Environmental Engineering FOURTH EDITION

Measurement of Air Quality 379

Air sample TCM So2-TCM + Pararosaniline

STEP 1 with SO2 - absohing - complex

solution

Colored

solution

Light Filter Sample Photocell Readout

Figure 19-5. Schematic diagram of the pararosaniline method for measuring S02.

SO2 may be measured by bubbling air through hydrogen peroxide, so that the following
reaction occurs:

The amount of sulfuric acid formed can then be determined by titrating against a base
of known concentration.
Figure 19-5 is a schematic diagram of the pararosaniline technique for measuring
S02, which is a standard method for measuring atmospheric S02. In this method, air is
bubbled through a solution of tetrachloromercurate (TCM). The SO2 andTCM combine
to form a complex that then is reacted with pararosaniline to form a colored solution.
The intensity of the color is proportional to the SO2 concentration, absorbs light at
wavelength 560 pm, and can be measured with a colorimeter or spectrophotometer.
A similar colorimetric technique, for measuring ammonia concentration, is described
in Chap. 5.
Most bubblers are not 100% efficient; not all of the gas bubbled through the liquid
will be absorbed, and some will escape. The quantitative efficiency of a bubbler is
established by testing and calibrating with known concentrations of various gases in
air. Gas chromatography is a newer and very useful second-generation measurement
method, particularly since trapping the pollutant in a bubbler is not necessary. Long
sampling times are not needed, and the air sample can usually be introduced directly
into the gas chromatograph.
One widely used third-generation device is nondispersive infrared spectropho-
tometry (nondispersive IR), used for measurement of CO, including CO measurement
for routine automobile inspection and maintenance. Like all asymmetric gas molecules,
CO absorbs at the specific infrared frequencies that correspond to molecular vibrational
and rotational energy levels. As shown in Fig. 19-6, the air sample is pumped into
one of two chambers in the detector. The other chamber contains a reference gas
like nitrogen. Infrared lamps shine through both the sample cell and the reference
cell. CO in the sample will absorb IR in direct proportion to its concentration in
the sample. After passing through the two cells, the radiant energy is absorbed by
the gas in the two detector cells, both of which contain CO. Absorption of radiant