Geoscience Reference
In-Depth Information
The equilibrium constant is (White, 1972)
+
−
[ [
HCl
][
HOCl
]
44810
4
K
H
=
=×
.
at 2
5°C
(23.99)
Cl
2(aq)
Henry's law is used to explain the dissolution of gaseous chlorine, Cl
2(aq).
Henry's law describes
the effect of the pressure on the solubility of the gases: There is a linear relationship between the
partial pressure of gas above a liquid and the mole fraction of the gas dissolved in the liquid (Fetter,
1999). The Henry's law constant,
K
H
(as shown in Equation 23.99), is a measure of the compound
transfer between the gaseous and aqueous phases.
K
H
is presented as a ratio of the compound's con-
centration in the gaseous phase to that in the aqueous phase at equilibrium:
P
K
=
(23.100)
H
C
water
where
K
H
= Henry's Law constant.
P
= Compound's partial pressure in the gaseous phase.
C
water
= Compound's concentration in the aqueous solution.
Note:
The unit of the Henry's law constant is dependent on the choice of measure; however, it
can also be dimensionless. For our purpose, Henry's law can be expressed as (Downs and
Adams, 1973)
=
Cl
Cl
(mol/L atm)
2(aq)
2(aq)
Cl
=
(23.101)
2(g)
H
⋅
P
Cl
2
where
[Cl
2(aq)
] = Molar concentration of Cl
2
.
H
= Henry's law constant = 4.805 × 10
-6
exp(2818.48/
T
), where
T
is temperature (K).
P
Cl
2
= Partial pressure of chlorine in the atmosphere.
The disinfection capabilities of hypochlorous acid (HOCl) are generally higher than those of
hypochlorite ions (OCl
-
) (Water, 1978). Hypochlorous acid is a weak acid and subject to further
dissociation to hypochlorite ions (OCl
-
) and hydrogen ions:
HOCl ↔ OCl
-
+ H
+
(23.102)
Its acid dissociation constant
K
a
is
−
+
[
OCl H
HOCl
][]
K
H
=
(23.103)
[
]
−
8
=×
=×
37 10
2611
.
.
at 25°C
−
0
at 20°C
The value of
K
a
for hypochlorous acid is a function of temperature (K) as follows (Morris, 1966):
ln
K
a
= 23.184 - 0.058
T
- 6908/
T
(23.104)
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