I was afraid that example 8 wasn't what you needed, but there is no way to easily turn off the charge balance, without getting into the program. I guess I still don't quite understand what you need. If you are running a "batch-reaction" calculation and you turn off charge balance, the pH will be fixed at the initial pH and will not depend on anything. This is because the unknown activity(H+) is associated with the charge balance equation in PHREEQC. It seems like you want the pH to vary with whatever equilibrium constants are being adjusted. I'm not sure you really have a problem; the pH of the reacted solution is independent of the initial charge imbalance. The calculations carry along whatever charge imbalance was calculated for the initial solution, so that each reacted solution has exactly the same charge imbalance. The variations in pH that you are calculating are due to the changes in your independent variables. However, if you are adding REACTIONS or KINETICS in your reactor, it is essential that the reactants be charge balanced; it is not possible to add a charged ion, like H+, you must add for example HCl. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 "Jon Schwantes" <plutonium@xxxxx To: "David L Parkhurst" <dlpark@xxxxxxxx> et> cc: In-Reply-To: <MABBJCOBOKMBDNHOOGCMGEBACBAA.plutonium@xxxxxxx> Subject: RE: turning off charge balance 04/27/01 10:52 AM Dr. Parkhurst, Thanks for your timely response. I am aware of that example. However, when using my inverse model to regress on "best" equilibrium constants, sets of measured data are used as dependent variables (including pH) and are assumed to contain a bit of error. In a hypothetical case where datasets used for basing the regression are not exactly defined to zero charge balance, it would be advantagous for the user to manually turn off charge balance during regression. In such cases where datasets are closely defined to within a few percent of charge balance, if charge balance was turned off regression could occur on dependent variables including pH. Currently, the user must define pH exactly as in example 8 and are not able to regress on pH in such cases. Thanks Jon Schwantes Environmental & Water Resources Engineering Civil Engineering Department, MS-3136 Texas A&M University College Station, Texas 77843-3136 h: 979-695-8264, w: 979-845-9772 e: plutonium@xxxxxxxx "That that you give to others, does not die with you" -----Original Message----- From: David L Parkhurst [mailto:dlpark@xxxxxxxx] Sent: Friday, April 27, 2001 8:35 AM To: plutonium@xxxxxxx In-Reply-To: <MABBJCOBOKMBDNHOOGCMGEBACBAA.plutonium@xxxxxxx> Subject: Re: turning off charge balance > I am currently working on an inverse model that uses PHREEQC to back calculate equilibrium and kinetic constants. In order for my model to work, an input file describing several reactor solutions (in the case of EQ constants) is described and saved and then equilibrated with several unknown solids. In the first equilibration, we charge balance on a fictitious species. However, during equilibration with the solids, pH is changed to satisfy charge balance. Is there a way we can "turn this off"? The general idea for my model is that it will call PHREEQC to perform equilibrium several times in order to regress on the real values of the constants for specified unknowns. To do this, we regress on several dependent variables, including pH, so it is imperative that pH is not affected by charge imbalance. There is a way to fix the pH, which is demonstrated in example 8 of the manual. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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