Membrane concentrate Versus Neutral AqueousSpecies

[Jicé]

Hello all,

I've recently noticed that, while:
- modeling a membrane process using PhreeqC Water speciation+"Phreeqc.dat" data source;
- comparing the results with real lab data;

PhreeqC calculated a concentrate pH slightly lower than real life's.

Then, I've tried removing all neutral AqueousSpecies from the database (CaCO3(aq), CaSO4(aq),...) and, after another PhreeqC calculation, I've obtained exactly the same concentrate pH calculated by PhreeqC as real life's.

My questions are:
- Is there a general rule to know when to enable/disable those neutral AqueousSpecies?
- What would be the risk(s) of disabling them permanently?
- For a given SolidPhase/AqueousSpecies (example: Calcite CaCO3(s)/CaCO3(aq) ), would the following rule be reliable:

If SaturationIndex(Calcite)<100% then
'We assume that CaCO3(aq) AqueousSpecies does EXIST in solution while Calcite does NOT precipitate
          CaCO3(aq).Enabled = TRUE       
Else
'We assume that CaCO3(aq) AqueousSpecies does NOT EXIST in solution while Calcite does precipitate
          CaCO3(aq).Enabled = FALSE
End if

Thank you very much,

[dlparkhurst]

I would not remove the ion pairs. They are present as the result of fitting some empirical data, so removing them would cause the fits of those data to be worse, even if it happens to look better for your data.

phreeqc.dat is simply an aqueous model. There may be other physical or chemical reasons for the difference in pH. Perhaps you want to try the pitzer.dat database, which has almost no ion pairs in its formulation. I am not sure that the lack of ion pairs in that model will result in a better fit, but you can try.