solubility of amorphous calcium carbonate

[aud]

Hello,

I am using PHREEQC to calculate the saturation indices of amorphous calcium carbonate (ACC), employing a solubility constant from the literature (log K = −7.64). These saturation indices are subsequently used to fit the Classical Nucleation Theory (CNT) model.

The fitting work well at low ionic strength; however, when increasing the ionic strength to 0.5 M, I encounter significantly overestimated saturation indices, which leads to a failure of the CNT fitting. According to the literature, the CNT model should still be applicable at this ionic strength.

As far as I understand, the Pitzer database is more suitable for high ionic-strength systems. I would therefore like to ask whether it is appropriate to apply the same solubility constant (log K = −7.64) when using the Pitzer database, by defining ACC in the PHASES block.

Thank you very much in advance for your support.
aud

[dlparkhurst]

You can use the same equilibrium constant for either database. However, for most electrolytes, I expect similar saturation indices for either database at 0.5 ionic strength.

You do say solubility constant, but Phreeqc is calculating activity corrected saturation indices, which could be different than what you expect.