Bismuth Trioxide, Bi₂O₃

Bismuth Trioxide or Bismuth Sesquioxide, Bi₂O₃, is found naturally as bismuth ochre. It is the product pbtained when bismuth burns in air ("flores bismuti"), or when steam is decomposed by metallic bismuth at white heat. It is usually obtained by the prolonged heating of molten bismuth in air, or by the action of heat upon the carbonate, sulphate or basic nitrate. Bismuth monoxide when heated in air yields the trioxide. The crystalline form is obtained by melting the powdered form with potassium hydroxide, by boiling the hydroxide with potassium or sodium hydroxide, by the action of potassium cyanide on a nitric acid solution of bismuth nitrate, or by adding sodium nitrate intermittently to a molten mixture of sodium hydroxide, bismuth and potassium chromate at 350° C. The oxide is also obtained when chlorine is passed into a fused mixture of bismuth and silver nitrate.

The mineral, bismuth ochre, occurs in massive form with an uneven, earthy fracture; its colour is yellowish-grey or green; it is very soft and friable. The crystalline form of the oxide, prepared as described above, is obtained in the form of bright yellow, transparent rhombic prisms:

a:b:c = 0.817:1:1.065

More usually, bismuth trioxide is obtained as a lemon-yellow powder which becomes darker on heating. When pure it is insensitive to light. The various values that have been given for the density and melting point of this oxide are probably due to the existence of polymorphism. Three varieties have been described. The first variety has a melting point at 820° C. and density 8.9. On cooling it passes to the second variety at 704° C. (density 8.2); supercooling frequently occurs at this transformation, followed by recalescence. This second variety varies from yellow to brown in colour. The third variety, melting at 860° C., and of density 8.5, is formed by heating the trioxide strongly in a porcelain crucible. Both the first and third varieties crystallise in the rhombic system, but the first modification cannot exist at ordinary temperatures.

The variation of the specific heat of bismuth trioxide with the temperature is shown by the following data:

Temperature, ° C.	50	100	200	300	400
Specific heat	0.0569	0.0593	0.0617	0.0631	0.0641

Temp., ° C.	+16.3	-10.9	-59.7	-106.6	-160.0	-204.3	-212.4
Specific heat	0.0563	0.0561	0.0512	0.0447	0.0350	0.0241	0.0204

The variation of electrical resistivity with temperature is:

Temperature, ° C.	225	425	645
Resistivity (ohm-cm.)	2.34×108	1.44×105	6.01×10-3

The thermoelectric power with reference to lead, between 500° and 800° C. and with the cold junction at 0° C., is given by

Thermoelectric power = (1.946 – 1.86t)×10^-6 volt per degree C.

where t is the temperature (° C.). The specific magnetic susceptibility is -0.170×10^-6. The dielectric constant is 182 and appears to be independent of the field strength.

Bismuth trioxide is volatile at high temperatures, the calculated boiling point being 1890° C. Volatilisation begins at 950° C.

In the spectrum of bismuth trioxide four band systems are found between 4300 and 6700 A.

The heat of formation of the trioxide from its elements is 136,600 gram-calories per mole. The heats of formation of the hydrated forms of bismuth trioxide have also been calculated.

The trioxide does not decompose when heated to 1750° C. It is reduced, partially or completely, by a number of reducing agents, such as hydrogen, carbon, carbon monoxide, silicon, sodium, potassium, methane, ammonia, ammonium chloride, potassium cyanide, aluminium carbide, and an alkaline stannous solution. It is oxidised by ozone to bismuth pentoxide, and by ozone in the presence of alkalies to bismuthates; it is only slightly oxidised by alkaline permanganate solution.

When dry bismuth trioxide is heated in a current of dry chlorine, a white, crystalline, deliquescent sublimate of bismuth trichloride is formed. When heated with an excess of bromine for several hours, and after exposing the product to the air, an oxybromide, Bi₁₁O₁₃Br₇, is obtained as a non-deliquescent, cream-coloured powder.

When heated with manganese dioxide, slight reaction takes place between 300° and 500° C. A violet-grey powder is formed, but the nature of the reaction has not been fully established.

Bismuth trioxide reacts with sulphur to form bismuth trisulphide, and with hydrogen sulphide to form a sulphide to which has been ascribed the formula Bi₄S₃. A rather complex reaction occurs with sulphur dioxide, among the products being a basic bismuth sulphate, and, possibly, bismuth monoxide. The reaction may possibly be represented by the equation

7Bi₂O₃ + 3SO₂ = 6BiO + 4Bi₂O₃.3SO₃

It begins at a temperature below visible red heat. At first a dark grey or black powder is formed; this, on prolonged heating, is converted into the white, crystalline, basic sulphate, 4Bi₂O₃.3SO₃. A little sulphur trioxide is also evolved. The dark grey powder contains a sulphate, but not a sulphide. It is not completely soluble in hydrochloric acid, and the black residue so produced is mainly bismuth. From this it is conjectured that the dark grey powder contains bismuth monoxide.

Bismuth trioxide does not appear to react with nitrogen, even when heated. It is reduced to metal by heating with ammonia at 250° C. There is no indication of the formation of a bismuth nitride, but as some water is produced during the reaction it would appear that some of the ammonia is decomposed. The reduction is accelerated in the presence of silver and quartz sand. With phosphorus trichloride at 160° C. a complex reaction takes place, the products including bismuth oxychloride, bismuth phosphate, phosphorus oxychloride and perhaps bismuth dichloride. The trioxide is reduced by arsenic in the presence of molten sodium hydroxide. It reacts readily with bismuth trisulphide according to the equation

2Bi₂O₃ + Bi₂S₃ = 6Bi + 3SO₂

The reaction begins at quite low temperatures in a current of carbon dioxide; it can also be conducted under molten sodium chloride. There remains, in addition, a residue which gives the reactions for a sulphate; probably partial reduction also takes place according to the equation

6Bi₂O₃ + Bi₂S₃ = 8Bi + 3(BiO)₂SO₄

Bismuth trioxide, when heated with silicon tetrachloride, yields bismuth trichloride and silica.

It reacts with potassium thiocyanate with the formation of bismuth trisulphide and a complex compound, K₂Bi₂S₄.

Freezing Point Diagram of the System Bi2O3-PbO.

Thermal examination of the system Bi₂O₃-PbO reveals the possible existence of the compounds 4Bi₂O₃.PbO (decomposing at 690° C.), 3Bi₂O₃.2Pb0 (M.pt. 686° C.) and Bi₂O₃.2PbO (M.pt. 625° C.) (see fig.).

Bismuth trioxide does not attack platinum below 1200° C. in a neutral atmosphere. At 1300° C. it is slowly decomposed and the liberated bismuth gradually absorbed by the platinum with the formation of a brittle, fusible alloy. At 1400° C. this action becomes rapid, and a platinum vessel is completely destroyed when heated with bismuth trioxide at this temperature.

Bismuth trioxide reacts mainly as a basic oxide, forming bismuth salts which, in most cases, are readily hydrolysed yielding, as the final product, basic salts. That, under certain conditions, it also exhibits very feeble acidic properties is shown by its slight solubility in aqueous solutions of alkali hydroxides, as indicated by the following data for the solubility in solutions of sodium hydroxide:

Concentration of NaOH (moles per litre solution)	1.0	2.0	3.0
Solubility of Bi2O3 (grams per 100 c.c. solution)	0.0013 ± 0.0002	0.0026 ± 0.0002	0.0049 ± 0.0005

The solutions, in each case, give no indication of being colloidal. It will be seen that the solubility increases approximately in proportion to the concentration of the alkali. The feeble acidity of bismuth trioxide is further supported by the fact that in the solid state it will react with barium oxide when heated, but not with oxides less basic.

Mixtures of bismuth trioxide and iron oxide (containing from 3 to 4 per cent, of bismuth trioxide) have been suggested as substitutes for platinum gauze in the process for the catalytic oxidation of ammonia. Similar mixtures of bismuth trioxide and cobalt oxide have also been employed. These oxide mixtures are used in the form of a loose, granular powder; they are active at about the same temperature as platinum gauze, but they act more slowly and their life is not so long.