Ferric fluoride, FeF₃

Ferric fluoride, FeF₃, may be obtained in the hydrated condition by dissolving ferric hydroxide in aqueous hydrogen fluoride, or by oxidation of ferrous fluoride with nitric acid in the presence of hydrofluoric acid. Upon evaporation crystals of the nonahydrate, 2FeF₃.9H₂O, separate out, which, at 100° C., yield the hexahydrate, 2FeF₃.6H₂O, and when gently heated in gaseous hydrogen fluoride, evolve water yielding the anhydrous salt, FeF₃.

The last-named is also obtained when hydrogen fluoride is passed over red-hot iron, ferric oxide, or ferric chloride; or by heating the double salt, ferric ammonium fluoride, FeF₃.3NH₄F, in an inert gas.

At 1000°C. ferric fluoride partially volatilises and partially crystallises to small green crystals, possibly triclinic, of density 3.18. When fused with alkali carbonates it yields ferric oxide, as also when ignited in air or steam. Hydrogen reduces the salt at dull red heat. Boiling water dissolves only traces of the anhydrous salt. Aqueous solutions of ferric fluoride are colourless, and their electric conductivity very small, so that but little ionisation takes place. Determinations of the freezing - points of dilute solutions indicate a molecular weight corresponding to the simple formula FeF₃. Addition of ammonium hydroxide causes the precipitation of a yellow basic fluoride.

It has been suggested that the formula for the hexahydrate would be more correctly represented as Fe₂F₄(HO)₂(HF)₂.4H₂O, since only two atoms of fluorine enter into double decomposition with barium chloride solution, although the reaction is complicated by the precipitation of barium ferrifluoride, Ba₃(FeF₆)₂. Again, on boiling the salt with alcohol, two molecules of hydrogen fluoride are readily evolved, whilst the remaining fluorine is expelled very slowly. On prolonged heating at 95 ° C. an oxyfluoride, Fe₂OF₄, is produced: -

Fe₂F₄(HO)₂(HF)₂.4H₂O = Fe₂OF₄ + 5H₂O + 2HF.

These reactions appear to indicate that four of the fluorine atoms are attached to the iron considerably more firmly than the remaining two atoms.

Some double salts have been described, and those of general formula

FeF₃.3MF, or M₃FeF₆,

are known as ferrifluorides or hexafluoferrates. They form an isomorphous series, likewise isomorphous with corresponding double fluorides of aluminium and chromium.