Oscillation phenomena during the electrolysis of alkaline sulfide solutions on platinum electrodes

The anodic oxidation of alkaline sulfide solutions on a platinum electrode results in a cyclical formation and removal of elemental sulfur. The formation and removal of sulfur is associated with regular current-potential oscillations. At low potentials (> -0.1V), a surface layer of platinum sulfide is formed. This process can be compared with the passivation of an active metal. The sulfide film inhibits the expected oxidation of sulfide ions and the deposition of elemental sulfur. The passivation of the platinum electrode is accompanied by an increase of the potential (similar to 1.5 V) and a decrease of the current (similar to 0 mA). However, the sulfide film can be removed by anodic oxidation to platinum oxide at these higher potentials (> 1 V). Now, sulfur formation is possible on the oxide surface layer. The current increases, too, but the potential decreases (similar to 0.7 V). At lower potentials ( < 1 V), the oxide film is slowly removed by formation of a passivating platinum sulfide layer; the potential increases (<1.5V) and the current decreases (similar to 0 mA). The thick sulfur deposition is disolved by formation of a yellow polysulfide solution until the platinum surface appears, the sulfide film is oxidized, and a new oscillation cycle begins.