LaMotte COD Plus Colorimeter 1922-EX2 - Manual (Page 11)

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AN INTRODUCTION TO COLORIMETRIC ANALYSIS
Most test substances in water are colorless and undetectable to the human eye.
To test for their presence we must find a way to “see” them. The COD PLUS
Colorimeter can be used to measure a test substance that is itself yellow or
green to blue, or can be reacted to produce these colors. 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. Some reactions have limitations or variances inherent to them that may
give misleading results. Many such interferences are discussed with each
particular test instruction. In the most basic colorimetric method the reacted
test sample is visually compared to a known color standard. However, accurate
and reproducible results are limited by the eyesight of the analyst,
inconsistencies in the light sources, and the fading of color standards.
To avoid these sources of error, 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. Only a small difference would be measured between white
light before it passes through a colored sample versus after it passes through a
colored sample. 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 the sample.
The COD PLUS Colorimeter passes one of two colored light beams through
one of two optical filters which transmits only one particular color or band of
wavelengths of light to the photodectector where it is measured. The difference
in 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 wavelength for testing is important. It is interesting
to note that the wavelength that gives the most sensitivity (lower detection
limit) 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 wavelength in the green region should be selected to
analyze this sample since a pinkish-red solution absorbs mostly green light.
COD PLUS COLORIMETER 6/02 11