Ferrous hydroxide, Fe(OH)₂

Ferrous hydroxide, Fe(OH)₂, may be obtained in the crystalline form as green hexagonal prisms by deposition from sodium hydroxide solution. Even after washing with alcohol and ether and drying out of contact with air, the prisms oxidise immediately they are brought into contact with oxygen, developing considerable heat and yielding a residue of ferric oxide.

In the amorphous condition ferrous hydroxide is usually prepared as a white precipitate by addition of sodium or potassium hydroxide to a solution of a ferrous salt in the complete absence of air. The precipitate is washed in an atmosphere of nitrogen and dried with ether. If air is present, the precipitate assumes a green colour which becomes greenish black in consequence of the formation of hydrated magnetic oxide, and ultimately turns brown owing to further oxidation to ferric hydroxide.

If, however, after precipitation, the supernatant mother liquor is poured off and solid potassium hydroxide is added to the residue, the ferrous hydroxide shrinks in volume to a greenish grey pulverulent precipitate which is considerably more stable, and oxidises in air direct to ferric hydroxide without the intermediate formation of magnetic oxide.

Such ferrous hydroxide suspended in potassium hydroxide solution is slowly oxidised by nitric oxide to ferric hydroxide, the nitric oxide being largely converted into ammonia.

Ferrous hydroxide is not completely precipitated from solutions of ferrous salts by ammonium hydroxide in the presence of ammonium chloride.

As ordinarily prepared, ferrous hydroxide undergoes oxidation upon exposure to moist air with such rapidity as to become incandescent. It readily dissolves in dilute acids yielding ferrous salts.

Towards iodic acid and the salts of such readily reducible metals as mercury and platinum, ferrous hydroxide acts as a reducing agent. It can even decompose water, slowly in the cold but more rapidly on boiling, the solution becoming turbid in air, and possessing a distinctly ferruginous taste. Solutions of the alkali hydroxides readily dissolve ferrous hydroxide; even ammonium hydroxide acts similarly, and in the presence of air or oxygen the ferrous hydroxide is rapidly converted by this means into hydrated ferric oxide. The heat of formation of ferrous hydroxide is given as follows: -

[Fe] + (O) + H₂O = [Fe(OH)₂] + 68,280 calories.

The dihydrate, FeO.2H₂O, has been found in clay near Cuxhaven, in a more or less impure form as hard, yellowish-brown lumps, which crumble upon exposure to air.