An Introduction To Colorimetric Analysis - LaMotte 1200 Instruction Manual

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 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. 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.
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.
10