Hydroiodic acid
Hydroiodic acid is an unstable, very strong hydrohalic acid, 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.
Contents
Properties
Physical
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.
Chemical properties
Hydroiodic acid is a very strong acid and a highly potent reducing agent. It is also used as a iodinating agent.
Sources
Hydroiodic acid is sometimes available in chemical reagent stores, but this availability varies because of the acid's use in drug synthesis. It is not used in any household chemicals.
Preparation
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 phosphoric acid can be used to produce hydroiodic acid with this method. Because of this, other methods are used, such as reducing elemental iodine with hydrogen sulfide or hydrolysis of phosphorus triiodide.
Handling
Safety
Hydroiodic acid is a very strong, corrosive acid. It is mostly non-toxic. However, protective gear is advised when handling it.
Legality
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.
Storage
Hydroiodic acid must be stored in a dark place, in bottles of dark glass, sealed from access to atmospheric oxygen.
Disposal
Hydroiodic acid should be neutralized before disposal by any non-toxic base.
