4
titrant. The titration endpoint is identi
fi
ed by the development of color resulting
from the reaction with an indicator, by the change of electrical potential or by
the change of pH value.
Ion Selective Electrode
An ISE is a transducer (or sensor) that converts the activity of a speci
fi
c ion
dissolved in a solution into an electrical potential, which can be measured by a
voltmeter or pH meter. The sensing part of the electrode is usually made as an
ion-speci
fi
c membrane, along with a reference electrode.
INDICATORS
The US EPA recommends testing for the following indicator parameters, based
on the MS4 (Municipal Separate Storm Sewer System) permitting process:
Ammonia
Ammonia is a good indicator of sewage, since its concentration is much higher
there than in groundwater or tap water. High ammonia concentrations may also
indicate liquid wastes from some industrial sites. Ammonia is relatively simple
and safe to analyze. Some challenges include the tendency for ammonia to
volatilize and its potential generation from non-human sources, such as pets or
wildlife.
Color
Color is a numeric computation of the color observed in a water quality sample,
as measured in cobalt-platinum units (APHA, 1998). Both industrial liquid
wastes and sewage tend to have elevated color values.
Unfortunately, some “clean”
fl
ow types can also have high color values. Field
testing has found high color values associated for all contaminated
fl
ows, but
also many uncontaminated
fl
ows, which yielded numerous false positives.
Overall, color may be a good
fi
rst screen for problem outfalls, but needs to be
supplemented by other indicator parameters.
Conductivity
Conductivity, or speci
fi
c conductance, is a measure of how easily electricity can
fl
ow through a water sample. Conductivity is often strongly correlated with the
total amount of dissolved material in water, known as Total Dissolved Solids.
as an indicator depends on whether concentrations are elevated in “natural” or
clean waters. In particular conductivity is a poor indicator of illicit discharge in
estuarine waters or in northern regions where deicing salts are used (both have
high conductivity readings).
Conductivity has some value in detecting industrial discharges that can exhibit
extremely high conductivity readings.
Hardness
Hardness is a measurement of the positive ions dissolved in water and
primarily includes magnesium and calcium in natural waters, but is sometimes
in
fl
uenced by other metals. Field testing suggests that hardness has limited
value as an indicator parameter, except when values are extremely high or
low (which may signal the presence of some liquid wastes). Hardness may be
