Bismuth Tribromide, BiBr₃

Freezing Point Curve of the System Bismuth-Bromine.

Bismuth Tribromide, BiBr₃, is the only compound of bismuth and bromine indicated on the freezing point curve (fig.). It may be obtained by the direct union of the two elements, an excess of bromine being employed. A variety of methods has been used by different investigators, the following being some of the more important. Bismuth is heated in bromine vapour; the two elements are heated together in a sealed tube; powdered bismuth is added to bromine and the mixture allowed to stand for several days, then being distilled and the tribromide purified by repeated distillation; or bromine may be dissolved in an organic solvent, such as ether, powdered bismuth added, the mixture filtered after standing for a time, and the solution evaporated in vacuo.

Bismuth tribromide may also be obtained from bismuth trioxide or bismuth trisulphide by heating in bromine vapour. In the former case some oxybromide is also formed, and in the latter some thiobromide.

Bismuth tribromide, by slow distillation, can be obtained in the form of large, flat, golden-yellow crystals. Its density is 5.604 at 20° C., and its molecular volume at -273° C. (calculated from the density at -194° C. and the coefficient of expansion) is 77.0. It melts at 218° C., forming a red liquid; the density of the liquid between 272° and 330° C. is given by the expression

D₄^t = 5.248 – 0.0026t

The surface tensions, together with other data for the density, at various temperatures, are as follows:

Temperature, ° C	250	281	299	320	346	370	389	417	442
Density, D4t	4.598	4.525	4.471	4.416	4.348	4.286	4.237	4.164	4.099
Surface Tension (dynes per cm.)	66.5	63.6	61.6	59.5	56.7	53.8	52.0	48.9	46.2

The boiling point lies between 454° and 498° C., or 278° C. under a pressure of 11 mm. mercury; the vapour is deep red in colour.

Bismuth tribromide is not soluble in water but is decomposed by it. It is soluble in both hydrochloric and hydrobromic acids, but it is decomposed by nitric acid. It will also dissolve in alcohol, ether, and to a certain extent in some hydrocarbons, but it is almost insoluble in most other organic solvents. It is also said to dissolve in heated arsenic tribromide and in certain fused metallic halides.

It sublimes in air almost without change, there remaining only a small non-volatile residue, probably of bismuthyl bromide. It absorbs moisture from the air, and is decomposed by excess of water forming bismuthyl bromide. Variation of temperature appears to have but little effect upon the hydrolysis of bismuth tribromide, and only one oxybromide is produced.

Nitrogen tetroxide reacts even at the ordinary temperature, producing bismuth oxybromide. The tribromide forms a number of ammoniates, among them being an olive-green compound, BiBr₃.2NH₃, and a light, straw-coloured, amorphous powder, BiBr₃.3NH₃, both of which are decomposed by water, probably with formation of oxybromide. A third compound, 2BiBr₃.5NH₃, is said to be formed as a greyish-green sublimate by the action of dry ammonia upon heated bismuth oxybromide; it is not deliquescent, nor is it decomposed by water. All these compounds react readily with hydrochloric acid, forming double salts of bismuth bromide and ammonium chloride: BiBr₃.2NH₄Cl.3H₂O, BiBr₃.3NH₄Cl.H₂O, and 2BiBr₃.5NH₄Cl.H₂O. In addition, an ash-grey, crystalline substance has been obtained which is stated to have the composition BiN₂Br.

When an ether solution of bismuth tribromide is gradually added to dry phosphide, a lustrous, black substance is formed which probably has the composition PBrH(BiBr₂)₃ or 2BiBr₃.HBr.BiP:

3BiBr₃ + PH₃ = 2HBr + PBrH(BiBr₂)₃

It is hygroscopic, and is decomposed by water with liberation of bismuth and formation of phosphine, hydrobromic and phosphoric acids. It is decomposed by a solution of potassium hydroxide with evolution of hydrogen and phosphine, and formation of potassium bromide and phosphate. Concentrated sulphuric acid attacks it only when heated, but concentrated nitric acid acts upon it very violently. It decomposes with violence when heated in air, yielding bromine, bismuth bromide and phosphorus pentoxide, but is stable when heated to 220° C. in a current of carbon dioxide.

Bismuth tribromide does not react readily with sulphur; on heating, only a small quantity of thiobromide, BiSBr, is obtained. It reacts with hydrogen sulphide at the ordinary temperature; at moderate temperatures bismuth thiobromide, BiSBr, and at higher temperatures bismuth trisulphide, Bi₂S₃, is formed. The tribromide is not altered by heating with sulphur dioxide.

The decomposition potential of bismuth tribromide in solution in fused zinc chloride is 0.46 volt. This places bismuth below mercury in the electrochemical series.

If a saturated solution of bismuth tribromide in concentrated hydrobromic acid is cooled to -10° C., a complex substance of an acidic nature separates out as yellow needles. Its composition is BiBr₃.2HBr.4H₂O or H₂BiBr₅.4H₂O. It is extremely deliquescent and unstable, losing hydrogen bromide when exposed to air.

Two organic compounds have also been obtained, with the compositions H₂BiBr₅.4O(C₂H₅)₂ and H₂BiBr₅.10O(C₂H₅)₂; both are hygroscopic and unstable. The alkali salts, Li₂BiBr₅, Na₂BiBr₅ and K₂BiBr₅, and the thallium salt, Tl₂BiBr₅ (the latter forming lemon-yellow crystalline plates), have been isolated. In addition, the ammonium salt, (NH₄)₂BiBr₅.2H₂O, has been obtained by heating bismuth bromide and ammonium bromide with alcohol and a little ammonium acetate in a sealed tube; it is a greenish-yellow, transparent substance, crystallising in the rhombic system and isomorphous with the corresponding chloride and with the double chloride of potassium and bismuth. It loses combined water completely when heated to 100° C.; it is dissociated when strongly heated, and is decomposed by water forming bismuthyl bromide. Several mixed halides of similar composition have also been reported, among them being K₂BiClBr₄, K₂BiCl₃Br₂, K₂BiCl₄Br, and (NH₄)₂Bi(Cl, Br)₃Br₂. Evidence for the existence of a complex of the type H₂BiBr₅ is also obtained from a study of the absorption spectrum of a solution of bismuth tribromide in hydrobromic acid.

An ammonium compound, NH₄BiBr₄.H₂O, which corresponds to the hypothetical complex acid HBiBr₄, has also been obtained by the action of bromine on bismuth in alcohol in the presence of ammonium bromide. It forms yellow, needle-like crystals of the rhombic system, soluble in alcohol, but decomposed by water. From thermal analysis there is evidence for the existence of the aluminium compound AlBiBr₆, but this substance does not appear to have been isolated. A very complex triple salt, Rb₅Au₂BiBr₁₄ or 5RbBr.2AuBr₃.BiBr₃, has been prepared, in which the gold is present in the tervalent form. It is a black substance, soluble in hydrobromic acid. It is also possible that complex compounds of bismuth and bromine of the type K₂Bi₃Br₁₁, etc., similar to the corresponding compounds of antimony may exist, but they have not yet been isolated.