PhreeqcUsers Discussion Forum

Conceptual Models => Database selection and modification => Topic started by: jungsdao on April 26, 2019, 08:37:36 AM

Title: Modeling high ionic strength solution: Pitzer & SIT?
Post by: jungsdao on April 26, 2019, 08:37:36 AM
Hello, I'm trying to model aqueous solution with high ionic strength(I>6~10)
To this end, either pitzer of sit parameters would be needed.
However, when I looked into the parameters given in sit.dat, I found several parameters for some ion pairs are missing which is present in my system.
For example, when modelling H2SO4 - Ca(OH)2 system, following parameters might be needed.
H+ - SO4-2
H+ - HSO4-
Ca+2 - SO4-2
Ca+2 - HSO4-
If there isn't value in the database file then it would be set as zero..
I'm concerned that this might fail properly calculating activity coefficent at the high ionic strength for abovementioned ion pairs.

In pitzer database, there are also several missing points especially for fluoride ions.
If there aren't parameters for ionic pairs in the database, then the result from PHREEQC can be unreliabe for high Ionic strength?
Plus, is there any recommendation between Pitzer and SIT for high ionic strength if there are some missing parameters?

Thanks in advance...
Title: Re: Modeling high ionic strength solution: Pitzer & SIT?
Post by: dlparkhurst on April 26, 2019, 04:47:04 PM
Fluoride is not included in the pitzer.dat database. You would have to find all of the appropriate parameters.

Ca, SO4-2, and HSO4- are included and should have relatively complete parameterizations, although HSO4- would only be important at pH below 3. Gypsum solubility in NaCl solutions was used as an example of the goodness of fit for the Pitzer model.

Given the extensive number of parameters available in the Pitzer approach, this approach can accurately represent almost any chemical system, provided it has been carefully fit. However, if a system has not been studied and parameterized, you are out of luck. SIT is more likely to have parameters for more systems, but with just a single interaction coefficient for each pair of ions, it is less likely to be accurate under all combinations and ionic strengths.

If you have solubility, osmotic, or other experimental results for you system, you can always compare calculations to the experimental data. Also, you can compare Pitzer versus SIT versus ion-association calculations to see how much simulations differ among the approaches.