Determination of Water in Nitrogen Tetroxide: Gas Chromatographic Method for Total Hydrogen Content R. F. Muraca, Edward Willis, C. H. Martin, and C. A. Crutchfield Stanford Research Institute, Menlo Park, Calf. A method utilizing a combination of a reduction furnace and a gas chromatograph has been developed for determination of the water content of nitrogen tetroxide. A measured amount of Nz04 carried by a stream of helium is passed over hot copper (800 “C); Nz04 is converted quantitatively to nitrogen, and HN03and HNOz are converted to water. The products are fed into a gas chromatograph and separated on a Porapak-Q column; the heights of the peaks are directly proportional to the nitrogen and water content of the nitrogen tetroxide sample. Less than 10 minutes are required for an analysis and as little as 0.1 pg of water (0.01 wt-%) can be determined reproducibly. Organic compounds or dissolved carbon dioxide in the sample can also be determined.
THEREACTION PRODUCTS formed by contact of only a few tenths of one per cent of water with nitrogen tetroxide, N204, seriously impair the propellant’s usefulness for rocketry because they increase its rate of attack on metals Consequently, military procurement specifications place stringent limitations on the amount of water which has come into contact with nitrogen tetroxide rocket propellant; typically, this amount of water is limited to O . l % , even though as much as 0.2% appears to be acceptable for rocketry ( I ) . Water usually can be determined easily when it exists essentially as the molecular specie, H 2 0; however, since nitrogen tetroxide reacts with water more o r less completely t o form a variety of new molecular and ionic species, procedures used t o determine “water” in N 2 0 4actually determine one or all of the reaction products. The water equivalents of the reaction products are reported as “water content,” and for the purposes of rocketry this value is considered t o be the amount of water with which the propellant has come into contact. The rate of reaction of small amounts of water with liquid nitrogen tetroxide has not been studied in detail; kinetic studies suggest that N z 0 4 is the reactive species, N z 0 4 2 N 0 2 . Small amounts of water may remain dissolved in NzO4 for some time at 0 “C (2), but it is generally recognized that within a short time essentially all the water is converted t o a mixture of nitrous acid, nitric acid, and dinitrogen trioxide ( 3 ):
+ Nz04 e H+ + NO3- + HNOz 2 HzO + NO + NOz 2 NO + N204 e 2 N203
H20
”02
(1)
(2)
processes the brown color of nitrogen tetroxide is restored by air or oxygen. Although the phase diagram has not been established for the system: N z 0 4 - H N 0 3 - H ~ 0 - N 0when Nz04 is by far the major constituent, available information suggests that an appreciable amount of unreacted water can be maintained in nitrogen tetroxide as H N 0 3 .xHzO; the ratio of water to nitric acid is undoubtedly influenced by the relative concentrations of all species and temperature. Thus, nitrogen tetroxide of commerce (brown) contains principally nitric acid in amounts essentially equivalent to the water with which it has come into contact, together with small amounts of nitrous acid, nitric oxide, and unreacted water (or water of equilibrium). Dissolved carbon dioxide, nitrosyl chloride, and dissolved o r suspended organic matter may also be present. The determination of the “water” content of nitrogen tetroxide by the method currently used to control the quality of propellant-grade material ( I ) is based o n the removal of readily-volatile nitrogen tetroxide by a stream of dry nitrogen and retention of a liquid residue which is assumed to consist of the entire water, nitrous acid, and nitric acid content; the procedure is time-consuming, highly empirical, lacks sensitivity, and yields results that are questionable. Whitnack (4) improved the procedure by vaporizing a sample of nitrogen tetroxide and allowing its nitric (and nitrous) acid content to react with sodium carbonate at 280 “C and metallic copper at 600 “C; free water (if present) and the water released by neutralization of nitric acid is absorbed by anhydrous calcium sulfate and weighed. A review of other methods has been published (5) and Sutton et al. (6) have developed an NMR method for determining the proton equivalent of nitrogen tetroxide (protons occur indistinguishably in HOH, HONOZ, and HONO); the method is straightforward and precise, but it remains unattractive as a quality control procedure because of the high cost of instrumentation and the intricacy of sample preparation. Numerous papers (7-10) describe gas chromatographic methods for the determination of a variety of impurities in nitrogen tetroxide and other corrosive gases, but a method for the direct determination of water has not been reported. The direct determination of water in nitrogen tetroxide is of little practical value because only a trifling amount of water is in equilibrium with the ionic and molecular species in the propellant and the relationship of molecular specie H 2 0 to the
(3)
When water is added to liquid nitrogen tetroxide a t room temperature, a greenish color develops almost instantly (N203); the same color is produced by addition of nitric oxide t o nitrogen tetroxide. Since dinitrogen trioxide is readily oxidized to nitrogen tetroxide, in many commercial
(4) G. C. Whitnack and C. J. Holford, ANAL.CHEM.,21, 801 (1949). ( 5 ) W. L. Clark, A. Nudo, and P. Yin, in “Humidity and Moisture,” Wexler, Ed., Vol. 4, Reinhold, New York, 1965, p. 55. (6) N. V. Sutton, H. E. Dubb, R. E. Bell, I. Lysyj, and B. C. Neale,
“Advanced Propellant Chemistry, American Chemical Society (1) Military Specification; Propellant, Nitrogen Tetroxide, Mil-P26539 USAF, July 18, 1960. (2) C. W. Alley, A. W. Hayford, and H. F. Scott, Jr., Corrosion, 17 (lo), 101 (1961). (3) J. J. Carberry, Chem. Eng. Sci., 9, 189 (1959).
No. 54,” 1956, p 231. (7) J. M. Trowell, ANAL.CHEM., 37, 1152 (1965). (8) R. A. Hagstrom, AFRPL TR 56-20, January 1966. (9) R. N. Smith, J. Swinehart, and D. G . Lesnini, ANAL.CHEM., 30, 1217 (1958). (10) F. D. Huillet and P. Urone, J . Gas Chromafogr.,4,249 (1966). VOL. 41, NO. 2, FEBRUARY 1969
295
'2
'
I
sAMPLE\I I / COOLING
X 1024
6
SYSTEM
5
REDUCT ION FURNACE
SIEVE
4
>-
e
J
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ELIUM
E 3 I-
5 2
'COOLING BLOCK AND SAMPLING VJPLVE
II
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-
Figure 2. Schematic diagram of analytical equipment for determination of hydrogen in nitrogen tetroxide I
0
0
I RETENTION T I M E
2
- minutes
Figure 1. Typical chromatograph record showing separation of gaseous products and clean resolution of water peak rates of corrosion of materials used in construction of rockets has not been established, More importantly, however, ambient equilibria in the sample would be shifted by removal of water in a chromatographic procedure, and thus the separated water would be a function of the sample composition as well as a function of chromatographic variables such as sample size, temperature, column length, and carrier gas flow. An extensive series of studies would be required t o establish the relationship of such empirical results t o the value of the propellant for rocket propulsion. We have examined at length the possibility of a gas chromatographic analysis depending o n the separation of components such as H N 0 3 and H N 0 3 xHzOfrom nitrogen tetroxide, and have found that confusing chromatograms are obtained when the column packings are attacked by nitrogen tetroxide and nitric acid; in these instances, we have observed a carbon dioxide peak amid a succession of ill-defined peaks that have defied correlation with the "water" content of nitrogen tetroxide and are intermingled with a tailing NsOc peak. Columns prepared from entirely inert materials, such as perfluorinated polymers and oils, or inorganic substrates and inert liquids that have been exhaustively nitrated, resist oxidation and degradation by nitrogen tetroxide, but up to the present they also have failed to provide clean separations or to yield reproducible and interpretable chromatograms with Nz04,"OB, or red fuming nitric acid (mixture of N 2 0 4 , HNO,, and H20). An example of a resistive column is KelF-300 with 15% Kel-F oil #10 at 80 "C (16-ft, 'i4-inch) with a flow of 25 ml/min of helium. The results we have obtained thus far with gas chromatography are in accord with statements made above which suggest that water does not exist per se in nitrogen tetroxide (except in trifling amounts), that the established equilibrium of mixtures of Nz04, H N 0 3 , and H 2 0 is altered by conditions 296
ANALYTICAL CHEMISTRY
in the chromatographic column, and that water present as such or formed by decomposition of H N 0 3 in the gas chromatograph is usually separated together with variable amounts of H N 0 3 (sometimes accompanied with NOz). We are continuing studies of the behavior of the N204-HN03-H20 system in gas chromatographic procedures. It has been found that problems associated with the determination of "water" in nitrogen tetroxide by gas chromatography can be circumvented by passing a sample through hot copper and copper oxide contained in a column situated before the inlet of the gas chromatograph, whereupon all nitrogen- and hydrogen-containing compounds are converted quantitatively to nitrogen and water, and if free water is present, it passes through unaltered. Because the resulting gases are inert, selection of the chromatographic substrate is greatly simplified for it need only provide adequate separation of water from nitrogen and other gases such as carbon dioxide. The reducing column is packed initially with copper, but after the first sample of N 2 0 4has been passed through, the packing consists of a mixture of copper and copper oxide; thus, when organic compounds are present in the nitrogen tetroxide sample, they will be converted to water and carbon dioxide by the copper oxide, as in the Dumas method for analysis of the nitrogen content of organic materials. Organic compounds may also be directly oxidized by nitrogen dioxide at the elevated temperatures in the column. (If dissolved carbon dioxide is present in the nitrogen tetroxide sample, it will pass through the column unchanged.) A chromatogram showing the retention times of the nitrogen, carbon dioxide, and the water peaks on a typical substrate (Porapak-Q) ( Z I ) , is shown in Figure 1. EXPERIMENTAL
Apparatus. A block diagram of the apparatus is given in Figure 2. The gas chromatograph is a dual-column F & M Model 810 with a thermal conductivity detector operating at a filament current of 155 mA. The chromatographic column is made from 6 feet of stainless steel tubing and is packed with Porapak Q (Waters Associates); it is operated a t 100 "C (isothermal). The injection port and detector temperatures are held at 230 "C. Helium is used as the carrier gas a t a constant rate of flow of 75 ml per minute, and it is (11) D. L. Hollis and W. V. Hayes, J. Gas Chrornatogr., 4, 235 (1966).
purified by passage over a moisture-hydrocarbon trap ( F & M Part No. 2-3660). The reduction system is a quartz tube column (4-mm id by 300 mm long) initially packed with copper wire (Coleman; #29-120 Cuprin) and operated at temperatures up to 800 "C by an electrically-heated furnace (Leco Model 507-300); the cool ends of the quartz tube are connected to the gas chromatograph and to the sampling valve by means of stainless steel Swagelok fittings. Suitable operating temperatures for the copper-copper oxide column are selected by injecting samples and observing the shape of the nirogen peak and the relative areas of the other peaks; the nitrogen peak should remain symmetrical, implying complete decomposition of nitric oxide (see below), and the ratios of the peaks should remain constant over the applicable temperature range. High temperatures ensure rapid decomposition of NzOl and complete oxidation of organic matter. Column temperatures are kept below 800 "C in order to subdue formation of carbon monoxide from carbon dioxide and hydrogen from water (12). At the flow rate indicated and at temperatures in the vicinity of 800 "C,the short column of copper and copper oxide apparently does not decompose water or carbon dioxide sufficiently to affect the analysis (=t2%); this was demonstrated by direct injection of water and carbon dioxide. Moreover, the analytical procedure is optimized and calibrated directly for the water content of N204; if carbon dioxide analyses are required, calibration with ethylene chloride will provide the required accuracy (see below). The liquid sampling valve (Wilkens Cat. No. 57-040 with core No. 57-049) is maintained at 0 to 2 "C by circulating icewater through a block fabricated from aluminum to fit around the valve; control of temperatures ensures reproducibility of the two-microliter samples. All connections are of 1/16N stainless steel tubing (316) and as short as possible between the sampling valve, the copper-packed combustion tube, and the gas chromatograph inlet in order to reduce peak broadening. It is also convenient to have similar stainless steel tubing connections lead to a cell which permits direct spectrophotometric determination of the NO content of N 2 0 4according to the method of Wright, Orr, and Balling (13). We have also found that preheating the carrier gas enhances the symmetry of the water peak, probably by inducing rapid vaporization of the sample. Standard Samples. Calibration vessels of about 75-ml capacity and approximately spherical in shape were constructed from thick-walled borosilicate glass. The tubulatures were connected to stopcocks fitted with Teflon plugs and terminated with glass-to-Kovar seals to which were affixed stainless steel Swagelok fittings. Each vessel was cleaned, dried, evacuated, and then weighed. The vacuum was released (dry air) and a measured volume of water at a known temperature was introduced through the stopcock bore with the aid of a hypodermic needle. The water was frozen by immersing the vessel in liquid nitrogen; the vessel was then attached to a vacuum manifold made of l / q n stainless steel tubing and fittings to which was connected a cylinder of nitrogen tetroxide. The manifold and the vessel were evacuated; about 50 ml of nitrogen tetroxide was allowed to enter the vessel. The vessel's stopcock was closed; its contents were allowed to come to room temperature and then thoroughly mixed. The vessel was reweighed at room temperature and the percentage of water added to the nitrogen tetroxide was computed. The contents were frozen again and the vessel vented to dry air; the stopcock was closed and the contents allowed to come to room temperature. (Standards prepared in this way have been maintained for months without appreciable change in water content.) (12) C. 0. Willits, ANAL.C H E M . 134(1949). ,~~, (13) C. M. Wright, A. A. Orr, and W. J. Balling, ibid.,40, 29 (1968).
0
I 0
aJ r V
I
PERCENT H 2 0 ADDED 0.10
I
0.20
I
I
3.0
.-C
I
% 2.0 2 W
L Y
a W
a [L
1.0
W I-
$ I 0
0.10 0.20 0.30 TOTAL HYDROGEN, AS PERCENT " W A T E R CONTENT"
Figure 3. Calibration curve for total hydrogen in nitrogen tetroxide Procedure. A sample vessel is attached to the system with stainless steel fittings. When thermal equilibrium is established in the chromatograph and the valve cooling block, a small amount of nitrogen tetroxide from the sample vessel at room temperature is vented through the sampling valve to force air out of the line between it and the sampling vessel. The valve is then rapidly turned to introduce the sample into the analyzing system as a single slug so that the helium carrier gas will sweep it through the reduction furnace and then into the gas chromatographic column where the products are separated. Peak heights are read directly from chromatograph records in the usual fashion. RESULTS AND DISCUSSION
The analytical procedure described above is essentially a Dumas combustion involving reduction of the N204,HN03, and HNOz content of the sample with copper (and oxidation or organic matter by CuO), followed by a gas chromatographic separation of the resulting inert products. Since the analytical accuracy and applicability of the Dumas procedure is documented, it was necessary merely to determine that appropriate experimental conditions (such as flow rate and temperatures) can be readily established and that the mechanical arrangement provides reproducible introductions of sample. It is to be recalled that the Dumas combustion converts all hydrogen compounds to water; thus, hydrocarbon greases and organic matter that may be present in the nitrogen tetroxide sample will contribute to the chromatographic water peak. Consequently, the gas chromatographic procedure described here is actually a determination of the total hydrogen content of a nitrogen tetroxide sample. As expected, pure nitric acid (99.9%) added to nitrogen tetroxide samples increases the water peak in proportion to the new hydrogen contents. Typical data obtained from standard samples prepared with pure N2O4as described above show a coefficient of variation of about 1% (eight replicates of a sample of nitrogen tetroxide containing 0.13% water). When plotted as peak height us. per cent added water, data points are linear and permit determination of the initial "water content" of the nitrogen tetroxide used to prepare the standards (see Figure 3). The day-to-day variation of analytical results obtained with two samples indicated that results were readily obtainable within the =k2% accuracy of delivery from the sampling valve; the height of the nitrogen peak is a sensitive indicator of the VOL. 41,NO. 2, FEBRUARY 1969
297
amount of sample delivered by the valve and it also was well within +2%. The limit of detection of water can be improved by using a valve which delivers a larger sample. AS indicated in Figure 1, carbon dioxide is also determinable. Carbon dioxide may be present per se in nitrogen tetroxide or may be formed in the analytical procedure by oxidation of dissolved or suspended organic matter. Since ethylene dichloride is freely soluble in nitrogen tetroxide, it was used as a calibrant for the carbon dioxide peak; samples of nitrogen tetroxide containing dissolved ethylene dichloride gave proportionally increased carbon dioxide and water peaks (stoichiometric). Perhaps ethylene dichloride may be useful as a calibrant for both carbon and hydrogen because it offers a weight advantage over the direct introduction of carbon dioxide or water. The halogen content of the ethylene dichloride does not appear in the gas effluent from the hot copper; apparently, it forms copper(1) chloride and remains in the column or condenses in the line leading to the chromatograph. The shape of the nitrogen peak is symmetrical as long as there is sufficient copper in the column packing t o reduce nitrogen tetroxide; when the copper is spent, nitric oxide will be formed and the nitrogen peak in the chromatogram appears unsymmetrical because nitric oxide has a longer retention time than nitrogen and is not cleanly resolved. The
column packing can be regenerated by repeated injections of small portions of methanol while the column furnace is maintained at 400 "C to 500 "C. During regeneration, the water peak is monitored; when it reaches a low, constant value, regeneration is complete. Alternatively, the combustion tube can be repacked with fresh copper. A calibration plot for water was also obtained with methanol-water mixtures that had been analyzed for their water content by the Karl Fischer procedure. The calibration plot obtained in this way (by-passing the furnace) was in all ways identical with the one obtained from mixtures of water and nitrogen tetroxide. A methanol-water mixture may be used for a periodic check of the calibration of the chromatographic system, once it is established that the operating conditions of a given copper-copper oxide column are such that the water content of a calibrated mixture of nitrogen tetroxide as indicated by the gas chromatograph record is the same as that obtained from a methanol-water mixture. RECEIVED for review June 30, 1967. Accepted October 31, 1968. This work was performed for the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the National Aeronautics and Space Administration under Contract NAS7-100.
Analysis of Tea Flavanols by Gas Chromatography of Their Tr imethylsilyl Derivatives Albert R. Pierce, Harold N. Graham, Seymour Glassner, Howard Madlin,' and Jorge G. Gonzalez Thomas J. Lipton, h e . , 800 Sylvan Ave., Englewood CIifs, N.J. 07632 An accurate quantitative gas chromatographic method for the determination of tea flavanols as their trimethylsilyl ethers is described. Two sets of isothermal conditions are used to separate the derivatives on an 8-foot glass column packed with 3% OV-1 supported on 60-80 mesh Gas Chrom Q. The method is convenient and accuracy and reproducibility are good. Pure TMS ether calibration standards were not commercially available nor could they be successfully prepared. The need for pure standards was overcome through the unique use of a gas density balance detector to determine directly the exact yield of the derivatization reaction.
THEORGANOLEPTIC properties of green tea (consumed mostly in Japan and China) are partially determined by the quantities and proportions of the various flavanols present: catechin (CAT), epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG) ( I ) . These flavanols are enzymatically oxidized during the fermentation of green leaf to black tea to produce the normal tea of commerce for this country and most of the tea drinking world. This black tea is manufactured by a process that crushes the green tea leaf which breaks the vacuoles and thereby causes the flavanols to come into contact with the tea polyphenolase enzyme. The resulting oxidations bring into
Present address, Foster D. Snell, Inc., 29 West 15th Street, New York, N. Y. (1) C . R. Harler, "Tea Manufacturer," Oxford University Press, London, 1963,2-3.
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ANALYTICAL CHEMISTRY
existence the characteristic black tea properties which are partially dependent on the original flavanol content (2, 3). The enzymic reactions are terminated by heat treatment when the process is considered to be complete. Leaf to be manufactured as green tea is made enzymatically inactive before maceration, thereby preventing most flavanol oxidation. Paper and column chromatographic methods for the separation and quantitative analysis of these chemicals have been reported (4-13). The methods are time consuming and require very carefully controlled conditions in order to obtain reproducible results. The flavanols cannot be directly de(2) E. A. H. Roberts, J . Sci. Food Agr., 9, 381 (1958). (3) G. W. Sanderson, Tea Quart., 36, 172 (1965). (4) A. E. Brandfield, M. Penny, and W. B. Wright, J . Chem. SOC., 1947, 32. (.5 .) R. A. Cartwright - and E. A. H. Roberts, Chem. Znd. (London), 1954, 1389. 16) ~,K. M. Dzhemukhadze and G. A. Shalneva, Biokhimiya, 20, 336 (1955). (7) A. L. Kursanov and M. N. Zaprometov, ibid., 14,466 (1949); Chem. Abstr.. 44. 9786 (1950). (8) E. A. H. Roberts and D. J. Woods, Biochem. J., 53, 332 (1953). (9) M. S . Shipalov, M. A. Bokuchava, and G. A. Soboleva, Biokhimiya, 23, 390 (1958); Chem. Absfr.,52, 17359e (1958). (10) M. N. Zaprometov, Fiziol. Rust., 5, 46 (1958); Chem. Abstr., 53, 131f(1959). (11) M. Nakagawa and H. Torii, Agr. Biol. Chem. (Tokyo), 28, (3), 160 (1964). (12) M. Nakagawa and H. Torii, ibid., 28, (8), 497 (1964). (13) L. Vuataz, H. Brandenberger, and R. H. Egli, J . Chromatogr., 2, 173 (1959).
