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Re: how to add NaOH as a solution species

You do not need to define "Lye". You need to define your second solution to
contain the appropriate amount of sodium and then either fix the pH at your
given value (which will result in a charge imbalance, although it should
not cause a problem) or allow the pH to vary to produce charge balance for
solution 2. The latter is probably better because it will generate the
appropriate molality of OH.

Note your input is  mol/L, so the density may be important. PHREEQC will
use the density to converting to mol/kgw, which are the units used for all
internal calculations (and all printout). Right now the density is 1.0.

Finally, I would use mixing fractions that sum to 1--0.998 and 0.002. It is
not required but it is better to keep approximately 1 kg water in the
reaction solution, especially if concentrations are extreme, as in your
case. Note that the lye solution speciation is probably not very good
because it is well beyond the range of the ion-association model. It is not
really a problem because you are only interested in adding the appropriate
amount of NaOH to the mixture and are not really interested in the lye
solution itself. You could also use REACTION form this simulation, without
explicitly generating the lye solution.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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