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RE: diolution of Air composition

> I tried to use the Ngas to simulate the air component disolution in
the result is very close (almost the same) to the simulation with only
CO2(g) in pH 2-9.5. I think this may be because the N2 (maybe O2 too) is
just not disoluted in the water  during the blowing process. However there
is no further analysis of the outlet component to verify. But it makes me
feel better that the simulation is somehow very close to the real world.

The O2 and N2 are not reactive in this simulation, so there is only a small
effect through the activity of water.

> However after pH increases over 9.5, the total CO2 (as well as CO3-2 and
HCO3-) seems to deviate (decrease sharply) from the "right track". Even
REACTION data block seems ineffective. Attached is the CO2 and carbonate
species distribution in solution.

There is an activity coefficient for uncharged species that is dependent on
the ionic strength. As the ionic strength gets larger, the activity
coefficient gets larger. The activity of dissolved CO2 is fixed, so the
molality of CO2 must decrease. If you plot the activity of CO2(aq) it
should be constant over the entire pH range. Above pH 10, NaCO3- is
calculated to be increasingly important. I'm not sure if this is reality or
we have passed the limits of the aqueous model. In summary, the effects you
see are the result of activity corrections and aqueous speciation. The
calculations are correct given the aqueous model, but the ion-association
model may not be adequate.


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

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