Hydrofluoric acid, HF, is a fuming industrial acid of low strength. It is a solution of hydrogen fluoride in water; pure hydrogen fluoride may also be called hydrofluoric acid, because it displays self-ionization and is strongly acidic even without water. It has incredible corrosive abilities, most notably towards glass. It is advised that all but the most experienced amateur chemists stay away from it.
Hydrofluoric acid is usually considered a weak acid, and it is indeed weaker than the other hydrohalic acids (pKa 3.17, mid-strength). However, in high concentrations it behaves like a strong acid, because it starts to protonate itself, forming H2F+ cations which are much more acidic than H3O+, and also HF2- anions which are a weaker base than F-. It is probably one of the most corrosive acids, even when very diluted. It is able to dissolve glass, forming hexafluorosilicic acid and water.
- SiO2 + 6 HF → H2SiF6 + 2 H2O
However, due to the tendency of certain fluorides to be insoluble in water, some metals and metal alloys are able to resist it effectively, including nickel and copper and many alloys containing either. Noble metals, like gold are also unaffected by concentrated HF.
Hydrofluoric acid is a solution of hydrogen fluoride in water, and thus has properties between those of water and pure hydrogen fluoride. Hydrofluoric acid forms an azeotrope with water, consisting of 37% HF by weight, with a boiling point of 120 °C.
Availability and Uses
Hydrofluoric acid is used as a source of the fluoride ion, and for making Chlorofluorocarbons, or CFCs. It is rarely used in the home lab or even in college laboratories because of its extreme corrosive-ness and hazard to humans.
Hydrofluoric acid is available in some places as a 3-4% solution to etch glass. It is also used in many aluminium cleaning products. However, OTC aluminium cleaning agents usually also contain other acids such as sulfuric acid, and distilling hydrofluoric acid from them is very dangerous.
Very dilute (1-3%) hydrofluoric acid is marketed in the United States under the brand name "Whink Rust Stain Remover". This is the only form of hydrofluoric acid that is even remotely practical for most amateur chemists to use. Do not confuse it with the similarly named "Whink Rust and Iron Stain Remover" and "Whink Rust Flush", which contain no hydrofluoric acid and are based on other chemicals.
In Europe, HF is cheaply available as this Italian-made rust removal product. It contains 10% HF by mass.
Although hydrofluoric acid is hazardous, it has been reported by sciencemadness.org members that it can be made by heating calcium fluoride (which can be obtained as a mineral) or sodium fluoride in sulfuric acid. Because HF acid dissolves glass or etches it, this is a very hard procedure. The set-up required to make hydrogen fluoride is usually made out of passivated copper or nickel. You can also use a cast lead retort, because lead is also passivated by hydrofluoric acid and is relatively easy to cast. To make the casting work easier, you can design the retort to consist of two parts, and seal the seam with PTFE tape.
- Glass etching
- Make fluorides
- Fluoroantimonic acid synthesis
The fluoride ion is able to precipitate calcium ions in the bloodstream, which can quickly cause hypocalcemia and death due to arrhythmia. Hydrofluoric acid will readily penetrate the skin, and if such an exposure occurs, calcium gluconate gel is the recommended treatment. Subcutaneous injections of calcium gluconate in the spot exposed to the acid are also recommended.
Besides the exposure to fluoride ions, hydrofluoric acid will readily burn the skin, though the burns may not be immediately evident until a day later. If you are crazy enough to have this acid, keep it in plastic dropper bottles and never pour it; only apply hydrofluoric acid by drops. This way you'll minimize the damage in case of a spill and ensure that one won't kill you, merely incapacitate for days or weeks.
A fume hood or a glovebox is very recommended if you want to experiment with this acid. If you use a glovebox, make sure it's made of plastic and has no glass walls, because fumes of hydrogen fluoride tend to etch glass and make it misty and opaque, hampering your ability to observe what's going on inside the box.
Hydrofluoric acid should be stored in a tightly sealed and thick polyethylene (PE) bottles, preferably in a box with calcium carbonate/hydroxide. Polytetrafluoroethylene (Teflon) bottles can also be used, however, PTFE is somewhat porous to the acid.
Hydrofluoric acid must be neutralized before disposal. Calcium hydroxide is a good neutralizing agent, as the reaction produces the insoluble calcium fluoride which has low toxicity. Aqueous suspension of Ca(OH)2 is a better choice than solid Ca(OH)2 (small pieces or powder).
However, due to HF's high toxicity and ability to easily penetrate the skin, neutralization of hydrofluoric acid is not as simple as with other acids. If the acid is diluted, cool the acid solution, then slowly add the slaked lime to the acid, as acid-base neutralization is exothermic, and if the temperature gets too high, very hazardous HF vapors might be released. If the acid you need to neutralize is concentrated, or worse, it's in the fuming concentration, you will first have to wear proper protection, which includes thick gloves, a gas mask and a full body apron, as well as a ready to use gel or syringe with calcium gluconate. Take a large plastic (PE) beaker/bucket, place it in a larger tupperware, which is placed in a fume hood or outside, in a place with a strong air draft. Add some crushed ice in the beaker/bucket, then carefully pour the acid to the ice. The ice will cool the HF, limiting its evaporation, and as the ice melts, the acid will become more and more dilute. If you can further dilute it, add more cool water. Then, slowly add the slaked lime and stir the solution with a plastic rod. Monitor the temperature with a thermometer, to make sure the temperature doesn't rise too much or too fast.
Avoid using carbonates/bicarbonates to neutralize hydrofluoric acid, since the neutralization produces lots of foaming, which will aerosolize the very hazardous HF.
A good trick to see any escaping HF fumes is to have a solution of concentrated ammonia nearby, the ammonia fumes will react with the gaseous HF and produce a fine mist of ammonium fluoride in air.