Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 1.—A thermodynamic investigation of the fluoride system

Abstract

The complexation of fluoride ions with lead (II) ions in molten equimolar (K, Na)NO₃ has been investigated in the temperature range 240–280 °C. Complexes with formal compositions PbF₂, PbF⁺ and Pb₂F³⁺ were identified and the stability constants for these species were determined at five temperatures from fluoride activity measurement in the systems (K⁺, Na⁺, Pb²⁺)–(NO^–₃, F^–) by means of a fluoride ion-selective electrode. ΔH^°_mn and ΔS^°_mn for the stepwise formation of Pb_mF^2m-n_n have been estimated, yielding : ΔH^°₁₂=– 7.0 kJ mol^–1, ΔS^°₁₂= 38.3 J K^–1 mol^–1 for PbF⁺+ F^–→ PbF₂; ΔH^°₁₁=– 15.3 kJ mol^–1, ΔS^°₁₁= 30.0 J K^–1mol^–1 for Pb²⁺+ F^–→ PbF⁺; ΔH^°₂₁=– 5.9 kJ mol^–1, ΔS^°₂₁= 1.9 J K^–1 mol^–1 for PbF⁺+ Pb²⁺→ Pb₂F³⁺. The thermodynamic results deviate from what is predicted by a quasi-lattice ideal configuration. These deviations are assumed to arise from coordination effects between lead (II) and nitrate and Pb–Pb interactions in Pb₂F³⁺. The solubility of PbF₂ in excess of Pb(NO₃)₂ in (K, Na)NO₃(l) was measured at 280 °C. The complexation model derived from the e.m.f. measurements in dilute melts, expressing all activities in mole-fraction units, does accurately describe the solubility of PbF₂ in the range C_PbC_F. In melts with an excess of fluoride over lead(II) the model fails though, probably because polynuclear, i.e. polyfluoric, species are formed.