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Alkalinity vs ANC PHREEQC question



Dr. Parkhurst:
I have been working recently to add a triprotic
representation of organic acids to the phreeqc.dat
database.  Because I will be applying this
representation to very dilute waters, where the
calculation of Acid Neutralization Capacity (ANC) is
important, I wanted to make sure that I understand how
to specify the "alk" value that is assigned to a
SOLUTON_MASTERS_SPECIES.  

ANC is usually determined by Gran titration to an
endpoint pH of 4.8 to 5.2.  Thus, it represents the
difference between the sum of H+ acceptors minus the
sum of the H+ donors by laboratory titration.  Hence,
I was thinking that the aqueous species that are
present at that pH range of 4.8 to 5.2 represent the
"zero-point" for the contribution of that particular
species to the ANC.  

Given that small bit of background, I notice that in
phreeqc.dat, that the "alk" value for Fe+3 is -2.0,
i.e.,

#element species        alk     gfw_formula    
element_gfw
Fe(+3)   Fe+3           -2.0    Fe

I guess that I was thinking that alk should be equal
to +2.0 based on reactions like the following that
would accept H+ in a titration:

Fe(OH)3  + H+   = Fe(OH)2+ + H20
Fe(OH)2+ + H+   = FeOH+2 + H2O

where FeOH+2 would be the dominant species at the
titration endpoint at pH 4.8 to 5.2.  In the same way,
I was also think that Al would need to have a similar
entry in the SOLUTION_MASTER_SPECIES list, e.g.,

#element species        alk     gfw_formula    
element_gfw
Al       Al+3          +2.0     Al             
26.9815

Likewise, for the triprotic organic acid that I want
to define, the predominant aqueous species at pH 4.8
to 5.2 is HOrgacid-2, hence I think that I need to
define Orgacid in the SOLUTION_MASTER_SPECIES as:

#element species        alk     gfw_formula    
element_gfw
Orgacid  Orgacid-3      2.0     Orgacid          218.2

in consideration of reactions like the following that
involve H+ acceptors:

       Orgacid-3 + H+ = HOrgacid-2 
and    HOrgacid-2 + H+  = H2Orgacid- 


I have the feeling that my logic about all this may be
flawed given that I am not certain why Fe+3 has a an
alk value of -2.0 rather than +2.0.  I suspect that
the problem is that I am thinking in terms of ANC and
not alkalinity as used by PHREEQC, but wondered if you
might be able to clarify if possible.  

Also, I think there is an alternative approach that I
can take, which would be to not worry about the "alk"
values for various master solution species and
calculate the ANC from the PHREEQC speciation results
by something like:

ANC = [HCO3-] + 2[CO3-2] + 2[Orgacid-3] + [HOrgacid-2]
+ [Al(OH)2+] + 3[Al(OH)4-] 
      + 2[Fe(OH)3] + Fe(OH)2+] + [OH-] - [H+]


Thank you for the time and consideration of my
question,

Ted Eary





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