Precipitation of lead sulfide for surface chemical studies

Although there seems to be a general agreement that the surface chemical pr operties of precipitated lead sulfide depend on the surface stoichiometry o f the precipitate, the effect of the precipitation conditions on the surfac e stoichiometry has not been studied in detail. In this work, the precipita tion procedure, per se, was studied by titrations with lead nitrate and sod ium sulfide solutions. The titrations were controlled potentiometrically us ing glass, redox, and ion-selective (Pb2+, S2-) electrodes and supplemented by analyses of the precipitates. It was found that the titration speed exe rcised a major influence upon the results. When the titrations of lead nitr ate solutions with sodium sulfide solutions were conducted slowly enough, t wo inflection points and slow achievement of equilibria at the titration po ints between them were obtained and were explained as being due to lead and sulfide ions adsorbing on the surfaces of the final crystals. Further, the position of the major inflection point was found to depend on the titratio n speed, which was explained by considering the coprecipitation of nitrate ions with lead sulfide. The coprecipitation was verified by analyzing the p recipitates, which were shown to contain up to 3% nitrate. Due to these cop recipitation and adsorption phenomena, the addition of an equivalent amount of sulfide to a lead nitrate solution results in a lead sulfide suspension bearing an excess of sulfide on the surfaces. Both in situ monitoring of t he precipitation by ion-selective electrodes and facilitated desorption of the excess adsorbed sulfide ions at a correctly adjusted pH are suggested a s better methods for producing lead sulfide suspensions. (C) 2000 Elsevier Science B.V. All rights reserved.