Optical Acidity and Basicity Diagnostics of Molten Salt for the Evaluations of Salt Properties and Local Structure.

Analogous to the aqueous solution where the pH of the solvent affects its multiple behaviors, the optical acidity and basicity of molten salts also greatly influence their thermophysical and thermochemical properties. In the study, we develop ion probes to quantitatively determine the acidity-basicity scale of molten NaCl-xAlCl₃ (x = 1.5-2.1) salt using in situ ultraviolet visible (UV-vis) spectroscopy. With the accumulation of acidity-basicity data of NaCl-AlCl₃ molten salt for a variety of compositions, the correlation between the acidity-basicity of salt and its measured fundamental properties is derived. To understand the physical and chemical features controlling the acidity-basicity variations, the local structures of NaCl-xAlCl₃ molten salts with different chemical compositions are investigated in terms of bonded complexes and coordination numbers. The comprehensive understanding of the correlation between composition, acidity-basicity, fundamental properties, and local structures of molten salt can serve for the full screening and online monitoring of salt melt in extreme environments by simply measuring the salt acidity-basicity as developed in this study.