Varying solid concentration for titration data

[nvgroeni]

Dear Phreeqc community,

I've been trying to fit an acid-base titration of a solid phase using PEST and defining the surface in the function SURFACE with sites, surface area, mass, etc.

Now I realize that the solid concentration you give is fixed over your whole titration which is not the case for my titration data.
Does anyone knows a way to vary the solid concentration over the titration?

Thank you very much for your help.

Natacha

[dlparkhurst]

Depends on how you are doing your titration. If you are actually mixing waters (MIX) then you have to explicitly define each mixture, and you could simply explicitly define you surface as well for each step.

If you are using REACTION, you could get fancy and relate the surface to a sparingly soluble mineral, say quartz. Then you can add SiO2 to increase the precipitation of quartz, which would increase the amount of the surface.

Another keyword that could be useful is SURFACE_MIX, which will allow you to change the amount of the surface by a factor.

Code: [Select]

SOLUTION 1
Na 1
P  1 charge
END
SURFACE 1
Hfo_wOH 0.001 1 600
-eq 1
END
SURFACE_MIX 2
1 2.0
END
USE solution 1
USE surface 2
END


[John Mahoney]

I just saw the post from David Parkhurst. Here are some other thoughts depending upon how you have set this up.

You should be able to use the equilibrium_phases Keyword to include the phase in the "beaker".  Then as the phase dissolves or precipitates, depending on how its solubility changes, the amount of the phase will also change.  But for this to work you will need the know the solubility product of the phase.   If it is a really unusual phase you might have some trouble getting the log_k value.

 if you also have surface sites to track then use a Surface setup such as:

Surfc_wOH   Fe(OH)3(a)  equilibrium_phase 0.1     1e5 

which links the Surfc_wOH concentrations with the equilibrium_phase concentration of Fe(OH)3(a), in this case; the above case assumes 0.1 moles of these sites per mole of Fe(OH)3(a) in the "beaker".  So as the amount of phase changes as the pH changes the site concentration will also change. 

Generally surface area remains a constant that is proportional to the amount of precipitate.  The 1e5  values above represent a surface area per mole of Fe(OH)3(a).    Look at SURFACE in the HELP file for more discussion. 

 Try not to use too many fitted parameters.

 Also you might want to consider using PhreePlot for the fitting.  If you are really concerned about changes to the surface area you might have to write some sort of numeric tag expression (this is a PhreePlot term not sure how that is handled in PEST) , but that might get into a circular argument. 

There are some other options using the KINETICS and RATES keywords that might work.