1.0 Scope:
1.1 This standard prescribes a method
for measurement of nitrogen oxides (as NOx) in air.
2.0 Principle
and Applicability:
2.1 Nitrogen oxides as nitrogen
dioxide are collected by bubbling air through a sodium hydroxide solution to
form a stable solution of sodium solution of sodium nitrite. The nitrite ion
produced during sampling is determined colorimetrically by reacting the exposed
absorbing reagent with phosphoric acid, sulphanilamide, and N (I-naphthyl)
ethylenediamine dihydrochloride.
2.2 The method is applicable to
collection of 24 hour sample in the field and subsequent analysis in the
laboratory.
3.0 Range and
Sensitivity:
3.1 The range of the analysis is 0.01
to 1.5 µg NO2/ml. With 50 ml absorbing reagent and a sampling rate
of 200 ml/min for 24 hours, the range of the method is 20 to 740 µg/m3 (0.01 to
0.4 ppm) nitrogen dioxide.
3.2 A concentration of 0.01 µg oxides
of nitrogen ( as NO2/ml) will produce an absorbance of 0.005 using
1-cm cells.
4.0 Interferences:
4.1 The interference of sulphur
dioxide is eliminated by converting it to sulphuric acid with hydrogen peroxide
before analysis.
5.0 Apparatus:
5.1 Absorber – Polypropylene tubes 164
x 30 mm, equipped with polypropylene two port closures. Rubber stopper cause
high and varying black valves and should
not be used. A gas dispersion tube with a fritted end of porosity B (70 to
100 µm maximum pore diameter) is used.
5.2 Spectrophotometer – capable of
measuring absorbance at 540 nm. Band width is not critical.
6.0 Reagents:
6.1 Absorbing Reagents – Disolve 4.0 g
of sodium hydroxide in distilled water and dilute to 1000 ml.
6.2 Sulphanilamide – Dissolve 20 g of
sulphanilamide in 700 ml of distilled water. Add, with mixing, 50 ml of
concentrated phosphoric acid (85 %) amnd dilute to 1000 ml. this solution is
stable for a month if refrigerated.
6.3 NEDA Solution – Dissolve 0.5 g of
N (l-naphthyl) ethylenediamine dihydrochloride in 500 ml distilled water. This
solution is stable for a month if refrigerated and protected from light.
6.4 Hydrogen Peroxide – Dilute 0.2 ml
of 30% hydrogen peroxide to 250 ml with distilled water. The solution may be
used for a month if protected from light.
6.5 Standard Nitrite Solution – Dissolve
sufficient desiccated sodium nitrite [ NaNO2 (assay of 97 % or greater)]
and dilute with distilled water to 1000 ml so that a solution containing 1000
µg NO2/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 NO2 into sodium nitrites; and
A = assay, percent.
7.0 Procedure:
7.1 Sampling – Assemble; the sampling
train including the absorber, critical flow control device and pump. Add 50 ml absorbing reagent to the absorber.
Disconnect funnel, insert calibrated flow meter, and measure flow before
sampling. If glow rate before sampling is less than 85 % of needle calibration,
check for leak or change filter as necessary. Remove flow meter and replace
funnal. Sample for 24 hours from midnight to midnight and measure flow at the
end of sampling period.
7.2 Analysis – Replace any water lost
by evaporation during sampling. Pipette 10 ml of the collected sample into a
test-tube. Add 1.0 ml of hydrogen peroxide solution, 10.0 ml of sulphanilamide
solution, and 1.4 ml of NEDA solution with through mixing through mixing after
the addition of each reagent. Prepare a blank in the same manner using
interval, measure the absorbing reagent. After a 10 min colour-development
interval, measure the absorbance at 540 nm against the blank. Read µg NO2/ml
from the standard curve (see 8.2).
8.0 Calibration:
8.1 Sampling
8.1.1 Caligration of Flow
meter – Using a wet test meter and a stopwatch, determine the rates of airflow
(ml/min) through the flow meter at several ball positions. Plot ball positions
versus flow rates.
8.2.2 Calibration of Hypodermic needle – Connect the calibration flow meter, the needle to be calibrated,
and the source of vacuum in such a way that the direction of airflow through the needle is the same as in
the sampling train. Read the position of the ball and determine flow rate in
ml/min from the calibration chart prepared in 8.1.1. Reject all needles not having flow rate of 190 to 210 ml/min
before sampling.
8.2 Calibration Curve – Dilute 5.0 ml
of the 1000 µg NO2/ml solution to 200 ml with absorbing reagent.
This solution contains 25 µg NO2/ml. Pipette 1, 2, 5 and 15 ml of
the 25 µg NO2/ml solution into 50, 50, 100 and 250 volumetric flasks
and dilute to the mark with absorbing reagent. The solutions contain 0.50,
1.00, 1.25, and 1.50 µg NO2/ml respectively. Run standards as
instructed in 7.2. Plot absorbance versus µg NO2/ml.
9.0 Calculation:
9.1 Calculation
the volume of air samples as follows:
F1 + F2
=
------------------------- x T x 10-6
2
where
V = volume of air sampled, m3;
F1 = measured flow
rate before sampling, ml/min;
F2 = measured flow
rate after sampling, ml/min; and
T = time of sampling, min.
9.2 Calculate the concentration of
nitrogen dioxide as follows:
(
µg NO2/ml) x 25
Mass
of nitrogen dioxide in µg per m3 = -------------------------------
V x 0.35
Where
25 = volume
of absorbing reagent used in sampling, ml;
V = volume of air sampled, m3;
and
0.35 = overall
average efficiency.
9.2.1 If desired,
concentration of nitrogen dioxide may be calculated as ppm NO2.
NO2,
ppm = ( µg NO2/m3) x 5.32 10-4 
No comments:
Post a Comment