LaMotte LTC3000wi Turbidity & Chlorine Lab Meter - Manual (Page 16)

 CHLORINE
WHAT IS CHLORINE
Chlorine is added to water systems to sanitize the water. There are
various forms of chlorine that are added to water. These can be gas,
liquid (commonly called bleach or sodium hypochlorite), calcium
hypochlorite mixtures, stabilized chlorine products and chlorine
generated from salt. When these forms of chlorine are added, they
react with water to form free chlorine, hypochlorous acid. If free chlorine
reacts with ammonia, it will form various types of combined chlorine
(chloramines). Depending on the chlorine to ammonia ratio, these can
be mono, di or tri chloramines.
Because free chlorine can react with precursors in the water to form
carcinogenic trihalomethanes (THMs), many water systems have
switched to chloramines. In these systems, free chlorine and ammonia
are added together and controlled to form monochloramine. Although
not as active a sanitizer as free chlorine, chloramine is less likely to form
THMs. Since it is a slower sanitizer, the concentration of chloramine
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 LTC3000we/
wi makes it ideal for such measurements. Because of its wide range,
the LTC3000we/wi 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 LTC3000we/wi 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 the combined form of chlorine.
The LTC3000we/wi 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 LTC3000we/wi uses the ratio of these two
measurements to calculate the concentration of chlorine and displays
the result. The LTC3000we/wi uses the EPA approved wavelength of
525 nm to make these measurements.
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