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Properties of Hydrofluoric Acid
Hydrofluoric acid, also known as hydrogen fluoride, has the formula HF.
Externally, it looks like a colorless liquid. It has a sharp, specific odor and fumes in open air — the higher the concentration, the more actively it fumes.
At 70% concentration, the melting point is minus 75 degrees Celsius, and the boiling point is 85.8 degrees.
It reacts actively with metals and glass, so it can only be stored in plastic containers. It also does not react with rubber, paraffin, or other plastics.
Another feature — when it comes into contact with skin, a burn forms, but it only appears after several hours; with a large contact area or long duration of contact, it can be fatal. When inhaled, it can cause burns to the respiratory tract, including fatal outcomes.
Hydrogen fluoride is considered the best solvent for proteins.
Interestingly, “dry” HF does not react with the overwhelming majority of metals and their compounds (oxides). But if a reaction with an oxide does occur, the process then self-accelerates, since water appears, as seen from the reaction:
MO + 2HF = MF2 + H2O, where M is the metal.
Industrial Use of Hydrofluoric Acid
In industry, hydrofluoric acid is used in chemical etching of glass (the glass surface becomes matte/frosted), which can be described by the formula:
SiO2 + 4HF(gas) = SiF4(gas) + 2H2O — a reaction that releases heat, here HF is gaseous
Or 6HF + SiO2 = H2SiF6 + H2O — a reaction at room temperature, here HF is concentrated
* for glass composition Na2O x CaO x 6SiO2, the full reaction would look like this:
Na2O x CaO x 6SiO2 + 28HF = Na2SiF6 + 4SiF4 (gas) + 14 H2O
Silicon etching can be described by the formula 4HF + Si = SiF4 + 2H2. The reaction proceeds with the release of heat, here HF is gaseous.
The same formula describes the use of hydrofluoric acid in the oil industry — it is used to treat wells with silica-containing formations, as well as to clean heating equipment and pipes where deposits contain silicon.
Technical-grade acid usually contains impurities. For primary (rough) purification, distillation is carried out in lead or platinum containers.
For even greater purification, hydrofluoric acid is converted into Potassium bifluoride, after which it is heated to decomposition and the resulting acid is diluted with distillate.
Production of Hydrogen Fluoride
Hydrofluoric acid can be obtained from fluorspar (CaF2) using the formula:
CaF2 + H2SO4 = 2HF + CaSO4 — this formula describes the Schwankwerder method, which was used to obtain the acid as far back as 300 years ago. The reaction temperature is 220-280 degrees.
This formula is greatly simplified, since the production process is more complex due to the possible formation of fluorosulfonic acid:
H2SO4 + HF = HSO3F + H2O
With an excess of water, the equilibrium shifts to the left.
However, fluorosulfonic acid itself can react with CaF2 and form Ca(SO3F)2, but at temperatures above 100 degrees this compound decomposes back into CaF2 and SO3. With an excess of sulfuric acid, CaSO4 and HSO3F are formed.
There are some other particular features of the process: with a higher water content in the sulfuric acid, the probability of obtaining HSO3F decreases, but increasing the water content also significantly increases corrosion of the entire mixture with equipment components. For this reason, the water content is limited to a range of 2-7%.
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