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Re: phreeqc-alkalinity

Dear David,

I agree completely with you, including ANC and BNC definition. Furthermore in real systems dynamics
and kinetics are very important to reach the so called reference point. However, why Fe(3) hasn´t
got -3 as alkalinity in the phreeqc.dat?


David L Parkhurst wrote:
> > I have a a problem with phreeqc calculation of alkalinity in strongly
> acidified mine pit lake water. The main constituents in well oxygenated
> surface water are Fe(3)-1.55mM, Al-1.24 mM and S(6)-13mM. The pH is given
> with 2.7. Phreeqc is calculating an alkalinity of -5.88e-03 eq/L. In my
> opinion this cannot be correct. Because of the infinite low CO2
> concentration the alkalinity (alk) is defined via
> alk=-[H+]-aci[Fe-species]-aci[Al-species]-[HSO4-],
> > isn´t it? I´m calculating -8.21e-03 eq/L.
> > What about your ideas to solve this problem?
> PHREEQC does just as you indicate, except that each aqueous species is
> assigned an alkalinity. Not all Fe+3 species have the same alkalinity, nor
> do all the Al+3 species. PHREEQC assigns an alkalinity to each of the
> master species (SOLUTION_MASTER_SPECIES data block). Alkalinities for the
> aqueous species are derived from the alkalinities assigned to the master
> species by using an equation that does not contain e- (probably can not
> have O2 or H2 either). For example:
> Fe+3 is a master species assigned an alkalinity of -2 in phreeqc.dat; H+ is
> -1; H2O 0. Using the eqn Fe+3 + H2O = FeOH+2 + H+, FeOH+2 has an alkalinity
> of -1. After you have the alkalinity assigned to each aqueous species, sum
> the alkalinity times the molality of each species to get the alkalinity
> that PHREEQC calculates.
> There is some ambiguity because of the pK of each hydrolysis species and
> the assignment of integral alkalinities to the master species; a species
> may not be completely titrated in an acidity titration. You may want to
> simulate a titration and look at the difference in the calculated
> alkalinities, which might give a more accurate value. In any event, there
> is some ambiguity in the definition of alkalinity, because it refers to an
> acid neutralizing capacity relative to a reference state (see Stumm and
> Morgan).
> David
> David Parkhurst (dlpark@xxxxxxxx)
> U.S. Geological Survey
> Box 25046, MS 413
> Denver Federal Center
> Denver, CO 80225
> Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled

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