Environmental Engineering Reference
In-Depth Information
Henry's law can be expressed as:
c
c
p
c
H
RT
g
=
e
H
=
;
H
=
(14.2)
e
e
l
a
l
where
H e is Henry's constant (given in concentration basis, in units of atmosphere per cubic meter per
mole)
p is the partial pressure
c l is the liquid concentration (molar)
Henry's constant can also be written in a nondimensional form by dividing by the universal gas
constant ( R ) and temperature ( T a ), so that it represents the ratio of the aqueous-phase concentration
( c l ) to the gas-phase concentration ( c g ).
Henry's law represents an equilibrium relationship in that it predicts the water concentration, as
a function of the partial pressure, when that concentration is in equilibrium with the atmosphere.
At equilibrium, the aqueous-phase concentration ( c l ) would be equal to the gas-phase concentration
( c g ), resulting in the saturation concentration ( c s ). As described earlier, the rate of movement ( R ) is
proportional to the gradient between c l and c g , or between the saturation concentration ( c s ) and the
liquid concentration ( c 1 ) and the luid (air and/or water) turbulence.
The saturation concentration can be computed from Henry's constant as
p
H
(14.3)
c
=
s
e
Henry's constants for selected compounds are provided in Table 14.2. For example, the atmo-
sphere is about 21% oxygen, so the partial pressure of oxygen is 0.21. The molecular weight of
oxygen is about 32 g mol -1 ; therefore, the saturation concentration of oxygen (at 20°C) is 0.21 atm/
(7.74E-01 atm m -3 mol −1 ) = 0.27 mol m -3 or 8.7 g m -3 = 8.7 mg L -1 . The oxygen concentration
decreases with altitude (decreasing barometric pressure), and decreases with increasing temperature
and salinity. For example, the solubility of oxygen in saltwater is, depending on the temperature,
16%-21% less than that in freshwater.
Henry's law accurately describes the behavior of gases dissolving in water (or other liquids) when
concentrations and partial pressures are reasonably low. Henry's constant varies with temperature;
therefore, as the temperature increases, so does Henry's constant, the result of which is that solubil-
ity decreases. Solubility also varies with other materials dissolved in water, and solubility is com-
monly related to salinity.
TABLE 14.2
Henry's Constant for Selected Compounds
Henry's Constant (20°C)
Compound
Formula
(Dimensionless)
(atm m -3 mol −1 )
Methane
CH 4
64.4
1.55E+00
Oxygen
O 2
32.2
7.74E−01
Nitrogen
N 2
28.4
6.84E−01
Carbon dioxide
CO 2
1.13
2.72E−02
Hydrogen sulide
H 2 S
0.386
9.27E−03
Sulfur dioxide
SO 2
0.0284
6.84E−04
Ammonia
NH 3
0.000569
1.37E−05
 
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