Cationic lead(II) halide complexes in molten alkali-metal nitrate. Part 2.—A thermodynamic investigation of the chloride, bromide and iodide systems

Abstract

The complex formation between lead (II) ions and chloride, bromide and iodide ions in molten equimolar (K, Na) NO₃ has been studied. The systems were investigated at four temperatures between 240 and 300 °C. The iodide system was only studied at one temperature, since I^– is oxidized to I₂ in melts rich in Pb(NO₃)₂. Halide-ion activities were measured in the systems (K⁺, Na⁺, Pb²⁺)–(NO^–₃, X^–), X^–= Cl^–, Br^– and I^–, by means of Ag/AgX electrodes. The formation of PbX₂, PbX⁺ and Pb₂X³⁺ was observed. Special interest was focused on the formation of cationic complexes with anions as coordination centres. The thermodynamic parameters ΔH^°_mn and ΔS^°_mn have been evaluated for the chloride and bromide systems. The results are : ΔH^°₁₁=– 7.9 kJ mol^–1, ΔS^°₁₁= 26.3 J K^–1mol^–1 for Pb²⁺+ Cl ^–→ PbCl⁺; ΔH^°₁₁=–4.6 kJ mol^–1, ΔS^°₁₁= 35.2 J K^–1 mol^–1 for Pb²⁺+ Br^–→ PbBr⁺; ΔH^°₂₁= 1.5 kJ mol^–1, ΔS^°₂₁= 16.1 J K^–1 mol^–1 for PbCl⁺+ Pb²⁺→ Pb₂Cl³⁺; ΔH^°₂₁=–7.1 kJ mol^–1, ΔS^°₂₁= 1.9 J K^–1 mol^–1 for PbBr⁺+ Pb²⁺→ Pb₂Br³⁺. The parameters are compared with previous literature data and recent results obtained from the lead (II) fluoride system in (K, Na)NO₃(l).