Optical Basicity for Understanding the Lewis Basicity of Chloride Molten Salt Mixtures

Currently, there is a lack of reliable measurement techniques for understanding the basicity of molten chloride salts. Optical basicity, an ultraviolet-visible (UV-vis) spectroscopic method for measuring Lewis basicity, is explored for its applicability to molten chloride salts. Shifts in probe ion (Pb²⁺ and Bi³⁺) electronic transitions are observed that show that cations in chloride salts follow the same basicity series as in oxide melts, and an increase in basicity is observed with increasing temperature. Pb²⁺ and Bi³⁺ are validated as effective probe ions in alkali, alkaline earth, and aluminum-sodium chloride molten salts. However, the utility of Bi³⁺ is limited by the volatility of BiCl₃ at high temperatures, posing a challenge for use in high-temperature molten salt applications. Since optical basicity measures the extent of electron donation, it may be a useful metric for electrochemical corrosion.