Hydrofluoric acid
A corrosive acid used in etching, cleaning, and fluoride production.
Hydrofluoric acid is a solution of hydrogen fluoride gas dissolved in water, forming a colorless, acidic, and highly corrosive liquid. First prepared in 1771 by Carl Wilhelm Scheele, it is now primarily produced by reacting the mineral fluorite with concentrated sulfuric acid at high temperatures. It also arises as a by-product during the manufacture of phosphoric acid from certain phosphate minerals. In dilute aqueous solution, hydrogen fluoride behaves as a weak acid, a property attributed to the strength of the hydrogen-fluorine bond and the tendency of the solute to form hydrogen-bonded ion pairs. However, at higher concentrations, the acid becomes dramatically stronger through a process called homoassociation, where HF molecules combine to form polyatomic ions like the bifluoride anion, greatly increasing acidity. The Hammett acidity function for pure hydrogen fluoride is comparable to that of pure sulfuric acid. This concentrated acid is so aggressive that it can protonate other strong acids. In industry, it is less important than anhydrous hydrogen fluoride, but it is a common laboratory reagent for producing organofluorine compounds and other fluorides. It is essential for manufacturing high-volume inorganic fluorides such as cryolite and aluminium trifluoride, used in aluminium production, as well as sodium fluoride and uranium hexafluoride. In the semiconductor industry, it is a key component of etchants used to clean silicon wafers. It etches glass by reacting with silicon dioxide to form gaseous or water-soluble silicon fluorides. Dilute solutions are used in dental ceramics to improve bonding, in household rust removers, and in wheel cleaners. It also dissolves rock samples for analysis and helps extract organic fossils from silicate rock. Because it attacks glass, hydrofluoric acid must be stored in fluorinated plastic containers, typically PTFE.
- prepared_by
- Carl Wilhelm Scheele
- common_concentration
- 49% (48–52%)
- chemical_formula
- HF in water
- primary_mineral_source
- fluorite (CaF2)
- storage_material
- fluorinated plastic (often PTFE)
Lore & Background
Hydrofluoric acid, a solution of hydrogen fluoride in water, is a colorless, acidic, and highly corrosive liquid. First prepared in 1771 by Carl Wilhelm Scheele, it is now mainly produced by treating the mineral fluorite with concentrated sulfuric acid at around 265 °C. The acid is also recovered as a by-product during the production of phosphoric acid from apatite and fluoroapatite. Because of its high reactivity toward glass, it is stored in fluorinated plastic containers, often PTFE. In dilute aqueous solution, hydrogen fluoride behaves as a weak acid, a property partly due to the strength of the hydrogen–fluorine bond and the tendency of HF and fluoride ions to form clusters. Infrared spectroscopy has shown that in dilute solution, dissociation involves the formation of a hydrogen-bonded ion pair, which has been characterized in the crystalline state at very low temperature. Further association occurs, and as concentration increases, polymerization is assumed to take place, supported by the isolation of a salt of a tetrameric anion. The Hammett acidity function for 100% HF has been reported as low as −11, comparable to values near −12 for pure sulfuric acid, making concentrated solutions extremely acidic. The species present in concentrated solutions are not all characterized, but the formation of other polymeric species is highly likely.
Reader's Guide
Hydrofluoric acid is a critical reagent in both laboratory and industrial settings. In industry, it is used to produce high-volume inorganic fluorides such as cryolite (Na3AlF6) and aluminium trifluoride (AlF3), which serve as solvents for metallic aluminium production. It also yields sodium fluoride and uranium hexafluoride. In the semiconductor industry, it is a major component of Wright etch and buffered oxide etch for cleaning silicon wafers. It etches glass by reacting with silicon dioxide to form gaseous or water-soluble silicon fluorides. Dilute hydrofluoric acid is used in dental restoration etching, household rust stain removers, wheel cleaners, and pre-commissioning boilers for high-pressure steam. It is also employed to dissolve rock samples and extract organic fossils from silicate rocks. Despite being a weak acid in dilute solution (pKa ≈ 3.18), concentrated solutions are highly corrosive and strongly acidic, with a Hammett acidity function comparable to that of pure sulfuric acid. Health risks are severe: it penetrates tissue, can cause delayed symptoms, and interferes with nerve function, making burns initially painless. Treatment includes calcium gluconate gel or hexafluorine.
Did You Know?
- In dilute solution, hydrogen fluoride behaves as a weak acid with pKa = 3.17, but concentrated solutions are as acidic as pure sulfuric acid.
- It is stored in fluorinated plastic (often PTFE) containers because of its high reactivity toward glass.
Frequently Asked Questions
Who is Hydrofluoric acid?
Hydrofluoric acid is a colorless, highly corrosive liquid formed when hydrogen fluoride gas dissolves in water. Fans typically encounter it as a roughly 49 % aqueous solution, with commercial concentrations falling between 48 and 52 percent.
What are Hydrofluoric acid's powers and role?
Its signature ability is dissolving glass and silica, which makes it indispensable for etching, surface cleaning, and large-scale fluoride manufacturing. It also appears as a secondary product when apatite or fluoroapatite is processed into phosphoric acid.
How does Hydrofluoric acid's story end?
Because it attacks virtually every common container material, its final chapter is spent stored in fluorinated plastics such as PTFE, where it remains stable until a chemist puts it to work. Once consumed in a reaction, it simply becomes fluoride ions dispersed in solution.
Why is Hydrofluoric acid important to the canon?
It is the go-to reagent for breaking down silicate minerals, a feat no other common acid can accomplish effectively. Without it, glass etching, semiconductor cleaning, and industrial fluoride production would all grind to a halt.
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