Hydroiodic acid (or hydriodic acid ) is an aqueous solution of hydrogen iodide (HI). It is a strong acid , which ionizes completely in an aqueous solution . It is colourless. Concentrated solutions are usually 48 – 57% HI.
Hydroiodic acid reacts with oxygen in air to give iodine:
4 HI + O 2 → 2 H2o + 2 i 2
Like other hydrogen halides, hydroiodic acid adds to alkenes to give alkyl iodides. It can also be used as a reducing agent, for example in the reduction of aromatic nitro compounds to aniline. 
The Cativa process is a major end use of hydroiodic acid, which serves as a co-catalyst for the production of methanol from acetic acid to carbonylation.
Hydroiodic acid is listed as a US federal DEA List I chemical, due to its use as a reducing agent related to the production of methamphetamine from ephedrine or pseudoephedrine (recovered from nasal decongestant tablets).
|other nameshydronium iodide|
|chemical formula||HI (AQ)|
|molar mass||127.91 g/mol|
|density||1.70 g/mL, aziotrope|
( 57% HI by weight)
|boiling point||127 °C (261 °F; 400 K) 1.03 bar, azeotrope|
|Solubility in water||aqueous solution|
|Acidity ( pKa ) _||-9.3|
|other ions||Hydrofluoric Acid|
|related compound||hydrogen iodide|
|Except where otherwise noted, data are given for materials at their standard conditions (25 °C [77 °F], 100 kPa).|
What is the difference between hydrogen iodide and hydroiodic acid?
However, hydrogen iodide and hydroiodic acid differ in that the first is a gas under standard conditions, while the second is an aqueous solution of the gas. They are interchangeable. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent.
Why is hydrogen iodide a strong acid?
Since the size of the iodine atom is much larger than that of the hydrogen atom, it makes the H-I bond larger and weaker. Therefore, hydrogen iodide readily donates its proton, which makes it a strong acid.