In Pitzer–Prausnitz model both activity and fugacity are
described by empirical parameters. The empirical parameters
are defined as ion–ion, ion–molecule, and molecule–molecule
interactions. The Pitzer–Prausnitz model can be used out of
the range of Debye-H¨uckel–Prausnitz model (0.005≤I≤20)
[20]. The empirical Pitzer interactions were superimposed on
the Debye-H¨uckel–Praunitz model.