DIC as Sum of Carbonate Species
In hydrochemistry, DIC or CT is defined as the sum of all dissolved carbonate species:
|(1)||DIC = CT = [CO2] + [HCO3-] + [CO3-2]|
where the rectangular brackets symbolize molar concentrations. The three carbonate species are:
|• CO2 = CO2(aq) + H2CO3||composite carbonic acid H2CO3*|
|• HCO3-||hydrogen carbonate (bicarbonate)|
Equation (1) applies only to the pure CO2-H2O-System. In real systems, additional carbonate species or complexes are present and 1 becomes:
|(2)||DIC = CT = [CO2] + [HCO3-] + [CO3-2] + carbonate complexes|
Typical carbonate complexes are: CaCO3(aq), CaHCO3+, MgCO3(aq), MgHCO3+, NaHCO3(aq), NaCO3-, FeCO3(aq), FeHCO3+ and others.3
Each equilibrium calculation provides the complete carbonate speciation. One example is given in the lower part of the right screenshot. It displays the four terms of 2 that add up to DIC.
More details, can be found in the output table Ions. It contains the molar and mass concentrations of each individual carbonate complex.
Example: The Closed Carbonate System
While each carbonate species vary with pH, their sum remains constant:
|(3)||DIC = CO2 + HCO3- + CO3-2 + NaHCO3 + NaCO3- = 1 mM|
The counterpart of C(4) is C(-4), i.e. carbon in the oxidation state -IV. To C(-4) belong methane CH4 which predominates under strong reducing conditions. ↩
Note the difference between the dissolved species such as CaCO3(aq) and the solid phase CaCO3(s) known as calcite. These are quite different entities. ↩