Potentiometric study of the interaction of Sm³⁺ and O²⁻ ions: thermodynamic properties of samarium compounds in molten NaCl-2CsCl eutectic

The reactions of oxygen and samarium ions in molten NaCl-2CsCl eutectic in the temperature range of 823-913 K have been investigated by potentiometric titration in an inert gas atmosphere. The activity of oxygen ions in the studied melts was monitored using a standard YZME (indicator) electrode based on Y₂O₃-stabilized ZrO₂ with a solid-electrolyte membrane. The titration of Sm³⁺ by O²⁻ ions demonstrated the formation of insoluble SmOCl compounds in the investigated region. The mechanism of formation of SmOCl and Sm₂O₃ was established by the analysis of potentiometric titration curves. Subsequently, the solubility of SmOCl and Sm₂O₃ in the melt was calculated using the material balance equation and solubility constant equation. Finally, the basic thermodynamic properties of SmOCl and Sm₂O₃ in molten NaCl-2CsCl eutectic were calculated, and the properties of samarium in the melt were summarized using the Pourbaix-type (E-pO²⁻) diagram to determine the kinds of stabilized samarium compounds. Furthermore, the optimal chlorination conditions can be obtained from the E-pO²⁻ diagram of Sm.