Railsback's Some Fundamentals of Mineralogy and Geochemistry
Variation in the dissociation constants of H2CO3 and HCO3- with temperature and salinity
As previous pages in this series have discussed, the behavior of dissolved
inorganic carbon (CO2, HCO3-, and CO32-) hinges on dissociation of H2CO3* and
HCO3-. The equilibrium "constants" for the dissociation of H2CO*3 to HCO3- and
H+ (conventionally designated K1) and of HCO3- to H+ and CO32- (K2) vary with
temperature and salinity. The plots below illustrate the extent of that variation.
The values shown were calculated from equations reported by Millero, F.J., et al.,
2006, Dissociation constrants of carbonic acid in seawater as a function of salinity
and temperature: Marine Chemistry, v. 100, p. 80-94.
10.8
10.6
10.6
10.4
S=0
10.4
HCO3- --> H+ + CO32-
10.2
K2 =
aH+ x aCO32aHCO3-
10.2
10.0
10.0
9.8
pK2
pK2
9.8
S=5
9.6
9.6
9.4
9.4
2°C
S = 20
9.2
10°C
9.0
S = 35
9.0
20°C
S = 50
8.8
30°C
8.8
8.6
8.6
0
0
10
20
30
40
5
10
50
15
20
25
30
35
40
35
40
Temperature (°C)
Salinity (ppt)
Greater dissociation
9.2
6.6
6.6
H2CO3* --> H+ + HCO3-
6.4
pK1
6.2
K1 =
aH+ x aHCO3aH2CO3 *
0
10
20
30
S=5
5.8
20°C
30°C
6.2
6.0
10°C
5.8
S=0
pK1
2°C
6.0
6.4
40
Salinity (ppt)
H2CO*3 is the sum of true H2CO3 (C in three-fold coordination with one O2- and
two OH-) and CO2(aq) (C in two-fold coordination and hydrated with one H2O).
50
5.6
S = 20
S = 35
S = 50
0
5
10
15
20
25
30
Temperature (°C)
LBR SFMGMilleroetalpKsvsT&S04 11/2009