Hydrogen fluoride

Hydrogen fluoride

Names
IUPAC name Hydrogen fluoride
Other names Anhydrous hydrofluoric acid
Properties
Chemical formula	HF
Molar mass	20.01 g/mol
Appearance	Colorless fuming liquid (<19.5 °C) Colorless gas (STP)
Odor	Pungent
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 in water	Miscible
Solubility	Reacts with amines Slightly soluble in diethyl ether, trifluoroacetic acid Poorly soluble in benzene, tetralin, toluene, xylene
Vapor pressure	783 mmHg (20 °C)
Acidity (pKa)	3.17
Thermochemistry
Std molar entropy (So298)	8.687 J·g-1·K-1 (gas)
Std enthalpy of formation (ΔfHo298)	−13.66 kJ/g (gas) −14.99 kJ/g (liquid)
Hazards
Safety data sheet	Matheson
Flash point	Non-flammable
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)
Related compounds
Related compounds	Hydrogen chloride Hydrogen bromide Hydrogen iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

Properties

Chemical

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.

Physical

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.

Availability

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.

Preparation

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 anhydrous hydrogen fluoride in a home setting 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.

Projects

• Alkylation catalyst
• Acyl fluorides

Handling

Safety

Hydrogen fluoride is extremely corrosive and toxic. Contact with skin or tissues will lead to necrosis and even death.

Storage

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.

Disposal

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.

References

Relevant Sciencemadness threads

• concentration Hydrogen fluoride gas
• Advice needed for HF synthesis