Determine the saturation index (SI)

[Aksenova]

Hello! Which method is used to determine the saturation index (SI) of CaSO4 and CaCO3 in the phreeqc.dat database? Oddo-Thomson, Pitzer or some other method? And which database (dat. file) is better for calculation of SI for temperature of 75?C and with an already known pH value of water solutions?

[dlparkhurst]

Phreeqc.dat is an ion-association model that accounts for non-ideality of solutions by the formation of ion-pairs and complexes and the use of Debye-Huckel activity coefficients. Phreeqc.dat does not use Oddo-Tomson or other simplified methods for calculation of saturation indices.

Pitzer.dat uses the Guggenhiem-Pitzer approach to solution non-ideality, which is a virial-expansion parameterization based on experimental data.

Pitzer is generally better for high ionic-strength waters. You can always make the calculations with different databases and compare.