LaMotte TC-3000 Tri-Meter - Manual (Page 8)

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 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
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 specifies the use of dilute chloroplatinate/cobalt color standards to
define 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 electonically with a spectrophotometer, or colorimeter
liekt the TC-3000.
The TC-3000 is calibrated with APHA color standards at 375 nm. This wavelength
was found to give the greatest sensitivity with chloroplatinate/cobalt color standards,
thus most natural waters. The meter electronically measures color in comparison to a
colorles water sample. First it measures the intensity if a light beam passing through a
clear colorless sample, the blank. Then it measures the intensity of light passing
through the colored sample. The TC-3000 uses the ration of these two measurements
to calculate the color and displays the result. The results are expressed in APHA color
units (cu). The TC-3000 can use signal averaging to improve accuracy when
measuring very low levels of color. Not valid, unless a correlation chart has been
constructed.
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 TC-3000 to
meters using wavelengths other than 375 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, meters using different wavelengths should not be
8 COLOR