Iron(III) Oxide - Hydrated Iron(III) Oxides

Hydrated Iron(III) Oxides

Several hydrates of Iron(III) oxide exists. When alkaline is added to solutions of soluble Fe(III) salts, a red-brown gelatinous precipitate forms. This is not Fe(OH)₃, but Fe₂O₃·H₂O (also written as Fe(O)OH). Several forms of the hydrated oxide of Fe(III) exist as well. The red lepidocrocite γ-Fe(O)OH, occurs on the outside of rusticles, and the orange goethite, which occurs internally in rusticles. When Fe₂O₃·H₂O is heated, it loses its water of hydration. Further heating at 1670 K converts Fe₂O₃ to black Fe₃O₄ (FeIIFeIII₂O₄), which is known as the mineral magnetite. Fe(O)OH is soluble in acids, giving 3+. In concentrated aqueous alkali, Fe₂O₃ gives 3-.