Some phase relationships between basic bismuth chlorides in aqueous solutions at 25 °C

Abstract

Observations are reported on the interconversion of solid α-Bi₂O₃, Bi₁₂O₁₇Cl₂, BiOCl, and a daubréeite-like phase tentatively identified as Bi₂O₂(OH)Cl, in aqueous chloride solutions at 25 °C. Equilibrium constants, K, for these interconversions are expressed as anion activity ratios, {Cl⁻}/{OH⁻}. Experimental values of K for equilibrium between Bi₂O₃ and each of the chlorides are 10^{0.56 ± 0.20} for Bi₁₂O₁₇Cl₂, 10^{1.5 ± 0.4} for Bi₂O₂(OH)Cl, and 10^{3.13 ± 0.04} for BiOCl; the fatter two represent metastable equilibria. These equilibrium constants yielded the following estimates of Gibbs energies of formation: Bi₁₂O₁₇Cl₂, −3141 ± 6 kJ mol⁻¹;"Bi₂O₂(OH)Cl", −696 ± 4 kJ mol⁻¹; BiOCl, −321.5 ± 1.3 kJ mol⁻¹. Phase relationships among these solids are discussed, with reference to natural occurrence, other bismuth oxychlorides, and the stability of other basic salts of bismuth.