Fluoroantimonic acid is a superacid formed by the combination of hydrogen fluoride (HF) with antimony pentafluoride (SbF₅). It is typically represented by the formula HF·SbF₅ and exists as a mixture in which the predominant species are the protonated fluoroantimonate ion (H₂SbF₇⁻) and the cationic hydrogen species (H⁺). The acid is one of the strongest known Brønsted acids, surpassing the acidity of pure sulfuric acid, triflic acid, and even other superacids such as magic acid (FSO₃H–SbF₅).
Chemical composition and structure
- Formula: Often written as HF·SbF₅ or H₂SbF₇.
- Molar ratio: The most acidic mixtures contain roughly a 1:1 molar ratio of HF to SbF₅, though the precise ratio can be varied to adjust acidity.
- Ionic species: In solution, the equilibrium can be represented as:
$$ \mathrm{HF + SbF_5 \rightleftharpoons H^+ + SbF_6^-} $$
or more accurately, the formation of the fluoroantimonic anion $\mathrm{SbF_6^-}$ and the protonated species $\mathrm{H_2SbF_7^-}$.
Physical properties
- State: Colorless, fuming liquid or gas at room temperature, depending on composition and temperature.
- Boiling point: Not well defined for the mixture; pure HF boils at 19.5 °C, while SbF₅ boils at 164 °C. The acid mixture typically exists as a highly reactive liquid at ambient conditions.
- Density: Approx. 1.7 g cm⁻³ for a typical 1:1 mixture, though values vary with composition.
- Acidity: Measured by the Hammett acidity function (H₀), fluoroantimonic acid can reach H₀ ≈ −31, indicating an acidity orders of magnitude greater than 100% sulfuric acid (H₀ ≈ −12).
Preparation
Fluoroantimonic acid is prepared by adding antimony pentafluoride to anhydrous hydrogen fluoride under an inert atmosphere (e.g., nitrogen or argon) to avoid moisture. The reaction is highly exothermic and must be performed in corrosion‑resistant equipment, typically made of polytetrafluoroethylene (PTFE) or other fluoropolymer materials.
Reactivity and applications
- Protonating agent: Its extreme acidity enables the protonation of compounds that are inert to conventional acids, such as alkanes, aromatic hydrocarbons, and certain Lewis bases.
- Catalysis: Utilized in certain laboratory-scale catalytic processes, including isomerization and alkylation reactions where a strong proton source is required.
- Research tool: Employed in fundamental studies of superacid chemistry, carbenium ion stability, and the behavior of weakly basic species under extreme protonation conditions.
Safety and handling
Fluoroantimonic acid is highly corrosive, toxic, and reacts violently with water, producing hydrofluoric acid and antimony oxides. Contact with skin or inhalation of vapors can cause severe chemical burns and systemic fluoride poisoning. Proper handling requires:
- Full protective clothing, including acid‑resistant gloves, face shield, and a positive‑pressure suit.
- Use of a certified fume hood with exhaust filtration capable of capturing fluorine‑containing gases.
- Storage in sealed PTFE containers, kept away from moisture, bases, and reducing agents.
- Immediate neutralization protocols using calcium gluconate gel for HF exposure and thorough decontamination procedures for equipment.
Historical context
The superacidic character of mixtures of HF and SbF₅ was first reported in the mid‑20th century by researchers investigating the limits of Brønsted acidity. The term “fluoroantimonic acid” reflects the involvement of fluorine (from HF) and antimony (from SbF₅).
Regulatory status
Due to its extreme hazards, fluoroantimonic acid is subject to strict regulations in many jurisdictions, classified as both a highly toxic substance and a corrosive agent. It is generally restricted to specialized research laboratories with appropriate licensing and safety infrastructure.