Thermodynamics of formation and the structure of polymetal alkaline-earth-metal(II) fluoride complexes in molten-nitrate media

Abstract

The complexation reactions between alkaline-earth-metal ions, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺, and fluoride in molten equimolar (K,Na)NO₃ have been studied by potentiometric titrations at temperatures between 240 and 300 °C, using a fluoride ion-selective electrode. A structural investigation at 280 °C was performed by detecting the liquid X-ray scattering from melts containing Sr²⁺ or Ba²⁺, with and without fluoride present.

Equilibrium constants were determined at temperatures between 240 and 300 °C. No polymetal complexes were detected in the magnesium(II) and calcium(II) systems owing to the high stability of the polyfluoride species MgF^–₃, MgF₂ and CaF₂. The standard enthalpy and entropy changes for the stepwise formation of M_mF^{2m– 1} in the strontium(II) and barium(II) systems have been evaluated from the temperature dependence of ΔG^°_m1, yielding: ΔH^°₁₁=–9.5 kJ mol^–1, ΔS^°₁₁= 25 J K^–1 mol^–1 for Sr²⁺+ F^–→ SrF⁺; ΔH^°₂₁=–27 kJ mol^–1, ΔS^°₂₁=–30 J K^–1 mol^–1 for SrF⁺+ Sr²⁺→ Sr₂F³⁺; ΔH^°₁₁=–4.7 kJ mol^–1, ΔS^°₁₁= 26 J K^–1 mol^–1 for Ba²⁺+ F^–→ BaF⁺; ΔH^°₂₁=–20 kJ mol^–1, ΔS^°₂₁=–21 J K^–1 mol^–1 for BaF⁺+ Ba²⁺→ Ba₂F³⁺. These results are compared with the predictions of the statistical quasi-lattice model for specific association in ionic melts. The structural investigation shows that new peaks arise in the radial distribution functions following the addition of fluoride to the Sr²⁺- and Ba²⁺-containing nitrate melts. The new peaks at 4.1 and 4.3 Å in the Sr and Ba systems, respectively, are considered as metal–metal correlations due to the presence of the M₂F³⁺ complexes in the melts.

The thermodynamic and structural parameters indicate that the dimetal complexes M₂F³⁺, M = Sr or Ba, might be stabilized by direct M–M interactions and/or bridging nitrates.