DIC as Sum of Carbonate Species
In hydrochemistry, DIC or CT is defined as the sum of all dissolved carbonate species:
||DIC = CT = [CO2] + [HCO3-] + [CO3-2]
where the rectangular brackets symbolize molar concentrations of the constituent in solution. The three constituents are:
|• CO2 = CO2(aq) + H2CO3
||composite carbonic acid H2CO3*
||hydrogen carbonate (bicarbonate)
DIC or CT is also abbreviated by C(4), which stands for carbon in the oxidation state IV. The latter is the favored notation in hydrochemistry codes and their thermodynamic databases.
Equation (1) holds for the pure CO2-H2O-System only. In real systems, additional carbonate species or complexes are present. Then 1 becomes:
||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.
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
In a closed carbonate system the total inorganic carbon (DIC) is a conserved quantity. This is demonstrated in the stacked-area chart of a titration calculation:
While each carbonate species vary with pH, their sum remains constant:
|| DIC = CO2 + HCO3- + CO3-2 + NaHCO3 + NaCO3- = 1 mM
[last modified: 2016-02-14]