What Is Hydrogen Fluoride?

Hydrogen fluoride is a compound of hydrogen and fluorine with the chemical formula HF. Fluorine is a member of a group of elements known as the halogens, all of which combine with hydrogen in a similar way to form hydrogen halides. At room temperature and normal pressure, hydrogen fluoride is a colorless gas with a boiling point of 67.1°F (19.5°C), which is much higher than those of the other hydrogen halide, and allows it to exist as a liquid at everyday temperatures. In water, it dissolves to form hydrofluoric acid. Liquid HF is also known as anhydrous — meaning water-free — hydrofluoric acid, and “HF” can be used to denote the gas, the liquid, or the aqueous acid.

In aqueous solution, hydrofluoric acid is a weak acid, due to hydrogen bonding between the HF and water molecules, which limits the degree of dissociation into ions. Hydrogen bonding between HF molecules accounts for the relatively high boiling point of hydrogen fluoride compared with the other hydrogen halides. The acid reacts with many metals, usually forming hydrogen gas and a metal fluoride, for example: Mg + 2HF -> MgF₂ + H₂. Unlike many acids, however, it also reacts readily with most metal oxides and with silicates, including glass, forming soluble compounds. For this reason, it cannot be stored in glass bottles.

Hydrogen fluoride can be produced by the reaction of a metal fluoride, for example calcium fluoride, with sulfuric acid: CaF₂ + H₂SO₄ -> CaSO₄ + 2HF. It is produced this way in the chemical industry, using fluorite, a common mineral form of calcium fluoride. The main industrial uses are in the production of polytetrafluoroethylene (PTFE), in the semiconductor industry for removing oxide from silicon, in the extraction of uranium from its oxide ore, in etching glass and as a catalyst in the petrochemical industry. It is also used to remove rust stains, as it reacts with metal oxides to form soluble fluorides. Fluorine is produced industrially by the electrolysis of liquid HF.

In the laboratory, HF in the form of aqueous hydrofluoric acid is used in mineral analysis due to its ability to dissolve silicates. It is also used in the analysis of pollen in soil samples. Soil consists mainly of organic and mineral material, with the minerals consisting mainly of carbonates and silicates. To identify any pollen present, this material needs to be removed, and, following treatment with other reagents to remove the carbonates and organic material, hydrofluoric acid is used to remove the silicate minerals.

Hydrogen fluoride and hydrofluoric acid are highly toxic and very corrosive. Inhalation of the gas damages the respiratory system and can cause pulmonary edema and death. Skin contact with hydrofluoric acid, even in very dilute solutions, can result in severe burns and allow fluoride ions into the bloodstream. The acid is absorbed very quickly through the outer skin and kills living tissue underneath, mainly due to the fluoride ion combining with calcium ions and precipitating insoluble calcium fluoride. Calcium is essential to cell metabolism and to the functioning of vital organs; its removal from the system can result in a condition known as hypocalcemia, which can lead to death from cardiac arrest or multiple organ failure.

Due to these hazards, hydrogen fluoride and hydrofluoric acid need to be handled very carefully and stringent safety precautions are normally observed where they are used. Ingestion, inhalation or skin contact with HF requires urgent medical attention, even if there are no immediate symptoms, as with dilute solutions the effects may be delayed. Spillages covering 2% or more of the body surface are considered life threatening, due to the risk of significant quantities of fluoride ions entering the bloodstream. Application of calcium gluconate gel to the affected area provides calcium ions which bind to the fluoride ions, helping to minimize damage and prevent hypocalcemia.