Lead orthophosphates-III. Stabilities of fluoropyromorphite and bromopyromorphite at 25°C

Abstract

The reactions of secondary lead orthophosphate with approximately 10 -1 M sodium fluoride and sodium bromide solutions have been investigated at 25°C. Interpretation of the solubility data resulted in solubility product constants for fluoropyromorphite and bromopyromorphite of 10 -71.6 and 10 -78.1 , respectively. According to these constants, the stability sequence for lead pyromorphites is Pb 5 (PO 4 ) 3 Cl > Pb 5 (PO 4 ) 3 Br > Pb 5 (PO 4 ) 3 OH > Pb 5 (PO 4 ) 3 F. The derived free energy data have been used to evaluate the respective stabilities of fluoro-pyromorphite and bromopyromorphite within the systems PbF 2 -PbO-P 2 O 5 -H 2 O and PbBr 2 -PbO-P 2 O 5 -H 2 O and to predict the equilibrium behavior of the Pb 5 (PO 4 ) 3 F-Pb 5 (PO 4 ) 3 OH solid solution under aqueous conditions. © 1973.