LaMotte DC1100 Colorimeter - Manual (Page 2)

AN INTRODUCTION TO COLORIMETRIC ANALYSIS
Most test substances in water are colorless and undetectable to the human eye.
In order to test for their presence, we must find a way to “see” them. The
LaMotte colorimeter can be used to measure any test substance that is itself
colored or can be reacted to produce a color. In fact, a simple definition of
colorimetry is “the measurement of color” and a colorimetric method is “any
technique used to evaluate an unknown color in reference to known colors.” In
a colorimetric chemical test, the intensity of the color from the reaction must
be proportional to the concentration of the substance being tested.
A colorimeter can be used to photoelectrically measure the amount of colored
light absorbed by a colored sample in reference to a colorless sample (blank).
White light is made up of many different colors or wavelengths of light. A
colored sample typically absorbs only one color or one band of wavelengths
from the white light. Little difference could be measured between white light
before it passes through a colored sample versus after it passes through. The
reason for this is that the one color absorbed by the sample is only a small
portion of the total amount of light passing through the sample. However, if we
could select only that one color or band of wavelengths of light to which the
test sample is most sensitive, we would see a large difference between the light
before it passes through the sample and after it passes through. 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 your 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.
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