Antimony Trioxide, Sb₂O₃

Antimony Trioxide, Sb₂O₃ was known in ancient times; it is probably referred to by Pliny under the name of stibia femina, and by Basil Valentine under the name flores Antimonii. The latter name was subsequently applied to the product derived from the roasting of antimony sulphide. Antimony trioxide occurs naturally in the minerals senarmontite and valentinite, and in certain other more complex minerals.

The trioxide may be prepared by the direct oxidation of antimony, by heating in air or in water vapour; by the action of concentrated nitric acid, in which case a mixture of oxides is obtained; or by fusion with potassium nitrate and potassium bisulphate. The higher oxides of antimony may be reduced to the trioxide by the action of sulphur dioxide or hydriodic acid.

When antimony trisulphide is roasted, a mixture of oxides of antimony is obtained, from which the trioxide can be separated by fusion with more antimony trisulphide. The trioxide may also be obtained from the trisulphide by treatment with concentrated sulphuric acid, followed by the addition of an alkali carbonate to the solution obtained.

Many antimony compounds may be decomposed by suitable reagents, yielding antimony trioxide. Thus, antimonyl chloride is completely converted to the trioxide by treatment with water at 150° C.; antimony salts are decomposed by alkali hydroxides and carbonates, and potassium antimonyl tartrate is decomposed by the action of salts of weak acids such as borates, acetates, thiosulphates, phosphates, sulphites, etc., trioxide being formed in each case.

Technical antimony trioxide, as used in the manufacture of paints, is frequently obtained direct from antimony ores or concentrates.

Vapour Pressure Curve of Antimony Trioxide.

Antimony trioxide is dimorphous, the two modifications crystallising in the cubic and rhombic systems respectively. Both modifications occur naturally, the cubic as senarmontite and the rhombic as valentinite; both can also be produced artificially, the cubic by sublimation and the rhombic by the hydrolysis of solutions of antimony trichloride. Although both modifications can exist unchanged for long periods at the ordinary temperature, the cubic modification is the stable form, the rhombic form being stable at higher temperatures. In support of this view, it is found that a specimen of antimony trioxide after prolonged heating at 550° C. yields cubic crystals only; after similar treatment at 590° C. rhombic crystals only are obtained. Rhombic crystals separate from the melt on crystallisation, but these are completely converted to cubic crystals by prolonged heating at 550° C. At a temperature of about 570° C. both cubic and rhombic crystals are in equilibrium under the pressure of their own vapour. The transition temperature is therefore at approximately 570° C., the cubic modification being stable below this temperature and the rhombic above. Further support for this view is afforded by an examination of the vapour pressure curves of the two modifications (fig.); the computed transition point, however, appears to be slightly lower than the value recorded above.

Cubic antimony trioxide (senarmontite) contains eight molecules of Sb₄O₆ in the unit cell a = 11.14 A.

Its density determined by pyknometer is 5.19, by X-rays, 5.49. Its hardness on Mohs' scale is 2.0 to 2.5. The specific heat is 0.093 gram-calories per gram. The melting point is approximately 650° C., and the calculated molar latent heat of fusion (assuming the molecule to be SbO₆) is 29,490 gram-calories. The vapour pressure (in millimetres) below the melting point is given by

logp = 12.195 – 10,357/T

The dielectric constant is 12.8, and is not affected by high field strengths.

Vapour pressure of Antimony Trioxide

Temperature, ° C.	Pressure (mm. Mercury).
450	0.0075
475	0.0224
500	0.0625
550	0.406
557	0.525
575	0.908
600	2.42
625	3.91
642	7.43
650	7.60
655	8.50
675	10.42
700	13.32
750	20.78
1456	760

Rhombic antimony trioxide, in the form of the mineral valentinite, has the crystallographic axial ratios

a:b:c = 0.3910:1:0.3364

The density of the synthetic form at 27.4° C. is 5.99; that of the mineral is 5.57. The hardness of the mineral on Mohs' scale is 2.5 to 3.0. The molar heat capacity of the prepared form at low temperatures (assuming the molecule to be Sb₂O₃) is as follows:

Temperature, °C.	-213.2	-198.3	-182.4	-164.3	-120.7	-92.6	-49.3	-20.1	+1.6	+17.5
Molar heat capacity (gram-calories)	6.893	9.114	10.97	13.11	16.97	19.05	22.54	22.59	23.57	24.49

The calculated molar latent heat of fusion is 13,250 gram-calories. The vapour pressure (in millimetres) is given by

logp = 11.318 – 9,635/T

and the molar heat of vaporisation is 22,040 gram-calories. The molar heat of transition from the cubic to the rhombic modification is 1,620 gram-calories.

Antimony trioxide, when heated in air, undergoes no change at temperatures below 360° C. Above that temperature it absorbs oxygen and is converted into the tetroxide. At a higher temperature the tetroxide dissociates into trioxide and oxygen; the dissociation begins at about 900° C. and is complete at 1,030° C.

Antimony trioxide is almost insoluble in both hot and cold water, and also in dilute nitric and sulphuric acids; it will dissolve in dilute hydrochloric acid. It is oxidised by concentrated nitric acid, a mixture of oxides being formed in which antimony pentoxide predominates. With concentrated sulphuric acid, antimony sulphate is formed. The trioxide is soluble in alkaline solutions, forming antimonites; it is also soluble in tartaric acid, in lactic acid, and in certain other organic compounds. The statement that antimony trioxide is oxidised when boiled with aqueous alcohol has been contradicted.

At a red heat antimony trioxide is reduced by hydrogen; reduction also occurs on treatment with hydrogen under the influence of the silent electric discharge. Hydrogen peroxide is without action.

A complex reaction occurs when the oxide is heated with chlorine, antimony tri- and penta-chlorides being formed in addition to antimony tetroxide. The last is decomposed on further treatment with chlorine. When antimony trioxide is melted with a little sulphur, a mixture of antimony trisulphide and antimony trioxide, known as "antimony glass," is obtained; with excess of sulphur, antimony trisulphide and sulphur dioxide are formed. A reaction also occurs between antimony trioxide and antimony trisulphide resulting in the formation of metallic antimony. Thermal investigation of the system Sb₂O₃-Sb₂S₃ indicates the formation of a compound Sb₂O₃.5Sb₂S₃ or Sb₄OS₅. When a current of hydrogen sulphide is passed over the trioxide a yellow coloration is produced in the cold; when heated an oxysulphide is obtained. With ammonium sulphide the trioxide reacts with the formation first of an orange-coloured oxysulphide which passes ultimately into the trisulphide. With sulphur monochloride, antimony trichloride is obtained:

6S₂Cl₂ + 2Sb₂O₃ = 4SbCl₃ + 3SO₂ + 9S

Phosphorus trichloride is decomposed by antimony trioxide with the formation of red phosphorus. The oxide dissolves slightly in phosphoric acid, some phosphate being formed.

The trioxide is reduced when heated with carbon and certain carbon compounds such as carbon monoxide, potassium cyanide, sodium formate, etc. From an examination of the equilibrium conditions of the reduction by carbon monoxide, according to the equation

Sb₂O₃ + 3CO ⇔ 2Sb + 3CO₂ between 502° and 596° C. the change in free energy is given by

ΔF = -33,461 + 34.286T log T – 0.01110T² + 0.00000093T³ - 88.65T

Reduction of the trioxide is complete at 500° C.

Antimony trioxide reacts also with silicon tetrachloride forming antimony trichloride and silicon; and with silicochloroform in the presence of sodium hydroxide, in which case metallic antimony and hydrated silica are obtained. It may be reduced to metal by the action of boron nitride.

The more active metals such as potassium, magnesium and aluminium act as reducing agents; fusion with alkali nitrates results in the formation of antimonates. When fused with caustic soda and sulphur a mixture of antimonate and thioantimonate is formed, but fusion with sodium hydroxide and arsenic leads to reduction to the metal.

The heat of formation of cubic antimony trioxide is 149,690 ± 200 gram-calories per mole.