Critical Evaluation and Calorimetric Study of the Thermodynamic Properties of Na₂CrO₄, K₂CrO₄, Na₂MoO₄, K₂MoO₄, Na₂WO₄, and K₂WO₄

This article critically evaluates crystallographic data and thermodynamic properties—including solid–solid transitions and fusion—of six pure salts: sodium chromate (Na₂CrO₄), potassium chromate (K₂CrO₄), sodium molybdate (Na₂MoO₄), potassium molybdate (K₂MoO₄), sodium tungstate (Na₂WO₄), and potassium tungstate (K₂WO₄). Using differential scanning calorimetry (DSC) and high-temperature x-ray diffraction (XRD), the study refines phase transition temperatures, enthalpy changes, crystal structures, and space groups, notably identifying three phases for K₂MoO₄ instead of four previously reported. The work serves as a foundational step toward developing a comprehensive thermodynamic model for the Na⁺, K⁺//Cl⁻, SO₄²⁻, CO₃²⁻, CrO₄²⁻, Cr₂O₇²⁻, MoO₄²⁻, Mo₂O₇²⁻, WO₄²⁻, W₂O₇²⁻, O²⁻ system, which is relevant for understanding and mitigating high-temperature corrosion in combustion environments containing alkali salts and corrosive gases. The article compiles recommended thermodynamic data from literature and new measurements to support future phase equilibrium calculations aimed at predicting corrosive deposit formation and improving equipment durability.