Re: PHREEQC Question on Charge Balance

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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