Aquatic Chemistry Controlling Nutrient Cycling: Redox and O - Freshwater Ecology: Concepts and Environmental Applications - page 208

sample is added to this carrier solution, and the sample is carried through
the column. The time needed for specific ions to pass through is called the
retention time and is known for ions of interest. Ions in the sample are de-
tected after they pass through the column and are separated according to
their retention times. A nonspecific detector for ions (e.g., a conductivity
meter) can be used to estimate the amount of material passing out the end
of the column. The method is rapid and can be used to analyze a large
number of ions simultaneously. However, it is generally less sensitive than
colorimetric methods.
Atomic absorption spectrometry is used to estimate concentrations of
dissolved and particulate metals. This method is based on the idea that in-
dividual elements emit light at very specific wavelengths when their elec-
trons are excited. This is the same concept used to detect the elemental
composition of distant stars. For the analysis, the water sample is injected
into a chamber, where it is subjected to high energy that causes excitation
of electrons. The intensity of the light at the wavelength specific to the ion
is directly proportional to the amount of the ion. The signal is compared
to a standard concentration, thus giving the concentration of the element
in the sample. This method is time-consuming because only one element
can be measured at a time.
Dissolved and particulate organic carbon concentrations are usually
analyzed by conversion to CO 2 followed by analysis of concentration of
this gas. Some controversy exists regarding the efficiency of different meth-
ods (e.g., UV, high temperature, or persulfate oxidation digestions) used to
decompose organic carbon to CO 2 (Koprivnjak et al., 1995). Given the
complex chemical composition of dissolved and particulate organic car-
bon, it is not surprising that analysis is difficult.
Several methods are available for analyzing particulate materials. One
involves degradation of particulate material into dissolved ionic forms and
then analysis by methods mentioned previously for dissolved ions. Another
method involves combustion at high temperature and analysis of the re-
sulting gasses (e.g., organic nitrogen is converted to N 2 gas). As with or-
ganic carbon, problems can occur with the efficiency of degradation.
mations can be plotted relative to each other (Fig. 11.6). The energy yield
of a chemical transformation is the distance between the head and tail of
the arrow. The transformation will yield potential energy if the redox po-
tential of the solution is less than the head of the arrow for the reductions
and greater than the head of the arrow for the oxidations.
It is important to distinguish the total potential energy to be gained or
lost during conversion among chemical forms, in addition to determining
the energy required for the reaction to occur. The energy required to make
the conversion is called the activation energy (Fig. 11.5). The reaction will
not occur rapidly if the activation energy needed is high. For example, am-
monium can exist in an oxidized solution without spontaneously convert-
ing to nitrate, even though ammonium has a significantly higher potential