McKay, Donald. "Front matter" Multimedia Environmental Models ...

Environmental thermodynamics or phase equilibrium physical chemistry applies
to a relatively narrow range of conditions. Tropospheric or surface temperatures
range only between –40° and +40°C and usually between the narrower limits of 0°
and 25°C. Total pressures are almost invariably atmospheric but, of course, with an
additional hydrostatic pressure at lake or ocean bottoms. Concentrations of chemical
contaminants are (fortunately) usually low. Situations in which the concentration is
high (as in spills of oil or chemicals) are best treated separately. These limited ranges
are fortunate in that they simplify the equations and permit us to ignore large and
complex areas of thermodynamics that deal with high and low pressures and temperatures,
and with high concentrations.
The presence of a chemical in the environment rarely affects the overall dominant
structure, processes, and properties of the environment; therefore, we can take the
environment “as is” and explore the behavior of chemicals in it with little fear of
the environment being changed in the short term as a result. There are, however,
certain notable exceptions, particularly when the biosphere (which can be significantly
altered by chemicals) plays an important role in determining the landscape.
An example is the stabilizing influence of vegetation on soils. Another is the role
of depositing carbon of photosynthetic origin in lakes.
A point worth emphasizing is that thermodynamics is based on a few fundamental
“laws” or axioms from which an assembly of equations can be derived that relate
certain useful properties to each other. Examples are the relationship between vapor
pressure and enthalpy of vaporization, or concentration and partial pressure. In some
cases, the role of thermodynamics is simply to suggest suitable relationships. Thermodynamics
never defines the actual value of a property such as the boiling point
of benzene; such data must be obtained experimentally. We thus process experimental
data using thermodynamic relationships. Despite its name, thermodynamics is not
concerned with process rates; indeed, none of the equations derived in this chapter
need contain time as a dimension.
It transpires that two approaches can be used to develop equations relating
equilibrium concentrations to each other as shown in Figure 5.1. The simpler and
most widely used is Nernst’s Distribution law, which postulates that the concentration
ratio C 1/C 2 is relatively constant and is equal to a partition or distribution coefficient
K 12. Thus, C 2 can be calculated as C 1K 12. K 12 presumably can be expressed as a
function of temperature and, if necessary, of concentration. Experimentally, mixtures
are equilibrated, and concentrations measured and plotted as in Figure 5.1. Linear
or nonlinear equations then can be fitted to the data. The second approach involves
the introduction of an intermediate quantity, a criterion of equilibrium, which can
be related separately to C 1 and C 2. Chemical potential, fugacity, and activity are
suitable criteria, with fugacity being preferred for most organic substances because
of the simplicity of the equations that relate fugacity to concentration. The advantage
of the equilibrium criterion approach is that properties of each phase are treated
separately using a phase-specific equation. Treating phases in pairs, as is done with
partition coefficients, can obscure the nature of the underlying phenomena. We may
detect a variability in K 12 and not know from which phase the variability is derived.
Further complications arise if we have 10 phases to consider. There are then 90
possible partition coefficients, of which only 9 are independent. Mistakes are less
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