Hydrogen iodide

  • 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)

The following are some synonyms of hydrogen iodide:

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

The oxidation number of hydrogen in hydrogen iodide is 1.


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

The table shows element percentages for HI (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%.


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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