Gas Chromatographic Bubble-Gas Analysis for Determination of Hydrazine Gel Decomposition Mechanism N. A. Kirshen a n d G. H. Olsen Poulter Laboratory for High Pressure Research, Stanford Research Institute, Menlo Park, Calif. THEUTILITY of hydrazine as a rocket fuel is enhanced by the suspension of powdered metals such as aluminum, following gelations of the hydrazine with organic compounds of high molecular weight. During initial stability studies ( I ) of aluminized and neat hydrazine gels, it was found that the gels swelled. This swelling, subsequently measured dilatometrically over a period of two years (2, 3), was determined to be caused by the entrapment of gaseous decomposition products that arise from hydrazine decomposition by one or a combination of the following known mechanisms ( 4 , 5):
+ 4NH3 2N2H4 = N2 + H2 + 2NH3 3NJH4 = NZ
EdNEEDLE 2.77mm I.D. x 4 8 in. LONG CAPILLARY TUBE
COUPLER REDUCING U N I O N OVEN POSITIONING TABS
(1)
2.5 L I T E R G L A S S G E L STORAGE VESSEL
w
(2)
T H E R M O C O U P L E WELL
A novel analytical technique employing gas chromatographic bubble-gas analysis was developed to determine which of these reactions predominated, the results clarifying the physical and chemical factors-e.g., pressure, temperature, reagent purity-as well as the kinetics of the gel decomposition. EXPERIMENTAL
Dilatometric Apparatus. The swelling of aluminized and non-aluminized hydrazine gels and the consequent trapping of decomposition gas bubbles were observed in sensitive 2.5-liter borosilicate glass dilatometers (see Figure 1). Approximately 20 dilatometric experiments were conducted at temperatures varying from 90 t o 110 OF. Analytical Apparatus. Because it is difficult t o separate the decomposition gases (nitrogen, ammonia, and hydrogen) with one column on a gas chromatograph, a Varian Aerograph 1520-1 dual-column gas chromatograph was fitted with the two columns in tandem (see Figure 2), and the gases were detected by the method of Murakami (6), in which both
Figure 1. Glass dilatometer
sides of a dual thermal conductivity cell are used alternatively as sensing elements. The Quadrol column ( 5 feet X 1/4 inch 0.d. stainless steel) separates the ammonia, while the molecular sieves column (15 feet X 1/4 inch 0.d. stainless steel) separates the nitrogen and hydrogen. Activation of the rnolecular sieves 5A column was necessary only once at the beginning of the analysis. A carrier gas flow rate of 50 ml/min was maintained and chromatograms were recorded with center zero. Calibration Procedure. The relative sensitivities of nitrogen, hydrogen, and ammonia were determined from the
-
14D.C.
?6oOcOETECTOR . SIDE A
COLUMN-8 M SIEVES O L E C U 5LA AR
23.C
(1) D. W. Brees, P. S. Grakle, K. Koshar, and G. E. Mason, “Chemical and Physical Properties of Alumizine,” Proceedings of the Metalized Gelled Propellants Conference, Edwards Air Force Base, California, Chemical Propulsion Information Agency Publication No. 64, November 1964, p 2 . ( 2 ) E. L. Capener, L. A. Dickinson, and N. A. Kirshen, “Formulation and Evaluation of Liquid Propellant Dispersions,” Final Report, Stanford Research Institute, Contract No. AF 04(611)9880, February 1966. (3) E. L. Capener, L. A. Dickinson, and N. A. Kirshen, “Formulation and Evaluation of Liquid Propellant Dispersions,” Final Report, Stanford Research Institute, Contract AF 04(611)9880, March 1967. (4) C. A. Clark, “Hydrazine,” Matheson Chemical Corp., Baltimore, 1953, p 1. ( 5 ) L. F. Audrieth and B. A. Ogg, “The Chemistry of Hydrazine,” John Wiley and Sons, New York, 1951, pp 66, 115-17, 145-8. (6) Y. Murakami, Bid/. Chern. SOC.Japan, 32, 316 (1958).
H2
u
Figure 2. Schematic diagram of gas chromatograph
Table I. Relative Sensitivities and Retention Times on Decomposition Gases Decomposition Relative Retention gas sensitivity time, min Nz 100.00 19.0 “3 81.21 4.6 H 2 1.97 3.6
VOL. 40, NO. 8, JULY 1968
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Neat Gel No. 115
Compound Nz
Table 11. Bubble Gas Composition Gel description, Neat Gel Aluminized Gel No. 116 No. 123
98.40 1.45 0.15
"3
Hz
F
99.50 0.52 0.05
99.58 0.33 0.10
Aluminized Gel No. 127
Standard deviation (Gel No. 127)
99.43 0.47 0.10
0.038 0.024 0.005
CAPILLARY
w
x w
REDUCING
UNION
SILICONE
EXCESS HYDRAZINE
BUBBLE
SILICONE OIL
GAS
0 STORAGE
5
10 TIME ( m i n u t e s )
15
Figure 4. Gel No. 127 bubble-gas chromatogram 0
b
G
Figure 3. Preparation for bubble-gas sampling (a) Dilatometer in storage (b) Storage vessel with silicone oil layer and septum (c) Storage vessel prior to sampling
slopes of the linear calibration curves, assuming nitrogen t o have a sensitivity of 100. These sensitivities are shown in Table I with related retention time data. Hydrazine was undetectable at concentrations that would be expected t o exist in the bubble gas. Sampling Procedure. The steps in the preparation of dilatometers for bubble-gas sampling are shown in Figure 3. Under a helium atmosphere, the capillary and reducing union are removed from the dilatometer. The excess hydrazine, which is used to top the gel and provide a means to follow gel swelling by capillary rise, is removed. Silicone oil is substituted for this extracted hydrazine and its level brought up to the top of the storage vessel. Next, a silicone rubber septum is clamped over the top of the storage vessel. The dilatometer is then vibrated t o allow suspended bubbles t o rise to the top of the dilatometer and coalesce under the silicone rubber septum, thereby displacing the silicone oil. I n a helium atmosphere, 100O-pl gas samples are then withdrawn from the dilatometer. RESULTS AND DISCUSSION
Results of the bubble gas analyses performed on aluminized and neat hydrazine gels are shown in Table 11. Figure 4 is
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ANALYTICAL CHEMISTRY
a typical chromatogram of bubble gas that evolved within Gel No. 127. The gas from this gel was sampled five times, to determine the precision of the sampling technique. Fairly close agreement for nitrogen, ammonia, and hydrogen resulted, as exhibited by small standard deviations (see Table 11). Reaction 1 is apparently the principal decomposition reaction occurring within the gel structure, as the primary decomposition product appearing in all of the analyses is nitrogen. The high solubility of ammonia in hydrazine ( 5 ) precludes its appearance in the gas phase, except in very small quantities. The trace amount of hydrogen found indicates that Reaction 2 takes place only to a very limited extent. If Reaction 2 were the principal decomposition mechanism, the amounts of nitrogen and hydrogen would have been more nearly equal. Comparison of the gas products arising from neat and aluminized hydrazine gels shows that the presence of aluminum powder does not alter the decomposition reaction mechanism. RECEIVED for review January 8, 1968. Accepted April 15, 1968. Research described was carried out as part of U. S. Air Force Contract No. AFRPL TR-66-267. Permission for publication was granted by the U. S. Air Force Rocket Propulsion Laboratory, Air Force Systems Command, Edwards Air Force Base, Calif.
