Question about Alkalinity Contribution in PHREEQC Databases

[Juan Sebastian]

Hello everyone,

I'm learning how to modify a PHREEQC database, but I don't fully understand how to estimate the correct value for the alkalinity contribution when adding a new element.

For example, in the wateq4f database, the following values are used for arsenic and iron:

As        H3AsO4    -1        74.9216   74.9216
As(+3)    H3AsO3     0        74.9216   74.9216
As(+5)    H3AsO4    -1        74.9216

Fe        Fe+2       0        55.847    55.847
Fe(+2)    Fe+2       0        55.847
Fe(+3)    Fe+3      -2        55.847

How do I determine whether the alkalinity contribution should be 0, -1, -2, or another value?

Thank you very much for your help!

Sincerely,

Sebastian

[dlparkhurst]

Alkalinity can be a bit arbitrary once you get past carbonate, hydroxide and hydronium ions. The usual approximation is to look at the predominant species at pH 4.5 and the master species for the element. If hydrogen ions are needed to convert the master species to the predominant species at 4.5, assign an alkalinity equal to the number of hydrogen ions needed. If hydroxide is needed, assign a negative value.