Hydroiodic acid (or hydriodic acid) is an unstable, very strong hydrohalic acid with chemical formula HI, formed by dissolution of hydrogen iodide in water. Pure hydroiodic acid is colorless, but it is very hard to obtain a pure sample and keep it pure, because it easily decomposes, liberating elemental iodine and forming the very unstable hydrotriiodic acid. In practice, solutions of hydroiodic acid are typically yellow or brown. It is one of the strongest mineral acids.
The physical properties of hydroiodic acid vary depending on its concentration. Azeotropic hydroiodic acid contains 57% HI and boils at 127°C. More common is 45% hydroiodic acid, which has a density of 1.48 g/cm3.
Hydroiodic acid is a very strong acid and a highly potent reducing agent. It is also used as a iodinating agent.
Availability and Sources
Hydroiodic acid is sometimes available in chemical reagent stores, but this availability varies because of the acid's use in drug synthesis. In USA it is classified as DEA List I chemical, and thus cannot be acquired by private individuals. Unlike hydrochloric or hydrofluoric acids, HI is not used in any household chemicals, thus there are no commercial products containing this acid.
Even in countries where there are fewer restrictions on the sale and ownership of HI, chemical suppliers generally stay away from this compound, as it's difficult to store and most chemists prefer to produce it in situ most of the time.
Hydroiodic acid is the most elusive hydrohalic acid, it generally cannot be prepared in satisfying amount by reactions of other strong acids and iodide salts. Only certain acids such as phosphoric acid or hydrogen sulfide can be used to produce hydroiodic acid with this method.
- H3PO4(aq) + KI → KH2PO4 + HI(aq)
- H2S(aq) + 2 KI → K2S + 2 HI(aq)
Hydrogen iodide can also be obtained from the reaction of hydrogen sulfide with elemental iodine.
- H2S(aq) + I2(s) → 2 HI(aq) + S(s)
Most strong acids, such as sulfuric acid will oxidize iodide salts to elemental iodine, and as such cannot be used.
Other methods used are the hydrolysis of phosphorus triiodide or aluminium iodide.
- PI3 + 3 H2O → 3 HI + H3PO3
- AlI3 + 3 H2O → 3 HI + Al(OH)3
- AlI3 + 2 H2O → 2 HI + Al(OH)2I
Hydroiodic acid is a very strong, corrosive acid. It is mostly non-toxic. However, protective gear is advised when handling it. Concentrated solutions release corrosive fumes.
Like many strong reducing agents, hydroiodic acid is a controlled substance in many countries, because it is used in illegal production of metamphetamine. In the US, it's a DEA List I chemical.
Hydroiodic acid must be stored in a dark place, in bottles of dark glass, sealed from access to atmospheric oxygen. Hypophosphorous acid is sometimes added as stabilizer.
Hydroiodic acid should be neutralized before disposal. Any non-toxic base can be used. Reducing agents, like sodium thiosulfate or metabisulfite can be used to neutralize any iodine traces resulting from the decomposition of HI.