SURFACE-REACTIONS IN AQUEOUS METAL SULFIDE SYSTEMS .3. ION-EXCHANGE AND ACID-BASE PROPERTIES OF HYDROUS LEAD SULFIDE

Potentiometric titrations for studying surface properties of hydrous lead sulfide in 0.1 MNaNO3 and 0.1 M NaClO4 media at 25-degrees-C were performed. Acid-base properties and an ion-exchange process between aqueous protons and surface Pb2+ sites at the PbS-H2O interface were investigated. Ion-selective electrodes were used to register -log [H+] (glass electrode), -log [S2-] (Ag/Ag2S electrode) and -log [Pb2+] (PbS/Ag2S electrode) during the titrations. Solubility studies were performed in which the total concentrations of H+, Pb2+, and sulfur species were measured as a function of -log [H+]. Synthetic PbS, precipitated under oxygen-free conditions, as well as the mineral galena were studied. Bronsted acidity constants, pK1s = 7.11 and pK2s = 10.0 for synthetic PbS and pK1s = 7.15 and pK2s = 10.2 for galena, were established resulting in -log [H+] of the zero-proton condition equal to 8.56 and 8.68 respectively. In addition to these amphoteric properties, an ion-exchange reaction: = SPb + 2H+ = = SH2 + Pb2+, log K = 9.48 (synthetic PbS) and log K = 10.21 (galena) was established within the range 4.5 less-than-or-equal-to -log [H+] less-than-or-equal-to 7. Experimental data were evaluated using the computer program FITEQL, assuming a constant capacitance model for the electric double layer. In the calculations the "best" fit to experimental data was obtained assuming no charge dependence of the constants K1s and K2s.