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Re: Phreeqc Alkalinity Question

> I have a question regarding how PhreeqC calculates alkalinity.  I am
a simple speciation calculation on a low pH mine water sample.  When I
the solution composition without indicating a value for alkalinity, Phreeqc
calculates a negative alkalinity (-2.551e-3 eq/kg).

Alkalinity is, for a first approximation, HCO3- - H+ - HSO4-. The HCO3- is
negligible at  pH 3, and HSO4- is probably small relative to H+ unless SO4
concentrations are really large, so the negative alkalinity is primarily
due to H+ ion.

> If I assign a
concentration of zero alkalinity, the alkalinity result is the same.

The program ignores concentrations of zero.

> If
however I specify a nominal amount of alkalinity (0.0001 mg/L as CaCO3),
Phreeqc adds a significant amount of total carbon to the solution (5.85
mol/kg).  The carbon is essentially all present as H2CO3 with a minor
of HCO3- (2.431e-3 mol).

You need to think about what you are requiring. You are fixing the pH at
about 3, but you are requiring  HCO3- - H+ to be about 2e-6 (.0001/50) or
essentially HCO3- = H+. At pH 3, the ratio of HCO3- to CO2 is less than
1e-3 (HCO3- + H+ -> CO2 + H2O, which has log K of -6.3, and do a little
math for pH 3), so you need moles of carbon, most of which is CO2, to have
a millimole of HCO3-.

> Is a negative value for alkalinity valid?

Yes. However, PHREEQC does not allow defining negative alkalinity, it
simply does not have this feature.

> Conceptually is this equivalent
to the amount of acidity associated with the sample?

Not quite. Acidity is the amount of base consumed in titrating a sample to
pH 8.3 (usually), so CO2(aq) would be titrated in addition to H+ and HSO4-;
Al, Fe, and anything else that has pK less than 8.3 will also get into the
act in an acidity titration.

> This water quality is
to be mixed with other waters to simulate the water quality of a pit lake.
Will inclusion of a negative alkalinity make the results invalid?

No. It should work correctly. You should consider how to handle carbon.

For the initial solution:
   C   1   CO2(g) -3.5

will add enough CO2 to be in equilibrium with the atmosphere; if this makes
geochemical sense.

For the mixture you need to add an EQUILIBRIUM_PHASES data block

    CO2(g) -3.5

to  add CO2 to atmospheric equilibrium.


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

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