Question for using Fix_H+

[Jeonghwan Hwang]

My name is Jeonghwan Hwang, and I have a question regarding the method for fixing pH using Fix_H+.
When using Fix_H+ in Equilibrium_Phases, I have configured the system to use a base (ex: NaOH or Ca(OH)2) if the pH is above 7 and an acid (HCl) if the pH is below 7 to fix the pH. I have attached an example code below for your reference.

     SOLUTION   1
     -units  mol/L
     temp 25
     density 1
     pe 8.218
     pH 3.8
     I 0.0001
     Ca  0.00333
     Cl  0.00666 charge
     -water 0.01
     END

     PHASES
     Fix_H+
     H+ = H+
     log_k  0.0
     END

     USE solution 1
     EQUILIBRIUM_PHASES 1
     Fix_H+ -3 HCl  1 # if pH <= 7
     Fix_H+ -10 Ca(OH)2 1 # if PH > 7
     END

I am curious to know if this approach is correct.
Or should I set HCl for all pH levels?

Thank you for your assistance.

Best regards,

Jeonghwan Hwang

[dlparkhurst]

You can't define two versions of Fix_H+ in EQUILIBRIUM_PHASES.

There is a kludge for adding either acid or base as needed in Example 8 of the manual. It involves adding a fictitious PHASE, in this case NaNO3. Either HNO3 or NaOH will be added, as needed as needed to achieve the desired pH.

Code: [Select]

PHASES
NaNO3
NaNO3 = Na+ + NO3-
log_K -20

Fix_H+
H+ = H+
log_K 0
END

SOLUTION 1
EQUILIBRIUM_PHASES
NaNO3 0 10
Fix_H+ -2 HNO3 10
END

SOLUTION 1
EQUILIBRIUM_PHASES
NaNO3 0 10
Fix_H+ -12 HNO3 10
END