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Determination of silicon dioxide. Metallurgical metallographic analysis. Silicon compounds. Manufacture of fluoride. Hydrofluoric acidIdentifiersPropertiesHazardsRelated compounds

Other names[hide] fluoric acid; fluorhydric acid
CAS number	7664-39-3 Y
ChemSpider	14214 Y
UNII	RGL5YE86CZ Y
EC number	231-634-8
ChEBI	CHEBI:29228 Y
RTECS number	MW7875000
Jmol-3D images	Image 1
SMILES[show] F
InChI[show] InChI=1S/FH/h1H YKey: KRHYYFGTRYWZRS-UHFFFAOYSA-N Y InChI=1/FH/h1HKey: KRHYYFGTRYWZRS-UHFFFAOYAC
Molecular formula	HF (aq)
Molar mass	not applicable(see hydrogen fluoride)
Appearance	Colorless solution
Density	1.15 g/mL (for 48% soln.)
Melting point	not applicable(see hydrogen fluoride)
Boiling point	not applicable(see hydrogen fluoride)
Solubility in water	Miscible.
Acidity (pKa)	3.17 [1]
MSDS	duPont MSDS
EU Index	009-003-00-1
EU classification	Very toxic (T+)Corrosive (C)
R-phrases	R26/27/28, R35
S-phrases	(S1/2), S7/9, S26, S36/37, S45
NFPA 704	042COR
Flash point	Non-flammable
Other anions	Hydrochloric acidHydrobromic acidHydroiodic acid
Related compounds	Hydrogen fluoride
Y (verify) (what is: Y/N?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Hydrofluoric acid is a highly corrosive acid, capable of dissolving many materials, especially oxides. Its ability to dissolve glass has been known since the 17th century, even before hydrofluoric acid had been prepared in large quantities by Carl Wilhelm Scheele in 1771.[2] Because of its high reactivity toward glass and moderate reactivity toward many metals, hydrofluoric acid is usually stored in plastic containers (although PTFE is slightly permeable to it).[3]

Acidity

Hydrofluoric acid is classified as a weak acid because of its lower dissociation constant compared to the strong acids. It ionizes in aqueous solution in a similar fashion to other common acids:[4]

HF + H2O H3O+ + F−

It is the only hydrohalic acid that is not considered a strong acid, i.e. it does not fully ionize in dilute aqueous solutions.

When the concentration of HF approaches 100%, the acidity increases dramatically because of the following equilibrium:

2 HF H+ + FHF−

The bifluoride (FHF−) anion is stabilized by the very strong hydrogen–fluorine hydrogen bond.

[edit] Production

Hydrofluoric acid is produced by treatment of the mineral fluorite (CaF2) with concentrated sulfuric acid. When combined at 265 °C, these two substances react to produce hydrogen fluoride and calcium sulfate according to the following chemical equation:

CaF2 + H2SO4 → 2 HF + CaSO4

Although bulk fluorite is a suitable precursor and a major source of world HF production, HF is also produced as a by-product of the production of phosphoric acid, which is derived from the mineral apatite. Apatite sources typically contain a few percent of fluoroapatite, acid digestion of which releases gaseous stream consisting of sulfur dioxide (from the H2SO4), water, and HF, as well as particulates. After separation from the solids, the gases are treated with sulfuric acid and oleum to afford anhydrous HF. Owing to the corrosive nature of HF, its production is accompanied by the dissolution of silicate minerals, and, in this way, significant amounts of fluorosilicic acid is generated.[3]

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