Environmental Engineering Reference
In-Depth Information
TABLE 3.4
ϕ
Values for Some Organic Compounds in Seawater
Molar Volume
(cm
3
/mol)
Organic Compound
ϕ
Naphthalene
125
0.00242
Biphenyl
149
0.00276
Phenanthrene
182
0.00213
Dodecane
228
0.000962
Tetradecane
259
0.000964
Hexadecane
292
0.00233
Octadecane
327
0.00290
Eicosane
358
0.00190
Hexacane
456
0.00488
Source:
From Aquan Yeun, M., Mackay, D., and Shiu, W.Y. 1979.
Journal of Chemical and Engineering Data
24, 30-34.
favorable for the solute. This is called the
salting-out
process and is the basis of the
widely used concept of purifying organic compounds by crystallization from their
mother liquor. Since ions tend to bind water molecules in their hydration layer, they
make less water molecules available for solubilizing organics and hence the organics
tend to fall out of water. The opposite effect of
saltingin
is caused when
k
is negative.
Aquan Yeun, Mackay, and Shiu (1979) suggested that the effect of ionic strength on
the activity coefficient
γ
i
of the neutral organic species
i
can be correlated using the
following form of the Setschenow equation:
log
γ
i
γ
0
= Φ
V
i
C
s
,
(3.15)
where
γ
0
is the activity coefficient of the neutral solute species
i
in pure water.
Φ
is a parameter that depends on the partial molar volume of the salt in solution
(
V
o
)
and the molar volume of the liquid salt (
V
s
)
. In the above equation
V
i
is the
partial molar volume of the organic solute species in solution (cm
3
/mol) and
C
s
is the molar concentration (mol/dm
3
)
of the salt in solution. The values of
for
several hydrocarbon compounds found in seawater are listed in Table 3.4. The val-
ues are found to range from a low of 0.000962 for dodecane to as high as 0.00488
for hexadecane. A mean value of 0.0025 is used in most estimation of activity
coefficients. Most environmental waters have salt concentrations varying from 0 to
0.5 mol/dm
3
, where the latter is the mean seawater salt concentration. In seawater,
the increase in activity coefficient over that of pure water is seen to be from 1.2 to 2.3
for compounds of molar volumes ranging from 7.5
ϕ
10
−
4
m
3
/mol.
The ionic strength effects are therefore not entirely negligible for saline waters.
10
−
5
to 2.0
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