Hydrogen: hydrogen iodide

hydrogen iodide is known by the following synonyms:

  • hydrogen iodide
  • hydrogen(I) iodide
  • hydroiodic acid

The oxidation number of hydrogen in hydrogen iodide is 1.

  • Formula: HI
  • Hill system formula: H1I1
  • CAS registry number: [10034-85-2]
  • Formula weight: 127.912
  • Class: iodide
  • Colour: colourless
  • Appearance: gas
  • Melting point: -51°C
  • Boiling point: -35°C
  • Density: 2850 kg m-3 (-47°C); 6.613 kg m-3 (gas)


Hydrogen iodide may be made in the laboratory by the reaction between purified hydrogen gas and iodine. This is catalyzed by platinized asbestos.

I2 + H2 [Pt] → 2HI

Less desirable in the laboratory, but a process used commercially, is the reaction between hydrogen sulphide, H2S (danger!) and iodine.

8I2 + 8H2S → 16HI + S8

Solid state structure

  • Geometry of hydrogen: 1 coordinate: terminus
  • Prototypical structure:

Crystal structure of hydrogen iodide

Element analysis

Element percentages in hydrogen iodide
Element %
H 0.79
I 99.21

Isotope pattern for HI

THe chart below shows the calculated isotope pattern for the formula HI with the most intense ion set to 100%.


Coming soon...


The data on these compounds pages are assembled and adapted from the primary literature and several other sources including the following.

  • R.T. Sanderson in Chemical Periodicity, Reinhold, New York, USA, 1960.
  • N.N. Greenwood and A. Earnshaw in Chemistry of the Elements, 2nd edition, Butterworth, UK, 1997.
  • F.A. Cotton, G. Wilkinson, C.A. Murillo, and M. Bochmann, in Advanced Inorganic Chemistry, John Wiley & Sons, 1999.
  • A.F. Trotman-Dickenson, (ed.) in Comprehensive Inorganic Chemistry, Pergamon, Oxford, UK, 1973.
  • R.W.G. Wyckoff, in Crystal Structures, volume 1, Interscience, John Wiley & Sons, 1963.
  • A.R.West in Basic solid state chemistry Chemistry, John Wiley & Sons, 1999.
  • A.F. Wells in Structural inorganic chemistry, 4th edition, Oxford, UK, 1975.
  • J.D.H. Donnay, (ed.) in Crystal data determinative tables, ACA monograph number 5, American Crystallographic Association, USA, 1963.
  • D.R. Lide, (ed.) in Chemical Rubber Company handbook of chemistry and physics, CRC Press, Boca Raton, Florida, USA, 77th edition, 1996.
  • J.W. Mellor in A comprehensive treatise on inorganic and theoretical chemistry, volumes 1-16, Longmans, London, UK, 1922-1937.
  • J.E. Macintyre (ed.) in Dictionary of inorganic compounds, volumes 1-3, Chapman & Hall, London, UK, 1992.

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