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H therefore has the units of Pa m 3 mol 1 and can be estimated by
ratioing a chemical's liquid-phase saturation vapour pressure (usually
the sub-cooled liquid vapour pressure) over its aqueous solubility, at
some reference temperature. Note that the vapour pressure (P) can be
readily converted into 'solubility' with units of molar concentration by
applying the ideal gas law
PV ¼ nRT
ð 6 : 9 Þ
Rearranging this equation to n/V (mol m 3 ) ¼ P/RT, where RT is the
gas constant - temperature product (8.314 Pa m 3 mol 1 K K), allows
the following expression
RT C w ¼ C a
P
C w ¼ H 0 or K aw
ð 6 : 10 Þ
where P/RT is the air concentration (C a ) with the same units as C w , and
H 0 is the 'dimensionless' H or K aw and can also be obtained by simply
dividing H by RT (note that the units cancel), where T is the relevant
environmental temperature in K.
Like K ow , values of H vary tremendously for the range of organic
compounds of interest in the environment. For example, short-chain
alkanes with their high vapour pressures and low aqueous solubilities
have values of H that are orders of magnitude higher than alcohols,
which are highly soluble in water and have lower vapour pressures. For
certain pesticides and other semi-volatile organic compounds, which
may persist in the environment, their lower vapour pressures generally
result in relatively low values of H, but often their very low solubility can
result in higher than expected values of H. As a consequence, these
chemicals (particularly sparingly soluble organochlorine pesticides) ap-
preciably partition to air from bodies of water in which they may be
residing (i.e. run-off from agricultural regions). In addition, they will not
be efficiently removed from the atmosphere by rainfall and as a result
may be transported long-distances by the prevailing wind. Table 1
provides examples of select chemicals and their calculated H values
using both saturated vapour pressure and aqueous solubility data.
Clearly, n-hexane has the highest value of H out of this group of
chemicals, but interestingly H for benzene is 557, B 200-fold greater
than DDT and B 4000-fold greater than chlorpyrifos, yet both pesticides
are relatively involatile, with vapour pressures some 600 million and 85
million times less than benzene, respectively. The low solubility of these
pesticides ensure that H is higher than may be expected and both
pesticides partition appreciably out of water.
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