Overview Activity Chem Equilibrium Acids Carbonate System Mineral Phases

DIC as Sum of Carbonate Species

In hydrochemistry, DIC or C_T 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)
•   CO3-2	carbonate

DIC or C_T is also abbreviated by C(4), which stands for carbon in the oxidation state IV.¹ C(4) is the favored notation in hydrochemistry codes and their thdyn databases.²

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

Carbonate Speciation

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:

(3)	DIC  =  CO2 + HCO3- + CO3-2 + NaHCO3 + NaCO3-  =  1 mM

Footnotes