Hydrogen fluoride

In aqueous solution, HF is a weak acid, with a pK_a of 3.17, again in contrast with the other hydrogen halides, which are all completely ionized in solution. But there is nothing weak about HF’s other properties. As a gas, as a liquid, or in solution, it is extremely aggressive toward a wide range of substances, including most metals, glass and other forms of silica, and animal and plant tissue.

French chemist Edmond Frémy discovered HF gas in the mid-19th century, although HF solutions had been known and used at least 100 years earlier. Its main source today is the mineral fluorite (calcium fluoride), which is treated with sulfuric acid to form HF gas. Significant amounts of HF also are obtained as a byproduct of phosphate fertilizer manufacture from the mineral apatite, which contains fluoride ion as well as phosphate.

The synthesis of almost all commercial fluorine-containing chemicals begins with HF. These include inorganics, such as molecular fluorine (F₂), cryolite (Na₃AlF₆) for aluminum production, and a wide range of metal fluorides; and organics as diverse as polymers (e.g., polytetrafluoroethylene, or Teflon), chlorofluorocarbon refrigerants, and perfluoroalkyl substances (PFAS), which currently are in the news as notorious environmental contaminants. HF itself is used as a catalyst in petroleum refining.

A more modern use of HF is in chemical lasers. George C. Pimentel at the University of California, Berkeley, a pioneer in the field, developed dozens of infrared lasers based on chemical reactions, including some that use the reaction of fluorine and hydrogen to form HF. HF lasers emit IR light in the 2.7–2.9 µm wavelength range; if the hydrogen is replaced by deuterium, the wavelength increases to ≈3.8 µm.

1. The hydrogen bonds are so strong that HF can form homopolymers, designated as (HF)_n.

Hydrogen fluoride hazard information

*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. 
Explanation of pictograms. 
**Not an official GHS code.