Thermodynamics of Aqueous Complex Solutions Containing 3/1 Rare Earth Electrolyte Pairs and Salting-Out Agents to Very High Concentrations

Isopiestic osmotic coefficients have been determined for the unsaturated and NH₄NO₃-saturated quaternary systems H₂O−NH₄NO₃−Y(NO₃)₃−Pr(NO₃)₃ and H₂O−NH₄NO₃−Pr(NO₃)₃−Nd(NO₃)₃ and their subsystems H₂O−NH₄NO₃−Y(NO₃)₃, H₂O−NH₄NO₃−Pr(NO₃)₃, and H₂O−NH₄NO₃−Nd(NO₃)₃ up to the maximum ionic strength I = 30 mol·kg^-1 at 298.15 K. As Y(NO₃)₃, Pr(NO₃)₃, and Nd(NO₃)₃ are 3/1 electrolytes and may form complexes (or double salts) with NH₄NO₃ at high concentrations, their highly unsymmetrical mixing up to NH₄NO₃ saturation throws light on the importance of ionic interactions and solute−solute interactions in such complicated solutions. A modified ion interaction (Pitzer) model extending to C⁽³⁾ and Ω_cc‘5X has been presented, which enables the new osmotic coefficient data for the ternary and quaternary systems and the literature data for their other subsystems to be represented quantitatively in the whole concentrations, where the C⁽³⁾ parameter is specific to 3/1 rare earth electrolytes while the Ω_cc‘5X parameter belongs to the (Ln³⁺)(NH₄⁺)(NO₃^-)₅ interactions. The solute−solute interactions have been analyzed for the unsaturated ternary and quaternary systems by a modified form of the Zdanovskii−Stokes−Robinson (ZSR) model or Wang's partial ideal solution (PIS) model and for the NH₄NO₃-saturated quaternary systems by Wang's ideal-like solution (ILS) model (Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1045), which indicate the nonzero interchange energies between NH₄NO₃ and each rare earth nitrate and the zero interchange energies between the unlike rare earth nitrates. The ILS model is completely verified by the isopiestic osmotic coefficient measurements and by the Pitzer model calculations for the NH₄NO₃-saturated quaternary systems.