Hydroiodic acid

Hydroiodic acid (or hydriodic acid) is a colorless liquid. It is an aqueous solution of hydrogen iodide with the chemical formula HI(aq). It is a strong acid, in which hydrogen iodide is ionized completely in an aqueous solution. Concentrated aqueous solutions of hydrogen iodide are usually 48% to 57% HI by mass.^[2]

Hydroiodic acid

Space-filling model of hydrogen iodide Space-filling model of water
The iodide anion Space-filling model of the hydronium cation
An oxidized solution of hydroiodic acid
Names
Other names Aqueous hydrogen iodide Hydriodic acid Hydrogen iodide, hydrous Hydronium iodide
Identifiers
CAS Number	10034-85-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:43451 Y
ChemSpider	23224 Y
EC Number	233-109-9
PubChem CID	24841
RTECS number	MW3760000
UNII	694C0EFT9Q N
InChI InChI=1S/BrH/h1H Y Key: CPELXLSAUQHCOX-UHFFFAOYSA-N Y InChI=1/BrH/h1H Key: CPELXLSAUQHCOX-UHFFFAOYAZ
SMILES [OH3+].[I-]
Properties
Chemical formula	HI(aq)
Molar mass	127.912 g·mol−1 (HI)
Appearance	colorless liquid
Odor	acrid
Density	1.70 g/mL, azeotrope (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 (HI)[1]
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314
Precautionary statements	P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501
NFPA 704 (fire diamond)	3 0 0 ACID
Flash point	Non-flammable
Related compounds
Other anions	Hydrofluoric acid Hydrochloric acid Hydrobromic acid
Related compounds	Hydrogen iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

Main article: Hydrogen iodide § Synthesis

Reactions

Hydroiodic acid reacts with oxygen in air to give iodine:

4 HI(aq) + O₂ → 2 H₂O + 2 I₂

Like 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 anilines.^[3]

Cativa process

The Cativa process is a major end use of hydroiodic acid, which serves as a co-catalyst for the production of acetic acid by the carbonylation of methanol.^[4][5]

The catalytic cycle of the Cativa process

Illicit uses

Hydroiodic acid is listed as a U.S. Federal DEA List I Chemical, owing to its use as a reducing agent related to the production of methamphetamine from ephedrine or pseudoephedrine (recovered from nasal decongestant pills).^[6]