LaMotte SMART 2 Electronic Soil Lab - Manual (Page 10)

CHEMICAL TESTING
n
AN INTRODUCTION TO COLORIMETRIC ANALYSIS
Most test substances in water or soil extract are colorless and undetectable to
the human eye. To test for their presence we must find a way to “see” them.
The SMART 2 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.
The SMART 2 Colorimeter passes one of four colored light beams through one
of four 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.
10 SMART2 SOIL MANUAL 1.07