In studying aquatic chemistry, there are many applications for chemical equilibria.
For instance, it is possible to be looking into the mechanisms that control the
movement of solutes in groundwater or surface water; or one could be interested in
controlling the pH, alkalinity or corrosivity of drinking water; or one may be interested
in the toxicological effects of dissolved metals on the biota. Actually, there are so
many possible applications of chemical equilibria to aqueous systems that it would be
hard to list them all.
For a person interested in aquatic systems (or soil systems, or marine systems, for
that matter) a few assumptions hold true: Dissolved ions in solution interact with each
other (form complexes), interact with particulate surfaces (adsorb) and possibly form
solid phases (precipitate). In a typical natural system, say a stream water, there may
be 10 to 20 major chemical components dissolved in solution. These components
have the potential to form hundreds of dissolved
chemical complexes, solids phases or adsorbed
species. Some of these chemical species may be biologically active or even toxic
while others may be inert. All of this depends on factors like the total concentration
of each component, the pH, pe, ionic strength and temperature.
This is where calculating chemical equilibrium is helpful. As the name implies,
chemical equilibrium assumes that all reactions have gone to completion and are
in equilibrium with one another. So time dependent reactions -- those reactions
that have kinetic restrictions -- are not addressed in this approach. In essence, the
chemical equilibrium approach provides you with a thermodynamic snapshot of
your system: the pH, ionic strength, the distribution of dissolved chemical species,
how much solid phase formed, etc. This is the type of information that is critical to
understanding what happens chemically in water.