Environmental Engineering Reference
In-Depth Information
The general principle is that the higher that constant, the greater the potential for the compound to
volatilize from the water surface. The Henry's law constant is a function of molecular structure and
air and water temperatures (Goss, 2006).
The Henry's law constant,
H
, is usually dei ned at a particular temperature and at equilibrium as
C
a
___
H
=
C
w
,
(3.6)
where
C
a
is the concentration of a compound in air and
C
w
is the concentration of that compound in
water.
H
is conventionally dei ned in terms of gas concentrations (in atm) and liquid concentrations
(in mol/m
3
); therefore, typical units for
H
are (atm · m
3
)/mol.
H
is usually reported from measure-
ments made at 25°C or 20°C and 1 atm pressure. It is important to take note of the temperature at
which
H
was measured to ensure that values for different compounds can be compared.
The dimensionless form of the Henry's law constant, noted as
H
C
, is obtained by converting gas
concentrations from partial pressures in atmospheres to moles per cubic meter. The ideal gas law
relates pressure, volume, temperature, and number of moles:
n
P
__
___
V
=
RT
,
(3.7)
H
___
H
C
=
RT
,
(3.8)
where
R
is the universal gas constant and is equal to 8.2 × 10
−5
(m
3
atm)/(mol K);
T
is the tempera-
ture (in K). Note that the dimensionless
H
C
actually has the units of concentration of the gas (in mol/
m
3
) per concentration of the liquid (in mol/m
3
); the units can also be volume (in m
3
of liquid) per
volume (in m
3
of gas).
The Henry's law constant is often stated as the simple ratio of a compound's vapor pressure to its
aqueous solubility. A limitation to this interpretation of Henry's law is the inherent assumption that
water does not dissolve into the compound. The vapor pressure of the pure compound is used in this
statement of Henry's law, but the solubility used is for the compound when saturated with water. The
Henry's law constant estimates that are calculated from the compound's vapor pressure and solubil-
ity fail where the solubility of water in the chemical exceeds a few percent (Corsi, 1998). An exhaus-
tive compilation of measured Henry's law constants is found in Sander (1999).
The Henry's law constant describes the tendency of a compound to escape the liquid phase and
move into the gaseous phase. The term for the potential to transfer from one medium to another is
fugacity
, or escaping tendency, and is a function of a compound's activity coefi cient (Arbuckle,
1983). Compounds such as dichloromethane with high vapor pressures (i.e., with high fugacity) and
high aqueous-phase activity coefi cients will tend to partition to the gas phase to equalize the chem-
ical potentials for the compound in water and air; thus such compounds have high values of
H
.
Conversely, compounds such as alcohols with low vapor pressures and high solubilities (i.e., low
activity coefi cients in water) will have low values of
H
and will tend to partition into the aqueous
phase (Schwarzenbach et al., 1993).
Where the ionic strength of a solution increases, as may occur when polluted water is discharged
to marine waters, a compound's solubility in the saline solution will be lower, while its vapor
pressure remains constant, leading to higher values of
H
and increased rates of volatilization. This
phenomenon, called electrostriction or “salting out,” is leveraged in laboratory analysis, as described
in Chapter 4.
Table 3.3
provides Henry's law constants in two forms for solvent-stabilizer and chlo-
rinated solvent compounds.
3.1.3.2 Mass Transfer Rates from Water to Air: Flux Density
Henry's law values can be used to estimate the rate of mass transfer, or l ux density, from a surface-
water body by evaluating the volatility of an aqueous solution in isolation from other factors. The
air-water interface can be idealized as a thin layer of static air above the water surface, above which
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