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).