Definition of acid dissociation in water

[Yongqiang]

Hi PHREEQC experts,

I want to model an organic acid (PTBBA) dissociation in the solution. I used the following method to do it. But no matter how much acid in the brine, the pH keeps at 7.0. Please share me some insights on this problem. Thank you very much.

Best Regards,
Yongqiang



DATABASE c:\phreeqc\database\LLNL.DAT
TITLE pH calculation

SOLUTION_MASTER_SPECIES
   A   A-   -1.0   177.231    177.231

SOLUTION_SPECIES
   A- = A-
   log_k 0
   A- + H+ = AH
   log_k -4.4


SOLUTION 1
   units mol/l
   A   0.07
   Na   0.355
   Ca   0.045
   Mg   0.045
   Cl   0.534
END

[MichaelZ20]

Add     pH 7 charge  to your SOLUTION 1

[Yongqiang]

Hi Michael,

Thank you for your reply.
Yes, adding the pH 7 charge works.

I want to know that the pH usually adjusts itself when we do solution species calculation. Can you share any insights on whether to use the pH charge function during the definition of solution conditions?

Regards,
Yongqiang

[MichaelZ20]

See: http://phreeqcusers.org/index.php/topic,70.msg167.html#msg167