Environmental Chemistry
John H. McFarlandl and C. S. Benton Westrnont College
Santo Barbora, California 93103
The Oxides of Nitrogen and their Detection in Automotive Exhaust
The oxides of nitrogen are receiving more and more attention as air pollutants. They are formed from a variety of sources, almost all involving combustion of fossil fuels (see Table 1). Nitrogen oxides are important because they participate in many photochemical reactions and are largely responsible for the formation of the "broxn haze" of smog and the accompanying eye irritation and other physiological effects. Some physical data on the important oxides of nitrogen are listed in Table 2. Of these oxides, only nitrogen oxide (NO) and nitrogen dioxide (NOz), together referred to as NO,, are of concern in air pollut,ion studies. Nitrous oxide (N20) has no effect a t low concentrations and emission sources are rare. Dinitrogen tetraoxide (NO& or NtOq is a polymer of NO, ~ ~ h i forms c h when NO? is compressed and liquefied.
shown that ultraviolet photolysis affects the conversion of NO to NO, (4). Also avariety of hydrocarbons are known to catalyze the reaction (5). Therefore, most investigations lump NO and NOp together as NO, for determination purposes since a t most concentrations and conditions they are easily interconverted in the atmosphere. The ratio of NO to NO2 varies with the time of day (decreasing during the day and increasing at night), depending on the action of sunlight, oxygen, and other oxidizing or reducing agents present. NO2 is a brown pungent gas. Concentrations of 20-50 pprn are irritable to the eyes. At only 150 pprn there is danger of strong local irritations especially to the respiratory organs (3). Table 1.
Total Estimated Oxides of Nitrogen Emissions from Mobile and Stationary Sources in United States, 1968 ( 1 )
2 N 0 1 F1 NsOc
At the concentrations usually encountered in the atmosphere, equilibrium is very quiclcly attained and the quantity of N204is SO small as to be of minimal importance in air pollution studies. NO is present in automobile exhaust in concentrations ranging from a few parts per million to several thousand ppm. It is a colorless, odorless gas. Little is known about the toxicological effectsof NO since a t high concentrations NO converts into NOz. During the combustion of fossil fuels, the nitrogen and oxygen in the air used combine a t high temperatures to form NO.
NO. estimated tons. 1968
Fuel type Gas and LPG Petroleum Autoleasoline
Indnstrinl non-combustion Agricultural, wood, m d forest fires
Table 2.
Pro~ertiesof the Oxides of Nitrogen (2) Spe+
The equilibrium concentration of the reaction depends on the flame temperature a t which combustion occurs, the length of time the combustion gases are maintained a t that temperature, and the amount of excess air present in the flame. Once NO is formed the decomposition rate is too slow under the ordinary conditions of rapid cooling to allow the NO to dissociate into oxygen and nitrogen. Thus the NO is "frozen" in the combustion products on leaving the high-temperature zone. In automotive exhaust gases at complete combustion and top speed the concentration of NO may be as high as 4000 pprn (see Table 3). While most of the NO, that is emitted in automobile exhaust is in the form of NO, nitrogen dioxide begins to form by the reaction of the NO uith excess oxygen ZNO
+ o2
2NOn
The reaction proceeds more slowly as the concentrat,ion of NO decreases so that a t low concentrations, NO levels may be stable for a long time. It has been Sellior research project, June 1971.
% total
~ravlty
10367a
Nitric oride NO or (NOb Niliogen Dioridc NO? or (NO& Nitrous ozide
1.448"
..=NO
1.530'
x.n
Solubility in 100 ts
Ha0
R O ~ mp, "C bp, 'C
7.34ml
ZGGml
decomposes 130.52 ml
soluble
-161
-151
-9.3 -102.3
21.3 -90.7
and NO* are paramagnetic
n
mitli reference t o air = 1.
Table 3.
-oxides
coz
H20 0 2
CO
H,
Com~ositionof Automobile Exhaust Gases (3)
Idline;
-
-----------
Full laad-Low Soeed Hieh h e e d
1000 pprn 0-.5O ppm 7-11% 6 . 5 - 8 ~017~ 9-11% 7-10 vol% 0.5-2% 1-1.5 vol% 3-8% 3-10 ~017, 0.2-1% 0.5-4 vol% 200-500 pprn
4000 pprn 12-13y0 10-11% 0.1-0.4% 1-5% 0.1-0.2% 100-300 pprn
Lead compounds (Ph) 60 mg/mJ, 3,4-Benzopyrene 0.001-0.01 mg/rna It is assumed t,hat nitrogen and small amounts of inert gases make up the remainingvolurne Volume 49, Number I, January 1972
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Environmental Chemistry Numerous wet chemical and instrumental methods have been reported in the literature which measure the oxides of nitrogen. In the remainder of this paper I would like to briefly describe some of those methods. Acidimetric Determination (3) Aeidimetric determination of NO, is common in industrial determination. The gas sample is introduced into a large evacuated flask. Water or dilute hydrogen peroxide is added and the mixture is allowed to stand for several houm before being titrated to a methyl red end point with NaOH. Ikaotians are as follows
-
3H*Oz+ 2NO -+ 2HNOa NaOH
+ NNO1
HIO
+ 2H10
+ NaNOa
The waiting time can he cut down by shaking the sample and hydrogen peroxide vigorously with a aentral foaming agent for 5 min.
Phenoldisulfonic Acid Method (3,6,7)
A sample is collected in an evacuated flask containing a dilute solution of sulfmic acid arid hydrogen peroxide. All nitrogen oxides iu the sample are oxidized to nitric acid. The resultant solution is made just basic with 1 N sodium hydroxide and evaporated to dryness. Phenoldisulfonio acid reagent and ammonium hydroxide are added t o form the yellow trialkali salt of 6-nitro-1-phenol-1,4-disulfonicacid. A colorimetric determination is then made a t 410 mp and compared to a. calibration curve dotted far known amounts of NaNO, treated the same u n y . The rn.h h n ~ l , n c k11, ?hi.; ~nrthudi, t h 111.1 ~ t h t il I. ~ d mI*~lc v e q trdi
