Encyclopedia of Environmental Science and Engineering, Volume I and II

TABLE 1

Solubility of gases

Examplea: CO 2 (g) CO 2 (aq)

Assumptions: Gas behaves ideally; [CO 2 (aq)]  [H2CO 3 *]

I. Expressions for Solubility Equilibriumb

(1) Distribution (mass law) constant, KD:

KD  [CO 2 (aq)]/[CO 2 (g)] (dimensionless) (1)

(2) Henry’s law constant, KH:

In (1), [CO 2 (g)] can be expressed by Dalton’s law of partial pressure:

[CO 2 (g)]  pCO 2 /RT (2)

Combination of (1) and (2) gives

[CO 2 (aq)]  (KD/RT)pCO 2  KH pCO 2 , (3)

where KH  KD/RT (mole liter^1 atm –1)

(3) Bunsen absorption coefficient, aB:

[CO 2 (aq)]  (B/22.414)pCO 2 (4)

where 22.414  RT/p (liter mole–1) and

aB  KH 
 22.414 (atm–1) (5)

Partial Pressure and Gas Composition

pCO 2  xCO 2 (PT – w) (6)

where XCO 2  mole fraction or volume fraction in dry gas, PT  total pressure and w  water vapor pressure

Values of Henry’s Law Constants at 25C

Gas KH(mole liter–1 atm–1)

Carbon Dioxide CO 2 33.8 
 10 –3

Methane CH 4 1.34 
 10 –3

Nitrogen N 2 – 0.642 
 10 –3

Oxygen O 2 1.27 
 10 –3

a Same types of expressions apply to other gases.

b The equilibrium constants defined by (1)–(4) are actually constants only if the equilibrium expressions

are formulated in terms of activities and fugacities.

Table from Stumm, W. and J. Morgan, Aquatic Chemistry, Wiley-Interscience, New York, 1970, p. 125.

and

[]

[]

CO.

3 CO H CO

(^22)
0
12
222


a
a
KP
KK
HHKp. (17)
It follows from these equations that in a logarithmic
concentration—pH diagram (Figure 4) the lines of H 2 CO 3 ,
HCO 32 , CO 32  have slopes of 0, 1, and 2, respectively.
If we equilibrate pure water with CO 2 , the system is
defined by two independent variables, for example, temper-
ature and Pco 2 , In other words, the equilibrium concentra-
tions of all solute components can be calculated by means of
Henry’s Law, the acidity constants and the proton condition
or charge balance if, in addition to temperature, one variable,
such as Pco 2 , [H 2 CO 3 ] or [H^ ^ ], is known or measured. Use
of the proton condition instead of the charge balance gener-
ally facilitates calculations because species irrelevant to the
calculation need not be considered. The proton condition
merely expressed the equality between the proton excess
and the proton deficiency of the various species with respect
to a convenient proton reference level. Figure 4 furnishes a
graphic illustration of its use.
Solubility Equilibria
Minerals dissolve in or react with water. Under different
physico-chemical conditions minerals are precipitated and
accumulate on the ocean floor and in the sediments of rivers
and lakes. Dissolution and precipitation reactions impart to
the water and remove from it constituents which modify its
chemical properties.
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