Bismuth Pentoxide, Bi₂O₅

Bismuth Pentoxide, Bi₂O₅, or a hydrated form of this compound, is probably formed, mixed with tetroxide, by most of the methods already mentioned. Bismuthic acid, HBiO₃, is said to be formed when a solution of bismuth oxytrifluoride, BiOF₃, is decomposed by water and nitric acid, and evidence of the formation of sodium bismuthate was also obtained when the same compound was treated with sodium hydroxide. A hydrated oxide, Bi₂O₅.H₂O, is obtained either by the action of chlorine on bismuth trioxide suspended in a boiling concentrated solution of alkali hydroxide or, in small quantities, by the action of ammonium persulphate on the trioxide suspended in dilute alkali at 40° to 60° C. for five or six hours. In both cases the pentoxide is mixed with hydrated tetroxide. Two varieties of the monohydrate of the pentoxide have been described, a red or brown variety soluble only with difficulty in concentrated nitric acid, and a brown variety, obtained from commercial sodium bismuthate after repeatedly grinding with glacial acetic acid, which is readily soluble in nitric acid (density 1.2). The hydrate rapidly decomposes at 100° C., leaving a residue of trioxide and tetroxide. It is possible that the anhydrous form is incapable of existence.

Bismuth pentoxide is described as a dark red powder of density (at 20° C.) 5.10; on heating it loses oxygen, yielding first the tetroxide and finally the trioxide. It is reduced by hydrogen and by carbon monoxide; it is an oxidising agent, but has only feeble acidic properties.

Bismuthates of sodium and potassium have been described. Sodium bismuthate can be prepared by adding basic bismuth nitrate gradually to caustic soda heated to redness; sodium peroxide is then added and the fused mass is allowed to cool. Many earlier investigators mentioned and described alkali bismuthates, obtained by the methods outlined for the preparation of higher oxides, but it is doubtful whether the substances obtained were pure, or were even true compounds. Sodium bismuthate as made commercially is a very unstable substance, being decomposed by hot water and by dilute acids. It is used as an oxidising agent in the analytical determination of manganese in iron and steel alloys, the manganese being oxidised in the cold to permanganate in the presence of nitric acid.