Biogeochemical Cycling of Chemicals - Principles of Environmental Chemistry - page 342

A further partition coefficient is the octanol-air partition coefficient,
which can be used to describe the partitioning between the organic matter
in soil, for which octanol is taken as a surrogate, and the atmosphere.
Thus,

K oa ¼ concentration in octanol
concentration in air at equilibrium
Values of these partition coefficients have been measured for a wide
variety of substances, but for those which measurements have not been
made, there are methods of calculation based upon fundamental proper-
ties of the molecule.
Frequently, the organic matter comprises only a fraction, f, of the
medium. Therefore for partition between air and soil containing 20%
organic matter content

K oa ¼ concentration in organic matter
concentration in air

¼ C om
p
¼ C soil
fp
and C soil ¼ K oa fp ¼ 0.2 K oa .p

Worked example
What concentration of naphthalene in a soil of 20% organic matter
will be in equilibrium with a concentration of 10 ppb vapour in air
(water solubility ¼ 31 g m 3 ; saturation vapour pressure ¼ 10.4 Pa
and log K ow ¼ 3.37).
In a saturated system,

K ow ¼ 2 : 35 10 3 ¼ C octanol

31

and

C octanol ¼ 7.28 10 4 gm 3 at equilibrium

Then,

K oa ¼ 7 : 28 10 4

10 : 4
¼ 7 : 00 10 3 gPa 1 m 3