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Re: Phreeqc Query

> 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.

      pH    7     charge
      Na    1

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.

      Na    1
      Alkalinity  1
      C(4)  1.5

(3) If you know alkalinity and PCO2 you can calculate pH. This calculation
assumes air equilibrium.

      pH    7     CO2(g) -3.5
      Alkalinity 1
      Na    1

(4) Variations of 3 could use other minerals.

      pH    7     Calcite     0.0
      Ca    0.5
      Na    1
      Alkalinity 2


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

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