EvaIuation of Six Methods for Determination of Nitrogen in Nitroguanidine MAE I. FAUTH and HARRY STALCUP
U. S.
Naval Powder Factory, lndion Head, Md.
,Data are presented for the determination of nitroguanidine by six methods: nitrometer, buffered titanium (111) chloride, transnitration with salicylic acid, Bowman-Scott, macro-Kieldahl, and micro-Dumas, The nitrometer method gives the greatest precision, but the results are slightly less than the theoretical values. Transnitration tends to give values slightly higher than theoretical. For the assay of nitrogroup nitrogen, the nitrometer, transnitration, and titanium(l1l) chloride methods are probably equally satisfactory, as operator and lot variability are of greater significance than differences in precision of the three methods. For the assay of total nitrogen, the Dumas method is preferable.
B
nitroguanidine is used in propellants and various laboratories are using different methods for determination of its purity, six different analytical methods were evaluated. Extensive references on methods for the analysis of nitro, nitrate, and nitramine groups are found in the literature. Becker and Shaefer ( I ) have given a comprehensive survey of the field. The methods employed were of two types: those which yield a value of the nitro-group nitrogen only, and those which give total nitrogen. The nitrometer, transnitration, titaniuni(II1) chloride, and Bowman-Scott methods belong to the first class; Kjeldahl and Dumas procedures, to the second. Four of these methods involve reaction in concentrated sulfuric acid. Davis (4) has stated that under these conditions nitroguanidine acts as if it consists of a mixture of nitramide and cyanamide and can produce nitric acid by hydration of the nitramide. ECAUSE
'NO,
If the accuracy of the above equations is assumed, the reactions involved in 1670
ANALYTICAL CHEMISTRY
methods utilizing sulfuric acid may be expressed as follows:
Bowman-Scott
+ 2"Oa
Nz03
+ 2HzSO4
+ 2Fez(SO& + 3Hz0 --f
Transnitration
" //
HzNC
+ C6Hi(OH)COOH
- NHNOz
-+ 25" C.
+
Oz?;C6H3(OH)COOH
p H yalues. Sternglanz, Thompson, and Save11 (IO), using a citrate buffer, obtained values of the order of 98% for nitroguanidine. By adjusting the concentration of iron(II), Zimmernian and Lieber (12) obtained a reduction equivalent of six titanium(II1) ions per NO, group. Brandt, DeVries, and Gantz (3) concluded that under their experimental conditions derivatives of hydroxylamine were formed as intermediates. For the acetate-buffered solution of pH 11.5described by TVegman and Roth ( I I ) , six equivalents of titanium(II1) are consumed per mole of nitroguanidine XH
//
NH2C
+
OZNCF,H~(OH)COOH 6TiC13 6HC1 + HzNC~H~(OH)COOH 6TiCh -k 2H20
+ +
Kjeldahl
" //
H2SC
- NHNOz
+ CGH~(OH)COOH
HZS0402NCsH3(OH)COOH d
HzNC~H~(OH)COOH :%NHiHSO, The over-all reaction for this method is : SH
//
HZNC - NHNOz +4"s The other two methods are not carried out in strongly acid solution. Here the formation of nitric acid as an intermediate is not presumed to occur. Dumas
" //
HSXC
- NHNOz
CO? cu
+ cuo COz + 2H20 + 2N2
Titanium(II1) Chloride. T h e course of the reaction of nitroguanidine with titanium(II1) chloride has been studied by various investigators. Kolthoff and Robinson (6) found t h a t the reduction potential of titanium(111) chloride increases with increasing
+ 6 Ti+++
+
\
- YH,
+ 6 Ti+-++
XHXO?
Whether the reduced form is aminoguanidine or some other compound has not been determinrd. PROCEDURE
The samples were obtained from various sources and used without further purification. Lots 1458 and HC-5366 were obtained from Korth American Cyanamid, lots 77 and 78-A were manufactured a t the Naval Powder Factory, and the fifth lot was Eastman White Label. Prior to analysis, each sample was dried a t 105' C. for 1 hour. Three determinations n-ere made of each lot by each method. Nitrometer. The method used differed from the established procedure (8) in that the sample was washed into the generator with 5 ml. of cold 80% by weight sulfuric acid, followed bv 20 nil. of 94.5% . " cold sulfuric acid. Dumas. The procedure of Niederl and Xiederl ( 6 ) was followed. Transnitration. The method used was t h a t of Stalcup and Williams (9). As 20 minutes were not sufficient for contact with the sulfuric-salicylicacetic acid solution, the time was increased to 1 hour. Kjeldahl. The procedure used, adapted from Scott ( 7 ) , differs from those in the literature. A 0.25- t o 0.30-gram sample is weighed into a 650ml. Kjeldahl flask. Thirty milliliters of concentrated sulfuric acid containing 1 gram of dissolved salicylic are added,
Table
Lot HC-5366 1458
Eastman cI I
78.1 Av. of 5 lots
Nitrometer Mean Std. dev. 13.43 13.42 13.40 13.41 13.42 13.42
0.010 0.006 0.010
0.006 0.006 0.008
I.
Data for Methods Yielding N02-Group Nitrogen
Transnitration Titanium(II1) Chloride Mean Std. dev. Mean Std. dev.
Bowman-Scott Mean Std. dev.
13.50 13.51 13.41 13.45 13.49 13.47
13.39 13.27 13.44 13.47 13.45 13.40
and the flask is closed with a stopper and set aside for 15 minutes. Ten grams of potassium sulfate, then 5 prams of sodium thiosulfate, and finally 0.2 gram of copper sulfate are added cautiously in a hood. A funnel is placed in the opening of the flask; the contents are digested until the solid has all been dissolved and the sample is a light straw or pale green color. The flask is cooled and the samples are carefully diluted with distilled n-ater equal to 3 times the sample volume. Several boiling beads are added and the flask is put into a freezer for several hours, or until very cold. L-sing 150 nil. of standard 0 . W sulfuric acid in the receiver, a distillation setup is prepared. Approximately 150 ml. of cold 50% sodium hydroxide are poured into the sample, layering it if possible. The distillation setup is quickly connected and the flask swirled gently t o effect complete mixing of sample and alkali. Care must be taken not to break any of the connections. As soon as mixing is complete, moderate heat is applied and the distillation continued until approximately 100 ml. of distillate are collected. The condenser and flask are thoroughly washed and the entire solution is titrated with 0.1N sodium hydroxide, using methyl red as the indicator. -4 blank is run following the same procedure used for the sample. A second blank is the run using sulfuric acid only. If satisfactory blanks are obtained, the per cent nitrogen is calculated by the following formula: Per cent nitrogen = (VI - V2) X N X 1.4008 grams of sample where T'i = nil. of blank KO. 1 T - 2 = ml. of sodium hydroxide required for sample 12' = normality of the sodium hydroxide Bowman-Scott ( 2 ) . The only modification mas t h a t the beaker containing the sulfuric acid solution of the sample iTas cooled in an ice bath during the titrations to prevent the temperature from rising above 303 C.
0.026 0.012 0.026 0.015 0.026 0.021
13.42 13.44 13.50 13.37 13.45 13.44
0.048 0.006 0.006 0,026 0.006 0.018
Per cent nitrogen = ( V - 0.2) X A- X 0.7004 grams of sample where T' = ml. of iron(I1) sulfate for the sample A' = normality of iron(I1) solution The 0.2 is a correction for the blank. Titanium(II1) Chloride Buffer. The procedure of Wegman and Roth (11) utilizes the buffer principle developed by Kolthoff and Robinson (6) and applied by Sternglaz et al. (10). The buffer solution consisted of 30% sodium hydroxide solution per 25 ml. of the acetate solution. This should give a pH value of 11.5 & 0.5. Per cent nitrogen =
A
0.2333iV ( A - B ) grams of sample
ml. of iron(II1) alum required for a blank titration B = ml. of iron(II1) alum required for a sample titration A' = normality of the iron(II1) alum standard solution =
DISCUSSION A N D RESULTS
The mean values and standard deviations for each lot for the four methods which yield only Norgroup nitrogen are shown in Table I. The nitrometer method gave the greatest precision and the smallest standard deviation. The average result of the titanium(II1) chloride method was closer to the theoretical value, but the standard deviation was more than twice as great as for the nitrometer data. Transnitration values tended to be higher than theoretical, and the standard deviation was larger than that of the other two methods. The BoJvman-Scott method, which inT-olves a titration with an end point that is difficult t o detect precisely, shon-ed considerable scatter and gave exceptionally low values for one lot, The standard deviation for all the analyses done by this method was 0.066, which was greater by a factor of 3 than
0.085 0.091 0.04i 0.092 0.015 0.066
Av. of
Means of 4 Methods
Av. of Means of Nitrometer, Transnitration, and Titanium-
13.44 13.41 13.44 13,42 13.45 13.43
(111)
Chloride 13,45 13.46 13.44 13.41 13.45 13.44
those for any of the other three methods in this group. For the four methods which give NO,group nitrogen there was a significant (99Oj, level) interaction between lots and methods. The lot average differed for the three methods. I n addition, the range of the lot averages for the nitrometer was 0.03, whereas it was 0.10 for transnitration, 0.13 for titanium(II1) chloride, and 0.20 for the BowmanScott. A statistical test of the averages of the nitrometer, transnitration, and titanium(II1) chloride methods did not shoxy these methods to differ significantly from each other. If any difference does exist among the three methods, it is clouded by the inconsistent pattern of the lots in each method-Le., the interaction of lots and methods. As a subsidiary piece of information, it was determined that the five lots of material are not exactly alike. Because the methods were tested by different operators, this variability is probably also a factor. For the methods which give total nitrogen (Table 11), there was no interaction between methods and lots a t the 95% level. There was a significant difference betn-een methods a t the 99% level. If the lots are typical, the per cent nitrogen found by the Dumas method is 0.64 higher than by the Kjeldahl method. At the 99% level there were significant dissimilarities among lots. KO important difference was noted in the precision of the two methods. The average m-ithinlot standard deviation which may be associated with either method is o.17y0 nitrogen. For comparison of the six methods the data were converted to per cent nitroguanidine (Table 111). From examination of this table the following conclusions may be drawn: The Bowman-Scott is the least precise and is probably not suitable for this type of analysis. VOL. 30, NO. 10, OCTOBER 1958
1671
Table II.
Data for Methods Yielding Total Nitrogen
Kjeldahl Lot
Mean
HC-5366 1458
Eastman 77 78A
Av. of 5 lots Table 111.
53 53 53 52 53 53
Dumas Std. dev. 0 0 0 0 0 0
42 39 00 94 38 23
Lot
1458
Eastman 77 78.4
Av. of 5 lots
99.78 99.78 99.70 99.70 99.78 99. 70 99.48 99.63 99.55 99.70 99.63 99.63 99.70 99. 70 99.63 99.67
15 18 12 22 17 17
53 54 53 53 54 53
Std. dev. 0 0 0 0 0 0
92 07 41 93 03 87
17 04 33 14 17 17
Per Cent Nitroguanidine by Various Methods
NiTrans- Bowmantrometer nitration Scott
HC-5366
Mean
100.15 100.22 100.52 100.30
100 45 100 45 99.48 99,85 99.55 100.00 100.15 99.93 100.45 100.07 100.52 100,14
The Kjeldahl consistently values. Transnitration trends to centages higher than the value. The nitrometer method
98.81 100.07 99.55 98.74 99 18 9 i 85 100 22 99 55 99 70 100 45 100 45 99 26 99 78 99 93 100 00 99 57
yields low give pertheoretical gives the
Titanium(111)
Chloride Kjeldahl 99.33 99.70 100.00 99.85 99 93 99 85 100 30 100 30 100 22 99 26 99 18 99 55 99 85 99 93 99 93 99 81
99.50 99 20 98.96 99.37 99 35 98 79 98 66 98 46 98 24 98 79 98 09 98 09 99 41 99 26 98 79 98 86
Dumas 100.35 100.32 99.80 100.43 100.37 100.50 99.67 98.51 99.41 100.00 99.93 100.45 100.35 100.04 100.65 100.05
Av. of 6 Methods 99.76 99.70 99.49 99 .59 99 .89
greatest precision, but if the compound is assumed pure, the results are slightly less than the theoretical value. For the assay of total nitrogen the Dumas is to be preferred. For the assay of KOz-group nitrogen,
the nitrometer values may be slightly lower and the transnitration ones slightly higher than theoretical. For the nitrometer, transnitration, and titanium(II1) chloride methods, operator and lot variability are probably of greater significance than differences in precision of the three methods. LITERATURE CITED
R. W:,Shaefer, W. E., in “Organic Analysis,” 5’01. 11, pp. 71122, Interscience, S e w York, 1954. (2) Bovman, F. C., Scott, IT. IT., Znd. (1) Becker,
Eng. Chem. 7,766 (1915). (3) Brandt, W. TT., DeVries, J. E., Gantz, E. St. C., ASAL. CHEX 27, 392 (1955). (4) Davis, Tenny L., “Chemistry of Powder and Explosives,” pp. 384-5, Kiley, Sen- York, 1943. (5) Kolthoff, I. bl., Robinson, C., Rec. trav. chzrn. 45, 169 (1926). (6) Siederl, J. B., Siederl, V., “Micromffthods of Quantitative Organic Analysis,” 2nd ed., pp. 79-99, Kiley, S e w York, 1942. (7) “Scott’s Standard Methods of Chemical iinalysis,” N. H. Furman, ed., 5th ed., pp. 632-3, Van Nostrand, New York. 1939. (8) Ibid., pp. 649-52. (9) Stalcup, H., Williams, R. JV., AXAL. CHEW27,543 (1955). (10) Sternglanz, P. D., Thompson, R. C., Savell, W,L., Ibid., 25, (1953). (11) Wegman, R. F., Roth, Milton, Picatinny Arsenal Tech. Rept. 2428, July 1957. (12) Zimmerman, R. P., Lieber, E., .4NAL. CHEN. 22, 1151 (1950).
RECEIVEDfor review January 8, 1958. Accepted May 10, 1958. Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
Reaction of Nitrates and Nitramines with Phosphoric Acid C. L. WHITMAN and MAE I. FAUTH Research and Development Department, ,Examination of the behavior of phosphoric acid as a reagent in the nitrometer reaction showed that, although inorganic nitrates reacted quantitatively, nitrate esters and nitramines were not soluble or the reaction was incomplete. A modification of the diphenylamine spot test for nitrates was found to differentiate nitrates, nitrites, and nitrate esters, from nitramines. All these classes of compounds gave positive results when a test reagent consisting of diphenylamine in sulfuric acid was used. When the diphenylamine was dissolved in phosphoric acid, nitramines failed to give the test. This difference in behavior permitted the detection of nitrates and nitrate esters in nitramines.
1672
ANALYTICAL CHEMISTRY
U. S.
Naval Powder Factory, Indian Head, Md.
A
SUMMARY of
the methods of analysis for nitrates and similar compounds, together with extensive references, has been given by Becker and Shaefer (1). The use of diphenylamine in sulfuric acid as a spot test reagent for nitrates and nitrites mas described by Feigl ( 3 ) . Kieland (4)studied the reaction of diphenylamine with nitric and nitrous acids and proposed the following set of equations: H I
W
-
D
HNOs T
HKOz
Diphenylamine (colorless)
S,S’-Diphenylbenzidine
(colorless)
--
HSOI
or
HSOs
Quinoidimonium salt of 11 (blue) 3 = C1-, SOr--, etc.
