When I ask pH to stay fixed for a solution in contact with atmosphere or in equilibrium with a mineral, why does the pH not stay fixed?
As a result of the way web-phreeq constructs the input files for phreeqc, phreeqc first equilibrates the solution at your chosen pH, then it reacts the solution with atmosphere (or a stable mineral). During this reaction phase, pH is changed to maintain charge balance. For pH truly to be fixed, a reaction such as a titration with HCl or NaOH (or with an appropriate mineral) would have to be inserted into the input file. Since web-phreeq is designed as a tool for teaching basic modeling, I suggest running a standalone version for this type of problem. Note: keeping pH fixed with no reaction (simple speciation) still works in web-phreeq. • Where can I get more information on how to run WEB-PHREEQ? A: Two places: The PHREEQC user’s guide is a great place to start. It’s also available in PDF form. Also, David Parkhurst of the USGS maintains a great Frequently Asked Questions for PHREEQC page. • WEB-PHREEQ doesn’t seem to be working properly. A: Please email me at bernhardt.saini-eidukat@nd
