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Re: Alkalinity vs ANC PHREEQC question

> 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.

You are on the right track. PHREEQC uses pH 4.5 as a reference state and
for Fe(3), the predominant species is Fe(OH)2+. From the equations, one
would assign Fe(OH)3 an alkalinity of 1 [one proton consumed to convert the
species to the reference state, Fe(OH)2+] and Fe(OH)2+ an alkalinity of 0.
If you keep going in the series, FeOH+2 gets an alkalinity of -1 and Fe+3,
-2. In other words to convert Fe+3 to Fe(OH)2+ releases two protons, which
gives it a negative alkalinity or negative acid neutralizing capacity.

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


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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