Antimony Trifluoride, SbF₃

Antimony Trifluoride, SbF₃, was first prepared by Berzelius in 1824 by evaporating a solution of antimony trioxide in hydrofluoric acid; Dumas prepared the same compound in 1826 by distilling a mixture of mercuric fluoride and powdered antimony. It has also been prepared by heating antimony trichloride with hydrofluoric acid in the presence of an organic solvent. Metallic antimony does not dissolve in concentrated hydrofluoric acid.

Antimony trifluoride is most conveniently prepared by the method of Berzelius. Pure antimony trioxide is dissolved in excess of hydrofluoric acid, and the solution is evaporated until a film forms on the surface. On cooling, long, needle-shaped crystals separate out. These may be dried between filter-paper and stored in vessels of gutta-percha or platinum.

Antimony trifluoride forms colourless, transparent, rhombic crystals of density (at 20.9° C.) 4.379. Its melting point is 292° C. or slightly lower; it sublimes when heated in a platinum vessel. The solubility in water is as follows:

t° C	0	20	25	30
SbF3 (grams per 100 grams water)	384.7	451.0	494.0	565.6

The solubility is increased by the presence of hydrofluoric acid and of alkali salts.

The heat of formation of antimony trifluoride is 144,300 gram- calories. It does not fume in air, but when heated it volatilises with partial decomposition, leaving a residue of antimony trioxide. It is very hygroscopic. If an aqueous solution is evaporated without the addition of hydrofluoric acid, some antimonyl fluoride is obtained; no hydrolysis is apparent, however, below 30° C.

Chlorine reacts with antimony trifluoride forming the compound 2SbF₅.SbCl₅.

Liquid ammonia reacts with antimony trifluoride to form the di-ammoniate, SbF₃.2NH₃. This is a yellow powder jvhich loses ammonia in the presence of moist air.

Antimony trifluoride shows a tendency to form double and complex salts. From the thermochemical examination of solutions in hydrofluoric acid, Guntz concluded that an acid fluoride is formed, but was unable to isolate it. Beck concluded that the most stable complex formation is of the type MSbF₄ or SbF₃.MF, in which the antimony is tervalent.

Numerous double compounds are formed with alkali fluorides, either by crystallisation from solutions of the mixed salts, or by addition of alkali carbonate to a solution of antimony trioxide in hydrofluoric acid. Salts in which the ratio SbF₃:MF has the following values have been obtained: 4:1, 3:1, 2:1, 7:4, 1:1, 1:2 and 1:3. They are colourless, crystalline compounds and contain no combined water; they are fairly stable in air, and dissolve readily in water without producing turbidity; the solutions are acid, and attack glass. The salts can be regained from these solutions by evaporation.

Double compounds with alkali chlorides and sulphates have also been obtained, having the general formulae MCl.SbF₃, M₂SO₄.SbF₃, 3M₂SO₄.4SbF₃ and M₂SO₄.2SbF₃, where M represents a univalent metal. They may be prepared by the first method mentioned above, by the action of basic antimony sulphate on the corresponding fluoride, or by the action of alkali sulphate upon antimony trifluoride in the presence of hydrochloric acid. In general they crystallise fairly well, without combined water; they are fairly stable and not hygroscopic. Their solutions in water are strongly acid and attack glass.

Other double compounds have also been obtained. In some cases there is evidence for the existence of a complex ion in solution, but not in others. Solutions containing potassium nitrate, potassium sulphate or oxalic acid give no evidence of a complex ion; while solutions containing normal sodium oxalate or tartrate, ammonium oxalate or potassium antimonyl tartrate indicate decidedly the formation of such ions. The following crystalline compounds have been obtained: 3KNO₃.SbF₃, 4(NH₄)₂C₂O₄.2SbF₃, 2Na₂C₂O₄.3SbF₃ and K₃SbO(C₂O₄)₂.SbF₃.8H₂O.

Salts containing antimony trifluoride have been used as mordants; but only those salts which yield complex ions in dilute solution are suitable for such purpose.