Geology Reference
In-Depth Information
where C
o
and C
a
represent the equilibrium concentrations in octanol
and air respectively (mol m
3
) and hence K
oa
is dimensionless. K
oa
may
be estimated as the ratio of K
ow
over K
aw
i.e. K
oa
¼
K
ow
/K
aw
.AsK
aw
¼
H/
RT then K
oa
can be expressed as
K
oa
¼
K
ow
RT
H
ð
6
:
12
Þ
As there are very few direct measurements of K
oa
, application of Equation
6.12 is useful, although this approach may have limited utility, as reliable
values of H and K
ow
for a particular chemical may be scare. Furthermore,
the validity of Equation 6.12 is brought into question when comparing
calculated K
oa
values with actual measurements
26
and extensive addi-
tional measurements of K
oa
arerequiredforawidenumberofchemicals
that occur in the environment. As with K
ow
and K
aw
,valuesofK
oa
may
vary over orders of magnitude within any particular chemical class, and
therefore K
oa
is often reported as a log value.
6.4.2 Temperature Dependence
Before proceeding, it is worth turning briefly to thermodynamics to
provide some fundamental aspects to partitioning coefficients and,
importantly, their relationship to temperature. Wide variations in tem-
perature experienced in the environment can greatly alter the partition-
ing behaviour of a chemical, and this will be demonstrated later when
comparing chemical behaviour between different latitudes with notable
temperature differences.
As with any chemical reaction, movement of a chemical from one phase
to another (i.e. liquid to gas) results in a change in the energy status of the
system. A partitioning coefficient is related to the (Gibbs) free energy of
transfer of the chemical between the two phases (DG,Jmol
1
). Where DG
relates both the enthalpic and entropic effects that result from the changes
occurring in either phase (i.e. intermolecular interactions), through both
the removal and addition of molecules of the chemical. The relationship
between DG and the change in enthalpy (or heat energy) of transfer (DH,J
mol
1
) and entropy (DS,Jmol
1
K
1
) is given by the Gibbs-Helmholz
equation: DG
¼
DH-TDS. Knowledge of the change in enthalpy for a
certain partitioning process is particularly useful and is sought by envi-
ronmental chemists to allow derivation of temperature-dependent parti-
tioning. The relationship between K and DG is given by
K
¼
constant
:
e
DG
=
RT
or lnK
¼
DG
RT
þ
ln
ð
constant
Þ
ð
6
:
13
Þ
