A considerable part of sodium sulfide used as sulfidizer for carbonate, oxide, and sulfate ores of ferrous metals is consumed on precipitation of metal cations from the solution in the liquid phase of the pulp to form the corresponding metal sulfides. The latter are formed on the surfaces of minerals undergoing sulfidization. It is therefore of practical interest to study the stability of solutions of sodium sulfide in the presence of these metal sulfides, especially freshly formed precipitates of the latter, because we know ['1-3] that the resistances of sodium sulfide solutions to oxidation depend markedly on the presence of finely crushed sulfide minerals in the liquid phase of the pulp.In the presence of aerated suspensions of such minerals as galena, pyrite, and chalcopyrite, sodium sulfide is oxidized far more rapidly, the rate depending on the state and area of the mineral surfaces [4,5]. A similar effect is obtained by adding freshly precipitated CuS to a solution of sodium sulfide [6]. Therefore the properties accelerating oxidation of sodium sulfide are inherent not only in sulfide minerals, but also in precipitates of metal sulfides which form these types of ores under sulfidization conditions. During their sulfidization, the surfaces of the metal sulfides being formed increase. This is due to partial removal of the latter from the mineral surface by the abrasive effect of the pulp and to the discrepancy between the lattice parameters of the oxidized minerals and the newly formed films of metal sulfides [7J.We studied the kinetics of oxidation of sodium sulfide in the presence of freshly formed precipitates of metal sulfides, and also the effect of their conditions of formation on the rate of oxidation of sulfide ions in the liquid phase of suspensions. We used the potentiometric method of measuring the residual concentration of S z-ions, using a silver sulfide electrode [8]. The sodium sulfide used in the experiments was purified by the method in [9].The potential of a silver sulfide electrode is related to the concentration of sulfide ions in the solution by a logarithmic law described by the Nernst equation; we can thus infer that the concentration of sulfide ions decreases with the electrode potential.The experiments were performed in a Nigrizoloto flotation cell of capacity 0.3 liter. A sodium sulfide solution with a concentration corresponding to an electrode potential of 775 mV was fed to the cell with mixing but without aeration. A tenth-normal solution of the metal salt was then added. During this process the electrode potential fell owing to combining of the sulfide anions by the metal cations.This method gave precipitates of metal sulfides of which the surfaces were unaffected by oxidation because the residual concentration of S z-ions at the end of the experiment was still fairly high (about 0.01 N). When the electrode potential reached 760 mV, aeration was begun and we recorded the change in potential with time. Figure 1 gives the results of oxidation of sodium sulfide in the presence of p...