Iron forms such compounds as oxides, hydroxides, halides, acetates, carbonates, sulfides, nitrates, sulfates, and a number of complex ions. It is chemically active and forms two major series of chemical compounds, the bivalent iron (II), or ferrous, compounds and the trivalent iron (III), or ferric, compounds. Ferrous sulfate heptahydrate, FeSO4·7H2O, sometimes called green vitriol, is a compound formed by the reaction of dilute sulfuric acid (formerly called oil of vitriol) with metallic iron; it is used in the manufacture of ink, in dyeing, and as a disinfectant. Ferric chloride hexahydrate, FeCl3·6H2O, is a yellow-brown crystalline compound used as a mordant in dyeing and as an etching compound. Ferric oxide, Fe2O3, is a reddish-brown powder used as a paint pigment and in abrasive rouges. Prussian blue, KFe2(CN)6, is a pigment containing the ferrocyanide complex ion. Iron rusts readily in moist air, forming a complex mixture of compounds that is mostly a ferrous-ferric oxide with the composition Fe3O4.