FeOH3

FeOH3, commonly referred to as ferric hydroxide or iron(III) hydroxide, is the empirical formula used for ferric hydroxide. In solid form it is not a single well-defined compound; it represents a family of hydrated iron(III) oxides and hydroxides with varying water content. The material typically appears as a gelatinous, yellow-brown precipitate when ferric ions in water undergo hydrolysis or are neutralized.

Formation and aging: Ferric hydroxide forms readily by the reaction of Fe3+ with hydroxide: Fe3+ + 3 OH− → Fe(OH)3(s). The precipitate is usually amorphous and poorly crystalline. With time and aging, Fe(OH)3 dehydrates and can transform into more crystalline iron oxyhydroxides such as goethite (FeO(OH)·H2O) or to iron(III) oxides like hematite (Fe2O3·xH2O).

Properties: It is very sparingly soluble in water. The material is typically yellow to brown in color and forms a gelatinous mass. It is stable under neutral to mildly basic conditions; in acidic solutions it dissolves to yield Fe3+.

Applications: Ferric hydroxide is widely used in water and wastewater treatment as a coagulant and adsorbent, helping remove contaminants such as arsenic and phosphate. It also serves as a pigment and as a precursor to iron oxide materials used in catalysis and magnetic applications after aging.

Occurrence: In nature, ferric hydroxide appears as part of rust-like coatings and as amorphous iron hydroxide or oxide phases in soils and sediments.