Chemical Analysis of Corrosive Oxidizers - Advances in Chemistry

22 Chemical Analysis of Corrosive Oxidizers

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II.

Instrumental Analysis of Nitrogen Tetroxide

Ν. V. SUTTON, H. E. DUBB, R. E. BELL, I. LYSYJ, and B. C. NEALE Rocketdyne Division, North American Aviation, Inc., Canoga Park, Calif.

Since nitrogen tetroxide (ΝΤΟ) is the most widely used oxi dizer in the U.S. space program, it has become necessary to develop sophisticated analytical chemical techniques to ensure the integrity of this system. Commercial ΝΤΟ con sists of

ΝΟ, 2

4

NO , 2

NO, 2

3

NO,

and H O (as HNO 2

3

and

HNO ). The techniques of NMR spectrometry as applied to the proton content are described in detail. Gas-solid chro matography was used to determine the nitrogen oxidizer. 2

T ^ r itrogen tetroxide is presently the work horse earth-storable oxidizer in liquid propellant systems. Present methods of analysis are wet chemical methods and are nonspecific. In order to fully understand the difficulties encountered in analyzing this compound, a brief review of its chemical and physical properties is presented. The equilibrium reaction N 0 ç± 2 N 0 is one of the fastest chemical reactions known. Nitrogen dioxide, N O 2 , is an intensely brown-colored gas. In the liquid state it is largely dimerized to nitrogen tetroxide, N 0 , and in the solid state it exists solely as colorless nitrogen tetroxide. The brown color of the liquid is solely the result of the equilibrium quantity of nitrogen dioxide present. When completely dry, nitrogen tetroxide is not significantly corrosive toward most common metals at room temperature. When it is moist, it becomes highly corrosive because of the formation of nitric acid. Nitrogen sesquioxide, N 0 (Also called dinitrogen trioxide), is formed by equimolar reaction of N O and N 0 . This oxide is, in turn, in equilibrium with the quite unstable nitrous acid, H N 0 , in the presence of water: 2

2

4

2

4

2

3

2

2

N O + N O , + H O ^± Ν,Οι + Η,Ο τ± 2HNO, t

231

232

ADVANCED PROPELLANT CHEMISTRY

N 0 is a dark blue liquid when boiling with decomposition into N O and N 0 at 38° F . Nitrous add is a paler blue liquid and is a rather weak acid but a fairly strong oxidizing agent. When the blue N 0 and brown N 0 are present in N 0 , a characteristic dark green color results. Starting with pure N 0 and pure H 0 , a final solution will be reached through a series of complex equilibrium reactions which may con tain all of the species found in Table I. 2

3

2

2

2

2


2

Table I. Species N1O4 NOj NO NjOf HNOt HNOj HsO

3

4

4

2

Chemical Species in ΝΤΟ State

Color

Liquid Gas and dissolved in liquid Gas and dissolved in liquid Liquid Liquid Liquid Liquid

Colorless Dark brown Colorless Dark blue Colorless l i g h t blue Colorless

Experimental The military specification for N 0 (Mil. Spec. P-26539A) requires among other things, 99.5% minimum N 0 and 0.1% maximum H 0 equivalent. In the assay, an excess of standard base is added to a known amount of N 0 , and the excess is back-titrated with standard acid. Cal culations are based upon a milliequivalent weight of 4.6008. If various percentages of the mixed oxides and acids are present, however, they may add or detract from the total acid value calculated as N 0 . The water equivalent is determined by evaporating a known amount of N 0 to a residue which is assumed to be a 70% nitric acid solution. This analysis requires an average of 12 hours. N M R Determination of Protons in ΝΤΟ. Because of the need for a rapid, reliable method for determining water in N 0 , attention was directed to using nuclear magnetic resonance (NMR) as a tool for total proton determination. N M R is advantageous in that only the pro tons may be observed, and no other nuclei interfere. In the liquid ΝΤΟ system, protons are mobile and exchange freely and rapidly with one another. This is an advantage in that only one resonance line will be observed, but it is a disadvantage in that infor mation as to the amounts of the different protons containing species ( H 0 , HNO3, and H N 0 ) cannot readily be obtained. Preparation of Sample. For this determination, a special N M R tube is required. The regular thin-walled N M R tubes are not satisfactory be cause of the danger of explosion from the pressure of ΝΤΟ gas and be cause of the difficulty of any glass blowing with the thin walled tubes. NMR grade, specially sized, borosilicate glass tubing may be purchased from Corning Class Works. This tubing is 4.80 mm. ±0.004-inch o.d. with 0.8 mm. ±:0.005-inch wall. An 8-inch tube is used with a standard taper 12/30 inner joint joined to the open end. 2

4

2

2

4

2

4

2

4

2

2

4

2

2

4

22.

SUTTON ET AL.

233

Nitrogen Tetroxide

Approximately 0.5-1.0 ml. of ΝΤΟ is run into a sample tube, and the tube is quickly capped with a 12/30 outer joint glass cap. Next, the tube is immersed in L N to freeze the ΝΤΟ. The cap is removed, and the taper joint is attached to a vacuum line as quickly as possible. Still frozen, the tube is pumped out and sealed off with a torch at the junction between the N M R tube proper and the taper joint. Samples so prepared may be stored indefinitely without deterioration or accumulation of mois ture. All of the ΝΤΟ samples examined so far contained protons in appre ciable quantities. To prepare standards, therefore, known amounts of water are added over and above that which is already present, and the calibration curve is extrapolated back to the base line. Standards are prepared by weighing a special N M R tube together with cap (preferably on an automatic semimicro balance to five places). A measured amount of water is added from a microliter syringe, and the tube, cap, and H 0 are reweighed. ΝΤΟ is again added, and the tube is weighed. The tube is then frozen, pumped, and sealed off as previ ously described. If the ΝΤΟ is first cooled to ice temperature and the tube kept capped, it is perfectly safe to remove the tube from the ice bath long enough to make the second weighing if an automatic balance is used. N M R Determination. Any N M R instrument capable of detecting protons and equipped with an integrator should be suitable. In the Rocketdyne work, the Varian DP-60 was used, although the Varian A 60 or HA-56/60 would be equally applicable. After the instrument is warmed up and aligned with an organic )roton-containing compound such as acetaldehyde, acetone, or cbJoroorm, integrals are taken consecutively of the unknown and of one or two standards using the same instrumental parameters. Table II and Figure


2

2

Î

Table II.

Calibration Data for NMR Determination off Proton* Water Added

Integral (Arbitrary Units)

0 0.078 0.36

22 31

67

§

PERCENT WATER Α00Ε0

Figure I . Calibration curve for NMR determination of protominNTO

234


1 show the reading of one unknown and two standards (prepared from the same unknown) all run at +40 decibel R F power and 0.30 X integral output. In Figure 1, the intercept on the horizontal axis falls at 0.19%. This represents the amount of protons (calculated as water) originally pres ent in the as-received ΝΤΟ. Thus, the standard to which 0.078% water was added actually contains 0.27% while the second standard actually contains 0.55%.


Results Comparison of the values obtained from samples by the N M R method and by the military specification method are given in Table ΠΙ together with a value for H 0-saturated ΝΤΟ. 2

Table HI.

Percent Water in Unknown

Sample

NMR, %

Military Specification, %

4-477 4-478 Bravo 5-390 6-33 6-164 Saturated

0.29 0.24 0.19 0.16 0.26 0.20 1.71

0.17 0.14 0.08 0.05 0.22 0.23 1.6 (Rcf. 7)

The value of 1.6% for saturated H 0 is a literature value at 25° C. (I). No value is available for the military specification method. The N M R value was taken at 28.3° C , which is the operating temperature of the Rocketdyne magnet. It is felt that this temperature difference could cause the higher value of 1.71%. 2

Discussion Insofar as precision of the N M R method is concerned, the small number of samples so far analyzed do not permit a firm statistical analysis. Values so far obtained have agreed within ± 0 . 0 1 % . The N M R values usually run consistently higher than the military specification values by about 0.11%. It will probably be necessary to accumulate more data comparing N M R and military specification results before this discrepancy can be reconciled. However, the N M R results appear to be quite selfconsistent, and we suspect that the assumption made in the military specification procedure—that no water is lost during the evaporation—may not be true. The N M R method appears quite attractive at this time. The repro ducibility of results is considerably better than with the military speci fication method. In addition, the time required per analysis, including the encapsulation of the sample, is no more than V 2 to I V 2 hours while the military specification procedure is considerably longer.

22.

SUTTON ET AL.

NUwgen Tetroxide

235

Gas Chromatography Analysis of ΝΤΟ. As previously mentioned, the presently accepted method for assaying N 0 is really only a total acid value and does not indicate the real composition of nitrogen tetrox ide. There were many unsuccessful attempts over the past decade to analyze oxides of nitrogen by gas chromatography; both gas-liquid and gas-solid attempts failed. The use of gas-liquid chromatography, which utilizes an organic sub strate to achieve separation, was precluded by the extreme reactivity of ΝΤΟ which will react with almost any organic substrate used in chro matography. The conventional gas-solid chromatography uses materials such as silica gel, molecular sieve, alumina, etc. to achieve separation re sulting from surface adsorption. Most of the materials mentioned, how ever, exhibit a high degree of polarity. A n attempt to analyze nitrogen oxides using such materials resulted in broad tailing peaks which pre cluded the possibility of using this approach as a basis for a quantitative analytical technique. A gas chromatographic analysis of ΝΤΟ, including separation of NO and N 0 , has been accomplished recently. The success of this work was a product of a fundamental study dealing with the principles of high temperature gas-solid chromatography. A number of candidate solid substrates were examined, and the best results were obtained with po rous glass. A detailed discussion of porous glass as a gas chromatographic medium, including surface area characteristics (pore volume, pore size, surface area, etc.) was published by MacDonell (3) and Lysyj and New ton (2). It is sufficient here to state that porous glass shows gas chroma tographic separating properties and is inert to the oxides and oxyacids of nitrogen. As the first step in this investigation, a gas chromatograph was de signed and built. The instrument incorporates a universal injection sys tem (made by Microtek Instruments) which can withstand attack by highly reactive specimens. Two 6-ft Vie-inch diameter, stainless steel columns packed with 3040 mesh porous glass (one separating and one reference) are placed in a temperature-programmed oven, which can be programmed at a high temperature rate. The two columns were necessary to provide a uniform change of pressure drop across the separating and reference gas stream, eliminating drifting base line when a temperature program is applied. The detector consists of a thermoconductivity cell with Teflon-clad hot wires (to prevent corrosion of the filaments), housed in a separately heated enclosure. For the read-out, a custom built bridge with potentiometric recorder and a printing integrator are used. The schematic of this system is shown in Figure 2.


2

2

4

4

Experiments which have been conducted to date indicate that gas

236


Λ

1

9r va

"β POWER SUPPLY REGULATION


0

Ο Pi P 2 - PRESSURE REGULATION V V - N E E D L E VALVES (

Ο

WH|ATSTONE

• ο

ο

Ε Ξ

RECORDER

2

U. I. - UNIVERSAL INJECTOR m — MOTOR E.P

- ELECTRICAL PANEL

O O P

INTEGRATOR

C | C - P O R O U S CLASS COLUMNS 2

T. C - THERMOCONOUCTIVITY CELL

P|r -FUmHETERS 2

M ^ M s

-HEATERS

Figure 2. Gas chromatograph for analysis of ΝΤΟ chromatographic analysis of nitrogen oxides is feasible, using the high temperature, gas-solid chromatographic principle. In order of elution, the following species appear at room temperature: air, nitric oxide, and nitrogen tetroxide. One temperature programming, an as yet unidentified peak elutes at at temperature of ~ 75° C , followed by water at ~ 165° C . Work is presently being carried out to optimize conditions for the reso lution of all of the apparent peaks in N2O4. Acknowledgment This work was supported, in part, under Contracts AF04(611)7023 and AF04(611)9377, Edwards Air Force Flight Test Center, Edwards, Calif., under the program monitorship of Forrest Forbes. Literature Cited (1) Briner, E., Burand, E. L . , Compt. Rend. 155, 583 (1912). (2) Lysyj, I., Newton, P. R., Anal. Chem. 36, 2514 (1964). (3) MacDonell, H. L., Noonan, J . M., Williams, J. P., Anal. Chem. 35, 1253 (1963). RECEIVED August 9, 1965.

Chemical Analysis of Corrosive Oxidizers - Advances in Chemistry

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