Density and EC calculations following dissolution

[Jacob]

Dear Phreeqc community,

1. Can someone please explain how a solution density is computed if the only input to the program is the major ion composition in molal units?

2. I would also appreciate if someone could refer me to the theory behind the calculation of density and EC used by Phreeqc with the Pitzer database.

Here is the background to these questions to those interested in more details:

I am using Phreeqc to react a given natural solution with NaCl at a constant temperature. The only input is the initial unreacted major ions composition of the solution in molal units (mol/kg H2O). Since it is a highly soluble salt the Pitzer database is used. The input file is set to incrementally dissolve the salt until thermodynamic equilibrium is reached. The program is asked to return both the solution density (RHO) and Electric conductivity (SC). The modelling results are then compared to an experimental dataset where both density and EC were measured. While the Phreeqc density output matches the measurments nearly perfectly, the SC deviate from the measurements above 250 g NaCl/Kg H2O presumably due to the complexity of calculating ion-ion interactions and ion-pairing in natural solutions at high ionic strengths. Below this value the match to the EC experimental data is great as well.

Thank you,

Jacob

[dlparkhurst]

There is a short description and some equations and references in the documentation for SOLUTION_SPECIES in the Version 3 manual.

Specific conductance is based on the diffusion coefficients (-dw parameters), and density is based on the -Vm parameters in SOLUTION_SPECIES. Only phreeqc.dat, Amm.dat, and pitzer.dat have values for these parameters.

[Jacob]

Thank you very much David, much appreciated.