Synthesis and solubility of brompyromorphite Pb₅(PO₄)₃Br

The bromide analogue of pyromorphite Pb₅(PO₄)₃Br was synthesized and characterized by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. The solubility of the brompyromorphite was measured at 25°C and pH values of 2.0, 2.6 and 3.2. For the 3 pH measurements, the average solubility product, log KSP, for the reaction Pb₅(PO₄)₃Br ⇔ 5Pb²⁺ + 3PO- ^3- + Br^- at 25ºC is -77.38 ± 0.70. The free energy of formation, ΔG°_f,298, calculated from this measured solubility product is -3724.7 ± 4.3 kJ mol⁻¹. These results confirm that brompyromorphite is more soluble than pyromorphite.