Boron sesqui-oxide, B₂O₃

Boron sesqui-oxide, boron trioxide, or boric anhydride, B₂O₃, the best-defined of the oxides of boron at present known, is formed when boron burns in air or oxygen, and is prepared by heating boric acid to redness in a platinum dish, water being eliminated and boron sesqui-oxide left behind.

Boron sesqui-oxide is an amorphous, colourless, glassy solid, which has no definite melting-point, but which is decidedly liquid at 557°. It vaporises rapidly in vacuo at a bright red heat. Its density is 1.877 at 0°, 1.848 at 1.2°, and 1.699 at 80°, the coefficient of cubical expansion being 0.001308. Between 16° and 98° its specific heat is 0.2374. It is a very bad conductor of electricity.

Boron sesqui-oxide is a very stable compound, but it can be reduced to boron by sodium, potassium, aluminium, and magnesium. It is attacked by hydrofluoric acid and reacts with heated metallic fluorides, boron fluoride and crystalline metallic oxides being produced. Being only slightly volatile at a red heat, boron sesqui-oxide is capable of decomposing nitrates, sulphates, etc., borates and volatile acids (or their decomposition products) being produced.

Boron sesqui-oxide is the anhydride of boric acid, into which it passes when dissolved in water. The oxide is very hygroscopic.

Boron sesqui-oxide combines with a number of other anhydrides to form what may be regarded as mixed anhydrides. The compound B₂O₃.P₂O₅, sometimes called borophosphoric acid or boron phosphate, was first prepared by Vogel by adding crystalline boric acid to a boiling solution of phosphoric acid. According to Mylius and Meusser, when these two acids are heated together at 80° to 100° the product has the composition B₂O₃.P₂O₅.2H₂O or (BO)H₂PO₄. Boric and phosphoric acids also condense together in sulphuric acid, acetic acid, or acetic anhydride as solvent, producing borophosphoric acid. This substance is a white powder, which, although readily hydrolysed by water when freshly prepared, is quite inert after it has been dried at 400°. At a red heat, it becomes distinctly crystalline. The mixed anhydrides SO₃.B₂O₃ and 2SO₃B₂O₃ may be prepared by heating sulphuric and boric anhydrides together in sealed tubes at 115°-120° and 230° respectively, a mixture of both being obtained at intermediate temperatures. They are colourless, amorphous, hygroscopic solids which, when strongly heated, dissociate into sulphur and boron trioxides, and which are readily hydrolysed by water to sulphuric and boric acids. The compound (2B₂O₃.3SO₃.3H₂O or BHSO₄)₃ is produced by the interaction of boric and sulphuric acids; other compounds of the type xB₂O₃.ySO₃.zH₂O have been described, but their existence has not been confirmed.

The existence of the preceding compounds is generally held to show that boron sesqui-oxide may act as a feeble base, in which case boron sesqui-oxide must be classed among the amphoteric oxides.