LaMotte LTC3000wi Turbidity & Chlorine Lab Meter - Manual (Page 17)

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. Since agitation will also decrease chlorine
concentrations, it is best to fi ll bottles completely to assure there is no air
space in the container. If sampling from a tap, allow the water to run for
a minute to assure a representative 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 defi ne color. If one examines
a water sample straight from a water source, the color of the water is
its apparent color. The color of the water without the contribution of
suspended substances is called true color. True color can decrease after
precipitation and increase 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 of water causing quick color
change. Major contributors are tannins, hemic acids and inorganic
minerals. Color can be critical, since as the color increases, the amount
of light that penetrates the water decreases, and thus submerged plant
life, that depend on this light for photosynthesis, will decrease.
HOW IS COLOR MEASURED?
Since most natural waters have color that is similar to a solution
of chloroplatinate and cobalt, the APHA specifi es the use of dilute
chloroplatinate/cobalt color standards to defi ne color values. In the
APHA method, the color of a water sample is compared visually to 6
to 9 chloroplatinate/cobalt standards. However, visual methods suffer
due to the subjective observations of the person judging the colors. To
eliminate this source of error, color can be measured electronically with
a spectrophotometer or a colorimeter like the LTC3000we/wi.
The LTC3000we/wi is calibrated with APHA color standards at 428 nm.
The meter electronically measures color 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 colored sample. The LTC3000we/wi
uses the ratio of these two measurements to calculate the color and
displays the result. The results are expressed in APHA color units (cu).
There is no standard wavelength for measuring color, and it is common
for meters to use different wavelengths. Since chloroplatinate/cobalt
standards will have different absorbance values at various wavelengths,
comparing results from the LTC3000we/wi to meters using wavelengths
other than 428 nm is not valid.
Meters using different wavelengths will only give the same reading
when measuring chloroplatinate/cobalt standards since they are
both calibrated to those standards. When measuring natural water,
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