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How to export solution density during SELECTED_OUTPUT + USER_PUNCH

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EBlackman:
Hi all,

I am trying to extract some data such as ionic strength from a number of fluids using SOLUTION_SPREAD, SELECTED_OUTPUT and USER_PUNCH. Everything works except that ionic strength is in mol/kgw not mol/L.

To fix this I decided to export the solution density as well, so that I can convert the units in excel manually. Unfortunately whenever I try to add density as a selected output, an error appears. Is there a way around this?

I am new to Phreeqc so I apologise if this is a rookie error.

Many thanks in advance.

Ed

dlparkhurst:
The Basic function RHO will provide the density, kg/L. Basic functions are described in the section The Basic Interpreter in the Version 3 manual; additional functions are described in the release notes (http://water.usgs.gov/water-resources/software/PHREEQC/Phreeqc_ReleaseNotes.txt).

Note that only the databases phreeqc.dat, amm.dat, and pitzer.dat have the molar volume data necessary to calculate density.

EBlackman:
Many thanks David.

May I ask which formula PHREEQC uses to calculate ionic strength so I may cite it? Also many of my solutions have very high ionic strengths (<16 M/L). Is there an alternative method I should consider? My data comes from a variety of deep oil and gas wells into aquifers of varying depths so high salinity is expected.

Best,

Ed

dlparkhurst:
Ionic strength is 0.5*z(i)^2*m(i) summed over all aqueous species, where z is the charge on a species and m is mol/kgw.

For high ionic strengths, you should probably use the pitzer.dat database. It uses a different activity-coefficient approach than the ion-association model. The Guggenheim-Pitzer approach uses a virial expansion that results in a parameterization with many coefficients, but, many fewer aqueous species.

You may get a substantial difference in the ionic strength calculated with phreeqc.dat an pitzer.dat. The same definition of ionic strength is used, but pitzer.dat has many fewer ion pairs and complexes. The result is that ion-association model will have some lower-charged species and a lower ionic strength relative to the Pitzer approach.

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