Environmental Engineering Reference
In-Depth Information
TABLE 4.1
Comparison of
K
aw
Values from the Literature
Technique
Reference
∗
Chloroform
Carbon Tetrachloride
Benzene
Lindane
Equilibrium
1
0.175
1.229
0.22
-
Batch stripping
2
0.141
1.258
0.231
-
Multiple equilibration
3
0.125
0.975
-
-
10
−
2
Vapor pressure
4
0.153
0.807
0.222
1.33
×
10
−
4
to
2.18
×
10
−
3
Direct measurement 5 0.153 1.14 0.227 -
Wetted wall 6 - - - 3.43
×
10
−
3
Fog chamber 7 - - - 3.54
×
10
−
3
K
aw
(mean) 0.149 1.082 0.225 7.17
×
10
−
3
Standard deviation 0.018 0.189 0.005 4.82
×
10
−
3
∗
(1) Ashworth et al. (1988); (2) Warner et al. (1987); (3) Munz and Roberts (1986); (4) Mackay and
Shiu (1981) and Suntio et al. (1987); (5) Leighton and Calo (1978); (6) Fendinger and Glotfelty (1988);
(7) Fendinger et al. (1989).
8
-
-
-
8.41
×
and solubility
4.1.3 E
FFECTS OF
E
NVIRONMENTAL
V
ARIABLES ON
K
aw
The air-water partition constants estimated at room temperature using binary aqueous
systems(solute+water)arenotparticularlyapplicabletonaturalsystems(wastewater,
atmospheric moisture, seawater, etc.). Several environmental variables affect
K
aw
in
different ways. In this section, we briefly review four important variables and their
effects on air-water partition constants. These are (i) temperature, (ii) presence of
other miscible solvents, (iii) presence of colloids, and (iv) variations in pH.
As noted above, since
K
aw
is a ratio of
P
i
to
x
i
, the effects of temperature on
K
aw
are directly related to the variations in vapor pressure and aqueous solubility
with temperature. As per the Clausius-Clapeyron equation, both
P
i
and
x
i
show
logarithmic dependence on temperature. Hence, the variation in
K
aw
with temperature
is given by the equation
B
T
,
ln
K
aw
(T)
=
A
−
(4.9)
where
A
and
B
are constants for a given compound. The values for
A
and
B
for a
variety of compounds have been tabulated (Table 4.2). In reality, the above equation
is a method whereby single values of air-water partition constants at a standard
temperature
T
0
are extrapolated over a modest range using the solute enthalpy of
solution. A thermodynamics expression associated with van't Hoff relates the air-
water partition constants at two temperatures (Smith and Harvey, 2007). The implicit
assumption is that the enthalpy of solution is constant over the temperature range and
this is valid only over a narrow range of temperature (
≈
20 K).
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