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partitioning). The net transport of an organic compound from one
phase to another (e.g. air to water) is limited by equilibrium constraints
and can be quantified according to a partitioning constant or coefficient
(K). This is simply defined as the ratio of the concentration (C) of the
chemical present in each phase (e.g. 1 and 2) at equilibrium; the point at
which the chemical potential in each phase is equal and there is no
longer net transfer in one direction. K can be represented as
K 1 ; 2 ¼ C 1
C 2
ð 6 : 1 Þ
In strict terms, K is expressed as the ratio of the mole fractions (x)ofa
chemical within each phase (for example, this could be the mole fraction
of a chemical present in a gaseous or liquid mixture). However, in
environmental chemistry, the common way of expressing chemical
concentration is not by mole fractions but as a molar concentration
(mol L 1 or molarity, M), although it is worth pointing out that the
volume (L or dm 3 ) may vary according to changes in temperature and
pressure and should be normalised to standard temperature (298 K) and
pressure (101,325 Pa (or 1 atm)). Furthermore, when dealing with one
phase that is solid, it is often more appropriate to deal with concentra-
tions as amount per mass to avoid having to estimate phase densities. A
mole fraction can be converted to molar concentration (M) according to
M ¼ x ð mol = mol Þ
V ð L = mol Þ
ð 6 : 2 Þ
where V is the molar volume of the mixture or solution. For aqueous
solutions relevant for environmental scenarios, it is possible to ignore
the contribution of the organic chemical to the molar volume of the
mixture due to the typically low concentrations of a chemical pollutant
present in a body of water. In this case, V is simply set as the molar
volume of water (V w ¼ 0.018 L mol 1 at 25 o C). 13
The term partitioning constant or coefficient refers to one chemical
species in each phase (for example, ionisable chemicals present in water
may exist in both neutral and dissociated forms and therefore each
would have a separate partitioning coefficient). For well-defined phases,
such as pure water or the pure liquid state of the chemical, then
partitioning with another equally well-defined phase such as air results
in the use of the term partitioning constant. The Henry's Law constant
describing chemical partitioning between pure air and water is one
example. Distribution ratios on the other hand, such as the soil-water
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