Quantitative Separation of Nitric Oxide from Nitrogen Dioxide at Atmospheric Concentration Ranges Dario A. Levaggi, Wayman Siu, and Milton Feldstein' Bay Area Air Pollution Control District, 939 Ellis Street, San Francisco, Calif. 94109 Evaldo L. Kothny Air and Industrial Hygiene Laboratory, California State Department of Public Health, 2151 Berkeley Way, Berkeley, Calif. 94704
Experimental N Extensive experimental trials have shown that an absorber
containing triethanolamine on firebrick is capable of quantitatively absorbing NOz from NO NO, mixtures in dilute gas streams. Various molecular sieves used for the same purpose showed either high NO generation from NO, or retention of NO. Soda lime and ascarite showed large losses of NO from mixtures of NO and NO,. While these latter materials may be used to obtain an NOn-free stream of NO, they do not quantitatively allow passage of NO, and are useless for precise analytical work. The triethanolamine-firebrick absorber can be used for the direct quantitative analysis of NO at ambient air concentrations in any ratio of NO/N02 from zero to infinity.
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D
ilute concentrations of NO and NO, in air are of importance to analysts involved in air pollution studies and ambient air monitoring. To obtain pure gas streams of NO in air for evaluating and developing analytical techniques it has been commonplace to use solid alkaline surfaces such as ascarite and soda lime, or reactive surfaces such as manganese dioxide or activated carbon which remove residual amounts of NO1 usually present in cylinders of NO. During a project to improve the existing technique of NO and NO2 determination, it was found that while a host of materials removed NO2 from a mixed gas stream, these materials also affected the resultant NO concentration either positively or negatively after NO2 removal. After checking numerous substances, an absorber using triethanolamine (TEA) on firebrick was found which completely removed NOn, while quantitatively passing NO in any NO/N02 ratio normally occurring in ambient air. The passage or removal of organic nitroxy compounds was not evaluated. Organic nitrates are the only class of compounds which could possibly interfere (Ripperton and Kornreich (1970), and their ambient concentrations are probably extremely low. The peroxyacyl nitrates (PAN)which can hydrolyze to yield nitrite ion have been found in the atmosphere (Darley et al., 1963). PAN, however, is an extremely reactive compound and its transport through an impregnated firebrick surface is highly improbable. To be analytically useful, an absorbing system which removes NO2 and quantitatively passes NO must meet three conditions: It must remove all NO? in the gas stream it must not generate any NO from NO, which may be in the gas stream it must allow the NO to pass quantitatively whether alone or in thepresence of NOz. 1
To whom correspondence should be addressed.
250 Environmental Science & Technology
Reagents. Saltzman NOn absorbing solution (Saltzman, 1954). SOLIDOXIDIZER. 100 % converter of NO to NO, at approximately 50% r.h. (relative humidity) (Siu and Levaggi, 1967). Soak 50-100 grams 4-8-mesh porous quartz chips (Hengar granules) in a mixture of equal weights of Cr03, HzO, and 85% H3P04.Drain off excess liquid, blot granules rapidly with tissue paper, and store in an airtight container. Place a layer of approximately 1 to 1.5 in. of the granules in a midget impinger. Replace the upper portion of the impinger and pass ambient air (30-7Oz r.h.) through for approximately 1 hr at 0.5 l./min to condition the chips. Oxidizer is now ready to use for oxidation of NO to NOz in gas streams of 40-70Z r.h. LIQUIDOXIDIZER. 2.5 % potassium permanganate in 2.5 % sulfuric acid (w/v). Use 50 ml in a 125-ml round-bottomed flask. This oxidizing solution has an efficiency of about 80% in the conversion of NO to NOn. TEA ON FIREBRICK. Prepare a 20% wjv solution of TEA in water. Add the solution to about 50 ml of 14-16-mesh celite-22 firebrick in sufficient quantity to completely cover the firebrick. Stir the mixture and allow to stand for approximately 15 min. Decant off as much surplus solution as possible and transfer to a porcelain dish. Allow to dry for 2 days in an operating hood. Transfer to an oven at 105°C for approximately 15 min. Cool and place in a stoppered bottle for storage. To prepare the absorber tube, fill a 100 mm X 15 mm polyethylene drying tube with the prepared TEA-firebrick using small glass wool plugs on both ends to hold the material in place. Other absorbers which were evaluated consisted of approximately 10-15 ml of material contained in a glass U tube. Stock NO gas mixtures were prepared by volumetric dilution in 34-liter stainless steel tanks at a concentration of approximately 50-ppm NO in nitrogen. Stock NOz gas mixtures were prepared by volumetric dilution in 34-liter stainless steel tanks at a concentration of approximately 100-ppm NOz in nitrogen. Sampling Systems. The sampling train used in evaluating the ability of an absorbing system to remove NOz is shown in Figure 1. NO, gas streams were used at concentrations from 0 to 1.0 ppm, and 50% r.h. Complete removal or leakage of NOn was determined by the Saltzman Method (Saltzman, 1954). Table I shows the results of the various materials tested. The sampling system described in Figure 2 was used to determine NO generation from NOn concentrations ranging from 0.20 to 1.0 ppm at 50% r.h. The use of a solid oxidant in the parallel sampling trains ensured the complete oxidation of residual traces of NO which may have been in the diluent air. A completely NO-free dynamic stream is mandatory when evaluating NO generation from NOn, for an NO background
NO:! IMPINGER
NEEDLE VALVES
DILUENT
STOCK
AIR PUMP
NO2 100 PPM
D I L U E N T STOCK AIR NO 50 P P M PUMP
Figure 1. System for evaluation of NOg absorption
Figure 3. System to determine quantitative passage of NO
concentration in the dilution air of 0.02 ppm would give an apparent 4 Z NO generation at an N O z concentration of 0.50 ppm. Additionally, air and/or light oxidation of the reagent by long sampling times was compensated for by control runs (Scaringelli et al., 1970). Table I contains results using this experimental procedure. Figures 3 and 4 illustrate the sampling trains used on promising materials for assessing the ability of passing quantitatively NO whether alone (Figure 3) or in the presence of NOa (Figure 4). The use of soda lime prior to the parallel sampling trains was for ensuring removal of all traces of NOz. Gas concentrations were varied from 0-0.8 ppm, with r.h. at 50 %. In all cases where acidified permanganate was used as the oxidizer, the same batch and amounts of chemicals were used in each leg of the sampling train. This added attention to sameness of reagent was to assure a comparable oxidation of NO to NOz in each leg of the sampling train. Figure 4 used for mixed streams of NO and NOz requires some explanation as to how the NO concentration was determined. The generation of NO from NOs in Saltzman reagent was first reported by Ellis (1964). He found an average of 14% generation of NO from NOs using fritted bubblers. Mueller and Tokiwa (1968) found a 10% generation using coiled columns used in continuous analysis instruments. Using the sampling train as described in Figure 2, replacing the absorber with a NO2 impinger, we obtained an average of 12% generation (based on NOz concentration), which is the mean of some 20 trials. A series system for NOz and NO then requires a correction
Table I. Absorption of NOs and Generation of NO by Various Absorbing Systems NOz absorbed, Absorber material
z
TEA-firebrick Silica gel, 20 mesh Alumina, activated, 40 mesh Soda lime, 8 mesh Ascarite, 14 mesh Nylon wool Molecular Sieve 3A (bone dry) Molecular Sieve 3A (humidified)b Molecular Sieve 5A (bone dry) Firebrick 10% hexamine Molecular Sieve 13X (bone dry) Molecular sieve 13X (humidified)b Silica gel-10 % sulfamic acid Alumina 10% Na2C03 Saltzman reagent (fritted bubbler) Molecular Sieve 4A (bone dry)
99-1 00 40 35 100 100 50-70 98
Q
b c
%a
0-2 10 18
