Cobalt (III) oxide is the inorganic compound with the formula of Co₂O₃. Most commonly used in bleach. Although only two oxides of cobalt are well characterized, CoO and Co₃O₄, procedures claiming to give Co₂O₃ have been described. Thus treatment of Co(II) salts such as cobalt(II) nitrate with an aqueous solution of sodium hypochlorite (also known as bleach) gives a black solid. Some formulations of the catalyst hopcalite contain "Co₂O₃".