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> In the PHREEQC database there is two equations for CaCO3:

This reaction is for the formation of the CaCO3 ion pair:

Ca+2 + CO3-2 = CaCO3
        log_k           3.224

This reaction is for the dissolution of calcite:

        CaCO3 = CO3-2 + Ca+2
        log_k           -8.480

> Which one of the equilibrium constants is used by PHREEQC to calculate
solubility constant?

The second one is for the solid, calcite.

> If both this constants are used doesn't the model became unstable.

You could remove the ion pair (give a small log K = -20), but it should not
cause any instability.

If you are equilibrating with calcite, you need the log K for calcite.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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