Should the known pKa for an acid be set in PHASES or in SOLUTION_SPECIES?

[MichaelZ20]

I have the pKa value for some organic acid.
Should it be set in PHASES or in SOLUTION_SPECIES?
I cannot reproduce the following manual calculation.

[dlparkhurst]

The SOLUTION_SPECIES definition is correct, but you missed the part about 200 mL of solution. If you add -water 0.2 you will be close.

There is also a difference in activity coefficients. To make the two calculations more consistent, you can eliminate the activity coefficients for Acetylsalicylate- and AcetylsalicylateH by defining "    -gamma    1e8 0" for each species, which will specify activity coefficients of nearly 1.0.

Code: [Select]

SOLUTION_MASTER_SPECIES
#element         species                 alk gfw_formula element_gfw
Acetylsalicylate Acetylsalicylate- 1 179.16         179.16          # # CH3COOC6H4COO-

SOLUTION_SPECIES
Acetylsalicylate- = Acetylsalicylate-
log_k 0
    -gamma    1e8 0

Acetylsalicylate- + H+ = AcetylsalicylateH
log_k 4.56 
    -gamma    1e8 0
END

SOLUTION 1
units mmol/kgw
pH 7 charge
Acetylsalicylate  100
-water 0.2
END

USE SOLUTION 1

REACTION 1
AcetylsalicylateNa 1
0.01484 mol  # 3 g at MW = 202.15
END