Optical Acidity\nand Basicity Diagnostics of Molten\nSalt for the Evaluations of Salt Properties and Local Structure

Analogous to the aqueous solution where the pH of the\nsolvent affects\nits multiple behaviors, the optical acidity and basicity of molten\nsalts also greatly influence their thermophysical and thermochemical\nproperties. In the study, we develop ion probes to quantitatively\ndetermine the acidity-basicity scale of molten NaCl–<i>x</i>AlCl<sub>3</sub> (<i>x</i> = 1.5–2.1)\nsalt using in situ ultraviolet visible (UV–vis) spectroscopy.\nWith the accumulation of acidity-basicity data of NaCl–AlCl<sub>3</sub> molten salt for a variety of compositions, the correlation\nbetween the acidity-basicity of salt and its measured fundamental\nproperties is derived. To understand the physical and chemical features\ncontrolling the acidity-basicity variations, the local structures\nof NaCl–<i>x</i>AlCl<sub>3</sub> molten salts with\ndifferent chemical compositions are investigated in terms of bonded\ncomplexes and coordination numbers. The comprehensive understanding\nof the correlation between composition, acidity-basicity, fundamental\nproperties, and local structures of molten salt can serve for the\nfull screening and online monitoring of salt melt in extreme environments\nby simply measuring the salt acidity-basicity as developed in this\nstudy.