How to Specify the Carbonate System?
There are three alternatives to determine the carbonate system:
- by entering the DIC (Dissolved Inorganic Carbon)
- by entering the alkalinity
- by entering the CO2 partial pressure (“Open CO2 System”)
The corresponding input panels are part of the main input window.
Input of DIC
The right screenshot shows the DIC input panel (in mg/L Carbon or in mmol/L). Note that DIC does not belong to the list of common water analysis parameters. An alternative is the alkalinity (if provided).
If neither DIC nor alkalinity data is available, then you can enter any initial DIC value (say, 1 mM or 1 mg/L) and let the program calculate the unknown DIC by charge-balance adjustment with DIC. [Warning: This method requires that the measured pH and all major ions are sufficiently accurate. Otherwise, the calculated DIC will not be very reliable.]
Input of Alkalinity
Instead of DIC the carbonate system can be specified by the measured alkalinity (also known as total alkalinity or M alkalinity). This quantity is closely related to the carbonate hardness (CH) and the Acid Neutralizing Capacity (ANC):
|Alkalinity = CH ≈ ANC to pH 4.3 … 4.5|
After calculation, all relevant parameters of the carbonate system (alkalinity, ANC, and the complete carbonate speciation) are displayed in the output tables.
The numerical treatment of alkalinity differs from the calculation of buffer capacities (ANC, BNC). The calculation of alkalinity is hard-wired in PhreeqC (which is the numerical solver in aqion). In contrast, buffer capacities are a result of reaction calculations (with HCl or NaOH) up to a certain equivalence point. The latter approach allows, for example, the consideration of mineral precipitation (in particular hydroxides).
Immediately after the first equilibrium calculation, the alkalinity (when entered by the user) is converted into a DIC value and stored as such (because the DIC, together with the pH, completely determines the alkalinity).