Optical vs. Thermodynamic Basicities: Probe Pb²⁺ Ion Spectra in Thermodynamically Characterized Molten Chloroaluminate Solutions

The energy of the outer-shell ³P₁ ← ¹S₀ transition of Pb²⁺ doped into various binary chloroaluminate solutions has been studied in an attempt to gain insight into the changes in electronic states of anions in a binary chloride solution state as the composition is changed through a region of abrupt thermodynamic changes (equivalence point). Sudden shifts in the band maximum of as much as 3000 cm⁻¹ are observed to occur at the stoichiometry of AlCl₄⁻, the range over which the change occurs closely matching the 0.5 mol % wide region of rapid chloride activity change observed in earlier measurements. In the case of the low-melting system A1C1₃ + ethylammonium chloride, spectra were taken at 0.05 mol % intervals at 90 °C and it was noted that the entire energy shift occurred across a 0.2 mol % gap in which the Pb²⁺ was quantitatively rejected from solution (presumably as PbCl₂ although Pb(AlCl₄)₂ has not been excluded). In this case a second composition region of spectral shift was found that corresponds with one in which NMR studies in a related system also indicate a second acid-base process. The correlation with thermodynamically determined basicity changes in these systems is good enough to justify the use of Pb²⁺ as a basicity indicator and to lend credence to the optical basicity scale proposed on the basis of the nephelauxetic effect for d¹⁰s² cations in acid media. However, measurements to be reported separately on the isoelectronic Tl⁺ and Bi³⁺ spectra in the same systems show that caution is necessary. An ion in this series will only be an effective and reliable indicator for basicity changes if, as for aqueous acid-base indicators, it is approximately midway in basicity between the acidic and basic species being reacted.