Hydrogen selenide

Hydrogen selenide is an inorganic compound with the formula H2Se. This hydrogen chalcogenide is the simplest and most commonly encountered hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound[3] with an exposure limit of 0.05 ppm over an 8-hour period.[4][5] Even at extremely low concentrations, this compound has a very irritating smell resembling that of decayed horseradish or 'leaking gas', but smells of rotten eggs at higher concentrations.

Hydrogen selenide
Structural diagram of the hydrogen selenide molecule
Space-filling model of the hydrogen selenide molecule
IUPAC name
Hydrogen selenide
Other names
Hydroselenic acid
selenium hydride
3D model (JSmol)
ECHA InfoCard 100.029.071
EC Number
  • 231-978-9
RTECS number
  • X1050000
UN number 2202
  • InChI=1S/H2Se/h1H2 Y
  • InChI=1/H2Se/h1H2
  • [SeH2]
Molar mass 80.98 g/mol
Appearance Colorless gas
Odor decayed horseradish[1]
Density 3.553 g/dm3
Melting point −65.73 °C (−86.31 °F; 207.42 K)
Boiling point −41.25 °C (−42.25 °F; 231.90 K)
0.70 g/100 mL
Solubility soluble in CS2, phosgene
Vapor pressure 9.5 atm (21°C)[1]
Acidity (pKa) 3.89
Conjugate acid Selenonium
Conjugate base Selenide
Occupational safety and health (OHS/OSH):
Main hazards
Extremely toxic and flammable
GHS labelling:
H220, H280, H330, H410
P210, P260, P271, P273, P284, P304+P340, P310, P320, P377, P381, P391, P403, P403+P233, P405, P410+P403, P501
NFPA 704 (fire diamond)
Flash point flammable gas
Lethal dose or concentration (LD, LC):
0.3 ppm (guinea pig, 8 hr)
5.9 ppm (rat, 1 hr)[2]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 0.05 ppm (0.2 mg/m3)[1]
REL (Recommended)
TWA 0.05 ppm (0.2 mg/m3)[1]
IDLH (Immediate danger)
1 ppm[1]
Safety data sheet (SDS) ICSC 0284
Related compounds
Other anions
Other cations
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Structure and properties

H2Se adopts a bent structure with a H−Se−H bond angle of 91°. Consistent with this structure, three IR-active vibrational bands are observed: 2358, 2345, and 1034 cm−1.[6]

The properties of H2S and H2Se are similar, although the selenide is more acidic with pKa = 3.89 and the second pKa = 11,[6] or 15.05 ± 0.02 at 25 °C.[7]


Industrially, it is produced by treating elemental selenium at T > 300 °C with hydrogen gas.[8] A number of routes to H2Se have been reported, which are suitable for both large and small scale preparations. In the laboratory, H2Se is usually prepared by the action of water on Al2Se3, concomitant with formation of hydrated alumina. A related reaction involves the acid hydrolysis of FeSe.[9]

Al2Se3 + 6 H2O ⇌ 2 Al(OH)3 + 3 H2Se

H2Se can also be prepared by means of different methods based on the in situ generation in aqueous solution using boron hydride, Marsh test and Devarda's alloy. According to the Sonoda method, H2Se is generated from the reaction of H2O and CO on Se in the presence of Et3N.[10] H2Se can be purchased in cylinders.


Elemental selenium can be recovered from H2Se through a reaction with aqueous sulfur dioxide (SO2).

2 H2Se + SO2 ⇌ 2 H2O + 2 Se + S

Its decomposition is used to prepare the highly pure element.


H2Se is commonly used in the synthesis of Se-containing compounds. It adds across alkenes. Illustrative is the synthesis of selenoureas from cyanamides:[11]

H2Se gas is used to dope semiconductors with selenium.


Hydrogen selenide is hazardous, being the most toxic selenium compound[3] and far more toxic than its congener hydrogen sulfide. The threshold limit value is 0.05 ppm. The gas acts as an irritant at concentrations higher than 0.3 ppm, which is the main warning sign of exposure; below 1 ppm, this is "insufficient to prevent exposure", while at 1.5 ppm the irritation is "intolerable".[5] Exposure at high concentrations, even for less than a minute, causes the gas to attack the eyes and mucous membranes; this causes cold-like symptoms for at least a few days afterwards. In Germany, the limit in drinking water is 0.008 mg/L, and the US EPA recommends a maximum contamination of 0.01 mg/L.[8][12]

Despite being highly toxic, no human fatalities have yet been reported. It is suspected that this is due to the gas' tendency to oxidise to form red selenium in mucous membranes; elemental selenium is less toxic than selenides are.[4]

See also

  • Hydrogen diselenide


  1. NIOSH Pocket Guide to Chemical Hazards. "#0336". National Institute for Occupational Safety and Health (NIOSH).
  2. "Hydrogen selenide". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  3. http://www.epa.gov/ttnatw01/hlthef/selenium.html, US Environmental Protection Agency, Air Toxins website
  4. "CDC - Immediately Dangerous to Life or Health Concentrations (IDLH): Hydrogen selenide (as Se) - NIOSH Publications and Products". www.cdc.gov. 2018-11-02. Retrieved 2023-01-09.
  5. https://www.cdc.gov/niosh/docs/81-123/pdfs/0336.pdf Occupational Health Guideline for Hydrogen Selenide, The National Institute for Occupational Safety and Health, 1978
  6. William M. Haynes; David R. Lide; Thomas J. Bruno, eds. (2017). CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data (97th ed.). Boca Raton, Florida. ISBN 978-1-4987-5429-3. OCLC 957751024.
  7. Levy, Daniel E.; Myers, Rollie J. (1990). "Spectroscopic determination of the second dissociation constant of hydrogen selenide and the activity coefficients and spectral shifts of its ions". The Journal of Physical Chemistry. 94 (20): 7842–7847. doi:10.1021/j100383a020.
  8. Bernd E. Langner "Selenium and Selenium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_525.
  9. Féher, F. In "Handbook of Preparative Inorganic Chemistry"; Brauer, E., Ed.; Academic: New York, 1963; 1, p 418.
  10. Sonoda, N.; Kondo K.; Nagano, K.; Kambe, N.; Morimoto, F. Angewandte Chemie International Edition English 1980, vol. 19, page 308
  11. Cohen, V.I. (1980). "A Convenient Synthesis of Mono-, N,N′-Di-, and Trisubstituted Selenoureas from Methyl Carbamimidothioates (S-Methylpseudothioureas)". Synthesis. 1980: 60–63. doi:10.1055/s-1980-28927.
  12. https://www.osha.gov/dts/chemicalsampling/data/CH_246700.html Archived 2017-05-02 at the Wayback Machine, OSHA GENERAL INDUSTRY PEL: 0.05 ppm, 0.2 mg/m3 ,OSHA CONSTRUCTION INDUSTRY PEL: 0.05 ppm, 0.2 mg/m3 TWA
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