FeOH3n

FeOH3n is a generic notation used to describe ferric hydroxide hydrates and polymeric iron(III) oxyhydroxides that exhibit variable hydration and degree of polymerization. It is not a single well-defined compound; rather, the formula represents a family of solids whose composition can be written as Fe(OH)3 with additional water of hydration or with networked Fe–O–Fe linkages. Commonly encountered related formulations include Fe(OH)3·nH2O and FeO(OH)·nH2O, while ferrihydrite is sometimes represented as Fe5HO8·4H2O.

Structure and composition of FeOH3n involve Fe3+ centers coordinated by hydroxide and oxide ligands, forming polymeric networks with Fe–O–Fe bridging. Such materials are often poorly crystalline or amorphous, especially in their hydrated forms, and they can display variable stoichiometry due to nonstoichiometric hydration and hydroxide content. The high surface area and reactive surface sites make these materials important in adsorption and catalytic contexts.

Formation and transformation typically begin with hydrolysis or precipitation of iron(III) salts (for example, FeCl3) in water, followed by aging. Under different conditions of pH, temperature, and ionic strength, FeOH3n can transform into more crystalline iron oxyhydroxides such as ferrihydrite, goethite, or hematite. In natural waters and soils, ferric hydroxide-like phases occur as colloids or coatings on mineral surfaces and can evolve over time.

Properties and behavior include insolubility at neutral to basic pH, amphoteric character, and a tendency to dissolve in acids to release Fe3+ or to form hydroxo complexes in strong base. They have high surface reactivity and can adsorb contaminants including heavy metals and phosphate, making them relevant to environmental remediation, groundwater chemistry, and materials science.

See also: ferrihydrite, goethite, ferric oxyhydroxide.