Ferrous fluoride, FeF₂

Ferrous fluoride, FeF₂, may be obtained as colourless monoclinic crystals of density 4.09 by heating reduced iron or anhydrous ferrous chloride to redness in a current of dry hydrogen fluoride. In the amorphous condition it results on passing hydrogen fluoride over ferrous chloride in the cold.

It volatilises at 1100° C., and is reduced by hydrogen at high temperatures to metallic iron. It dissolves slowly in water, and the solution gradually deposits hydrated ferric oxide.

The tetrahydrate, FeF₂.4H₂O, results in the form of white or faintly green prisms when iron is dissolved in aqueous hydrogen fluoride and the solution allowed to crystallise at the ordinary temperature. On warming gently the crystals lose their combined water, and the anhydrous salt is left. The crystals are more soluble in hydrogen fluoride solution than in pure water. The solution decolorises permanganate, the ferric salt being obtained, and the same result accrues, albeit more slowly, by simply exposing the aqueous solution to the atmosphere.

The mineral acids dissolve the crystals completely. When ignited in air, ferric oxide is obtained, as also when the crystals are fused with an alkali carbonate. Double salts formed by union with the fluorides of the alkali and certain other metals have been described. These may be termed

Thus, for example, with ammonium fluoride two salts have been obtained - namely ammonium trifluoferrite, NH₄FeF₃.2H₂O or FeF₂.NH₄F.2H₂O (green); and the tetrafluoferrite, (NH₄)₂FeF₄ or FeF₂.2NH₄F (brown). With potassium fluoride the flesh-coloured trifluoferrite, KFeF₃.2H₂O or FeF₂.KF.2H₂O, is formed. The tetrafluoferrite, K₂FeF₄, is also known.