Antimony tetroxide is an inorganic compound with the chemical formula Sb₂O₄. It is a mixed‑valence oxide containing antimony in both the +3 and +5 oxidation states, often described as antimony(III,V) oxide. The compound appears as a dark brown to black solid and is one of several antimony oxides, the others being antimony trioxide (Sb₂O₃) and antimony pentoxide (Sb₂O₅).
Structure and Bonding
Sb₂O₄ crystallises in a monoclinic structure (space group P2₁/c). The solid consists of Sb³⁺ ions coordinated in a distorted trigonal pyramidal geometry and Sb⁵⁺ ions in an octahedral environment, linked through shared oxygen atoms. This mixed‑valence arrangement gives rise to its characteristic brown‑black colour.
Synthesis
Antimony tetroxide can be prepared by controlled oxidation of antimony trioxide. Typical laboratory routes involve heating Sb₂O₃ in air at temperatures between 500 °C and 650 °C:
$$ 2,\text{Sb}_2\text{O}_3 + \text{O}_2 ;\longrightarrow; 2,\text{Sb}_2\text{O}_4 $$
Alternatively, direct combustion of elemental antimony in oxygen at elevated temperature yields the same product.
Physical Properties
| Property | Value |
|---|---|
| Appearance | Dark brown to black powder |
| Density | ~5.0 g cm⁻³ (depending on preparation) |
| Melting point | Decomposes before melting; begins to decompose above ~800 °C |
| Solubility | Insoluble in water; slowly reacts with strong acids, releasing Sb³⁺ and Sb⁵⁺ ions |
| Electrical conductivity | Poor conductor (insulating solid) |
Chemical Reactivity
- Acidic dissolution: In strong mineral acids (e.g., HCl, H₂SO₄) antimony tetroxide dissolves, producing a mixture of Sb³⁺ and Sb⁵⁺ salts.
- Reduction: Reducing agents such as carbon or hydrogen can convert Sb₂O₄ back to Sb₂O₃ or elemental antimony, depending on temperature and stoichiometry.
- Oxidation: Further oxidation at temperatures above 800 °C can convert Sb₂O₄ to antimony pentoxide (Sb₂O₅).
Applications
Antimony tetroxide has limited commercial use compared with other antimony oxides. Documented applications include:
- Flame retardants: As a component in certain flame‑retardant formulations, exploiting antimony’s ability to promote char formation and inhibit combustion.
- Catalysis: Employed as a catalyst or catalyst support in specific oxidation reactions, particularly those involving organic substrates where mixed‑valence behavior is advantageous.
- Pigments: Occasionally used as a dark pigment or filler in ceramics and glass, though its usage is less common than antimony trioxide.
Safety and Environmental Aspects
Antimony compounds are classified as toxic. Inhalation of fine particulate Sb₂O₄ may cause respiratory irritation, and chronic exposure can affect the lungs, gastrointestinal tract, and skin. The material is considered a possible carcinogen by some regulatory agencies. Appropriate personal protective equipment (PPE) such as respirators, gloves, and eye protection is recommended when handling the powder. Waste disposal must follow local hazardous waste regulations.
References
- Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed., Butterworth‑Heinemann, 1997.
- G. Brauer (Ed.), Handbook of Preparative Inorganic Chemistry, 2nd ed., Academic Press, 1963.
- R. J. Gillespie, Inorganic Chemistry, 5th ed., Pearson, 2013.
This entry summarizes the established scientific knowledge on antimony tetroxide as of the latest available literature.
