Solubility of pyromorphite Pb5(PO4)3Cl at 5–65 °C and its experimentally determined thermodynamic parameters

• Solubility of pyromorphite Pb5(PO4)3Cl was determined in dissolution experiments.
• The new solubility constant for pyromorphite (PY) is Ksp,298 = 10−79.6 ± 0.15.
• Thermodynamics of PY dissolution reaction were determined; it is endothermic.
• The Gibbs free energy of formation for PY is ΔG°f = −3764.3 ± 3.5 kJ·mol−1.
• The enthalpy of formation for PY is ΔH°f = −4108.4 ± 7.9 kJ·mol−1.

The solubility of synthetic pyromorphite Pb5(PO4)3Cl was determined in a series of dissolution experiments conducted at 5–65 °C and at pH = 2.0. The equilibrium was established within 4 months. The dissolution of pyromorphite was congruent at all the temperatures, and the measured solubility product log Ksp,298 for the dissolution reaction:Pb5(PO4)3Cl⇔5Pb2++3PO43-+Cl-was determined to be −79.6 ± 0.15. The equilibrium ion activity product of pyromorphite increased with temperature, indicating a positive enthalpy of the dissolution reaction in the temperature range from 5 to 65 °C. The temperature dependence of the log Ksp was nonlinear: log Ksp = A − B/T + D log(T), where A = 478.77 ± 136.62, B = 29,378 ± 6215, and D = −185.81 ± 46.77. This allowed for calculation of ΔG°r = 454.0 ± 1.7 kJ·mol−1, ΔH°r = 101.8 ± 6.0 J·mol−1·K−1, ΔC°p,r = −1545 ± 388.9 J·mol−1·K−1, and ΔS°r = −1181 ± 382 J·mol−1·K−1 of the dissolution reaction. Using these values and the published standard state quantities for constituent ions, the values of ΔG°f = −3764.3 ± 3.5 kJ·mol−1, ΔH°f = −4108.4 ± 7.9 J·mol−1·K−1, S°f = 622 ± 382 J·mol−1·K−1, and C°pf = 402 ± 398 J·mol−1·K−1 were calculated for synthetic pyromorphite Pb5(PO4)3Cl.