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Hydrogen iodide

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Hydrogen iodide
IUPAC name Iodane
Other names Hydroiodic acid (aqueous solution)
Iodine hydride
CAS number 10034-85-2
PubChem 16109203
RTECS number MW3760000
Molecular formula HI
Molar mass 127.91 g mol-1
Appearance Colorless gas
Density 2.85 g/mL (−47 °C)
Melting point

-50.80 °C, 222 K, -59 °F

Boiling point

-35.36 °C, 238 K, -32 °F

Solubility in water approximately 245 g/100 ml
Acidity (pKa) −10 (in water, estimate);[1] -9.5 (±1.0) [2]

2.8 (in acetonitrile)[3]

Molecular shape Terminus
Dipole moment 0.38 D
Main hazards Toxic, corrosive, Harmful and Irritant
NFPA 704

NFPA 704.svg

R-phrases R20, R21, R22, R35
S-phrases S7, S9, S26, S45
Flash point Non-flammable
Related compounds
Other anions Hydrogen fluoride
Hydrogen chloride
Hydrogen bromide
Hydrogen astatide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Hydrogen iodide (HI) is a chemical compound. Its chemical formula is HI. It is a colorless odorous gas. It reacts with oxygen to make iodine and water. It can be made by reducing iodine with hydrazine. It can also be made by reacting iodides with non-oxidizing acids such as phosphoric acid. It can also be made by reacting hydrogen sulfide with iodine. Very pure hydrogen iodide can be made by combining hydrogen and iodine, but this is very slow. It will break down if exposed to air. It can be used to make illegal drugs and various chemical compounds of iodine. It dissolves in water to produce hydroiodic acid. Hydroiodic acid is a strong acid that is used to make iodides.

Related pages


  1. Bell, R.P. The Proton in Chemistry. 2nd ed., Cornell University Press, Ithaca, NY, 1973.
  2. Trummal, A.; Lipping, L.; Kaljurand, I.; Koppel, I. A.; Leito, I. "Acidity of Strong Acids in Water and Dimethyl Sulfoxide" J. Phys. Chem. A. 2016, 120, 3663-3669. doi:10.1021/acs.jpca.6b02253
  3. Raamat, E.; Kaupmees, K.; Ovsjannikov, G.; Trummal, A.; Kütt, A.; Saame, J.; Koppel, I.; Kaljurand, I.; Lipping, L.; Rodima, T.; Pihl, V.; Koppel, I. A.; Leito, I. "Acidities of strong neutral Brønsted acids in different media." J. Phys. Org. Chem. 2013, 26, 162-170. doi:10.1002/poc.2946

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