Aquatic Chemistry Controlling Nutrient Cycling: Redox and O - Freshwater Ecology: Concepts and Environmental Applications - page 207

METHOD 11.1.
Analysis of Concentration of Specific Dissolved Ions and Other Dissolved
and Particulate Materials in Natural Waters
Many methods are used to detect the concentrations of specific ions in nat-
ural waters. Only a few of these will be discussed here. For a more defin-
itive treatment of a wide variety of methods, the reader is referred to S tan-
dard Methods for the Determination of Water and Wastewater (Eaton
et al., 1995). Probably the simplest method of analysis is the use of ion-
specific probes. The probes are calibrated in solutions with known ion con-
centrations. The calibrated probes can be immersed in the unknown solu-
tion and the concentration of the ion read directly from the meter. Most
biologically important ions are found in concentrations too low for analy-
sis with ion-specific probes; thus, other methods of analysis are required.
Colorimetric methods are used most commonly to analyze concentra-
tions of specific ions. These methods are based on formation of a specific
colored compound from the ion of interest. The concentration of the col-
ored compound can then be determined with a spectrophotometer (a ma-
chine that measures the absorption of light at specific wavelengths). Beer's
law states that the relationship between the absorption of light by a col-
ored compound and concentration of that compound is linear at interme-
diate concentrations. Beer's law allows the laboratory worker to make a
standard curve from which the unknown concentration in natural waters
can be read directly. Autoanalyzers are used routinely in laboratories that
need to analyze a large number of water samples. These machines auto-
matically take a small sample, add the appropriate chemical mixtures for
a colorimetric reaction to occur, measure absorption at the appropriate
wavelengths, and use a computer program to calculate concentration au-
tomatically from absorption.
Ion chromatography is also used in many laboratories. This technique
is based on the idea that dissolved ions will pass at different rates through
specific materials. A column filled with an appropriate material is used and
a carrier solution is passed through the column. A small amount of the
potential for natural aquatic habitat is difficult because of the myriad of
chemical compounds that co-occur and because the different compounds
may not be at equilibrium. Some elements common in the earth's crust
combine to form different organic and inorganic compounds that have a
different affinity for electrons from each other. Although these elements
should transfer electrons until they all have the same redox potential, in
reality some electron transfers are blocked, leading to redox potentials that
may vary for different elements.
The central idea that links the concept of redox potential to biogeo-
chemical cycling is that chemical compounds have potential energy when
they have a redox significantly different from their surrounding environ-
ment. Gibbs free energy diagrams are the best way to visualize the poten-
tial energy (Fig. 11.5). The redox potential of different chemical transfor-