# Ammonium bifluoride

Ammonium bifluoride is an inorganic compound with the formula [NH$_4$][HF$_2$] or [NH$_4$]F·HF. It is produced from ammonia and hydrogen fluoride. This colourless salt is a glass-etchant and an intermediate in a once-contemplated route to hydrofluoric acid.

## Structure

Ammonium bifluoride, as its name indicates, contains an ammonium cation ([NH$_4$]$^+$), and a bifluoride (or hydrogen difluoride) anion ([HF$_2$]$^−$). The triatomic bifluoride anion features a strong three-center four-electron bond (specifically, a symmetrical hydrogen bond) with a bond energy greater than 155 kJ/mol, and an H-F length of 114 pm.
In solid form ([NH$_4$][HF$_2$]), ammonium biflouride is similar to other fluoride salts. Its crystal system is considered orthorhombic, with each cation coordinated with four anions in a tetrahedron (and vice versa). Hydrogen atoms in the ammonium ion form hydrogen bonds with the fluorine atoms, and in the resulting structure, N-H-F are roughly colinear. As a result of these hydrogen bonds, this crystal structure varies from those of other bifluoride salts, such as potassium bifluoride and rubidium bifluoride. Its crystals are unstable, and decompose rapidly when exposed to air.

## Production and applications

Ammonium bifluoride is a component of some etchants. It attacks silica component of glass:
    SiO$_2$ + 4 [NH$_4$][HF$_2$] → SiF$_4$ + 4 [NH$_4$]F + 2 H$_2$O
Potassium bifluoride is a related more commonly used etchant.
Ammonium bifluoride has been considered as an intermediate in the production of hydrofluoric acid from hexafluorosilicic acid. Thus, hexafluorosilicic acid is hydrolyzed to give ammonium fluoride, which thermally decomposes to give the bifluoride:
    H$_2$[SiF$_6$] + 6 NH$_3$ + 2 H$_2$O → SiO$_2$ + 6 [NH$_4$]F
    2 [NH$_4$]F → NH$_3$ + [NH$_4$][HF$_2$]
The resulting ammonium bifluoride is converted to sodium bifluoride, which thermally decomposes to release HF.
Ammonium bifluoride is also used as an additive in tin-nickel plating processes as the fluoride ion acts as a complexing agent with the tin, allowing for greater control over the resulting composition and finish.

## Toxicity

Ammonium bifluoride is toxic to consume and a skin corrosion agent. Upon exposure to skin, rinsing with water followed by a treatment of calcium gluconate is required. In water, ammonium bifluoride exists in chemical equilibrium with hydrofluoric acid and heating releases hydrogen fluoride gas. Consequently, there is an equivalent toxicological risk as is present with hydrofluoric acid, and the same safety precautions apply.
Ammonium bifluoride is used in some automotive wheel cleaning products. Many injuries have resulted in users not being aware of the risks posed by the products. Ammonium bifluoride based products are often considered a safer alternative to hydrofluoric acid, yet still pose clear risks to the handler. Ammonium bifluoride, ammonium fluoride, and hydrofluoric acid have been described as "too dangerous for any use in a car wash environment" by Professional Car Washing and Detailing magazine, advice that accords with a 2015 report from the U.S. Centers for Disease Control and Prevention.