> 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: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The URL of this page is:
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 1824 since Jan 22, 1998.