LaMotte 1964-EPA, Manual

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in water is higher than the concentration of free chlorine in water distribution systems.
The present EPA limit of chlorine in water systems is 4.0 ppm. The amount of chlorine
used to process waste may be higher than this.
Many states also establish limits on the amount of chlorine that can be discharged into
a body of water after waste processing. These usually are less than 0.1 ppm. The low
detection limit of the TC-3000 makes it ideal for such measurements. Because of its
wide range, the TC-3000 can be used to measure the water used in the wastewater
process, in a distribution system and for many low level discharge requirements.
How is Chlorine Measured?
The most common methods for measuring chlorine are colorimetric methods. In
colorimetric methods, chlorine reacts with reagents added to a water sample. The
reaction of the chlorine with the reagents produces a color. The intensity of the color
produced is proportional to the concentration of chlorine in the sample. The intensity
of the color can be measured by visual comparison with a calibrated color chart or
other types of visual color comparators. Visual methods suffer due to the subjective
observations of the person judging the colors.
The TC-3000 uses EPA approved DPD reagents to react with chlorine. In the absence
of iodide, free available chlorine reacts instantly with DPD to produce a pink color.
Subsequent addition of potassium iodide (DPD 3) causes a reaction with combined
forms of chlorine. The TC-3000 electronically measures the color produced in these
reactions in comparison to a colorless water sample. First it measures the intensity of a
light beam passing through a clear colorless sample, the blank. Then it measures the
intensity of light passing through the pink reacted sample. The TC-3000 uses the ratio
of these two measurements to calculate the concentration of chlorine and displays the
result. The TC-3000 uses the EPA approved wavelength of 525 nm, to make these
measurements.
Taking Chlorine Water Samples
Chlorine solutions are not stable and should be analyzed immediately. Samples may
be collected in glass. Amber or opaque bottles are recommended since exposure to
sunlight or agitation will decrease chlorine concentrations. It is best to fill bottles
completely to assure there is no air in the container. If sampling from a tap, allow the
water to run for a minute to assure a proper sample.
COLOR
What is Color?
Many different dissolved or suspended materials contribute to the color of water. These
can include industrial wastes, plant materials, metals and plankton. There are two
terms used to define color. If one examines a water sample straight from a water
source, the color of the water is its apparent color. To ascertain the color of the water
without the contribution of suspended substances, the water may be filtered. The color
of filtered water is due to dissolved substances and is called true color. True color can
increase after precipitation, and decrease in drier weather.
Some bodies of water can change color quickly, depending on the runoff conditions
and plant life around them. Wind can also stir up substances more in shallower bodies
22 INTRODUCTION