Use the Carbonic Acid node to define all carbon dioxide species in contact with water (CO2(aq), H2CO3, HCO3-, and CO32-). The node automatically sets up equilibrium reactions and mass transport properties.

For the pKa there is two options: Built in (default), which offers temperature dependent values for carbonic acid in pure aqueous solution, or User defined, which lets you set values of your choice.

See also Species Nodes in the Aqueous Electrolyte Transport Interface and Diffusion and Migration Settings.