Elimination of the interferences from isotopes of relatively long half-life such as sodium and chlorine was accomplished by subtracting an 80-sec decay spectrum from a 20-sec decay spectrum. The elimination of the interferences far outweighed the possible loss of activity. A subtracted spectrum of NBS Bovine Liver No. 1577 is shown in Figure 4 and the subtracted spectrum of the same serum sample used in Figures 2 and 3 is shown in Figure 5. It is important to note that the oxygen and neon are still present; and, consequently their contributions must be subtracted from the area under the selenium peak. By utilizing y-ray spectra obtained from pure neon and water, the 23Ne and 1 9 0 contributions in the region of the 77mSe photopeak were obtained as a function of the counts in the respective photopeak of the contaminant. A Fortran computer program was then written for an IBM 1620 so that merely by feeding the 20- and 80-sec spectra (obtained on paper tape from the analyzer) into the computer, the channel sums are taken, contributions subtracted, and corrections made for the inherent error existing when counting short-lived activities in a period comparable to the radioisotope half-life. The selenium standards were prepared from dissolved and lyophilized Johnson Matthey spectrographically standardized selenium shot containing 0.03-ppm sodium and no reportable oxygen. Interfering ion corrections were not applied to the selenium standard.
niques. The National Bureau of Standards Standard Reference Material No. 1577 Bovine Liver had a reported selenium content of 1.1 f 0.1 pg/gram of sample (95% confidence limit). The reported selenium content was determined by neutron activation analysis employing the 75Se isotope ( 2 ) and isotope dilution mass spectrometry. Presented in Table I1 is a comparison between our analyses of the NBS Bovine Liver Standard employing both the Ge(Li) detector system and the 5-mm X 3-in. NaI (TI) crystal assembly. We can readily see that the results compare quite well to the NBS reported values. It should be pointed out that the Bovine Liver Standard contained quantities of selenium a factor of 10 higher than those found in typical serum samples. At high selenium concentrations, there is no apparent difference using a Ge(Li) detector or a 5-mm x 3-in. NaI (Tl) detector assembly. However, for typical biological systems using a low-power nuclear reactor, the 5-mm X 3-in. NaI (Tl) is preferred. As can be seen from a comparison of the actual values of selenium content in identical samples presented in Table 11, the relative standard deviation is less than 10%. The sensitivity of our described procedure is L0.6 pg of selenium, using a reactor power level of -3 x 10’1 n/cm2 sec. During a normal working day, the number of selenium analyses that can be run-including spectral analyses and data reduction-approaches 150 samples.
APPLICATION OF PROCEDURE It was of importance to compare analytical results using our procedure with a sample analyzed by different tech-
Received for review October 12, 1972. Accepted January 8, 1973.
Nuclear Magnetic Resonance Determination of Water and an Oxygen Titration for Nitric Oxide in Liquid Nitrogen Tetroxide Stephen P. Vango and Stanley L. Manatt Space Sciences Division and Propulsion Dwision. Jet Propulsion Laboratory. California lnstitute of Technology. Pasadena, Calif. 97 703
An NMR procedure is described for the rapid quantitative analysis of water in nitrogen tetroxide oxidizer. This technique is capable of detecting as little as 0,001 wt YO of H20 (10 p g of water per gram) and gives results in the concentration range of 0.01 to 0.02 wt YO with a precision of about f0.002 wt YO.Because many samples of oxidizer contain NO, a procedure for 02 titration of NO prior to N M R analysis for H 2 0 is described which gives the NO concentration accurate to f0.05 wt YO.
The quantity of water present in nitrogen tetroxide profoundly affects its rate of attack on metals. A few tenths of one per cent of water in nitrogen tetroxide to be utilized as an oxidizer in a propulsion system cannot be tolerated. As part of a quality control program of propellantgrade nitrogen tetroxide, we have had to consider carefully the merits of the various methods for detection and quantitation of water in the latter (1-4). Very recently, it has been stated that an NMR method for this analysis 1060
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
“remains unattractive as a quality control procedure because of the high cost of instrumentation and the intricacy of sample preparation” (4). We feel this statement is not warranted at the present time, as NMR instrumentation is very wide-spread in control laboratories and the sample preparation requires no more effort than that involved in sampling for the analyses of other components of a Nz04 oxidizer ( i e . , for total N204, NOC1, and ash). Thus, we describe here the details of our NMR water procedure for nitrogen tetroxide oxidizer which has been in common usage in our laboratories for some years. It is believed that a mixture of a small amount of water in nitrogen tetroxide a t ambient temperature is converted (1) Military Specification; Propellant, Nitrogen Tetroxide, Mil-P-26539
USAF, July 18, 1960. (2) G. C. Whitnack and C.J. Holford, Anal. Chem., 21, 801 (1949). (3) N. V. Sutton, H. E. Dubb, R . E. Bell. I . Lysyj, and B. C. Neale, “Advanced Propellant Chemistry,” Advan. Chem. Serios, 54, 231
(1966).
(4) R. F. Mufaca, E. Willis, C. H. Martin, and C. A . Crutchfield, Ana/. Chem.. 41, 295 (1969).
la) 0.12% H2C
SWACELCU FITTINGS7 ABSORPTION PEAK
I NIEGRA L
(bl 0.016% H20
A R,EA
& ABSORPTION PEAK
K
~
l
i
1 "
Figure 1. Proton NMR spectra of liquid on an A56/60 spectrometer
G
N204
-
SS VALVE; TEFLON PACKING i n KOVAR . TO GLASS SEA1 100-mi KJELDAHL FLASK USED FOR NITRIC OXIDE ASSAY
7
solutions recorded
4
- / C 2 s T i R R i N G
GLASS ENCLOSED MAGNETIC
BAR
Figure 2. Sampling assembly for NO determination
(ai 0.12 wt YO H20 and typical integral. / b ) 0.016 wt YO H 2 0 and typical integral: liquid N204 resulting from P205 treatment N I T R I C OXIDE DETERMINATION FLASK
to a mixture of nitric acid and dinitrogen trioxide (5, 6).There is some feeling that perhaps some unreacted water exists in nitrogen tetroxide as HN03.XH20 (3). No matter what the form of water present, there is an overwhelming amount of information which indicates that the protons in an acidic solvent such as the nitrogen tetroxide system will be exchanging so rapidly on an NMR time scale that one can expect to see only a single proton signal. Initially there was some doubt in our minds whether this single proton NMR line could be observed, because of strong relaxation by the free radical NO,. This species is present in relatively large amounts because of the equilibrium in the liquid phase, N20, 2 NO, (1) which has an equilibrium constant in the gas phase of 1.1 X 10-5 a t 25 "C a t 1 atm (6). In liquid nitrogen tetroxide a t 25 "C, it can be roughly estimated that NO, will be present at a concentration of a t least one molar. It is well known that radical concentrations of the order of 10-5molar can severely broaden proton NMR signals. The fact that we indeed could observe the sharp single proton signal, as shown in Figure 1, on which this analytical procedure is based, indicates that the protons must not experience relaxation by the odd electron on NO,. This observation suggests that the unpaired spin is severely quenched in solution or that the relaxation time of the latter is very short. Some of the nitrogen tetroxide oxidizer samples which we have analyzed are mixed oxides of nitrogen which are referred to as MON-10. This material contains about 10% by weight of NO dissolved in N204. We found that it was impossible to observe proton NMR signals of such mixtures as received. Thus, unlike NOz, the paramagnetic NO drastically affects the nuclear spin resonance. However, after titration with 0 2 , which converts the NO to NOz, we could subsequently proceed with analysis for water by NMR. For operational reasons, it is most practical to perform an NO analysis by an 0 2 titration procedure on about 50 grams of the oxidizer and then prepare NMR' samples from the product resulting from the titration. The details of this procedure are described herein.
EXPERIMENTAL Apparatus. Figure 2 shows the type of glass flask used in the nitric oxide determination. T h e flask can be constructed from a 100-ml Kjeldahl flask. T h e wall in t h e Kovar tubing of t h e Kovar t o borosilicate glass seal should be a t least 0.010 in. thick. Such seals can withstand several hundred psi, although in this applica(5) J. J . Carberry, Chem. Eng. Sci.. 9, 189 (1959). (6) P. Gray and A . D. Yoffe. Quart. Rev. /London).9, 363 (1955).
RESERVED M A T E R I A L FROM N I T R I C O X I D E DETERMINATION
v
NOTE: ALL HARDWARE I S STAINLESS
118 in. S W A G E L M F I T T I N G WITH TEFLON SLEEVE
Figure 3. Sampling assembly for preparation of from material resulting from 02 titration
NMR
samples
tion the pressures experienced will be much less. T h e valves used are modified ys-in. stainless-steel Hoke valves. The Ys-in. male pipe threads were replaced, by welding, with Yk-in. male Swagelok fittings. T h e packing in t h e valves is Teflon (Du Pont). T h e Hoke catalogue number for this valve is 321A. T h e flask is attached to a y,-in. oxygen line previously purged. Figure 3 shows t h e hardware used t o connect t h e N M R sample tubes to the flask of m a terial from t h e nitric oxide determination. N M R spectra were recorded on Varian A60 and A56/60 spectrometers. Careful attention was given t o establish precise calibrations of t h e spectrum amplitude switch and the integral a m plitude switch. T h e latter calibrations enabled the extension of the method over about two orders of magnitude in concentration and was useful for other analytical work in progress. Samples were sealed into 5-mm 0.d. precision N M R tubes either purchased from Varian Associates or Wilmad Glass Co., Inc. ( P a r t No. 507-PP). T h e tubes were inspected t o ensure uniform wall thickness and o.d., both visually with calipers a n d with a set of machined plastic shims. In general we have had to reject only a very small percentage of these tubes a s received, and also we have been able t o reuse t h e tubes many times. Typical spectrometer settings for the analysis are in the following ranges: r.f. field 0.03-0.05 mg, sweep width 250 or 500 Hz, and sweep times 100250 sec. T h e usual signal-to-noise ratios of our A56/60 spectrometer with which most of this work has been done range from 15:l t o 20:l for the Varian 5 m m 0.d. 1% ethylbenzene standard. Figure 1 shows some spectra a n d integrations which display typical signal-to-noise ratios observed with two oxidizer samples. Reagents. Commercial tank oxygen from several suppliers was used from which traces of moisture were removed by passing it through a n in-line glass tube VZ in. long packed with "Anhydrone" (J. T. Baker Chemical Co.). Reagent chloroform ( J . T. Baker Chemical Co.) was used as received. Phenylacetylene (Farchan Research Laboratories) was distilled under vacuum: it showed no detectable impurities by NMR. The nitrogen tetroxide used t o prepare the standard samples was obtained from Hercules Inc. I t was purified as follows: A quantity of t h e liquid nitrogen ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1061
tetroxide was transferred to a flask provided with a bubbler containing about a 5 wt % of PpO5 and a glass enclosed stirring bar. The contents of the flask were stirred for several hours a n d then left overnight. Anhydrone dried 0 2 was bubbled through the nitrogen tetroxide so as t o carry the vapors through a borosilicate glass tube heated to 100 "C and subsequently condensed a t -78 "C in the first of a series of traps. This operation serves t o convert any N O to NO2 a n d any C o p initially in t h e starting material passes the -78 "C trap. Any excess oxygen was pumped off from the distilled nitrogen tetroxide during a series of thaw-pumpfreeze cycles. This material was next transferred by bulb-to-bulb distillation with liquid Np to a series of ampules possessing break seals and these were sealed off. Analysis by the modified Whitnack-Holford Procedure (2, 7) of the material purified in this manner could detect no water. However, a sharp NMR signal indicative of about 0.016 wt % water was readily detected (see Figure 1 and procedure described below). Procedure for NO Analysis. The determination of nitric oxide is based on the reaction of NO with 0 2 t o give 5 2 0 4 . The NO content is calculated from the weight of 0 2 consumed in oxidizing the NO. Our procedure is based, in principle, on a military procedure, but differs in some details (8). A glass reaction vessel rather than a metal one is used (see Figure 2). T h e glass vessel permits visibility but is potentially more hazardous to handle. T h e glass vessel is lighter and more suited for weighing on a regular analytical balance. Also glass vessels are easier t o fabricate, clean, and free from the last traces of moisture. The glass unit is evacuated initially as well as after completing the reaction with oxygen. This procedure eliminates t h e need for a correction due t o the weight of excess oxygen present in the vessel. The evacuated weighed flask is attached to the sample holder a s shown in Figure 2. About 35 to 45 ml of liquid oxidizer are admitted into the flask via the evacuated tubulation between the sample holder a n d the flask. The liquid oxidizer is frozen by immersing the portion of the flask containing the liquid phase in liquid nitrogen. After freezing of the liquid phase, t h e whole flask is immersed in the liquid nitrogen in order to freeze out the oxidizer present in t h e gas phase. T h e valve on t h e flask is closed and the contents are thawed in water. After thorough drying and allowing time for equilibrium with t h e laboratory air, the flask is reweighed. The gain in weight represents the sample weight. Prior to admitting oxygen t o t h e flask, t h e magnetically stirred oxidizer in the flask is cooled with ice water. With the regulator on the oxygen cylinder set to 40 psig, oxygen is admitted t o the cooled flask by opening t h e flask valve. The dark green color, due to the N 2 0 3 present, gradually lightens and the oxygen flow indicated on an in-series flow meter tapers off. When the green color has disappeared and the oxygen flow has all b u t ceased, the valve on the flask is closed. With the valve closed, nitrogen dioxide fumes drift into the neck of the flask. From time to time, additional oxygen is admitted which causes the neck of the flask to be cleared of brown fumes, and the flow meter reading briefly reaches a high value followed by a rapid decay. The periodic opening of the valve in the flask is repeated until it no longer causes the brown fumes to disappear from the neck of the flask. The contents of the flask are stirred for an additional five minutes. T h e flask is disconnected from the oxygen line and immersed in liquid nitrogen. When the flask contents are completely frozen, as indicated by the quiescence of the liquid nitrogen, t h e excess oxygen is pumped off. T h e pressure in the flask is reduced t o 100 pm or less. The valve on t h e flask is closed, t h e flask is removed from the vacuum line, the contents are thawed in water, and after thorough drying and allowing for equilibrium with the laboratory air. the flask is reweighed on a n analytical balance. T h e gain in weight represents the weight of oxygen consumed in oxidizing the nitric oxide to nitrogen tetroxide. In order to realize the maxim u m precision, weighings are made t o 0.1 mg. T h e per cent nitric oxide is calculated as follows:
Wt % NO =
Wt of O2 Consumed
X
1.875
Wt of Oxidizer Sample
X
100
(2)
The contents of the flask from the nitric oxide determination are reserved for the assay of nitrogen tetroxide, nitrosyl chloride, water, and ash. Reliable procedures for performing these subsequent analyses by previously reported techniques have been described ( 7 , 9). ( 7 ) S. P. Vango, "Improved Sample and Analytical Procedure for Nitrogen Tetroxide and Mixed Oxides of Nitrogen," JPL Technical Report 32-1282, October 1 5 , 1968. (8) NASA Specification MSC-PPD-2.
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
Procedure for HzO Analysis. A portion of t h e material titrated above with oxygen is next transferred to one or more N M R sample tubes. T h e glass reaction vessel is attached t o t h e sample tube as shown in Figure 3. The sample tube and interconnecting tubulation are warmed with a heat gun, and the sample tube also plied with a spark coil while pumping t o remove adsorbed traces of water. Pumping is continued, using a mechanical p u m p protected against contaminants with a liquid nitrogen trap, until t h e pressure as read on a Pirani or thermocouple gauge drops t o 100 pm or less. When t h e desired vacuum has been obtained in t h e s a m ple tube, t h e valve t o the vacuum p u m p is closed and the nitrogen tetroxide is admitted to the sample tube by slowly opening the valve on the sample holder. Cooling the sample tube slightly by momentary immersion in liquid nitrogen facilitates t h e flow of oxidizer into the sample tube. With the desired sample volume in the sample tube (usually 0.3-0.6 ml), the tube contents are frozen in liquid nitrogen. Initially, the tube is immersed in liquid nitrogen only u p t o the level of the contained liquid oxidizer. A laboratory jack is a convenient way of adjusting the height of the liquid nitrogen dewar. With the oxidizer frozen, the sample tube is immersed in liquid nitrogen well above the frozen NOp. When the liquid nitrogen has become quiescent, the tube is sealed off with a hand torch. The sealed-off sample tube is thawed by heating in a stream of warm air from a heat gun. There is a slight temperature dependence in the area measurements and we ensure that our unknown and standard samples always reach the same ambient probe temperature before recording any data. Usually this can be achieved for the samples while the spectrometer is being tuned u p or while other work is being recorded by placing the samples in a small holder close t o the magnet gap inside the A56/60 magnet case. Usually 5-10 integrals are recorded, measured, and averaged in only a few minutes. T h e integration results present in Table I are the averages of 10 separate integrals for each sample. A precisions of &0.002 wt Yc in the 0.001 to 0.2 wt % range is usual.
RESULTS To establish useful NMR procedures for quantitative analysis, it is necessary t o demonstrate that there is a reasonably simple and smooth relation between signal area or height and concentration of the species which are to be measured. Table I contains the data for the N204-NO2 liquid standards which we made up and analyzed. For the samples above 0.2 wt % H20, it was feasible to transfer weighed amounts of H20 to the driest N 2 0 4 - N o ~we could make. Because the addition of the water causes generation of NO (5) via the following set of reactions:
H20
+
N20,
HNO2
Z=
HS + NO3H2O
+
NO
+
+
HNO,
NO,
(3) (4)
each of the samples was subsequently titrated with dry 0 2 to destroy the NO. Three standard samples of nitrogen tetroxide containing between 0.1 and 0.2 wt % H2O were prepared using purified nitrogen tetroxide. The water content was determined by multiple analysis using the more time-consuming, modified Whitnack-Holford analysis method (2, 7). Results reproducible to kO.01 wt 3' % were obtained. It was our experience on trying to add weighed, small amounts of moisture to give 0.1-0.2wt 7% standards that we could not obtain reproducible results. We suspect that these small quantities of H2O were being partially lost in the vacuum system. The data in Figure 4 show that a satisfactory linear relationship does exist between NMR peak area and concentration of protons. The peak heights were not utilized because we noted early in this work that there appeared to be rather significant, random variations of peak width from sample to sample. This could arise from different (9) S. P. Vango, "Techniques for Determining Gases Dissolved in
Oxidizer and Liquids in General," JPL Technical Report 321236, April 1968.
Fuels,
Table I. Data Obtained in Establishment of Relation of Weight % H20 to Proton NMR Integration for Set of Standard Samples Wt % H20 concentration Apparent, based on wt H20 added to "dry" oxidizer
Corrected for wt YO H20 in "dry" oxidizer
Electronic integration area of NMR signal, arbitrary units
0.01 5 f 0.001 0.11 1 f 0.002 0.114 f 0.003 0.138 f 0.002 0.21 0 f 0.003 0.325 f 0.004 0.572 f 0.008 0.776 f 0.013
f6.7 1.5 f 0.7 2.1 f 0.5 1.2 f 0.8 1.3 f 1.3 1.4 f 2.3 1.4 f4 1.7 AV 2-8 f 1.5% a Yo H 2 0 obtained by gravimetric analysis, ;.e. Whitnack-Holford method of reference 2 modified as described in reference 9 1 2a 30 4a 5 6 7 8
0.12 f 0.01 0.12 f 0.01 0.15 f 0.01 0.214 0.305 0.576 0.736
0.016 0.12 f 0.01 0.12 f 0.01 0.15 f 0.01 0.230 0.329 0.592 0.752
amounts (very small) of paramagnetic metal impurities in the various sample batches. The line presented in Figure 4 was determined by least squares fit to the seven standard samples and the origin. Thus, it appears that the driest NzO4-NOz we can make by our P205 treatment is about 0.016 f 0.002 wt % H2O. We performed our drying procedure on several batches of material and each time found proton concentrations by NMR within experimental error of this same magnitude. The largest error in establishing this procedure is in the standard sample preparation. It was our experience on trying to introduce small, weighted amounts of water into a sample through a vacuum system, that small and unpredicable amounts of HzO can be lost or liberated. The errors for the NMR integrations are much smaller (equivalent to f0.002 wt %) than the errors either in the modified Whitnack-Holford method (2, 7) (fO.01 wt %) or arising in the preparation of the weighed standard samples. To provide a further check on the total proton content, the peak areas of several of the standard samples were compared with peak areas of known proton contents in several pure liquid organic compounds, CHC13 (0.844 w t 70 H) and C&C=C-H (acetylenic proton, 0.987 wt 70 H ) . The agreement for these comparisons was within experimental error; i.e., f0.006 wt 70. The most significant source of systematic error for this NO and H2O analysis we believe stems from our observations that samples of the same batch of oxidizer (from a large tank) which were delivered to us in two separate sample containers and collected at different times, in most cases exhibit 5 1 0 % variation in proton concentrations and significant differences in NO concentrations. However, our sample preparation technique and the NMR spectral measurements can be done reproducibly to f0.002 wt % absolute in the concentration range of 0.01 to 0.2 wt %. Typically, we make two NMR samples from any N204-NOz sample from a given metal container. Then 5-10 integrated spectra of each of these samples are compared with 5-10 integrated spectra of one of our standard N204-NOz samples and the concentrations of the unknowns are calculated from the linear relation between concentration and area which can be expressed by the following simple equation: Wt % water = (Wt % H 2 0 in Standard)(Integral H 2 0 in Unknown) (Integral H,O in Standard) (Integration Gain for Unknown) (Integration Gain for Standard) (5) Usually we have selected a standard of the order 0.1 w t % H2O because most of our unknowns have had water conX
4.5 32.6 33.6 40.6 61.9 95.8 168.8 229
Wt % H 2 0 calculated from least squares fit
Average error of integration, %
f 0.3 f 0.5
0.9
I
o.at
l
I
:::I
I
WEIGHT Lk H 2 0
l
=
l
l
l
l
I
l
A
0.W340 x ARE4
0.5
WEIGHT Lk H20
0.3
o*21 0.1
40 80 120 160 200 AREA FROM ELECTRONIC INTEGRATION, RELATIVE U N I T S
240
Figure 4. Linear relation between wt % H20 in liquid nitrogen tetroxide and integration of proton NMR signals. Data from Table I. Error bars (ur)shown for area integrations where significant; error bars ( 1) shown for wt % H20 in three samples analyzed gravimetrically. Errors in wt % H20 in samples made up by adding weighed amounts of H20 to "dry" oxidizer can be as large as f0.03 wt YO H20. The point 0 represents wt % H20 in "dry" oxidizer
Table I I . Some Typical Analyses for NO and H 2 0 in Oxidizer. Each Letter Represents a Particular Tank Which Was Sampled Six Times during a Series of Propellent Loading Operations Wt % H20 Gravimetric (;.e., Whit- Wt % H20 By nack-Hoiford pro- NM average edure) ( 2 , 7) for two samples
Sample
Wt % NO
1A 2A 3A 4A 5A 6A
10.12 10.06 9.94 9.98 10.14 9.95
0.08 0.16 0.17 0.12 0.1 2 0.15
0.07 0.14 0.17 0.12 0.12 0.14
1B 38 48 58 6B
9.80 4.71 9.57 9.54 9.52 9.50
0.16 0.13 0.16 0.15 0.16 0.16
0.15 0.13 0.13 0.12 0.13 0.12
1c 2c 3c 4c 5c 6C
9.71 9.66 9.64 9.71 9.86 9.70
0.16 0.14 0.1 1 0.1 1 0.15 0.1 1
0.13 0.12 0.12 0.1 1 0.1 1 0.1 1
28
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1063
centrations from 0.02-0.14 wt % and all the integrations could be done a t a single gain setting. Thus the factor a t the right in the equation above did not enter. However, when establishing the linear relationship presented in Figure 4, we did have t o use three different gain settings. We have found with A60 and A56160 spectrometers that the use of standards and switching of samples does not introduce any measurable error and allows for any slight or major day-to-day variation of the instrument gain. We have found that our standard N204-NOz samples have exhibited stable proton concentrations for a t least five years. Table I1 gives typical results for oxidizer analyses for both NO and HzO. Each series of analyses with a particular letter were performed on different portions of oxidizer being passed from a large tank into several spacecraft oxidizer storage tanks. The variations observed are typical of the sampling and dispensing errors we have found characteristic from large tanks which are not stirred and not always a t the same temperature when tapped. Results for the H20 content were obtained both by the modified Whitnack-Holford method (2, 7) and the KMR method. Comparison of these HzO analyses shows that the NMR results are about 0.02 wt % lower than the former method.
DISCUSSION The detailed procedure for the N O analysis described here has been in general use in our laboratories for some time. Another procedure for determination of nitric oxide in dinitrogen tetroxide has been described (20). This spectrophotometric method was verified for the concentration range of only 0% to 1.5 wt % nitric oxide. The oxygen titration technique described here has been applied to samples containing 0.5-10 wt YO NO. The spectrometric procedure required a specially designed cell capable of being maintained a t 0 "C. The oxygen titration procedure requires less special equipment and interfaces more smoothly with subsequent analyses usually required from a sample aliquot of liquid nitrogen tetroxide oxidizer. The handling of the samples in the manners we described allows easy preparation of samples for NMR analyses and is also consistent with the transfer of samples t o borosilicate (10) C. M Wright. W. A . Orr. and W. J. Balling, Anal. Chem.. 40, 29 ( 1968).
glass sample bulbs for subsequent total N 2 0 4 assay and nitrosyl chloride determination (7, 9). Our sampling handling procedures do not encounter any difficulties due to hydroscopicity and high volatility because the material is always in a closed system. Also operator exposure to the oxides of nitrogen is not a probIem. Some time ago, a NMR method for measuring H2O in X 0 2 was suggested (3). The procedure described for this determination required t h a t one take a n unknown and add several carefully incremented amounts of H2O to several separate N M R sample tubes which were weighed subsequently. As mentioned above, it is difficult and timeconsuming to do this accurately for increments of water in the 0.1-0.2 wt % range. (Also, the N O formed should be removed.) From spectra of the samples so prepared, the initial amount of H2O present was ascertained (3). This procedure requires much more effort than preparation of one or two sample tubes which do not have to be weighed. We found that the sample preparation procedure and the NMR technique described here, once permanent standards were prepared. is rapid, precise, and convenient for subsequent analyses of other components in nitrogen tetroxide oxidizer (7, 9) In our hands, the time required for a NMR water determination as described here (both sample preparation from product of a n oxygen titration and recording of sample and standard spectra for duplicate samples) can be reduced to about half a n hour. The NMR water results are reproducible to *0.002 wt % in the concentration range 0.01 to 0.2 wt YO water on duplicate samples prepared from the same large sample. We have found t h a t the NMR method gives results about 0.02 wt % lower than the modified Whitnack-Holford method in use in our laboratories (7). The latter procedure requires much more effort to set up and stabilize (more than a day's time). T o accomplish duplicate analyses which require appropriate blanks and standards is much more time-consuming (requires about 5 hours). On multiple analyses of the same sample, this procedure appears to be accurate to about *0.01 wt % in the 0.01 to 0.2 wt % range which is less accurate than the NMR method. Received for review August 28, 1972. Accepted December 27, 1972. Research sponsored by the National Aeronautics and Space Administration under Contract No. NAS 7-100.
Protonation of Weak Bases in Sulfolane as Solvent J.
F. C o e t z e e '
and R J. Bertozzi
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pa. 15213
Predominantly anhydrous solutions of perchloric acid in sulfolane were titrated conductimetrically with a variety of mono- and bifunctional weak bases, including water, alcohols, ketones, and amines, and also substances that are themselves important dipolar aprotic solvents, including acetonitrile, nitrobenzene, dimethylformamide, and dimethylsulfoxide. Titration curves of 4 general types were
IPlease address all correspondence to this author. 1064
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7 , JUNE 1973
obtained. Since some water is always present, bases that are stronger proton acceptors than water in sulfolane (S) give separate inflections corresponding to successive protonation by S H + and H 3 0 + . Titrations of this type are useful for the differential determination of anhydrous acid and water. Even bases as weak as acetonitrile and nitrobenzene can be protonated in sulfolane. Homoconjugation of 2,6-dihydroxybenzoic acid was studied in some detail. Uncertainties in a provisional calibration of an acidity scale for sulfolane are discussed.
