Precipitation of lead sulfide for surface chemical studies

Although there seems to be a general agreement that the surface chemical properties of precipitated lead sulfide depend on the surface stoichiometry of the precipitate, the effect of the precipitation conditions on the surface stoichiometry has not been studied in detail. In this work, the precipitation procedure, per se, was studied by titrations with lead nitrate and sodium sulfide solutions. The titrations were controlled potentiometrically using glass, redox, and ion-selective (Pb2+, S2−) electrodes and supplemented by analyses of the precipitates. It was found that the titration speed exercised a major influence upon the results. When the titrations of lead nitrate solutions with sodium sulfide solutions were conducted slowly enough, two inflection points and slow achievement of equilibria at the titration points 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 titration speed, which was explained by considering the coprecipitation of nitrate ions with lead sulfide. The coprecipitation was verified by analyzing the precipitates, which were shown to contain up to 3% nitrate. Due to these coprecipitation 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 the precipitation by ion-selective electrodes and facilitated desorption of the excess adsorbed sulfide ions at a correctly adjusted pH are suggested as better methods for producing lead sulfide suspensions.