LaMotte DCL-20 Colorimeter - Manual (Page 3)

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. 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 standards. 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).
Why measure colored light? White light is made up of many different colors or wavelengths of light. A colored sample
typical absorbs only one color or one band of wavelengths from the white light. Not much 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 which the test sample is most sensitive to, 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 complimentary 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.
With the reagent blank in the colorimeter chamber, scan the blank then perform the unknown tests as described.
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 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.
The tubes that are included with the colorimeter have an index mark to facilitate this.
3
1985-01 •10/01