Ammonium hydrogen fluoride is the inorganic compound

Inorganic compound

Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula NH₄HF₂ or NH₄F·HF. It is produced from ammonia

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and hydrogen fluoride

Hydrogen fluoride

Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...

. This colourless salt is a glass-etchant and an intermediate in a once-contemplated route to hydrofluoric acid

Hydrofluoric acid

Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

.

Structure

Ammonium bifluoride, as its name indicates, contains a bifluoride, or hydrogen(difluoride) anion: HF₂⁻. This centrosymmetric triatomic anion features the strongest known hydrogen bond

Hydrogen bond

A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...

, with a F

Fluoride

Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

−H

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 length of 114 pm. and a bond energy greater than 155 kJ mol⁻¹.

In solid [NH₄][HF₂], each ammonium

Ammonium

The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 cation is surrounded by four fluoride centers in a tetrahedron, with hydrogen - fluorine hydrogen bonds present between the hydrogen atoms of the ammonium ion and the fluorine atoms. Solutions contain tetrahedral [NH₄

Ammonium

The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

]⁺ cations and linear [HF₂]⁻ anions.

Production and applications

Ammonium bifluoride is a component of some etchants. It attacks silica component of glass:

SiO₂

Silicon dioxide

The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

  + 4 [NH₄][HF₂] → SiF₄

Silicon tetrafluoride

Silicon tetrafluoride or Tetrafluorosilane is the chemical compound with the formula SiF4. This tetrahedral molecule is notable for having a remarkably narrow liquid range...

  + 4 [NH₄]F

Ammonium fluoride

Ammonium fluoride is the inorganic compound with the formula NH4F. It crystallizes as small colourless prisms, having a sharp saline taste, and is exceedingly soluble in water.-Crystal structure:...

  + 2 H₂O

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

Potassium bifluoride

Potassium bifluoride

Potassium bifluoride is the inorganic compound with the formula KHF2. This colourless salt consists of the potassium cation and the bifluoride anion. The salt is used in etchant for glass...

 is a related more commonly used etchant.

Ammonium bifluoride has been considered as an intermediate in the production of hydrofluoric acid

Hydrofluoric acid

Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

 from hexafluorosilicic acid. Thus, hexafluorosilicic acid is hydrolyzed to give ammonium fluoride, which thermally decomposes to give the bifluoride:

H₂SiF₆ + 6 NH₃

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

+ 2 H₂O → SiO₂ + 6 NH₄F

2 NH₄F → NH₃ + [NH₄]HF₂

The resulting ammonium bifluoride is converted to the sodium bifluoride, which thermally decomposes to release HF.