> I have tried to calculate the protons produced and consumed by the moles of phases dissolved and precipitated (I have attached an example calculation as a spreadsheet). By my understanding of the inverse model, (i) mass should always be conserved (i.e. the number of protons produced and consumed should be ~ equal), and (ii) pH in solution definition is only used to speciate the initial solutions, and has no bearing on the mass balance calculation itself. However when I perform these calculations (see attached spreadsheet). (i) Unless the pH is the same in the initial and final solution, protons are not conserved. The program does not have a mass balance on protons, but it does have mass balance on H and O and a charge balance constraint. Each of these states: initial + reactions = final. There is also a constraint that electrons are conserved, which is a similar equation to the other balances with the stipulation that initial and final are 0. Another constraint is conservation of alkalinity, which may be what you are thinking about. There is a section on the alkalinity balance in the manual. It also states that initial alkalinity + alkalinity from reactants = final alkalinity. Alkalinity of the reactants is calculated by summing the alkalinities of the aqueous species in the chemical equation defining the reactant. (ii) The pH is used to distribute the species in the initial solution, however this does affect the mass balance calculations. For a specified alkalinity, a pH of 12 versus 7 would make a factor of two difference in the total carbon for example, which would figure into the mole balance on carbon. Even a difference from pH 2 to pH 3 affecting no other element concentration would affect the mass balance for alkalinity and for charge. Hope this helps. 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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