Environmental Engineering Reference
In-Depth Information
The preceding discussion indicates that metals and elements only absorb or emit one
wavelength of light. This is not the case. All elements absorb and emit many different
wavelengths of light. In some cases, wavelengths from two different elements may
overlap or be so close together that they interfere with each other. For this reason it is
important for the person doing the analysis to know all the elements likely to be present
so that these possible interferences can be controlled or eliminated.
10.7.3.2. Molecular Spectroscopic Methods
Colorimetry includes all the physical-chemical methods that make use of the light
absorption of colored solutions for the quantitative analysis of dissolved substances.
Colorimetry is based on the direct color comparison without any measuring device
beyond the human eye. When light passes through a colored solution its intensity is
reduced due to absorption of light of a specific wavelength by the colored substance. This
change is used in direct color comparison between the test solution and either vials
containing known concentrations of the component of interest or a color chart. The
amount of material present is recorded from the reference vial or chart to which it most
closely compares.
Two types of colorimetry are common—one in which the solution is colored and one
in which a reagent that reacts with the component of interest to form a colored complex is
produced. This second method is commonly used in pH determination of soil and water
and the levels of chlorine in swimming pool water.
10.7.3.3. Spectrophotometry
Spectrophotometry is similar in principle to colorimetry, except an instrument is used to
measure or compare the amount of light absorbed. The instrument consists of a light
source and an optical system to isolate the wavelengths of light of interest and pass the
light through the sample to a detector. In Spectrophotometry light in both the UV and IR
regions of the spectrum is used in addition to visible light. This means that components of
interest do not have to have perceivable color in order to be detected. In UV
spectroscopy, the UV region of the spectrum is used, and absorption of light is caused by
the movement of electrons in the molecule. In the IR region of the spectrum, the vibration
of bonds in molecules is determined.
10.7.3.4. General Spectroscopy Considerations
From the above discussion it is obvious that any additions, subtractions, or changes in the
sample can change the absorption of light and thus change the spectrophotometric results.
This is especially the case for colorimetry, in which contamination of the sample with a
dye or other colored compound can have a dramatic effect on the quantitative results.
Also in such analyses, if the cells used to hold the sample in the instrument are dirty, they
can interfere with the analysis. Likewise, a cloudy sample will give an erroneous result.
All analytical methods that depend on the absorption or emission of light have several
requirements in common. They all need blank solutions that contain all the components
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