> I'm calculating equilibriums in seawater and, as you say in the manual, this is a bit beyond the possibilities of PHREEQC. On the other side, there are examples provided with PHRREQC which include seawater. So, what do you think is the approximate error I have to expect ? The best way to answer that is to compare PHREEQC results to results with a Pitzer model (EQ3/6, TEQUIL, Geochemists Workbench, PHRQPITZ) or, for seawater, the conditional constant approach like you have done. Pitzer models generally lack trace metals, redox, aluminum, and silica. PHREEQC will work best with the major ions. Minor elements are worse, probably much worse. > For example, if I compare calculated ion speciations to Millero's "Chemical oceanography", my results agree with Millero's for most ions (like Na, Mg, K, Ca). But for Cu++, I calculate 93.2 % Cu(OH)_2 and 5.5 % CuCO_3, while Millero says 73.8 % CuCO_3, 14.2 % Cu(CO_3)_2-- and 2.2 % Cu(OH)_2. > Do you have an explanation for that result ? First, systematic analysis of thermodynamic data is nearly absent. Second, whichever approach is chosen, the selection of ionic complexes is likely to be arbitrary. Ideally, there is spectroscopic data that defines the aqueous complexes, but usually, the researcher picks species because they fit the data best with the model that he or she is using. For ion association models, different researchers use different models and different complexes, perhaps to describe the same phenomenon. Then a collator comes along and puts all the constants together into a "selected" database, but there is usually no attempt to put the "selected" constants into a model and reproduce the original experimental data, solubility data, titration data, etc. So you have a terrible mess. The Pitzer approach systematically fit a collection of experimental data, so it is a more consistent and believable model where it is applicable. > By the way, is there a database which includes iodine ? wateq4f.dat, minteq.dat, and llnl.dat have I-. llnl.dat also has other oxidation states of iodine. 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
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 2886 since Jan 22, 1998.