An aqueous thermodynamic model for a high valence 4∶2 electrolyte Th⁴⁺−SO ²⁻₄ in the system Na⁺−K⁺−Li⁺−NH ⁺₄ −Th⁴⁺−SO ²⁻₄ −HSO ⁻₄ −H₂O to high concentration

Abstract

An aqueous thermodynamic model that is valid from zero to high concentration is proposed for the Na⁺−K⁺−Li⁺−NH ⁺₄ −Th⁴⁺−SO ²⁻₄ −HSO ⁻₄ −H₂O system. The model is based on the aqueous ion-interaction model of Pitzer and coworkers. The thorium sulfate complex species Th(SO₄)₂(aq) and Th(SO₄) ²⁻₃ are also included in the model. The final thermodynamic model presented here accurately predicts all reliable thermodynamic data, including solvent extraction and solubility data, for the Na⁺−K⁺−Li⁺−NH ⁺₄ −Th⁴⁺−SO ²⁻₄ −HSO ⁻₄ −H₂O system to high concentration. The aqueous thermodynamics of high-valence (3:2, 4:2), electrolytes are complicated by very strong specific ion interactions or ion pairing in dilute solution and by an effective redissociation of aqueous complex species at high concentration. Methods of treating these complications, in terms of valid aqueous thermodynamic models, are discussed in detail for the high-valence Th⁴⁺−SO ²⁻₄ −H₂O system.