subtracted from each. The logarithms of the corrected water count times at XI and Xz were subtracted from the logarithms of the corrected sample count times at XI and XZ. The same counting procedure was repeated for niobium and zirconium. Using Equation 2, these net corrected logarithms of count time were used for the calculation of the uranium, niobium, and zirconium concentrations in the sample. RESULTS
Estimates of precision for the analyses of metal chips were obtained by an 8-day test which included metal chip inhomogeneity and instrumental and analytical sources of variation. The chip batch for the test consisted of 200 grams of machine turnings which were chopped into small chips and blended to provide the most homogeneous sample possible. On 8 random days during a 1-month period, portions of the batch were submitted for analysis. Precision data are summarized in Table I. The relative standard deviation based on within-day and between-day variability is just slightly larger than the relative standard deviation based on withinday variability only.
Estimates of precision omitting metal chip inhomogeneity were obtained by analysis of a standard solution. The solution was extracted and counted as a routine sample during the same time period as the metal chip analyses. The relative standard deviations for 10 metal chip analyses are somewhat larger than the relative standard deviations of the standard solution. This comparison shows the degree of inhomogeneity of the metal chip batch. ACKNOWLEDGMENT
The authors acknowledge the assistance of R. E. Barringer in manuscript review and thank E. E. Johnson for the statistical evaluations. RECEIVED for review July 20, 1967. Accepted October 2, 1967. Work performed at the Oak Ridge Y-12 Plant, Post Office Box Y, Oak Ridge, Tenn. 37830, which is operated by Nuclear Division, Union Carbide Corp., for the United States Atomic Energy Commission under Contract W-7405eng-26.
Spectrophotometric Determination of Nitric Oxide in Dinitrogen Tetroxide Charles M. Wright and Allen A. Orr Research Center, Hercules, Inc., Wilmington, Del. 19899 William J. Balling Hercules, Inc., Hercules, Calg. 94547
Small amounts of nitric oxide, when dissolved in liquid dinitrogen tetroxide, exist as the combined form, dinitrogen trioxide. The nitric oxide content of such solutions can be determined by measuring the intensity of the green color of the dinitrogen trioxide at a temperature near Oo C. The method requires a specially designed cell constructed of materials resistant to dinitrogen tetroxide, capable of being cooled to and maintained at temperatures near Oo C, leakproof under moderate pressure and vacuum, and having a path length suitable for analysis of samples containing 0% to 1.5% nitric oxide. Design and construction details of this cell are given. The effects of temperature and various possible contaminantsviz., water, nitrosyl chloride, iron, and oxygen-on the method are discussed.
SIGNIFICANT QUANTITIES of the dinitrogen tetroxide now being manufactured contain a small, but closely specified, amount of nitric oxide to control corrosivity. The analysis of this material is of considerable industrial interest. The only previously available method (1) for the determination of small amounts of nitric oxide in liquid dinitrogen tetroxide is based on the gravimetric determination of the oxygen required to oxidize nitric oxide to nitrogen dioxide. This method is cumbersome, time-consuming, and of uncertain accuracy. A variety of methods for the analysis of gaseous mixtures of nitric oxide and nitrogen dioxide have been reported (2-6) but none of them is applicable here. Solutions of nitric oxide in liquid dinitrogen tetroxide are green in color-a murky green at room temperature, but becoming a clear emerald green as the temperature is lowered
to near 0’ C. This color is due to dinitrogen trioxide, formed from the combination of nitric oxide and nitrogen dioxide.
Nz04 NOz
+ NO
2 NO2
(1)
Nz03
The chemistry of dinitrogen trioxide (7) and dinitrogen tetroxide (8) has recently been reviewed and the above equilibria have been discussed. Although little data are available in the literature on the liquid dinitrogen tetroxide system, the work to be discussed here indicates that, when small amounts (up to 1.5x) of nitric oxide are dissolved in dinitrogen tetroxide at temperatures near 0” C, the nitric oxide is essentially all present as dinitrogen trioxide. The visible spectrum of dinitrogen trioxide in various organic solvents has been investigated by Mason (9). She (1) NASA Specification MSGPPD-2. (2) H. L. Fawcett, T. C. McRight, and H. G. Graham, Jr., ANAL. CHEM., 38, 1090, 1966. (3) B. E. Saltzman, Zbid.,26,1949 (1954). (4) M. D. Thomas, J. A. MacLeod, R. C. Robbins, R. C. Goettelman, R. W. Eldridge, and L. H. Rogers, Ibid.,28, 1810 (1956). (5) J. M. Trowell, Ibid.,37, 1152 (1965). (6) G. C. Whitnack, C. J. Holford, E. S. Gantz, and G. B. L. Smith, Zbid.,23, 464 (1951). (7) I. R. Beattie, “Progress in Inorganic Chemistry,” Vol. 5, p. 1, Interscience, New York, 1963. (8) P. Gray and A. D, Yoffe, Quart. Rev. (London),9, 363 (1955). (9) J. Mason, J. Chem. Soc., 1959, p. 1288. VOL 40, NO. 1, JANUARY 1968
29
found a weak broad absorption band in the red region that shifted toward the blue with increasing polarity of the solvent. There is no report in the literature of the use of this absorption band for analytical purposes. The present report describes the development of a rapid, precise, and accurate method for the determination of nitric oxide in dinitrogen tetroxide on the basis of the measurement of the intensity of the absorption band of dinitrogen trioxide at 700 mp at temperatures near 0" C. The design and construction of a special spectrophotometric cell are discussed in detail. EXPERIMENTAL
Design and Construction of Spectrophotometric Cell. The cell designed and built is shown schematically in Figure 1. It consists of a stainless steel block about 31/2 X 31/2 X 1 inch with the optical path through the center. A passage was drilled through three of the sides to permit circulation of ice water. A collar in the center of the cell supports and separates the windows and establishes the 2-mm path length. The sample entrance and exit ports were drilled through this collar from the outside of the cell perpendicular to the optical path. Stainless steel tubing, '/*-inch o.d., was threaded into these ports and silver-soldered in place. Stainless steel female Luer hubs were threaded onto the tubing and onto these were fitted Hamilton valves of Teflon and Kel-F. The windows were sealed with 2-mil Teflon gaskets between the collar and the window. Sealing pressure is applied by the end plate through the O-ring when the end plate cap screws are tightened. The cell and accessories permit transferring the sample directly from the sample vessel with only inert materials-i.e., Teflon, Kel-F, stainless steel, and silica windows-contacting the sample. As the cell assembly and tubing can be purged with dry inert gas prior to sampling, and the sample is always under positive pressure during transfer to the cell, there is no danger of contamination with moisture or air. This cell is readily adaptable for use with the Beckman Model B, Cary 14, and Cary 15 spectrophotometers, and most likely other models, although no other instruments were used. A limited number of detailed drawings of the cell are available from the authors on written request. Recommended Method for Determination of Nitric Oxide in Dinitrogen Tetroxide (Spectrophotometric). Apparatus. Cary 14, Cary 15, and Beckman Model B (red sensitive phototube) spectrophotometers were used with a Corning No. 4015 filter calibrated for difference in absorbances of 700 mp and 550 mp ( A A ) equivalent to a stated concentration of nitric oxide in dinitrogen tetroxide in a 2.00-mm cell. A limited number of these calibrated filters are available from the authors on written request. A special, jacketed, absorption cell of 2-mm light path, fitted with Hamilton valves, No. 2661 (Luer-Lok fittings) on the inlet and outlet parts (see Figure 1) was used with Luer-Lok adaptors (for the sample containers) and Teflon tubes with Kel-F female Luer fittings at each end (Hamilton Co.) for transfer of sample to the cell. Calibration Procedure. Measure the absorbance of the calibrated Corning Filter No. 4015 at 700 mp and 550 mp. Subtract A S S m pfrom A700mpto obtain the P A value which is equivalent to the nitric oxide value in a 2.00-mm cell supplied with the calibrated filter. If the path length of the cell to be used is not exactly 2.00 mm, correct the nitric oxide equivalent value as follows: corrected value = calibration value X 2.00 mm/actual path length. Obtain the absorbances of a sample of the pure, dry oxygentreated nitrogen tetroxide at 700 mp and at 550 mp as described under Procedure. Subtract the absorbance at 550 mp from the absorbance at 700 mp to obtain the A A value for nitrogen tetroxide. This AA value will be a negative number and represents 0.0% nitric oxide. Prepare a chart with -0.2 to f1.0 absorbance units as the 30
ANALYTICAL CHEMISTRY
F'L rCAP v T c -END PLATE --"O"
RING
i=
TEFLON GASKET--
-
-
HAMILTON VALVE STAINLESS STEEL TUBING
FIT TIN^ j
HAMILTON VALVE STAINLESS STEEL FEMALELUER
WELL
PASSAGE
STAINLESS STEEL FEMALE ,, ,, LUER FITTING COOLING WATER PASSAGE SAMPLE OUTLET PORT
....I- ._._ i, ~
-I.,--.
STAINLESSSTEEL
SI LlCA WINDOW
1 ,THERMOMETER
~
..o"---
@
SAMPLE INLET PORT
, 1
Figure 1. Temperature controlled spectrophotometric cell for determination of NO in N204
ordinate and 0.0 to 1.5 nitric oxide as the abscissa. Plot the two A A values against % nitric oxide on this chart and draw a straight line through them to give the calibration curve. This line will not pass through the origin. PROCEDURE. Precool the cell, then connect to the sample container. Bleed several milliliters of sample through the cell using the outlet valve to control the flow. When the cell is free of bubbles, close first the outlet valve, then the inlet valve and disconnect the sample supply tube. Place the cell assembly in the cell compartment of the spectrophotometer with the ice water circulating through the block. Connect dry air or nitrogen to the antifogging system and allow only enough gas flow to keep the cell windows fog-free. When the indicated temperature is between 2" and 4' C, measure the absorbance at 700 mp and 550 mp. Subtract Assomp from A700 m p to obtain A A , and read the corresponding per cent nitric oxide from the calibration curve. Preparation of Known Samples of Nitric Oxide in Dinitrogen Tetroxide. The procedure used in preparing all the known solutions of nitric oxide in dinitrogen tetroxide is given below. PROCEDURE.Several 30- to 60-1111, heavy-walled glass flasks were constructed and fitted with Teflon Fischer-Porter Labcrest needle valves, No. 795-609-0004. The flasks were filled with dinitrogen tetroxide directly from a cylinder; the amount added was determined by weight. A known amount of nitric oxide that had been purified by low temperature fractional distillation and shown to be better than 99 % pure by mass spectrometry was condensed at liquid nitrogen temperature into the flask from a calibrated all-glass vacuum line. The nitric oxide added was determined by the change in pressure in the line before and after transfer. SAMPLING HANDLING PROCEDURE.The sample was transferred from the container to the cell using the procedure above. The cell was connected to the ice water circulator and placed in the sample compartment of a Cary 14 spectrophotometer. The sample compartment of the Cary 14 was purged with dry nitrogen to prevent fogging on the cold cell windows. The cell mount for the Beckman Model B incorporates two nitrogen jets directed at the cell windows for this purpose. When the temperature dropped to about 3.5" C, the spectrum from 800 to 450 mp was scanned. The cell was cleaned between samples by applying dry nitrogen to the outlet valve and blowing the sample out the inlet valve, through a Teflon tube and into a waste receiver.
1.0
0.9
0.8
0.7
0.6 w U
z 2
0.5
9 0.4
0.3
0.2
0.1
09
I
I
I
I
450
500
I
I
550 600 WAVELENGTH
I 650
-m p
P 700
I 750
I 800
I 850
Figure 2. Effect of temperature on spectra of NO-Nz04solution (0.945%NO in Nz04)
Figure 3. Effect of temperature on spectra of pure NzOl
Cell thickness, 2.21 mm, Cary 14 spectrophotometer
Cell thickness, 2.21 mm, Cary 14 spectrophotometer
RESULTS AND DISCUSSION
A spectrophotometric cell suitable for determining nitric oxide in dinitrogen tetroxide requires the following features. It would have to be constructed of materials that would not be attacked by dinitrogen tetroxide; capable of being cooled to and maintained at a temperature in the neighborhood of 0' C (see later section on effect of temperature); capable of withstanding moderate pressure and vacuum without leaking; and readily dismantled for cleaning. Further the path length would have to be suitable for determining nitric oxide in the 0 to 1.5% range. No such cell is commercially available. The construction of the cell designed and built for this determination is described in the Experimental Section. Preparation of Standard Curve. No alternative method of proved reliability was available for the determination of small amounts of nitric oxide in liquid dinitrogen tetroxide. For this reason, in the development and evaluation of the colorimetric method, calibration using standards analyzed by other methods was not feasible, and it was necessary to prepare absolute standards by making known mixtures from pure nitric oxide and dinitrogen tetroxide. Eleven standards were prepared, covering the range from 0 to 1.3% nitric oxide, in an all-glass vacuum line. The standards were transferred to the cell and scanned over the region 800 mp to 450 mp with the Cary 14 spectrophotometer. Usually, each sample of a particular standard was scanned twice and each standard was sampled twice. The spectra of these solutions show a broad absorption band with a maximum at 700 mp and a minimum which shifts to slightly shorter wavelengths as the concentration of nitric oxide is increased. The spectra of a solution of 0.94z nitric oxide in dinitrogen tetroxide and of pure dinitrogen tetroxide
at a variety of temperatures are shown in Figures 2 and 3. These spectra show that dinitrogen tetroxide contributes very little to the absorbance at 700 mp but is responsible for a significant amount of the absorbance at 500 mp. The absorbance of the various standards at 700 mp and 550 mp was read from the curves and summarized in Table I. Reproducibility was excellent; the maximum variation observed for a particular standard was 0.01 absorbance unit. The data show that the variability from sample to sample of the same standard was not significantly greater than the variability from scan to scan on the same sample. Thus, the sample handling system was shown to be satisfactory. The absorbance values obtained at 700 mp were plotted against the nitric oxide concentration. A good straight-line calibration curve with origin near zero was obtained. The linearity of the calibration curve shows that the nitric oxide is essentially all present as dinitrogen trioxide and the equilibrium shown in Equation 2 is far to the right (10). As will be discussed later, the optimum temperature for scanning the spectra is in the range of 0-4' C. The use of an absorbance difference between readings taken at two wavelengths rather than a single wavelength reading is strongly recommended because it compensates for the cell blank, instrumental base line drift, and any neutral residues which may be on the cell windows including minor fogging resulting from condensation on the cold cell windows. Serious fogging was prevented by p r g i n g the cell compartment with dry air (Cary Models 11, 14, and 15) or nitrogen or by directing a jet of the dry air onto the windows (Beckman Model
B). (10) P. G . Ashmore and B. J. Tyler, J . Chem., SOC.,1961,p. 1017. VOL 40, NO. 1, JANUARY 1968
31
The difference in absorbance between 700 mp and 550 mp was calculated and plotted against the nitric oxide concentration for the eleven standards. This plot proved to be a good straight-line calibration curve that was satisfactory for use in determining the nitric oxide content of samples of dinitrogen tetroxide. Because of the special equipment needed and difficulties involved in preparing accurate calibration standards ofknown concentrations of nitric oxide in dinitrogen tetroxide, it was desirable to relate the experimental data obtained here to a stable standard. A green glass filter (Corning No. 4015) was found whose spectrum closely resembles that of dinitrogen trioxide. The absorbance difference between 700 mp and 550 mp ( A A ) was obtained on the Cary 14. The equivalent nitric oxide concentration was then read from the calibration curve prepared from the experimental data. The absorbance difference (700 mp to 550 mp) of this filter was then obtained on a Beckman Model B. As the nitric oxide equivalent of the filter was known (0.93%), one point on a A.4 nitric oxide concentration plot was established. The other point necessary to determine the linear calibration curve was obtained by measuring A A of pure, dry dinitrogen tetroxide and plotting this point as 0.0 nitric oxide. Effect of Temperature. In liquid and gaseous dinitrogen tetroxide, an equilibrium exists between the colorless dinitrogen tetroxide and the brown nitrogen dioxide. As the
Table I.
0
#-
+
Absorbance Valuesa Obtained with Standard Solutions of Nitric Oxide in Dinitrogen Tetroxide Nitric oxide Standard No. added, Sample No. at A i o o m r at A 550 mp Difference 1 0.640 1 (Scan 1) 0.600 0.210 0.390 2 0.525 1 (Scan 1) 0.505 0.200 0.305 1 (Scan 2) 0.505 0.195 0.310 3 1.29 1 (Scan 1) 1.220 0.320 0.900 1 (Scan 2) 1,220 0.320 0.900 4 0.205 1 (Scan 1) 0.190 0.125 0.065 1 (Scan 2) 0.190 0.130 0.060 2 (Scan 1) 0.195 0.135 0.060 2 (Scan 2) 0.195 0.135 0.060 5 0.728 1 (Scan 1) 0.680 0.230 0.450 1 (Scan 2) 0.680 0.230 0.450 2 (Scan 1) 0.670 0.225 0.445 2 (Scan 2) 0.675 0.225 0.450 6 0.510 1 (Scan 1) 0.490 0.190 0.300 1 (Scan 2) 0.500 0.200 0.300 2 (Scan 1) 0.500 0.190 0.310 2 (Scan 2) 0.500 0.190 0.310 7 1.12 1 (Scan 1) 1,090 0.295 0.795 1 (Scan 2) 1.095 0.295 0.800 8 0.743 1 (Scan 1) 0.700 0.230 0.470 1 (Scan 2) 0.700 0.235 0.465 2 (Scan 1) 0.700 0.230 0.470 2 (Scan 2) 0.700 0.230 0.470 9 0.328 1 (Scan 1) 0.315 0.160 0.155 1 (Scan 2) 0.315 0.155 0.160 2 (Scan 1) 0.310 0.160 0.150 2 (Scan 2) 0.310 0.155 0.155 10 0.945 1 (Scan 1) 0.900 0.250 0.650 1 (Scan 2) 0.900 0.250 0.650 11 Ni I 1 (Scan 1) 0.015 0.090 -0.075 1 (Scan 2) 0.015 0.090 -0.075 2 (Scan 1) 0.015 0.095 -0.080 2 (Scan 2) 0.015 0.095 -0.080 All values obtained using a Cary 14 spectrophotometer, a cell with a path length of 2.21 mm., and a cell temperature between 3.2" and
3.6' C.
32
temperature is lowered, the equilibrium shifts more to the left and the appearance changes. At slightly below room temperature liquid dinitrogen tetroxide is muddy brown; at temperatures below 10' C the liquid is a clear yellow, becoming lighter as the temperature drops. At temperatures below the freezing point, solid dinitrogen tetroxide is colorless. To determine the optimum temperature for spectrophotometric determination, a study of the effect of temperature on the spectra of pure dinitrogen tetroxide and nitric oxidedinitrogen tetroxide solutions in the range 3.0' C to 11' C was carried out. Standard 10, Table I, containing 0 . 9 4 z nitric oxide, was used as the nitric oxide-dinitrogen tetroxide solution. The spectra of this solution at various temperatures are shown in Figure 2. A similar series of spectra of pure dinitrogen tetroxide is shown in Figure 3. These spectra show that temperature has little effect on the absorbance due to dinitrogen trioxide. At 700 mp, the absorbance of 0.94 % nitric oxide varies by only 0.015 absorbance unit over the temperature range -2.0" to +6.8" C. This small change may be related to the contraction in volume as the color is slightly more intense at lower temperatures. The effect of temperature on dinitrogen tetroxide is more pronounced. As the temperature is raised, the absorbance at 550 mp increases, probably because of the larger amounts of nitrogen dioxide from dissociation of the dinitrogen tetroxide at the higher temperatures.
0
ANALYTICAL CHEMISTRY
Table 11. Sample
Water added, % 0.32
1
2 3
0.59 0.71
4
0.98
5
1.48
6
0.36 0.39 0.36
7 8
Addition of Water and Nitric Oxide to Dinitrogen Tetroxide Total Nitric oxide Yieldc of Nitric oxide nitric oxide found, nitric oxide, added, % found, % 0. 235
0.210
40 43
0.41b
36
0. 38b 0. 524
32 29 27
0.495 0.665 0.23" 0.224 0.22d
Recovery of added nitric oxide, %
0.36
0.56
95
0.33
0.79
91
0.30
0.50
93
27
38 36 37d
Scanned 2 hours after preparation. * Scanned 21 hours after preparation. Calculated on the basis that at 100% yield, 1 mole of water produces 1 mole of nitric oxide (Equation 3). In this sample the nitric oxide was added before the water and the amount of nitric oxide formed from the water calculated from the results of Runs 6 and 7 (37 % yield assumed). These studies show that temperature does have an effect on the spectra, but for analytical purposes, highly precise temperature control is not necessary. In the 0 to 10' C range an error of 3 " C in temperature would result in an error of less than 0.02 % nitric oxide in an analytical determination. The simplest means of cooling is to circulate water from an ice bath through the cell, and these results show that no closer temperature control is necessary. Analysis of Samples. With the calibration curve established and the effect of temperature on the spectra determined, a plant sample was analyzed by the Sample Handling Procedure given in the Experimental. Eight separate samples of this material were withdrawn from a 250-1111 cylinder over a period of several days. Some of the samples were analyzed on three different instruments, the Beckman Model B, the Cary 14, and the Cary 15. All values obtained were either 0.86 or 0.85% nitric oxide. The complete spectrophotometric curves obtained with the sample showed no anomalies when compared with the spectra of the standard solutions. The standard deviation of the values obtained was 10.01. These results show that the method is highly precise. An additional fact of significance is that repeated sampling from the same container resulted in no significant decrease in dinitrogen trioxide due to loss from the solution to the vapor inside the cylinder. Effect of Water. The effect on the method of several possible contaminants was investigated. Water is known (8) to react with dinitrogen tetroxide in a complex reaction to give nitric oxide as one of the products. The overall reaction can be represented as 3 Nz04
+ 2 H20 S 2 NO + 4 HNOs
(3)
To investigate the effect of small amounts of water on the determination, a series of known mixtures of dinitrogen tetroxide and water; and of dinitrogen tetroxide, water, and nitric oxide were prepared as shown in Table 11. In mixtures numbers 2, 3, 5, 6, and 7, the water was added to frozen dinitrogen tetroxide (liquid nitrogen temperature); sample was allowed to warm to room temperature and analyzed spectrophotometrically. These results show that under the conditions used here, water is not quantitatively converted to nitric oxide. The yield of nitric oxide, calculated on the basis of Equation 3, decreases with increased water addition.
In mixtures numbers 1 and 4, water was added to dinitrogen tetroxide and the nitric oxide formed determined as in the previous mixtures. The mixtures were scanned two hours after mixing and twenty-one hours after mixing with essentially the same results obtained at both times, showing that equilibrium was established relatively rapidly. Known amounts of nitric oxide were then added to mixtures 1 and 4 and the samples were again analyzed spectrophotometrically to determine the amount of the added nitric oxide that could be recovered. The recovery was greater than 90% for both samples. Reverse addition of nitric oxide followed by addition of water, mixture 8, gave a similar recovery. The conclusion drawn from these results is that water does affect the spectrophotometric determination by forming nitric oxide as in Equation 3 and by setting up complex equilibria that result in slightly low recovery of added nitric oxide. Effect of Oxygen. The effect of dissolved oxygen on the spectrum of dinitrogen tetroxide had to be established to show that no unusual absorbance would occur during the calibration of the Model B, in which dinitrogen tetroxide is used without degassing. Liquid oxygen was condensed into a sample of solid dinitrogen tetroxide (at liquid nitrogen temperature), the mixture allowed to warm up, the pressure relieved, and the visible spectrum scanned. The spectrum of this dinitrogen tetroxide was the same as the spectrum of a dinitrogen tetroxide sample that was free of oxygen. Effect of Chlorine. If chloride is present in dinitrogen tetroxide, it will most likely be present as nitrosyl chloride. To investigate the effect of this compound, a sample of dinitrogen tetroxide containing 0.95 nitric oxide and approximately 1.3 % nitrosyl chloride was prepared. Analysis by the spectrophotometric method found 0.97 % nitric oxide, indicating no significant interference from nitrosyl chloride. Effect of Iron. Another possible contaminant in dinitrogen tetroxide is iron, most likely as some form of Fe+3. A sample of dinitrogen tetroxide was prepared containing 0 . 6 0 z nitric oxide and 0.7% anhydrous ferric oxide. Spectrophotometric analysis found 0.60 % nitric oxide present. Visual observation indicated the FeaOc was unchanged and largely undissolved. RECEIVEDfor review September 13, 1966. Resubmitted August 25,1967. Accepted October 13,1967. VOL 40, NO. 1, JANUARY 1968
33
