Hydrogen Iodide

Title: Hydrogen Iodide
CAS Registry Number: 10034-85-2
CAS Name: Anhydrous hydriodic acid
Molecular Formula: HI
Molecular Weight: 127.91
Percent Composition: H 0.79%, I 99.21%
Literature References: Prepd by catalytic union of the elements: Caley, Burford, Inorg. Synth. 1, 159 (1939); Powell, Campbell, J. Am. Chem. Soc. 69, 1227 (1947). May also be prepd by treating concd hydriodic acid solns with P2O5: Schmeisser in Handbook of Preparative Inorganic Chemistry vol. 1, G. Brauer, Ed. (Academic Press, New York, 2nd ed., 1963) pp 286-289. Lab prepn: Hoffman, Inorg. Synth. 7, 180 (1963). Purification: A. Klemenc, Die Behandlung und Reindarstellung von Gasen (Vienna, 2nd ed., 1948) p 239; Irving, Wilson, Chem. Ind. (London) 1964, 653. Reviews of prepn and properties of HI and other hydrogen halides: Hills in Mellor's vol. II, suppl. 1 (originally published as suppl. II, part 1) 857-869 (1956); Downs, Adams, in Comprehensive Inorganic Chemistry vol. 2, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1280-1329.
Properties: Colorless, acrid, non-flammable gas. Fumes in moist air. Decomposed by light. mp -50.8°. bp760 -35.1°; bp2 atm -18.9°; bp5 atm 7.3°; bp10 atm 32.0°; bp60 atm 127.5°. d0 5.66; d25 5.23 g/l. Crit temp 151.0°, crit press. 82.0 atm. Sp heat (25°) 0.0545 cal/g/°C. Extremely sol in water. Soly (g/100 g H2O): 234 (10°); 900 (0°). Soly in organic solvents: Gerrard, Chem. Ind. (London) 1969, 295; Ahmed et al., J. Appl. Chem. 20, 109 (1970). Forms an azeotrope with water, see Hydriodic Acid. Reacts with the lower aliphatic alcohols forming the corresponding iodo compds. Forms a colorless liquid at atm pressure when cooled with dry ice and ether or similar cooling mixture. Attacks natural rubber.
Melting point: mp -50.8°
Boiling point: bp760 -35.1°; bp2 atm -18.9°; bp5 atm 7.3°; bp10 atm 32.0°; bp60 atm 127.5°
Density: d0 5.66; d25 5.23 g/l
CAUTION: Strong irritant.
Use: Manuf of hydriodic acid, organic iodo compds, to remove iodine from iodo compds.
Hydrogen Selenide Hydrogen Sulfide Hydrogen Telluride Hydrogen Tetracarbonylferrate(II) Hydrohydrastinine

Hydrogen iodide
Hydrogen iodide
Hydrogen iodide
CAS number 10034-85-2 YesY
RTECS number MW3760000
Molecular formula HI
Molar mass 127.904 g/mol
Appearance Colorless gas
Density 2.85 g/mL (-47 °C)
Melting point –50.80 °C (184.55 K)
Boiling point –35.36 °C (237.79 K)
Solubility in water approximately 245 g/ml
Acidity (pKa) –10 (in water, estimate)[1]

2.8 (in acetonitrile)[2]

Molecular shape Terminus
Dipole moment 0.38 D
MSDS External MSDS
R-phrases R20, R21, R22, R35
S-phrases S7, S9, S26, S45
Main hazards Toxic, corrosive.
NFPA 704
NFPA 704.svg
Flash point Non-flammable
Related compounds
Other anions Fluoride
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Hydrogen iodide (HI) is a diatomic molecule. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under standard conditions; whereas, the other is an aqueous solution of said gas. They are interconvertible. HI is used in organic and inorganic synthesis as one of the primary sources of iodine and as a reducing agent.