Determination of Nitroguanidine by Reduction with Buffered Titanous Chloride MILTON ROTH and RAYMOND F. WEGMAN Felfman Research and Engineering Laboratories, Picafinny Arsenal, Dover,
b The quantitative reduction of nitroguanidine by means of titanous chloride in a buffered solution proceeds readily a t room temperature, with the gain of six electrons per mole. The method has been applied to the determination of nitroguanidine both as a raw material and as a propellent ingredient. In both cases, a considerable time saving is effected. Order of addition of solutions to the nitroguanidine sample is a critical factor. When the sample is added to the buffered titanous solution, rather than vice versa, the theoretical six-equivalent reduction is readily achieved. The type of buffer is not significant. Statistical analysis of the results obtained indicates that the method is accurate, sensitive, and precise.
N
ITROOUANIDINE is
used extensively
in cannon propellants because of
its cool-burning properties. In order to ensure maximum stability and energy in the propellant, it is necessary to incorporate materials of high purity. I n determining the purity of the raw material, low results were obtained when the nitrometer method prescribed in the specification (9) was used. After nitroguanidine is incorporated into a propellant, it must be analyzed again for purposes of quality control. In this case, the nitrometer method is inapplicable because other propellent ingredients interfere. The method generally used is a water extraction procedure (16), which is unreliable because of its lack of specificity. Thus, inconsistent methods of analysis are used as the nitroguanidine is processed frqm the raw material into the finished products, and it becomes difficult to interpret the results. The titanous chloride reduction method of Knecht and Hibbert (10)has found wide use in the analysis of explosives. Although this method is satisfactory for nitro groups in which the nitrogen is attached to carbon, it fails when the nitrogen is attached to another nitrogen, as in the nitroamino group. Theoretically, the course of reduction for both the nitro and nitroamino groups is essentially the same and proceeds through the following steps (8):
2036
ANALYTICAL CHEMISTRY
-KO*-
2H
+ -NO+
N. 1.
2H’
+
Each step involves a gain of two electrons, with an over-all change of six electrons. Upon reduction with titanous chloride, nitro groups readily undergo a six-electron change as shown in the following equation: -KO*-
+ 6 T i t f + + 6H+ -NHt- + 6Ti+4 + 2H20 +
In the case of nitroamino compounds, however, this reaction does not readily occur. Methods have been developed on the basis of a four-electron change ( I ) , but these methods are erratic and actually vary, with nitroguanidine, from 3.8 to 4.2 equivalents of titanous chloride (22). The best approach to achieving a 6-electron reduction by titanous chloride was the method developed by Sternglanz and coworkers (IS’), using a 200% excess of titanous chloride in a titration medium buffered to p H 3.5 to 4.5 with potassium citrate. The authors noted, however, that the results for all nitroaminocarbonic acids tended to be low. This was attributed to two possibilities: that a slight hydrolysis occurred and that a side reaction occurred, leading to a hydroxylaminetype product requiring only four equivalents of titanous ion. In view of the progress of Sternglanz and coworkers in assaying nitroguanidine by the titanous chloride buffer method, an attempt was made to adapt this method for determining the nitroguanidine content of raw materials as well as propellants.
Indicator, 20% ammonium thiocyanate. Titration flask. A 500-ml. flat-bottomed boiling flask, with a 24/40 female ground-glass joint and a gas inlet tube sealed into the body of the flask. PURITY ANALYSIS
Keigh accurately a representative portion of approximately 2.7 grams of dried nitroguanidine and transfer to a 1-liter volumetric flask. Add about 750 ml. of hot distilled water and swirl to dissolve the sample. Cool to room temperature, dilute to volume with distilled water, and mix well. Transfer a 50-ml. aliquot t o a titration flask which contains 50 ml. of standard titanous chloride solution plus 25 ml. of the buffer solution, and from which the air has been previously displaced by a flow of carbon dioxide. Maintain the carbon dioxide atmosphere throughout the determination. Place the flask on a magnetic stirrer and stir the contents for approximately 3 niinutes. Add 25 ml. of 1 to 1 hydrochloric acid solution and 5 ml. of the indicator solution. Titrate the excess titanous chloride with standard ferric alum solution. Concurrently, run a blank determination using the same reagents used for the sample. Nitroguanidine, %
=
1.7345N ( A
- B)
W (1)
where ’
A = ml. of ferric alum required for a blank titration B = ml. of ferric alum required for a sample titration N = normality of ferric alum U7 = weight of sample in aliquot, grams. PROPELLANT ANALY S1S
REAGENTS A N D APPARATUS
Titanous chloride standard solution, 0.2N, prepared, standardized, and stored in accordance with MIL-STD286, Method 601.1. Ferric ammonium sulfate standard solution (ferric alum) 0.15N, prepared, standardized, and stored in accordance with MIL-STD-286, Method 603.1. Buffer solution. A 1 to 1 (w./v.) aqueous solution of sodium acetate trihydrate plus ap roximately 7 ml. of 30% sodium hy roxide solution per 25 ml. of sodium acetate solution to give a pH of 11.5 0.5.
cp
*
Separation Procedure. Extract an accurately weighed portion of approximately 2 grams of the sample with an azeotropic mixture (df) of pentanemethylene chloride (2 to 1 by volume) or with carbon tetrachloride in a suitable extractor. (A Roweg extractor, Roweg Apparatus Co., Ledgewood, N. J., was found satisfactory.) Reserve the insoluble residue for another extraction by one of the following methods: METHOD 1. Transfer the insoluble residue to a 150-ml. beaker containing 50 to 60 ml. of boiling water and extract for about 20 minutes in a wet mill.
Table 1. Effect of O r d e r of Addition and Type of Buffer on Determination Analysis of Nitroguanidine Order of Additions 1 2 Buffer
Nitroguanidine, yo
Acetate
2 Citrate
%. ?
99.34 99.22 98.94 99.34 99.21 99.25 98.67 99.25 98.67 98.96
99.60 99.60 99.67 99.67 99.64 99.21 99.87 99.62 99.46 99.54
Order 1 = titanous chloride and buffer added to sample solution; order 2 = sample solution added to titanous chloride
IiH PH t 7 1 - J H2N ,“NO Catalyst1
A
a
and buffer.
hT U
I\ I1
+ 3Hz
H2P\’dXHNO2
(A Sargent centrifugal wet mill, Model 1, E. H. Sargent & Co., Chicago, Ill., has been found satisfactory.) Decant the resulting solution into a 500-ml. volumetric flask. Repeat the water extraction twice more. Transfer the solution and the ground propellant to the 500-ml. volumetric flask, add hot n-ater to adjust the volume to about 450 nil., and then cool the solution to room temperature. Adjust to volume with Tvater. mix well. and allow the residue t o settle. METHOD2. Transfer the insoluble residue to a 250-ml. beaker containing about 200 ml. of water and boil for 15 minutes. Decant the resulting solution into a 500-ml. volumetric flask. Repeat this treatment of the insoluble residue tivice more. Transfer the solution and residue to the volumetric flask and dilute to about 450 ml. with hot water. illlow the solution to cool to room temperature and adjust to volume. Mix well and allow the residue to settle. Analysis of Solution. Transfer a 50-ml. aliquot of the supernatant solution obtained by either method 1 or 2 to a titration flask containing 50 ml. of standard titanous chloride solution plus 25 ml. of the buffer solution, and from n-hich the air has been previously displaced by a flow of carbon dioxide. Continue the determination in the same way as described above for the purity analysis. Calculate the per cent nitroguanidine according to Equation 1. PREVIOUS WORK
Previous experience in this laboratory, as well as the findings of Butts and coworkers (a), indicated that more efficient buffering action is obtained by using sodium acetate instead of potassium citrate (19). As this may be a factor contributing to the relatively low, erratic results that have been obtained ( I I ) , a substitution of buffering agents appeared desirable. Another factor that was considered to have a significant effect on the accuracy and precision of the method was
I n order to evaluate both factors, an experiment was designed to facilitate statistical analysis of the data. A homogeneous sample representing a single lot was used for all the tests. The results of the chemical analysis are shown in Table I. The statistical analysis-of-variance test for means was conducted on the basis of a fised-constants model (4), after a test of the data for homoscedasticity by the Cochran test (6) indicated that it would be valid to do so. The fixed constants model was selected because it was not intended t o consider other kinds of buffers or
hydrolysis of the nitroguanidine. Sternglanz and coworkers discussed this possibility but did not attempt to investigate the effect, although considerable information is available in the literature. In general, nitroguanidine is reported as being relatively stable in neutral, aqueous solution, but hydrolysis is accelerated by acids and alkalies (3, 16). Catalytic reduction has been shown to be more efficient when conducted in an acid medium (12, 13). The effect of the pH of the medium on the reduction products has been summarized by A h Kay (14)inthe followingreactionscheme:
I--+
J.
pH
Presumably, a similar reaction scheme would apply to reduction produced by other means. If the reduction is preceded by hydrolysis, or if hydrolysis is a competing reaction, results calculated on the basis of a six-electron change would tend to be low. Another method proposed recently for the volumetric determination of nitroguanidine is the salicylic acid transnitration method of Stalcup and Williams (18). This method is rather lengthy and requires the preparation of a number of special solutions that are useful only for this procedure. The most serious difficulty, however, is that this method, like the nitrometer, tends to give low results when applied to finely divided nitroguanidine because of the rapid decomposition that occurs when the sample is dissolved in concentrated sulfuric acid. RESULTS AND DISCUSSION
I n the usual procedure for the buffer method the sample is mixed with the buffer solution, forming an alkaline medium. Subsequent addition of the titanous chloride solution moves the p H to the acid side where, presumably, the nitroguanidine can be reduced with a six-electron change without side reactions. However, in the interim during which the nitroguanidine is exposed to the alkaline medium, slight hydrolysis may occur. During the addition of the titanous chloride there is some reduction occurring as the pH is changing from the alkaline to the acid side. Hon-ever, if the titanous chloride and buffer solutions are placed in the titration flask before the sample is added, the reduction proceeds under conditions that are essentially constant. Thus, the order of addition of the solutions may have a significant effect on the results.
