Hydroiodic acid

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Hydroiodic acid
Space-filling model of hydrogen iodide
Space-filling model of water
The iodide anion
Space-filling model of the hydronium cation
Other names
Hydronium iodide
3D model (Jmol)
ECHA InfoCard 100.030.087
EC Number 233-113-0
RTECS number MW3760000
Molar mass 127.91
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
Aqueous solution
Corrosive (C)
R-phrases R34
S-phrases (S1/2), S26, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
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

Hydroiodic acid (or hydriodic acid) is a highly acidic aqueous solution of hydrogen iodide (HI) (concentrated solution usually 48 - 57% HI). It is the strongest hydrohalic acid. Hydroiodic acid is a commonly used chemical reagent and is one of the strong acids that ionize completely in an aqueous solution. Concentrated hydroiodic acid has a pH of less than 0.[citation needed]


Hydroiodic acid readily reacts with oxygen in air, contributing to the deep colours associated with old samples;

4 HI + O2 → 2 H
+ 2 I2
HI + I2 → HI3

Like other halogens, hydroiodic acid will perform addition reactions with unsaturated hydrocarbons such as alkenes.

Illicit uses[edit]

Lab using the HI/P method

Hydroiodic acid is currently listed as a Federal DEA List I Chemical. Owing to its usefulness as a reducing agent, reduction with HI and red phosphorus has become the most popular method to produce methamphetamine in the United States. Clandestine chemists react pseudoephedrine (recovered from nasal decongestant pills) with hydroiodic acid and red phosphorus under heat. HI reacts with pseudoephedrine to form iodoephedrine, an intermediate which is reduced primarily to methamphetamine.[1] This reaction is stereospecific, producing only (d)-methamphetamine.

Due to its listed status and closely monitored sales, clandestine chemists now use red phosphorus and iodine to generate hydroiodic acid in situ.[2]


  1. ^ Skinner, Harry F. "Methamphetamine Synthesis via HI/Red Phosphorous Reduction of Ephedrine". Forensic Science International, 48 128-134 (1990)
  2. ^ Skinner HF. "Identification and quantitation of hydriodic acid manufactured from iodine, red phosphorus and water". Journal of the Clandestine Laboratory Investigation Chemists Association 1995;5(4):12; Microgram 1995;28(11):349

External links[edit]