This document discusses methods for monitoring nitrogen oxide pollutants nitric oxide and nitrogen dioxide. It describes both manual and automated techniques for measuring oxides of nitrogen either separately or collectively. The modified Na-Arsenite method is a manual technique that involves sampling air using glass absorbers, converting nitrogen dioxide to nitrite ions, and then using colorimetric analysis to measure absorbance and determine concentration. Calibration procedures and equations for calculating nitrogen dioxide concentration from absorbance measurements are also provided.

1. Nitrogen Oxide Monitoring
S.D. Joshi
Scientist
Air Pollution Control Division, NEERI, Nagpur- 440020
Nitric oxide (NO) and nitrogen dioxide (NO2) are the most prevalent
nitrogen oxide air pollutants. These pollutants are chiefly emitted from high
temperature combustion processes, with the major oxide in combustion
emissions being NO. Nitric oxide is oxidized to NO2 in the atmosphere.
Nitric oxide is a colorless, odorless gas that is slightly soluble in water.
Nitrogen dioxide is a reddish-orange-brown gas with a characteristic pungent
odour. Nitrogen dioxide is corrosive and highly oxidizing.
Nitric oxide and nitrogen dioxide may be measured either separately or
collectively as oxides of nitrogen (NOx) by either manual or automated
techniques. Manual techniques are mechanical methods (e.g., gas bubbler
devices) in which sample collection and analysis are performed separately.
Automated methods are those in which sample collection, analysis and
measurement are performed automatically. Procedure for manual techniques
currently practiced in NAQM project has been given in detail. The automated
method is the chemiluminescence atmospheric pressure. Nitrogen dioxide is
measured as NO^ in this technique after conversion of the NO2 to NO. The
manual method (Saltzman method) is based on a specific reaction of nitrite ion
with diazotizing-coupling reagents to form a deeply colored azo dye, which is
measured colorimetrically. Nitric oxide can be measured by this method by
oxidizing it to NO2 prior to sampling. Although various solid and liquid oxidants
for the conversion of NO to NO2 have been used and tested, their reliability for
field use is not wholly satisfactory. Consequently the measurement of NO by this
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2. method is not recommended. The detector tube method is applicable only to the
semiquantitative measurement of NO2. There are currently no detector tubes
available for the measurement of NO; however, tubes are available for the
measurement of NOx (NO + NO2).
A large number of other methods are available for measurement of NO
and NO2 in the ambient air. The Jacobs-Hochheiser procedure is a manual
method applicable to 24 hour sampling with subsequent analysis for NO2 up to 1
month after collection of samples. This method has several deficiencies and is
not recommended. There are two methods that are currently being evaluated.
These are the arsenite method and the TGS-ANSA method. Several automated
methods (continuous analyzers) are available for measurements of NO and NO2
based on colorimetric, electrochemical, spectrophotometric, and fluorescent
principles.
Continuous colorimetric analyzers based on the Saltzman procedure have
been used extensively. However, the use of analyzers based on this principle is
not encouraged for several reasons.
Modified Na-Arsenite Method
This method is based on a specific reaction of nitrite ion with diazotizing-
coupling reagents to form a deeply colored azo dye, which is measured
colorimetrically. Sampling is conducted in all-glass absorbers utilizing preferably
fritted bubbler. The NO2 in the ambient air is converted to nitrite ion which reacts
with the diazotizing-coupling reagents (Griess-Saltzman reagents) to give a
stable pink color within 15 minutes and is measured by a colorimeter or,
preferably, a spectrophotometer.
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3. The method is intended for the manual determination of NO2 in the
atmosphere in the concentration range of a few to about 9400 ug/m3
(5 ppm) for
sampling periods up to 2 hours at flow rates up to 0.6 liter/minute. Sulfur dioxide,
ozone, and other gases may interfere; but in urban air, the extent of the
interference is normally minimal.
The method can be standardized (calibrated) statistically by using sodium
nitrite (NaN02) standards. Standardization is based upon the empirical
observation that 0.74 mole of NaNC>2 produces the same color as 1 mole of
NO2 Using NaNC>2 for standardization is much more convenient than
preparing accurately known gas samples.
Apparatus
Absorber : The absorber should be all glass, impinger preferably with
fritted disperses
Air-metering device - A flow meter capable of accurately measuring a flow
between 0.3 and 0.6 liter/minute is recommended.
Air pump - An appropriate suction pump capable of drawing the required sample
flow for intervals of up to 2 hours is suitable.
Pipettes - 1 ml, graduated in 0.1-ml divisions; 10 ml, volumetric.
Graduated cylinders - 50 ml, graduated in 1-ml divisions; 100 ml,
graduated in 1-ml divisions; 1000-ml, graduated in 10-ml divisions.
Volumetric flasks - 25, 150,250, 1000 ml.
Beaker - 2 liters or larger.
Reagent bottle -1 liter, dark Pyrex glass.
Mechanical stirrer.
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4. Spectrophotometer or colorimeter - A laboratory instrument suitable for
measuring the pink color at 550 nm, with stoppered tubes or cuvettes, is
recommended.
Reagents
All reagents should be from analytical-grade chemicals in nitrite-free
water, prepared, if necessary, by redistilling distilled water in an all-glass still after
adding a crystal each of potassium permanganate (KMn04) and barium
hydroxide (Ba(OH)2). The reagents are stable for several months if kept well-
stoppered in brown bottles in the refrigerator. The absorbing reagent should be
allowed to warm to room temperature before use.
Absorbing media : 0.1N sodium hydroxide + 0.1% sodium arsenite.
Dissolve 4.0 g of sodium hydroxide i water and 1.0 g of sodium arsenite and 2.0
ml of n-butyl alcohol and dilute to 1 litre. The n-butyl alcohol promotes frothing
and improves absorption efficiency of the solution.
Hydrogen peroxide solution (0.5%) Dilute 1 ml hydrogen peroxide (30%
w/r) to 60 ml with distilled water. Prepare a fresh solution every time during
analysis.
Sulfanilamide Solution (Diazotizing reagent) (melting point 165-167°C
pharmaceutical grade). Dissolve 40.0 g sulfanilamide in about 500 ml water
containing 100 ml orthophosphoric acid (sp.gr. 1.75) and dilute to 1 litre with
distilled water. This solution is stable for 1 month if refrigerated.
Coupling reagent (N-1naphthyl-ethylene diamine dihydrochloride)
Analytical grade. Dissolve 0.100 gms of N-1naphthyl ethylene diamine
dihydrochloride in distilled water and dilute to 100 ml.
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5. Standard sodium nitrite solution (stock solution) : Dissolve sufficient
desiccated sodium nitrite [NaNC^] (assay of 97 percent or greater)] and dilute
with distilled water to 1000 ml so that a solution containing 1000 ug NC>2/ml is
obtained. The amount of sodium nitrite to use is calculated as follows :
1.500
G = x 100
A
Where,
G = amount in g of sodium nitrite;
1.500= gravimetric factor in converting N02 into sodium nitrite;
A= assay, percent
Standard sodium nitrite solution (working standard) : 0.015 g/liter -
Prepare a working solution fresh just before use by pipetting 10 ml of the stock
solution into a 1000-ml volumetric flask and diluting to the mark with water.
Sampling Procedure
Assemble fritted impingers, a flow meter, and a pump as in the case of
SO2 sampling. Use ground-glass connections upstream from the absorbers.
Butt-to-butt glass connections with slightly greased tygon or pure gum rubber
tubing may also be used for connections without losses if lengths are kept
minimal. In areas of high particulate concentrations, a filter is recommended at
the intake of the absorber to remove particulate matter.
Calibration Curve
Take 12 sample impinger tubes and number them in series marking one of
then as control or blank. Add Step-lll solution in the volume ranging from 0.5 to
18 ml i.e. 0.5 to 18 ug NO2. Make up the volume to 20 ml by adding appropriate
quantities of absorbing media in each tube. Add 2 drops of hydrogen peroxide to
each impinger to remove possible sulfur dioxide interferences. Sulfur dioxide
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6. present in the sample will be converted to soluble sulfate. Add 5 ml of diazotizing
reagent and shake well and add 1 ml of coupling reagent adjust the final volume
to 30 ml with absorbing media shake well. Wait for 30 minutes. Using 1-cm path
length cells, the absorbance of the sample in each bubbler and the absorbance
of unexposed reagent is measured with a colorimeter or spectrophotometer at
550 nm, using distilled water as a reference. The absorbencies of the test
solutions are then corrected for the reading given by the unexposed reagent; the
corrected absorbance values for each pair of bubblers are then added together.
Plot the curve on graph paper taking transmittance/absorbance on Y axis and
NC>2 (ug) concentrations on X axis.
For convenience, standard atmospheric conditions are taken as 760 mm
of mercury and 25°C; thus, only slight correction by means of the well-known
perfect gas equation is ordinarily required to get the standard volume of the air
sample.
It has been determined empirically that 0.74 mole of NaNC>2 produces the
same color as 1 mole of NO2. The concentration of NO2 in the samples is
calculated from :
F1 + F2
V = x T x 10"6
2
Where,
V = volume of air sampled, M^
F-j = measured flow rate before sampling; ml/min;
F2 = measured flow rate after sampling, ml/min; and
T = time of sampling, min.
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7. Calculate the concentration of nitrogen dioxide as follows :
(ug/N02/ml) x 50
Mass of nitrogen dioxide in ug/ per m3
=
V x 0.35
Where,
50 = volume of absorbing reagent used in sampling, ml;
V= volume of air sampled, m2
;and
0.35 = overall average efficiency
If desired, concentration of nitrogen dioxide may be calculated as ppm
N 0 2
N02 , ppm = (ug N 0 2 /m3
3) x 5.32 x 10"4
Calibration Procedure
Add graduated amounts (0.2, 0.4, 0.6, 0.8, and 1.0 ml) of freshly prepared
standard NaNC>2 solution (0.0203 g/liter) to a series of 25-ml volumetric flasks,
and dilute to marks with absorbing reagent. Good results can be obtained with
these small volumes of standard solution if they are carefully measured. If
preferred, however, larger volumes may be used with correspondingly larger
volumetric flasks. Mix well and allow 15 minutes of unexposed reagent using
distilled water as a reference.
Plot the absorbances of the standard nitrite solutions, corrected for the
reagent blanks, against the nitrite concentrations.
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