> I've been a little perplexed on a few things involving charge balance in PHREEQC. I am trying to model a simple mixing of 2 solutions, very similar in chemistry, to yield a new pH. The two initial solutions are charge balanced with Cl. The mixed solution is charged balanced (based on the output) by pH (or H+). The charge balance for the original solutions are listed to be very small (10-17), as is the charge balance for the mixed solution (10-12). However, when I manually calculate the charge balance based on the concentrations given in the output for the mixed solution, I get a value of 10-3 to 10-4??? Is this due to rounding of the displayed results? (I've attached an example scenario). I didn't do the calculation, but you should do a little better than 1e-04. You must sum the molality times charge for of all the aqueous species listed in the distribution of species. This has been checked many times, so I doubt there is an error in charge balance that is printed. > I'm currently trying to do a similar calculation in Microsoft Excel - using the charge imbalance of the mixed solution to define my H+ and pH but am not obtaining realistic results. How does PHREEQC use H+ to obtain charge balance? Conceptually, if the pH is adjusted to below pH 3, more and more H+ will be present in solution and the charge balance is shifted toward the positive. If the pH is adjusted to 11 or greater, OH- will be present and the charge balance is shifted toward the negative. In near neutral waters, a shift in pH has very little effect on H+ and OH- (~1e-7 equivalents), instead, the shift in pH changes the ratios of CO2:HCO3- and HCO3-:CO3-2. However, these changes in ratios act in the same sense as before, lower pH shifts the charge toward positive and higher pH shifts charge toward negative. The appropriate pH is found when the charge balance of the solution is zero (in your case)or a specified charge imbalance (if the initial solutions were not charge balanced). The appropriate pH may cause formation of H+ and OH- or it may shift in ratios of weak acids and bases. 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
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