| IUPAC name
| Other names
Anhydrous hydrofluoric acid
|Appearance|| Colorless fuming liquid (<19.5 °C)|
Colorless gas (STP)
|Density|| 1.15 g/L, gas (25 °C)|
0.99 g/cm3, liquid (19.5 °C)
|Melting point||−83.6 °C (−118.5 °F; 189.6 K)|
|Boiling point||19.5 °C (67.1 °F; 292.6 K)|
|Solubility|| Slightly soluble in diethyl ether|
Poorly soluble in benzene, tetralin, toluene, xylene
|Vapor pressure||783 mmHg (20 °C)|
|8.687 J·g-1·K-1 (gas)|
Std enthalpy of
| −13.66 kJ/g (gas)|
−14.99 kJ/g (liquid)
|Safety data sheet||Matheson|
|Lethal dose or concentration (LD, LC):|
LC50 (Median concentration)
| 1276 ppm (rat, 1 hr)|
1774 ppm (monkey, 1 hr)
4327 ppm (guinea pig, 15 min)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Hydrogen fluoride is highly dangerous chemical compound with the chemical formula HF, the anhydride of hydrofluoric acid. Unlike the other hydrogen halides, HF has a high boiling point, of 19.5 °C, due to the strong intermolecular hydrogen bonds between the hydrogen and fluorine atoms.
Hydrogen fluoride will react with most common materials in the presence of moisture, including glass, forming silicon tetrafluoride and hexafluorosilicic acid.
Hydrogen fluoride reacts with metals to form a thin layer of metal fluoride on their surface, which protects the metal from further corrosion. If the layer is broken, HF will corrode the metal beneath. The resulting passivation is used to protect metal tanks used to store compressed HF.
Hydrogen fluoride is a fuming colorless liquid that boils at near-room temperature, giving off a colorless gas, with a strong, pungent acid smell, highly corrosive and toxic. It is miscible with water.
Anhydrous hydrogen fluoride is sold by various chemical companies, but due to its great hazards, only industrial or authorized entities can purchase it.
Hydrogen fluoride is more readily available in aqueous solution as hydrofluoric acid.
While hydrofluoric acid is somewhat safer to handle as it does not volatilize easily, hydrogen fluoride boils at near room temperature, which makes preparation, handling and storage extremely difficult and dangerous. Since it reacts with glass, lab glassware is inadequate and the installation for the production of HF must be made from certain plastics, like PTFE, or passivated carbon steel or other metals, such as nickel, copper or lead. Synthesizing and storing hydrogen fluoride is therefore expensive, dangerous and may not worth the risk.
The industrial route involves refluxing concentrated sulfuric acid (90-98%) with calcium fluoride or sodium fluoride between 100-300 °C, in a PTFE or PTFE clad reactor. The HF gas evolution is slow and endothermic, which means that stopping the heating stops the gas evolution, which keeps the dangerous HF inside the vessel in the event of an accident. The resulting HF gas is condensed and collected in a chilled container or cooled and injected in a passivated metal cylinder. One SM user managed to perform this process in a small scale, and obtained a small amount, by heating the mixture at 200°C.
Since producing elemental fluorine is next to impossible in a home setup, the H2 + F2 route is not viable.
- Alkylation catalyst
- Acyl fluorides
Hydrogen fluoride is extremely corrosive and toxic. Contact with skin or tissues will lead to necrosis and even death.
Anhydrous hydrogen fluoride is almost always stored in passivated compressed metal tanks. Since it will slowly react with the metal of the tank, hydrogen pressure will build up inside the tank, which may lead to explosion, if the tank is severely corroded. Check the tank periodically for any pressure build-up or leaks
While HF's boiling point is close to that of room temperature and you might be tempted to think that simply keeping it in a bottle in a very cold place, like a freezer, would be good enough, it should NEVER be stored in the freezer, as it will give off highly toxic and corrosive fumes even when cold.
Hydrogen fluoride can be neutralized in liquid form by reacting it with calcium ions to form the insoluble calcium fluoride, which is safe to handle, in either a well ventilated area or in a special cabinet, to reduce the amount of HF being aerosolized. In gaseous form, excess ammonia can be used to neutralize it to ammonium fluoride, though this will cause a very fine mist of NH4F to form in air. Since ammonium fluoride can slowly hydrolyze back into HF in the presence of moisture, further treatment with calcium hydroxide will convert it to the more stable calcium fluoride, which can be safely removed and discarded.