pitzer database CaCO3 aqueous species

[dbjevans]

I wondered if anyone is able to explain why the CaCO3 aqueous species is commented out in the pitzer.dat file by default? Is there some application of the database that performs better without the inclusion of this? I searched through the forum and maual and didn't find an explanation, but apologies if I missed it. Thanks in advance.

[dlparkhurst]

I don't know any details. The same data are in frezchem.dat, which means you may be able to track down the source in frezchem documentation. ColdChem.dat, which is a descendant of frezchem.dat does not include the ion pair.

CaCO3(aq) is not a strong pair, and, in general, weak pairs tend to be accounted for in the Pitzer parameters rather than an explicit pair. My guess is that the fitting is acceptable in most cases without the pair, but you may be right that the pair helps in fitting some systems.