OSCILLATION PHENOMENA DURING THE ELECTROLYSIS OF ALKALINE SULFIDE SOLUTIONS ON PLATINUM-ELECTRODES

The anodic oxidation of alkaline sulfide solutions on a platinum elect rode 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 p assivation 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 (simil ar to 0 mA). However, the sulfide film can be removed by anodic oxidat ion to platinum oxide at these higher potentials (> 1 V). Now, sulfur formation is possible on the oxide surface layer. The current increase s, too, but the potential decreases (similar to 0.7 V). At lower poten tials ( < 1 V), the oxide film is slowly removed by formation of a pas sivating platinum sulfide layer; the potential increases (<1.5V) and t he current decreases (similar to 0 mA). The thick sulfur deposition is disolved by formation of a yellow polysulfide solution until the plat inum surface appears, the sulfide film is oxidized, and a new oscillat ion cycle begins.