Ambient Air quality Monitoring Guidlines. - Maharashtra Pollution ...

efore use, against 0.025 M Na2S2O3. The sodium thiosulfate is standardized
against primary standard biiodate [KH(IO3)2] or potassium dichromate (K2Cr2O7).
8.4.1 0.001 M I2 Solution - Pipet exactly 4.00 ml of the 0.025 M Stock solution into a
100 m low actinic volumetric flask and dilute to the mark with absorbing solution.
Protect from strong light. Discard after use.
8.4.2 Calibrating Iodine Solution - For calibration purposes exactly 5.11 ml of the
0.001 M I2 solution (or equivalent volume for other molarity) is diluted with
absorbing solution just before use to 100 ml (final volume) to make the final
concentration equivalent to 1 µl of O3/ml. This solution preparation accounts for
the stoichiometry described in Section 3.7 at standard conditions of 101.3 kPa
and 25 o C. Discard this solution after use.
8.5 Sulfur Dioxide Absorber - Flash-fired glass fiber paper is impregnated with
chromium trioxide, as follows (2) : Drop 15 ml of aqueous solution containing 2.5
g chromium trioxide and 0.7 ml concentrated sulfuric acid uniformly over 400 cm 2
of paper, and dry in an oven at 80 to 90 o C for 1 hour; store in a tightly capped jar.
Half of this paper suffices to pack one absorber. Cut the paper in 6 x 12 mm
strips, each folded once into a V-shape, pack into an 85 ml U-tube or drying tube,
and condition by drawing air that has been dried over silica gel through the tube
overnight. The absorber is active for at least one month. When, it becomes
visibly wet from sampling humid air, it must be dried with dry air before further
use.
9.0 PROCEDURE
9.1 Assemble a train consisting of a rotameter, U-tube with chromium trioxide paper
(optional), midget impinger, needle valve or critical orifice (10) and pump.
Connections upstream from the impinger should be ground glass or inert tubing
but joined with polyvinyl tubing. Fluorosilicon or fluorocarbon grease should be
used sparingly. Pipet exactly 10 ml of the absorbing solution into the midget
impinger. Sample at a rate of 0.5 to 3 L/min for up to 30 min. The flow rate and
the time of sampling should be adjusted to obtain a sufficiently large
concentration of oxidant in the absorbing solution. Approximately 1 µl of ozone
can be obtained in the absorbing solution at an atmospheric concentration of
0.01 ppm by sampling for 30 min at 3 L/min. Calculate the total column of the air
sample. Also measure the air temperature and pressure. Do not expose the
absorbing reagent to direct sunlight.
9.2 Measurement of Color - If, appreciable evaporation of the absorbing solution
occurs during sampling, add water to bring the liquid volume to 10 ml.
9.3 Within 30 to 60 minutes after sample collection, read the absorbance in a cuvette
or tube at 352 nm against a reference cuvette or tube containing water.
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