Determination of Iron in Nitrogen Tetroxide by Atomic Absorption Spectrophotometry C. E. Wilson, T. T. Bartels, and J. H. Taylor Engineering Laboratories, McDonnell Aircraft Co., S t . Louis, Mo. NITROGEN TETROXIDE (N204) has found widespread use as a rocket fuel oxidizer because of its high energy of reaction with amines such as hydrazine, and because of its storability. One drawback to using N 2 0 4 in rocket propulsion has been the periodic occurrence of flow decay phenomena in certain oxidizer facilities. Flow decay phenomena were the subject of an intensive investigation by the National Aeronautics and Space Administration (NASA), and it was concluded that a deposit of iron nitrate solvated with N 2 0 4(produced by a temperature drop in the N 2 0 4 was ) responsible for the flow decay ( I ) . The iron content of N 2 0 4must be determined prior to use to ensure that spacecraft oxidizer systems will not be affected by flow decay. Existing methods for the determination of iron in N 2 0 4are based on the extraction or hydrolysis of the NzOl sample, with subsequent analysis by colorimetric or atomic absorption methods. Such methods are time consuming because of the extraction or hydrolysis process, and the colorimetric methods are not valid for nonionic and distillable forms of iron (2-4). This article describes the development of an analytical test method employing atomic absorption spectrophotometry with direct aspiration and determination of total iron in liquid NO4.
EXPERIMENTAL Instrumentation. The atomic absorption spectrophotometer used in this work was a Perkin-Elmer Model 303 with a premix burner. A platinum-titanium nebulizer assembly was used throughout the investigation. Experimental parameters are presented in Table I. The drain tube from the burner system was submerged in a caustic solution t o neutralize any N2O4or nitric acid which might collect. Preparation of Standards. Nitrogen tetroxide-iron standards were prepared by dissolving appropriate quantities of iron octoate in known volumes of ethyl acetate. An iron octoate standard, 5.23 iron, was obtained from Angstrom,
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(1) E. F. L. Cain, A. E. Akworthy, J. Gerhauser, C. Fujikawa, and S.Rodriguez, “Method for Elimination of Corrosion Products of Nitrogen Tetroxide,” Rocketdyne Report (AFRPL-TR-67-277), July 1967. (2) B. L. TuWy, Rocketdyne, Canoga Park, Calif., Personal Communication, October 13, 1965. (3) S. R. Restivo, Chief, PAFB Branch Chemical Laboratory, Personal Communication, October 12, 1965. (4) H. E. Dubb, H. Schultz, and J. E. Sinor, “Inhibited N204 Engineering Data,” North American Rockwell Report (AFRPL TR-67-279), August 1967.
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Table I. Experimental Parameters Instrument Perkin-Elmer Model 303 Wavelength 248.3 and 248.8 mfi 40 mA Current Fuel Acetylene-9~ Oxidizer Air-8.7a Slit 0 . 3 mm Flow meter settings.
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ANALYTICAL CHEMISTRY
0.1
0
1
2
3
4
5
6
Iron Concentration (PPM)
Figure 1. Calibration curve of iron in NrOc and water, measured at 248.3 mp
Inc. Aliquots of the ethyl acetate-iron stock solutions were transferred to volumetric flasks which were cooled in an ice bath and diluted to volume with N 2 0 1which met the purity requirements of NASA specifications MSC-PPD-2. Nitrogen tetroxide-iron standards were prepared containing 1 by weight ethyl acetate and up t o 6 ppm by weight iron. Procedure. The standards and samples were maintained in an ice bath prior to and during direct aspiration into the flame. To determine the temperature coefficient of absorption, the temperature of a single N 2 0 4solution was varied from -10 to +10 “C, and the absorptions were subsequently obtained at the various temperatures. Aspiration rates also were noted. Absorptions were measured at two frequencies throughout the concentration working range and were compared to aqueous standards. The effect of matrix composition was investigated, and the accuracy and relative standard deviation were calculated.
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RESULTS AND DISCUSSION Using the 248.8 mp iron resonance line, a linear calibration curve was obtained through 6 ppm iron. The method appears very promising as the absorbance follows Beer’s law and, in addition, the sensitivity is five times greater than that of the aqueous standards. Figure 1 is a calibration curve obtained a t the iron resonance frequency of 248.3 mp. The sensitivity of iron in NzOl at this frequency is two and one-half times better than at 248.8 mp, at 3 ppm and below. However, at concentrations above 3 ppm, the relationship becomes nonlinear, and consequently the calibration curve prepared at 248.8 mp should be employed when the sample concentration exceeds 3 ppm. Next, the various parameters and possible interferences were investigated. Because N204boils at 70 “F,the temperature of the samples was expected to be very critical. A study was conducted to determine to what extent the temperature of N 2 0 4would affect the per cent absorption. Because of the practicality of working at or near 0 “C,the absorption of a standard was recorded at temperatures from - 10 to +10 “C. Ten readings were taken at each temperature investigated, and the data obtained are presented in Figure 2 . There was n o
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0.51
0.4
0
-
0 18
0
0.3r
Average Absorbance
+4
0
Temperature of
-4
N204
-8
-12
Iron Concentration
(OC)
Figure 3. Determination of iron in N204by atomic absorption using method of additions
Figure 2. Temperature effect of N 2 0 4 on absorbance
significant variation among the averages of the absorption readings taken from -10 to $10 "C; however, the relative standard deviation (based on replicate determinations of the same solutions) was much lower at the higher temperatures, as shown in Figure 2 . The aspiration rates of N 2 0 4were also measured, and these data are presented in Table 11. Most of the iron contamination in NsOl is thought t o be N204 solvated iron nitrate. This compound has a solubility of less than 3 ppm at 30 "C; therefore, to ensure an accurate analysis, 1 % ethyl acetate was added t o dissolve any solid iron nitrate particles. Ethyl acetate was chosen based on work done by Cain ( 1 ) who established that ethql acetate is compatible with N201and dissolves solvated iron nitrate. To determine if the ethyl acetate concentration was critical, solutions of N20, were prepared which contained 1.4 ppm iron and from 0 to 1 ethyl acetate; the iron absorption of each solution was measured. No differences in absorption were observed. The addition of water was found to cause erratic absorption. Consequently, an investigation was conducted to determine the effect of water on the analysis. To accomplish this, a series G f nitrogen tetroxide-iron standards were spiked with various quantities of water, and the absorption of each s o h tion was measured. Water at concentrations in excess of 0.25 % by weight interfered with the analysis causing absorption suppression and nonreproducibility. However, this concentration is two and one-half times the maximum allowable according to MSC-PPD-2. To test the accuracy of the analysis of iron in N204by this method, recovery studies were conducted. Samples of Nz04 containing 1 ethyl acetate were spiked with various quantities of iron and were then analyzed by direct aspiration into the flame. Ten replicate absorption readings were recorded for each sample. The results of the recovery studies and the relative standard deviation of each determination (based on replicate determinations of the same solutions) are presented in Table 111. When the analysis of iron first became of interest, several reports indicated that some of the iron was distillable. I n a n effort to determine if any of the iron was distillable, a sample of N 2 0 Jwas distilled three times, and the resulting distillate was aspirated directly into the flame. An absorption corresponding to 0.40 ppm iron was obtained; however, on checking standards and triply distilled N 2 0 4at a nonresonance wavelength, it became obvious that this absorption was caused by the N2O4and not by iron. Consequently, all data in this
(PPM)
Table 11. Aspiration Rates of NpOl and Water at Various Temperatures Sample Nitrogen tetroxide Nitrogen tetroxide Water Water
Temperature, "C - 10 $5 Ambient
Rate, ml/min 4 5
6.6 5.4
$5
Table 111. Precision of Analyses of Iron in N 2 0 r
a
Iron, ppm Added Recovered 1.19 1.20 2.19 2.23 5.63 5.69 Each solution was aspirated 10 times.
Re1 stda dev, Z 5.9 4.3 4.5
Table IV. Typical Iron Values of N s 0 4 Sample Iron content, ppm Premium 0.49 Premium 0.65 Premium 0.31 Nonpremium 1.05 Nonpremium 1.35
report have been corrected for the absorption attributable t o the blank. The concentration of samples is determined by the method of additions. The sample of N204is diluted to contain 1 ethyl acetate, cooled to approximately 0 "C, and aspirated directly into the flame. Figure 3 presents a curve extrapolated t o determine the original concentration of a typical sample. Table IV presents a few N204samples and their corresponding iron concentrations. Visual inspection did not reveal any damage t o the nebulizer or the burner assembly after several hundred direct aspirations of N 2 0 , into the flame. Adequate induced draft ventilation is required over the burner of the atomic absorption spectrophotometer because N 2 0 d is toxic and must not be inhaled.
RECEIVED for review June 10, 1968. Accepted August 26, 1968. VOL. 40, NO. 13, NOVEMBER 1968
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