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FILTRATION
When testing natural waters that contain significant turbidity due to
suspended solids and algae, filtration is an option. Reagent systems,
whether EPA, Standard Methods, LaMotte or any others, will generally
only determine dissolved constituents. Both EPA and Standard Methods
suggest filtration through a 0.45 micron filter membrane, to remove
turbidity, for the determination of dissolved constituents.** To test for
total constituents, organically bound and suspended or colloidal
materials, a rigorous high temperature acid digestion is necessary.
**LaMotte offers a filtering apparatus: syringe assembly (Code 1050)
and membrane filters, 0.45 micron, (Code 1103).
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.
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SMART COLORIMETER
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