> I guess that I was thinking that alk should be equal to +2.0 based on reactions like the following that would accept H+ in a titration: Fe(OH)3 + H+ = Fe(OH)2+ + H20 Fe(OH)2+ + H+ = FeOH+2 + H2O > where FeOH+2 would be the dominant species at the titration endpoint at pH 4.8 to 5.2. You are on the right track. PHREEQC uses pH 4.5 as a reference state and for Fe(3), the predominant species is Fe(OH)2+. From the equations, one would assign Fe(OH)3 an alkalinity of 1 [one proton consumed to convert the species to the reference state, Fe(OH)2+] and Fe(OH)2+ an alkalinity of 0. If you keep going in the series, FeOH+2 gets an alkalinity of -1 and Fe+3, -2. In other words to convert Fe+3 to Fe(OH)2+ releases two protons, which gives it a negative alkalinity or negative acid neutralizing capacity. Fe+3 + 2H2O = Fe(OH)2+ + 2H+ 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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