> I was wondering if you could help us out. We want to do a pretty simple calculation with phreeqc. We have a set of solution data and want to know what phases will deposit from that solution. We don not however, have a measured pH and want the programme to calculate this for us. If we do not specify the pH, it is calculated as 7.0. We know that the pH will be quite high in these solutions so a pH of 7.0 is not realistic, especially as some of our phases are very pH dependant. > We have read in a paper that the pH can be calculated by charge balancing the species in solution with ?OH? Can you let us know if this is possible with phreeqc or if there is some other way of calculating an unknown pH. There are several ways to estimate pH, all less palatable than measuring the pH in the field without loss of CO2. Here are several options you could use depending on data availability or plausible assumptions. (1) You can use charge balance to estimate pH. It is a little risky because you are lumping all analytical errors and unanalyzed species (trace elements) into the estimate of pH. You would have to be very careful if you are also missing some major ion data. The calculation is simple, just include "charge" on the line estimating pH. SOLUTION pH 7 charge Na 1 END The pH will be adjusted to charge balance. (2) If you have both alkalinity and total inorganic carbon, you can use this information to calculate pH. Just include both Alkalinity and C(4) in the solution definition and pH will be calculated. SOLUTION Na 1 Alkalinity 1 C(4) 1.5 END (3) If you know alkalinity and PCO2 you can calculate pH. This calculation assumes air equilibrium. SOLUTION pH 7 CO2(g) -3.5 Alkalinity 1 Na 1 END (4) Variations of 3 could use other minerals. SOLUTION pH 7 Calcite 0.0 Ca 0.5 Na 1 Alkalinity 2 END 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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