Bismuth Tetroxide, Bi₂O₄

Bismuth Tetroxide, Bi₂O₄, hydrated with one or two molecules of water, is formed when sodium bismuthate is decomposed with nitric acid; the anhydrous substance has not been obtained by this method, as oxygen is lost when water is removed.

Four different modifications have been obtained by the action of various oxidising agents upon a suspension of bismuth trioxide in boiling solutions of dilute alkali hydroxides. Two of these modifications are anhydrous, the others being modifications of the monohydrate, Bi₂O₄.H₂O. A dihydrated Bi₂O₄.2H₂O, is obtained by the action of chlorine upon a suspension of trioxide in a boiling, concentrated solution of alkali hydroxide; it is always contaminated with a hydrated pentoxide, from which it can be separated by treatment with boiling, concentrated nitric acid.

Anhydrous bismuth tetroxide is brown or purplish-black. Its density (at 20° C.) is 5.60 to 5.75, that of the dihydrate being 5.80. The anhydrous substance is stable at 100° C., but loses oxygen at 160° C. The monohydrates begin to lose water at 100° C., and are decomposed at 160° C., whilst the dihydrate is decomposed at 100° C., losing both oxygen and water. The tetroxide is not attacked by dilute nitric or sulphuric acid. The crude substance as usually prepared is partially dissolved by more concentrated nitric acid (density 1.2) at 70° to 90° C., with evolution of oxygen, the insoluble residue having a composition corresponding to that of bismuth tetroxide. From this it is deduced that the usual preparations are mixtures, probably of tetroxide and pentoxide. The tetroxide reacts with concentrated oxygen acids to form tervalent bismuth salts with evolution of oxygen. It is very sparingly soluble in alkali hydroxide.

Bismuth tetroxide can be reduced by hydrogen and by carbon monoxide. Hydrogen peroxide is decomposed by it. It is a powerful oxidising agent; hydrochloric acid is oxidised to chlorine even at -15° C., and manganous salts are oxidised to permanganate immediately in the cold in the presence of nitric acid.

With hydrochloric acid there is no indication of the formation of a chloride of quadrivalent bismuth.