Environmental Engineering Reference
In-Depth Information
where {H
} is the hydrogen ion activity, which is closely related to con-
centration expressed in moles per liter. Hydrogen ions interact with other
ions at all but the most dilute concentrations, so concentration does not
exactly correspond to hydrogen ion activity. The smaller the pH value, the
larger the hydrogen ion activity. On this log scale, each change of one unit
of pH corresponds to a 10-fold change in hydrogen ion activity. Pure wa-
ter has a pH of 7.0 (1.0
10
7
mol H
liter
1
). Vinegar and beer have
a pH of about 3, stomach acid has a pH of 2, and household ammonia has
a pH of about 11. The actual range found in most aquatic ecosystems is
near neutrality (several orders of magnitude in activity; pH
1). Acid
precipitation causes devastation in many aquatic systems because it both
lowers the pH several orders of magnitude (a hundred- to thousandfold in-
crease in H
activity) and increases solubility of toxic metals. The prob-
lem will be discussed in Chapter 14.
Additional bulk chemical parameters that are commonly measured as
indicators of water chemistry or quality include color, taste, odor and
tur-
bidity
(the light absorption from suspended particles).
Alkalinity
or
acid-
ity
indicate the capacity of water to react with a strong acid and base, re-
spectively, and are measured routinely.
Hardness
is the sum of the
magnesium and calcium ions present. Hardness is an indicator of the abil-
ity of water to precipitate soap and is of particular interest for domestic
water supplies. These bulk parameters are traditionally the first methods
used to characterize general water quality. Further tests are needed when a
more specific question is being asked.
Other dissolved materials will be discussed in more detail in this and
other chapters. The concentration of specific dissolved ions, other dissolved
compounds, and suspended particulate material can be analyzed by a wide
variety of methods. Some of these methods may be of interest to the stu-
dent (Method 11.1).
7
REDOX POTENTIAL, POTENTIAL ENERGY, AND
CHEMICAL TRANSFORMATIONS
The relative availability (concentration) of electrons for chemical reac-
tions in solution is referred to as
oxidation-reduction potential
or
redox
potential
. This parameter is important because it quantifies potential en-
ergy requirements or yields during biotic or abiotic processes that trans-
form chemical compounds in the environment. Even though the concept of
redox can be difficult to grasp, it is worth the effort because it provides an
understanding of the way that nutrients cycle and the behavior of pollu-
tants in the environment (Christensen
et al.,
2001). Building a rational
framework based on principles of chemistry is essential to learn the cycles
that underlie ecosystem function.
Redox potential of natural systems is simple to measure with elec-
trodes that assess availability of transferable electrons relative to the avail-
ability of electrons in hydrogen gas. Such sensors read in millivolts, with
low values (e.g., below 100 mV) denoting large numbers of transferable
electrons and high values of redox denoting transferable electrons. Even
though redox potential is easy to measure, prediction of a single redox
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