Iron(II) hydroxide is formed as a white flocculent precipitate when iron(II) salts
are treated with alkali under air-free conditions. In the presence of air it rapidly darkens to
a dark green and thence to an almost black colour; at this stage iron(II) and iron(III) are
both present in the precipitate. With excess of oxygen the red-brown iron(III) hydrous
oxide is formed. Ammonia incompletely precipitates iron(II) hydroxide due, at least in part,
to the formation of iron(II) ammonia complexes.
white, amorphous powder(s) or white to pale green, hexagonal crystal(s); oxidizes in air [MER06]
Pale green hexagonal crystals (in partially oxidized form) or white amorphous powder (when pure); density 3.4g/cm3; decomposes on heating; insoluble in water (1.5 mg/L at 20°C), KSP 8.0 x 10-16; soluble in acids; moderately soluble in ammonium salt solutions; insoluble in alkalies.
The compound is used in abrasives; and in pharmaceutical applications.
Iron(II) hydroxide may be prepared by precipitation of an iron(II) salt solution with caustic soda or caustic potash in the absence of air. Pure compound may be obtained by mixing solutions of caustic potash and iron(II) sulfate— both the solutions made in freshly boiled water—in a reducing atmosphere of hydrogen:
Fe2+ + 2OHˉ → Fe(OH)2
.
Structure and conformation
Iron(II) hydroxide crystallizes with the CdI2 layer structure in which every iron atom is
surrounded by six OH groups.
