Ferrous iodide, FeI₂

Ferrous iodide, FeI₂, is obtained when the constituent elements are allowed to react in the presence of water, heat being evolved 3 when iron filings are triturated with iodine; and by heating iron filings in a crucible to which small quantities of iodine are continually added. When the mass reaches a red heat, excess of iodine is added and the whole heated until iodine vapour ceases to escape. On cooling, a grey crystalline mass is obtained, which melts at 177° C. Ferrous iodide also results when clean iron wire clippings are heated in nitrogen gas saturated with iodine vapour. The salt is formed as deep red plates which appear almost black when thick. It is very deliquescent, but when kept in a desiccator turns white and subsequently evolves iodine, becoming grey or black. The white compound is believed to be the dihydrate, FeI₂.2H₂O. The anhydrous salt readily absorbs ammonia, forming the hexammoniate, FeI₂.6NH₃, as a voluminous powder. It is decomposed by water, and is converted by bromine vapour into ferric bromide, ammonium bromide, and ammonium bromo-iodo-bromide, NH₄Br.IBr.

When heated, the hexammoniate yields the diammoniate, FeI₂.2NH₃. No monammoniate has as yet been obtained. The dissociation pressures and heats of dissociation of the two compounds have been determined as follow: -

	Temperature. °C.	Dissociation Pressure, mm.	Heat of Dissociation. Cals.
FeI2.6NH3	153	206	16.5
FeI2.2NH3	215	6.1	22.5
FeI2.2NH3	231	12.1	22.6
FeI2.2NH3	278	85.4	22.8

An aqueous solution of ferrous iodide is readily prepared by warming iron filings and iodine together in water, when a colourless solution is obtained. This, however, is not stable in air, as it absorbs oxygen, liberating free iodine. If a little sugar be added to the clear solution, oxidation is retarded, and the crystalline pentahydrate, FeI₂.5H₂O, may be isolated.

A solution containing ferrous iodide and water in the proportion of 1 to 8.5 molecules respectively, deposits a deliquescent hexahydrate, FeI₂.6H₂O, at -16° C. This, on warming to +8° C., yields the tetra- hydrate, FeI₂.4H₂O. The nonahydrate, FeI₂.9H₂O, and monohydrate FeI₂.H₂O, are also stated to exist.