples CN and PETN by the Dumas Method, the Schlosing Method, and the Modified Devarda Method
CN Sample A, CN Sample B, CN Sample C, %N % N % N
13.50 13.45 13.43 13.47 13.45 13.39 13.43 13.39 13.43 13.39 13.43 0.04 13.53
12.84 12.84 12.79 12.79 12.76 12.79 12.80 12.81 12.81
11.93 11.92 11.95 11.90 11.88 11.90 11.90 11.94 11.94
12.80 0.03 12.95
11.92 0.02 12.02
Nitrate ester
Schltking method, % N
CN Sample A CN Sample B CN Sample C PETN
Dumas method, % N
Modified Devarda method ,(corrected for hydrolysis error) ,% N
13.53= 12.87b 12.03d 17.70"
13 .516 12.871 11.99J 17.70d
13.53a 1 2 .955 12.02a
. . .c
Mean value of two determinations. Mean value of five determinations. Value not determined. Mean value of four determinations. e Mean value of ten determinations. Mean value of nine determinations.
'
Table IV. Results of the Analysis of GN and PETN Samples by the Modified Devarda Method Table 11. Results of Nitrogen Determination in 3 CN Samples by the Devarda Method, without DMSO in the Conversion Reaction Nitrate ester
Mean Absolute std dev Modified Devarda method a
CN Sample A, CN Sample B, CN Sample C, % N %N % N
13.46 13.44 13.42 13.48 13.42 13.46 13.41 13.42 13.45 13.45 13.46 13.45 13.44 0.04
12.81 12.78 12.81 12.82 12.81 12.84
11.95 11.97 11.91 11.91 11.89
12.81
. . .a
11.93 . . .,l
13.43
12.80
11.92
Value not calculated.
compound gave 17.70%. This indicates that the application of hydrogen peroxide, which according to Howard should prevent loss of nitrogen-containing compounds, does not totally exclude these losses. This error did not originate from imperfect performance of the apparatus as could be proved by comparing the normalities of the titrant found by acidimetry and analyzing pure samples of potassium nitrate. So it seemed necessary to use a correction factor, indicating the ratio between the Dumas and Devarda value of PETN. Table I11 shows the Devarda results which are corrected in this way. As may be seen, the agreement between the Schlosing method, the modified Devarda method, and the Dumas method is satisfactory. Glyceryl Trinitrate and Pentaerythrite Tetranitrate. The results of these analyses are presented in Table IV. As in practice GN often occurs in combination with 2-nitrodiphenylamine and sometimes with diphenylamine, we also checked the influence of these compounds on the GN analysis. From Table IV, it is evident that 2-nitrodiphenylamine interferes to some extent with the GN determinations. As mentioned earlier, the conversion of the nitrate esters into sodium nitrate is accompanied by a small
Recovery Sample, Nitrate ester
mg
Glyceryl trinitrate
Glyceryl trinitrate (+12.7 mg diphenylamine) Glyceryl trinitrate ($3.6 mg 2-nitrodiphenylamine Glyceryl trinitrate (+9.7 mg 2-nitrodiphenylamine PETN
Uncorrected, Corrected, mg mg
45.9 45.9 100.2 69.0
45.4 45.3 99.5 68.3
45.7 45.6 100.1 68.6
48.3
47.7
48 .O
63.0
61.7
62 . O
60.2 110.1 80.1 78.0 81.2
59.1 109.7 79.5 77.3 80.6
59.5 110.3 80 .O 77.7 81.1
systematic error, as was evident from the results of the analysis of PETN. This error is ca. 0.4% (relative). By using pure PETN as a standard, it is possible to apply a correction factor for this systematic error in the GN determination in the same way as for CN.
CONCLUSIONS The procedure described is accurate (mean absolute standard deviation 10.03%). The exactness of the nitrogen determinations in CN is a little difficult to judge, because one has to make a comparison with the results of another method of nitrogen determination. From Table 111, it is clear that the Dumas results can be used as a reference as appears from the results obtained with this method for PETN. As may be seen from Table 111, the agreement between the corrected results of the modified Devarda method and the Dumas method is satisfactory. The application of DMSO in the hydrolysis medium combines well with the Devarda method and is especially convenient for nitrate esters such as PETN and CN which hydrolyze slowly in aqueous alkaline medium. The conversion of the samples into inorganic nitrate is completed in ca. 10-15 minutes. The following Devarda reaction and the titration of the distilling ammonia take ca. 15-20 minutes. If more than one sample has to be analyzed, the hydrolysis step can easily be performed for a series of samples.
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The combination of a rapid hydrolysis of the nitrate ester with the Devarda reaction and a simple titration of the distilling ammonia has resulted in a rapid method, which is less sensitive to environmental influence and "operator experience" than the gas-volumetric techniques such as the Lunge method and the Schlosing method. It is pointed out that the application of an automatic titration of the distilling ammonia could further reduce the possibility of human errors.
ACKNOWLEDGMENT The technical assistance of W. R. Urban is gratefully acknowledged. Thanks are also due to the department of analytical chemistry of the Technical University Twente for performing the Dumas analyses. Received for review April 6, 1973. Accepted September 4, 1973.
Consecutive Titration of Calcium and Magnesium in Ethanol-Water Mixture Bo Wallen Department of Analytical Chemistry, University of Uppsala, S-751 21 Uppsala 1, Sweden
Recently Rorabacher et al. ( 1 ) suggested that the result of an analytical procedure normally carried out in water might be improved if performed in a mixed solvent. This suggestion was based on the results of a determination of the stability constants of some metal ions with polyamines and polyaminopolycarboxylic acids in water-methano1 mixtures. The values of the stability constants generally increased with the methanol content of the medium. End-point determinations in complexometric titrations may hence be improved in mixed solvents [See also ( 2 ) and ( 3 ) ] .It was also observed that the changes in the stabilities of the complexes varied from one metal ion to another, a fact that might be used to increase the selectivity ofa titration. Some years ago a method based on these effects was developed in this laboratory for the simultaneous determination of calcium and magnesium. It will now be briefly presented as an example of what can be achieved by changing the medium. With a mercury indicator electrode, two potential breaks are obtained when a mixture of calcium and magnesium is titrated with ethylene glycol bis-(Pamhoethylether)-N,N-tetraacetic acid (EGTA) a t pH 10 in an aqueous medium containing 7040% (v/v) ethanol or methanol. In water, only the break corresponding to the titration of calcium is observed. The magnesium endpoint break is obtained, however, a t the expense of the quality of the calcium end-point break. To avoid this negative effect, calcium is titrated as usual in water a t pH 8.5-9.0. When the end-point break for the calcium titration has been obtained, the titration is interrupted and alcohol added to make the solution 70-8070 (v/v) with respect to this component. The pH is raised a t the same time to about 10 and the titration continued until the magnesium end-point break is obtained.
EXPERIMENTAL Apparatus. A fiber-tip. mercury(1) sulfate reference electrode and an amalgamared silver wire indicator electrode ( 4 ) were used. (1)
D. 8. Rorabacher. B.
J.
Blencoe, and D. W . Parker, Anal. Chem.,
44, 2339 (1972).
(2) G. Schwarzenbach and H . Ackermann, Helv. Chim. Acta, 31, 1029 (1948). (3) T. A. Kiss, F. F. Gaal, T. M. Suranyi, and I . J . Zsigrai, Anal. Chim. Acta, 43, 340 (1968). ( 4 ) C. N. Reilley. R. W. Schmid, and D. W. Lsmson, Anal. Chem.. 30, 953 (1958)
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The titration curves were recorded with a Metrohm Potentiograph E 336. A Metrohm pH meter E 396 equipped with a Metrohm 109 UX glass electrode and a mercury(1) sulfate reference electrode were used in the adjustment of pH. The instrument was calibrated against NBS standard buffers. Solutions. Standard solutions (0.01M) of calcium and magnesium were prepared by dissolving weighed amounts of primary standard calcium carbonate and magnesium metal in nitric acid and sulfuric acid, respectively. The calcium carbonate (Merck) was dried for 24 hours a t 150 'C before use. The magnesium metal (Johnson, Matthey & Co., Ltd.) contained not more than 0.01% by weight of total metallic impurities. The metal was washed successively in 5M hydrochloric acid, water, ethanol, and acetone and was allowed to dry a t room temperature. A 0.01M solution of EGTA (Eastman) was prepared by dissolving the acid in the appropriate amount of sodium hydroxide. The solution was standardized against calcium at pH 9.0. A solution (0.002M) of EGTA-mercurate(I1) was prepared by mixing equivalent amounts of mercury(I1)acetate (Mallinckrodt p.a) and EGTA (Eastman). Ammonium nitrate (1M) and concentrated ammonia solution were used for buffering. End Point. The end point was determined either from the inflection point of the curve or in the case of asymmetrical titration curves from the point of steepest potential break. The uncertainty in the end-point volume was estimated to +ti.til cm3 or less. Procedure. Add 2 cm3 of 1M ammonium nitrate to the sample, which contains 0.005-0.1 mmole of each of calcium and magnesium, and introduce concentrated ammonia dropwise until the pH is 8.8-9.0. Add a few drops of 0.002M mercury(I1)-EGTA and titrate with 0.01M EGTA using the amalgamated silver wiremercury(1) sulfate electrode combination. Stop the titration shortly after the first end-point break, which corresponds to the amount of calcium present, and make the solution 80% (v/v) in ethanol. Add a few drops of concentrated ammonia solution, so that the p H meter reading is 10. Continue the titration until a potential break is obtained. The EGTA consumed between the first and second potential break corresponds to the amount of magnesium present.
RESULTS AND DISCUSSION
A representative selection of results is shown in Table I. The errors in the calcium titrations are probably due to the uncertainty in the location of the end point, since the theoretically calculated titration errors (5, 6) are less than 0.1% assuming ApM = 0.1. The differences, which appear to be systematic in nature, correspond to an overtitration (5) L. G . Sillen in "Treatise on Analytical Chemistry," I . M . Kolthoff and
Elving, E d . , Part I, Vol. 1. The Interscience Encyclopedia, Inc., New York, N.Y.. 1959. A. Johansson and E. Wanninen, Sw. Kern. Tidsltr., 77, 492 (1965) P. J.
(6)
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