There are two types of carbonic acid: true carbonic acid (H2CO3) and composite carbonic acid (H2CO3*). True carbonic acid involves the direct reaction of CO2 and H2O, while composite carbonic acid treats CO2(aq) and H2CO3 as a single entity. Each acid has its own equilibrium constant (Ktrue and K1). K1 is around 500 times smaller than Ktrue due to the inclusion of dissolved CO2 in the composite definition. The reaction kinetics between the three species (CO2(aq), H2CO3, HCO3-) were also described, noting that the direct reaction between CO2 and H2O is much slower than

1. H2O + CO2
Carbonate Kinetics
aqion.de

2. True and
Composite Carbonic Acid
Part 1

3. Three components are
involved in the carbonic acid
formation:
CO2(aq)
H2CO3
HCO3
-
true carbonic acid
dissolved CO2
hydrogen carbonate

4. Two of them are bundled
into one entity:
CO2(aq)H2CO3
true carbonic acid dissolved CO2
the composite carbonic acid H2CO3* =
+
It‘s just the composite carbonic acid (and not the
true carbonic acid) that is commonly known as
the carbonic acid.

5. True and Composite Carbonic Acid
true carbonic acid: H2CO3
composite carbonic acid: H2CO3* = CO2(aq) + H2CO3
There are two types of carbonic acid:
Each of these two acids is characterized by
its own equilibrium constant :
Carbonic acid Reaction formula
Equilibrium
constant
true H2CO3  H+ + HCO3
- Ktrue
composite
(apparent)
H2CO3
*  H+ + HCO3
- K1
Note:Thisisnot
areactionequation.

6. True and Composite Carbonic Acid
true carbonic acid: H2CO3
composite carbonic acid: H2CO3* = CO2(aq) + H2CO3
There are two types of carbonic acid:
Each of these two acids is characterized by
its own equilibrium constant :
Carbonic acid Reaction formula
Equilibrium
constant
true H2CO3  H+ + HCO3
- Ktrue = 2.0·10-4 M
composite
(apparent)
H2CO3
*  H+ + HCO3
- K1 = 4.4·10-7 M
500 times
stronger !

7. }COH{}OH)aq(CO{
}HCO}{H{
K
3222
3
1



}COH{
}HCO}{H{
K
32
3
true


Law of Mass Action
Reaction Kinetics
log Ktrue = -3.7
log K1 = -6.35
?Equilibrium Constant
?

8. The underlying
Reaction Kinetics
Part 2

9. Three components are
involved in the carbonic acid
formation:
CO2(aq)
H2CO3
HCO3
-
true carbonic acid
dissolved CO2
hydrogen carbonate
ReactionKinetics

10. Reaction
Kinetics
CO2(aq) + H2O
true carbonic acid
HCO3
- + H+
H2CO3
1
2 3
k21 k13
k31
composite carbonic acid
k23
k32
k12
slow
1 = HCO3
- + H+
2 = H2CO3
3 = CO2(aq) + H2O2k1k3)kk(
dt
3d
3k1k2)kk(
dt
2d
3k2k1)kk(
dt
1d
23133231
32122321
31211312



1
2
3
1 2 3
2 1 3
3 1 2
1
2
3

11. Reaction
Kinetics
CO2(aq) + H2O
true carbonic acid
HCO3
- + H+
H2CO3
1
2 3
k21 k13
k31
composite carbonic acid
k23
k32
k12
slow
1 = HCO3
- + H+
2 = H2CO3
3 = CO2(aq) + H2O2k1k3)kk(
dt
3d
3k1k2)kk(
dt
2d
3k2k1)kk(
dt
1d
23133231
32122321
31211312



1
2
3
1 2 3
2 1 3
3 1 2
1
2
3

12. k13 , k31 (slow) << k12 , k21 (fast)
4-5 orders of magnitude
smaller
From Experiment:

13. k13 , k31 (slow) << k12 , k21 (fast)
2k1k3)kk(
dt
3d
3k1k2)kk(
dt
2d
3k2k1)kk(
dt
1d
23133231
32122321
31211312



1
2
3
1 2 3
2 1 3
3 1 2
X
X
CO2(aq) + H2O
true carbonic acid
HCO3
- + H+
H2CO3
1
2 3
k21 k13
k31
composite carbonic acid
k23
k32
k12
slow
X
2k1k
dt
1d
2112 
1
1 2

14. Fast Reaction  Equilibrium
02k1k
dt
1d
2112 
1
1 2
true
32
3
12
21
K
}COH{
}HCO}{H{
2
1
k
k


1
2
Law of Mass Action
2K1 true1 2

15. 2)kKk(3)kk(
dt
3d
2k1k3)kk(
dt
3d
23true133231
23133231


3
3 1 2
3
3 2
2K1 true1 2
ab kk
... simplify

16. 0
32
22
b
a
K
}COH{
}OH)aq(CO{
2
3
k
k



3
2
Law of Mass Action
Equilibrium
02k1k
dt
1d
ab 
3
3 2

17. CO2(aq) + H2O
H2CO3
ka kb
HCO3
- + H+
k12
k21
CO2(aq) + H2O
H2CO3 HCO3
- + H+
K0
Ktrue
Kinetic Rates
Equilibrium Constants

18. Composite Equilibrium Constant K1
The two species CO2(aq) and H2CO3 are treated
together as if they were one substance
(denoted by H2CO3*) with
}COH{}OH)aq(CO{
}HCO}{H{
K
3222
3
1



}COH{
}HCO}{H{
K *
32
3
1



19. 1}COH/{}OH)aq(CO{
K
}COH{}OH)aq(CO{
}COH}{H{K
}COH{}OH)aq(CO{
}HCO}{H{
K
3222
true
3222
32true
3222
3
1








1K
K
K
0
true
1


CO2(aq) + H2O
H2CO3
HCO3
- + H+
K1K0
Ktrue
H2CO3
*

20. Equilibrium Constants
k12 = 5·1010 M-1 s-1 [1]
k21 = 1·107 s-1 [1]
ka = kH2CO3 ≈ 18 s-1 [2]
kb = kCO2 ≈ 0.04 s-1 [2]
[1] Pocker & Blomkquist 1977
[2] Stumm & Morgan 1996: kH2CO3 = 10 ... 20 s-1, kCO2 = 0.025 ... 0.04 s-1
K0 = ka /kb = 450  log K0 = 2.65
Ktrue = k21 /k12 = 2.0·10-4 M  log Ktrue = -3.7
K1 = Ktrue /(K0 + 1) = 4.4·10-7 M  log K1 = -6.35
Kinetic
Rates

21. Résumé
H2CO3
* = H+ + HCO3
- log K1 = -6.35
Carbonate kinetics metamorphosed
into equilibrium thermodynamics:
... enters hydrochemistry models
and programs (PhreeqC, etc.)

22. Ref
aqion.de
www.aqion.de/site/175