ELECTROCHEMICAL BEHAVIOR OF SURFACE OF LEAD SULFIDE CRYSTALS AS REVEALED BY POTENTIODYNAMIC, REFLECTANCE, AND ROTATING-ELECTRODE STUDIES

Interest in the surface electrochemistry of lead sulfide arises from studies of the mechanism of flotation of such minerals as galena by means of addition agents such as xanthate in the presence of oxygen. In the present work, cyclic-voltammetry and individual linear potentiodynamic sweep experiments have been conducted over various potential ranges at polished galena crystal electrodes and corresponding determinations of relative reflectance changes, ΔR/R, have been made over the same ranges of potential in order to establish the electrochemical surface processes which arise. The overall anodic/cathodic cycle can be repeated reproducibly but the processes within it are not reversible. Two main regions of oxidation of the PbS surface arise in the potential range -0.45 to +0.40 V vs H2 electrode. Rotating PbS disk experiments indicate the role of formation of a solution-soluble species in the first potential range of electroactivity of PbS from -0.45 to -0.05 V EH. Evidence is given that this species is the S2- ion which subsequently becomes oxidized to another species that changes the surface properties of PbS as indicated by ΔR/R measurements. In the second potential range, -0.05 to +0.4 V EH, oxidation of the surface to Pb(OH)2 occurs in a process that is little influenced by stirring but is affected by species generated in the first region. The ΔR/R experiments enable surface processes which involve Faradaic film formation and reduction to be distinguished from those involving formation and reduction of solution-soluble species generated from the PbS surface during the potential cycling. Various mechanisms for the anodic and cathodic reactions observed, and for the film-forming processes, are tentatively proposed. © 1978.