Confusion regarding SIT and Pitzer

[GCanciani]

Hello all!
So i've been looking to try out some simple speciation calculations and have managed to confuse myself as i read through various manuals and posts.
 First things first:
i'm using the phreeqc for windows v.2.18.00 (based off of phreeqc-2 version 2.18.3-5570)

As i've been learning to use the program i've been using the user's guide to version 2. However this does not include any information on SIT or Pitzer interactions.
Reading around it seems that this was a feature that was introduced into phreeqc as of version 2.17 (which would explain why it isn't in the v2 manual) and some info on these two models is then found in the manual for V3.

I have a couple of questions which i wanted to clarify with more experienced users:

- is my understanding correct in that since the program i'm using is based off V 2.18.... the version of phreeqc i'm using is capable of using the SIT or Pitzer interactions when running its calculations (rather than just using the D-H theory, which would be the only option in versions before 2.17)?

- Am i correct in understanding that the way to specify which system to use (SIT or Pitzer) is simply by adding the appropriate line in the database file?

-Will the program then decide on its own if and when to apply the chosen model rather than D-H or will the fact that i've specified a model in the database mean that D-H isn't used at all?

- Beyond an improvement in calculation speed, would shifting to V3 of phreeqc improve the quality of speciation calculations?

Thank you all for your kind help!

[dlparkhurst]

Pitzer functionality was added at version 2.12, with minor enhancements in later versions. SIT was implemented at version 2.17, but there have been some subsequent bug fixes. The modifications are described in the RELEASE NOTES (http://water.usgs.gov/water-resources/software/PHREEQC/Phreeqc_ReleaseNotes.txt, also in the doc directory of the installation).

We are not supporting older versions of PHREEQC programs, only the current versions at the https://www.usgs.gov/software/phreeqc-version-3 and Tony Appelo's versions at  https://hydrochemistry.eu/. New versions tend to be released first at the Appelo site before they are migrated to the other site.

For simplicity, let's just say that the choice of database file determines the model that is used (essentially by containing PITZER, SIT, or neither). With the batch and interactive version of PHREEQC, only one activity model can be used in a run, basically D-H, SIT, or Pitzer. It is not possible to switch among activity models within the same run. (If you are using IPhreeqc or IPhreeqcCOM, it is possible to load new databases, however, the PHREEQC instance is completely re-initialized when a database is loaded, but it is possible to have multiple instances of IPhreeqc objects, each with its own database.)

The calculation may or may not be faster with the latest version 3, but using the latest version would ensure that all known bugs have been fixed and that you have access to all new capabilities (documented in green at https://water.usgs.gov/water-resources/software/PHREEQC/documentation/phreeqc3-html/phreeqc3.htm). Also, the versions at the websites given above are the only versions that we will consider for support and bug fixes.

[GCanciani]

Thank you for the reply that was exactly the information i was looking for.