A colorimeter passes a white light beam through an optical filter which
transmits only one particular color or band of wavelengths of light to the
photodetector where it is measured. The difference in the amount of
colored light transmitted by a colorless sample (blank) and the amount of
colored light transmitted by a colored sample is a measurement of the
amount of colored light absorbed by the sample. In most colorimetric
tests the amount of colored light absorbed is directly proportional to the
concentration of the test factor producing the color and the path length
through the sample. However, for some tests the amount of colored light
absorbed is inversely proportional to the concentration.
The choice of the correct optical filter and therefore the correct color or
wavelength of light is important. It is interesting to note that the filter
that gives the most sensitive calibration for a test factor is the
complementary color of the test sample. For example, the
Nitrate-Nitrogen test produces a pink color proportional to the nitrate
concentration in the sample (the greater the nitrate concentration, the
darker the pink color). A green filter is used since a pinkish-red solution
absorbs mostly green light.
REAGENT BLANK
Some tests will provide greater accuracy if a reagent blank is determined
to compensate for any color or turbidity resulting from the reagents
themselves. A reagent blank is performed by running the test procedure
on 10 mL of demineralized water. Use sample water to
SCAN BLANK
.
Insert the reagent blank in the colorimeter chamber and select
SCAN
SAMPLE
. Note result of reagent blank. Perform the tests on the sample
water as described. Subtract results of reagent blank from all subsequent
test results.
COLORIMETER TUBES
Colorimeter tubes which have been scratched through excess use should
be discarded and replaced with new ones. Dirty tubes should be cleaned
on both the inside and outside. Fingerprints on the exterior of the tubes
can cause excessive light scattering and result in errors. Handle the tubes
carefully, making sure the bottom half of the tube is not handled.
LaMotte Company makes every effort to provide high quality colorimeter
tubes. However, wall thicknesses and diameter of tubes may still vary
slightly. This may lead to slight variations in results (e.g. if a tube is turned
while in the sample chamber, the reading will likely change slightly). To
eliminate this error put the tubes into the colorimeter chamber with the
same orientation every time.
SMART COLORIMETER
9
