Fe 3+ Hydrolysis through various pH

[ajm1992]

Hi There,
I am trying to illustrate the effects of pH on Fe3+ hydrolysis and graph the various iron hydroxide species to duplicate some figures I've found in textbooks. I have used HCl and NaOH to manipulate the pH and used equilibrium phases and the Fix_H+ function. The issue I am running into is that Iron Chloride species begin to form in the lower pH solutions reducing my concentration of Fe3+. Is there a way to manipulate pH in Phreeqci without adding an acid or base and get it plot? I've attached the basic script I've got, the plots I've produced and what I'm aiming for.
Many Thanks!
-Austin

[John Mahoney]

If you just want to show the ferric  hydroxides you can define a dummy acid like HXl.  You will need to define Xl in Solution_Master_Species and the Xl- component in Solution_Species. Since there are no other reactions with Xl it will not form complexes.   But HXl will then behave as a strong acid.

[dlparkhurst]

Note also that PHREEQC will consider ionic strength effects, even with HXl, which may be ignored in the textbook figures.