> We would very much appreciate if you could help us sort out why the acidity of Fe3+ and Al3+ are handled so differently in the PHREEQC code and how we should run the model to account also for the aluminium acidity. Buried in the definition and operation of alkalinity is a reference state. For example, the reference state for sulfate is usually SO4-2, not HSO4-, because the reference state is usually chosen to coincide with carbonate equilibria and an alkalinity titration to approximately pH 4.5, not a titration to pH 1 (which would titrate SO4-2). We have used pH 4.5 as our reference state and at pH 4.5 Al+3 is the predominant species for aluminum and Fe(OH)+ is the predominant species for Fe(3) (using the phreeqc.dat database). Consequently we assigned 0 as the alkalinity for Al+3 and -2 for Fe+3. Not all of the Al is Al+3 and not all of the Fe(3) is Fe(OH)2+ at pH 4.5 and the endpoint of a titration may not be 4.5, so there is some uncertainty in these definitions. It might be more accurate to simulate a titration, and subtract the calculated alkalinity at the end point from the initial alkalinity; but then you have simulated the amount of acid added to reach the end point and that should also be the alkalinity. Acidity is a different matter and has not been addressed in PHREEQC. Conceptually you can assign acidities of the master species in place of the alkalinities. Acidity is more difficult with Fe+3 because of precipitation and oxidation of Fe+2. Some analytical methods include oxidation. Negative alkalinities will be calculated in some instances, but it is not possible to enter negative alkalinities in the SOLUTION data block. 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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