Environmental Engineering Reference
In-Depth Information
electronic spectrophotometer that measures the amount of light absorbed
by the treated sample at a 543-nm wavelength. The absorbance value con-
verts to the equivalent concentration of nitrate against a standard curve.
Methods for making standard solutions and standard curves are presented
in Standard Methods .
Before each sampling run, the sampling/monitoring supervisor should
create this curve. The curve is developed by making a set of standard con-
centrations of nitrate, reacting them, and developing the corresponding color,
then plotting the absorbance value for each concentration against concentra-
tion. A standard curve could also be generated for the color wheel. Use of the
color wheel is appropriate only if nitrate concentrations are greater than 1
mg/L. For concentrations below 1 mg/L, use a spectrophotometer. Matching
the color of a treated sample at low concentrations to a color wheel (or cubes)
can be very subjective and can lead to variable results. Color comparators
can, however, be effectively used to identify sites with high nitrates.
This method requires that the samples being treated are clear. If a sample
is turbid, filter it through a 0.45-µm filter. Be sure to test to make sure the
filter is nitrate free. If copper, iron, or other metals are present in concentra-
tions above several milligrams per liter, the reaction with the cadmium will
slow down and the reaction time must be increased.
The reagents used for this method are often prepackaged for different
ranges, depending on the expected concentration of nitrate in the stream. For
example, the Hach Company provides reagents for the following ranges: low
(0 to 0.40 mg/L), medium (0 to 4.5 mg/L), and high (0 to 30 mg/L). Determining
the appropriate range for the stream being monitored is important.
Nitrate Electrode Method
A nitrate electrode (used with a meter) is similar in function to a dissolved
oxygen meter. It consists of a probe with a sensor that measures nitrate activ-
ity in the water; this activity affects the electric potential of a solution in the
probe. This change is then transmitted to the meter, which converts the elec-
tric signal to a scale that is read in millivolts, and the millivolts are converted
to milligrams per liter of nitrate by plotting them against a standard curve.
The accuracy of the electrode can be affected by high concentrations of chlo-
ride or bicarbonate ions in the sample water. Fluctuating pH levels can also
affect the meter reading.
Nitrate electrodes and meters are expensive compared to field kits that
employ the cadmium reduction method. (The expense is comparable, how-
ever, if a spectrophotometer is used rather than a color wheel.) Meter and
probe combinations run between $700 and $1200, including a long cable to
connect the probe to the meter. If the program has a pH meter that displays
readings in millivolts, it can be used with a nitrate probe and no separate
nitrate meter is needed. Results are read directly as milligrams per liter.
Search WWH ::




Custom Search