> 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: Calcite CaCO3 = CO3-2 + Ca+2 log_k -8.480 > Which one of the equilibrium constants is used by PHREEQC to calculate the 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 David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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