Ε. Μ . Carlson P. G. L e F e v r e R. C . W i l l i a m s B. F. Goodrich Chemical Group Avon Lake Technical Center Avon Lake, Ohio 44012
Insidious Vapors Infrared Determination of N 0 2 Generated in a High-Voltage Electric Arc N o r m a l activities in daily life re quire all of us to venture into t h e p u b lic domain, into structures of all shapes a n d constructional quality. Often we find ourselves squeezed sar dine fashion into an elevator or fight ing t h e crowds to leave an establish m e n t . These situations and t h e knowl edge t h a t high-voltage electrical ca bles are often located in these build ings may elicit concern about safety during emergencies. Because polyvinyl chloride (PVC) is one of the primary insulating materi als used in wire and cable insulation (2), considerable effort has been de voted to assessing the relative toxicity of P V C off gases under various test conditions (2). We were particularly concerned about a recent report (3) t h a t low-ppm levels of phosgene were generated when a PVC-clad wire was subjected to an electric arc. W h e n we a t t e m p t e d to duplicate the experi ment, we observed a dark brown gas, generated in concentrations high enough to obscure the visibility of t h e electrodes in a 1-L flask. S u b s e q u e n t experiments with bare wires also dem onstrated the generation of significant levels of this dark brown gas. It is known t h a t NO2 can be generated in a n electric arc {4-6), and Fourier transform infrared (FT—IR) analysis confirmed t h a t the brown gas was m a i n l y NO2. Nitrogen dioxide is an in sidious gas t h a t causes d e a t h by as phyxia in animals in an average of 19 min a t t h e 1000-ppm level (4). Be
cause it attacks the lung tissue, caus ing pulmonary edema, d e a t h can occur several days after exposure. T o assess the m a g n i t u d e of t h e potential hazard from phosgene relative to t h a t from NO2, we needed to determine how m u c h NO2 can be generated in a highvoltage arc. Method selection and development Several m e t h o d s were considered to q u a n t i t a t e t h e concentrations of NO2 generated in the electric arc experi m e n t . Chemiluminescence was consid ered and ruled out because the equip m e n t available was designed for use only in flowing gas streams; our pro posed arc experiments were static. Gas c h r o m a t o g r a p h y was rejected because even large a m o u n t s of N 0 2 were al most quantitatively adsorbed and/or reacted on a variety of column coat ings a n d substrates. Mass spectrometric analysis was evaluated and rejected because the typical mass spectrometer source will create NO2 in an air m a trix, as the source electrons traverse t h e sample gas. Finally, a variety of wet chemical analyses were ruled out because they were too sensitive—anal yses designed to detect p p m levels were totally swamped by the copious quantities of N 0 2 generated in prelim inary experiments. At this point, we examined the pos sibility of using infrared (IR) spectros copy, which we had already used as a qualitative screening tool. Vapor phase IR appeared to be uniquely well
1454 A • ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
suited. T h e IR spectrum of N 0 2 gas is simple and distinct, a n d because air is IR t r a n s p a r e n t , there did not appear to be any interferences. During our final preparations for the arc experi m e n t s , however, we ran into a snag. W h e n high levels of NO2 were intro duced into the gas cell, a band near 1380 c m - 1 was observed to grow with time; moreover, this band, which is commonly observed in inorganic ni trites and nitrates, remained after the analysis cell had been evacuated. T h e NO2 was reacting with t h e KBr win dows of the analysis cell! T h e first a t t e m p t to overcome this problem involved saturating all the re active sites on t h e cell windows with NO2. T h i s failed because the reaction between NO2 gas and the K B r window was, to some extent, reversible. T h e second a t t e m p t consisted of covering t h e inside of the cell windows with a thin polyethylene (Gladwrap) film. Al though this approach initially ap p e a r e d successful, a gap between t h e film a n d t h e window changed each t i m e the cell was filled. This caused irreproducible interference fringes t h a t were significant relative to t h e height of t h e NO2 b a n d and could not be nulled out by normal F T - I R ratioing techniques. We next focused on coatings for the K B r crystal. After several failures, we found t h a t commercial paraffin, ap plied as a solid smear, heated with a hot-air gun to liquidity, and t h e n cooled to room t e m p e r a t u r e , was satis0003-2700/84/A351 -1454$01.50/0 © 1984 American Chemical Society
The Analytical Approach Edited by Jeanette G. Grasselli
factory. This technique appeared to give the best coverage of the crystal and inhibited formation of the 1380 c m - 1 band. Although a small band was observed after exposure to 10,000 ppm N 0 2 in N2 for 30 min, no band was observed for this same time period at lower concentrations. The interaction between NO2 and the windows was thus judged to be too small to affect our analysis results significantly. Experimental The experimental apparatus is shown schematically in Figure 1. For both calibration standards and the actual arc experiments, an aliquot of the contents of the 1-L flask (which was initially at atmospheric pressure) was withdrawn into the previously evacuated (less than 40 Pa) IR gas cell. The N 0 2 absorption maximum near 1628 c m - 1 was then measured using a Nicolet 7199 FT-IR spectrometer equipped with a liquid-nitrogencooled Hg-Cd-Te detector. Operating parameters were 4 c m - 1 resolution (4K data points, 8K transform points), Happ-Genzel appodization, and 32 scans per spectrum. Spectra were taken within 3 min of sample transfer to minimize any possible time-dependent effects. All measurements were made using a 10-cm path gas cell (nominal volume 140 mL) equipped with the paraffin-coated KBr windows. Background spectra were acquired using the same gas cell filled to the same total pressure as the samples
with either air or a mixture of air and dry N2. The mixture of air and dry N2 was necessary to match the water vapor content of the calibration standards so that the water vapor bands were minimized upon ratioing. A separate background spectrum was recorded prior to each sample spectrum. The region 2970 cm~ 1 -2760 cm" 1 was blanked out because it contains strong paraffin bands. The 1628 cm" 1 N 0 2 peak maximum was measured from the absorbance spectrum, which had been plotted from 4000-470 cm- 1 .
N 0 2 generation Prior to a given sequence of runs, the electrodes were adjusted, and the point-to-point electrode gap was measured to the nearest 0.5 mm; this configuration was held constant for all subsequent experiments in that run. Before igniting the arc, the reaction flask was sequentially evacuated to a pressure of less than 250 Pa, then flushed with air, for a total of five iterations. The arc was ignited by increasing the voltage to between 6000 and 8000 V. Once the arc was established,
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 • 1455 A
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Figure 1. Arcing apparatus for N 0 2 study
the voltage was immediately reduced to the desired potential and main tained for the desired period of time. After the arc was terminated, the reac tion flask was left undisturbed for 60 s to allow the flask contents to come to thermal and kinetic equilibrium. To accommodate pressure changes caused by thermal expansion of the flask con tents, the valve between the flask and the outside atmosphere was left open during the arcing operation and the subsequent equilibrium period. After equilibration, this valve was turned to seal the flask and transfer an aliquot to the attached IR gas cell simulta neously. Calibration A 1.09% N 0 2 Matheson-certified gas standard was used for calibration. The cylinder was rolled prior to use (as a check against possible stratifica tion) and reanalyzed by Matheson after this study (as a check against de composition and NO2 chemisorption on the cylinder walls). The value used was the average of the two analyses, which differed by
