Bismuth Monoxide, BiO

Various methods for the preparation of Bismuth Monoxide, BiO, were described by early investigators, who frequently referred to it as a suboxide, but in most cases the product was either impure or later proved to be a mixture of bismuth and its trioxide. It is probable that the monoxide was first obtained in a fairly pure condition by dissolving equivalent proportions of bismuth trioxide and stannous chloride in hydrochloric acid, pouring the mixture into a moderately concentrated solution of potassium hydroxide, filtering and washing the precipitate with a cold solution of potassium hydroxide. The product should be dried in a vacuum over sulphuric acid.

The substance produced when one of the basic oxalates of bismuth, Bi₂O(C₂O₄)₂, is heated is a mixture, probably of bismuth monoxide and bismuth, but when the basic oxalate Bi₂O₂.C₂O₄ is heated in a current of carbon dioxide, bismuth monoxide alone is stated to be obtained:

Bi₂O₂.C₂O₄ = 2BiO + 2CO₂

This method of Tanatar is now usually employed.

Reduction of Bismuth Trioxide by Carbon Monoxide at 300° C.

Bismuth trioxide is reduced by heating in a current of hydrogen at 267° C., or in carbon monoxide. In the former case it is possible that the monoxide is formed. In the latter case, the reduction being carried out at 300° C., it will be seen from the following data and the accompanying time-reduction curve (fig.) that there is a break at a point at which the composition of the reduction product corresponds approximately to that of bismuth monoxide.

Time (hours)	0	3	4½	6	9	12	15
Weight of Bi2O3 (grams)	3.000	2.934	2.920	2.890	2.868	2.850	2.831

The above methods and conclusions have, however, been criticised, and considerable doubt has been thrown on the existence of an oxide of bismuth lower than the trioxide.

Bismuth monoxide is described as a greyish-black, finely crystalline powder, having a mean density of 7.5 at 20° C., values lying between 7.17 and 8.55 having been reported. This value is lower than that of a corresponding mixture of bismuth trioxide and bismuth; but this does not prove the existence of a separate compound, as the low density may be due to the presence of hydroxide in the samples investigated.

When dry, the oxide is quite stable in air, but in the presence of moist air it is slowly oxidised to a white hydrated trioxide, Bi₂O₃.2H₂O. It is also oxidised on heating in air, oxidation beginning at about 180° C. It is decomposed slowly by cold water, and more rapidly on boiling.

It is attacked by warm dilute acids with formation of salts of tervalent bismuth and precipitation of bismuth. With hydrochloric acid this reaction may be represented by

3BiO + 6HCl = 2BiCl₃ + Bi + 3H₂O

It is oxidised by nitric acid, but in the presence of excess of acid the reaction proceeds as with hydrochloric acid.

When heated in oxygen, oxidation begins at 140° C. and is complete at 240° C. The oxide is also oxidised slowly when heated in a current of carbon dioxide. It is easily reduced to metal by heating on a charcoal block, or in a current of hydrogen or carbon monoxide; reduction in hydrogen takes place at 300° to 310° C., and in carbon monoxide begins at about 250° C.

The monoxide is readily oxidised on boiling with a solution of potassium hydroxide and bromine with the formation of the pentoxide, Bi₂O₅.xH₂O. It is converted to metal by reaction with aqueous potassium hydroxide alone. It will reduce Fehling's solution, and potassium permanganate.

It is weakly basic; the halide salts and the sulphide are the only salts derived from it that have been reported. They are unstable and difficult to obtain pure.

By comparing the heats of reaction of hydrochloric acid with bismuth monoxide and with a corresponding mixture of bismuth and bismuth trioxide, the calculated heat of formation of one mole of monoxide is given as 3938 gram-calories, that is:

Bi₂O₃ + Bi = 3BiO + 11,814 calories

As has been stated previously, most of the facts enumerated here have been adversely criticised. Arguments have been brought forward in favour of the view that, on theoretical grounds, it is very improbable that bismuth monoxide can exist. Comparisons have been made between bismuth monoxide, prepared by various methods, and a corresponding mixture of bismuth and its trioxide; and the numerical values obtained for the heat evolved in the reaction with hydrochloric acid, the solubility in an aqueous solution of sodium hydroxide and the specific magnetic susceptibility have been found to be approximately identical in each case. In further support of this view, it is stated that metallic bismuth can be extracted from bismuth monoxide by shaking with mercury; and that the monoxide reacts with dry hydrogen sulphide as if it were a mixture of bismuth, bismuth trioxide and bismuth hydroxide. When prepared by the action of potassium hydroxide upon a solution of bismuth trioxide and stannous chloride in hydrochloric acid, it reacts with sulphur dioxide to form basic bismuth sulphate, 4Bi₂O₃.3SO₃. Since this substance is also obtained by the action of sulphur dioxide upon bismuth trioxide, it has been suggested that bismuth monoxide may be a compound Bi.Bi₂O₃, but the possibility of its being merely a mixture of bismuth and trioxide is not eliminated.

Much of the confusion regarding the existence of bismuth monoxide undoubtedly arises from the difficulties encountered in the preparation and purification of specimens of the substance; the products obtained by the various methods described frequently vary in composition. In addition, these products are for the most part unstable, especially in the presence of moisture.