Composite and True Carbonic Acid
As CO2 gas dissolves in water, less than 0.3 % of the CO2 is hydrated to H2CO3. Since it is difficult to distinguish between the unionized species CO2(aq) and the pure acid, H2CO3, both quantities are lumped together to one single component:
| (1) | composite carbonic acid: H2CO3* = CO2(aq) + H2CO3 |
which is also named “apparent carbonic acid”. It is just the apparent or composite carbonic acid that is generally accepted as the carbonic acid. The pure carbonic acid, however, is much stronger than the apparent acid — as it can be seen from the pKa values (the lower the pKa the stronger the acid):
| carbonic acid | reaction formula | log K | pKa | |
| (2) | true (pure) | H2CO3 = H+ + HCO3- | -3.7 | 3.7 |
| (3) | composite (apparent) | H2CO3* = H+ + HCO3- | -6.35 | 6.35 |
Pure H2CO3 could not be synthesized and isolated for a long time. It is an unstable species that decays rapidly (with reaction rate k21 as shown in the diagram below). Thus, in chemical modeling, almost always the apparent carbonic acid H2CO3* with its pKa value of 6.35 is used (rather than H2CO3).
In aqion, the composite carbonic acid, H2CO3*, is abbreviated by CO2. It is the same nomenclature as used in the thdyn database wateq4f. The quantity CO2 should not be confused with CO2(aq) or CO2(g).
Reaction Kinetics. The interrelation between the equilibrium constants (i.e. the log K values in 2 and 3) and the underlying reaction kinetics is explained here and here.