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 PbmF2m-nn 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 CPbCF. In melts with an excess of fluoride over lead(II) the model fails though, probably because polynuclear, i.e. polyfluoric, species are formed.