Dr. Parkhurst: I have been working recently to add a triprotic representation of organic acids to the phreeqc.dat database. Because I will be applying this representation to very dilute waters, where the calculation of Acid Neutralization Capacity (ANC) is important, I wanted to make sure that I understand how to specify the "alk" value that is assigned to a SOLUTON_MASTERS_SPECIES. ANC is usually determined by Gran titration to an endpoint pH of 4.8 to 5.2. Thus, it represents the difference between the sum of H+ acceptors minus the sum of the H+ donors by laboratory titration. Hence, I was thinking that the aqueous species that are present at that pH range of 4.8 to 5.2 represent the "zero-point" for the contribution of that particular species to the ANC. Given that small bit of background, I notice that in phreeqc.dat, that the "alk" value for Fe+3 is -2.0, i.e., #element species alk gfw_formula element_gfw Fe(+3) Fe+3 -2.0 Fe 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. In the same way, I was also think that Al would need to have a similar entry in the SOLUTION_MASTER_SPECIES list, e.g., #element species alk gfw_formula element_gfw Al Al+3 +2.0 Al 26.9815 Likewise, for the triprotic organic acid that I want to define, the predominant aqueous species at pH 4.8 to 5.2 is HOrgacid-2, hence I think that I need to define Orgacid in the SOLUTION_MASTER_SPECIES as: #element species alk gfw_formula element_gfw Orgacid Orgacid-3 2.0 Orgacid 218.2 in consideration of reactions like the following that involve H+ acceptors: Orgacid-3 + H+ = HOrgacid-2 and HOrgacid-2 + H+ = H2Orgacid- I have the feeling that my logic about all this may be flawed given that I am not certain why Fe+3 has a an alk value of -2.0 rather than +2.0. I suspect that the problem is that I am thinking in terms of ANC and not alkalinity as used by PHREEQC, but wondered if you might be able to clarify if possible. Also, I think there is an alternative approach that I can take, which would be to not worry about the "alk" values for various master solution species and calculate the ANC from the PHREEQC speciation results by something like: ANC = [HCO3-] + 2[CO3-2] + 2[Orgacid-3] + [HOrgacid-2] + [Al(OH)2+] + 3[Al(OH)4-] + 2[Fe(OH)3] + Fe(OH)2+] + [OH-] - [H+] Thank you for the time and consideration of my question, Ted Eary __________________________________ Do you Yahoo!? Protect your identity with Yahoo! Mail AddressGuard http://antispam.yahoo.com/whatsnewfree
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