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Selenium dioxide

Selenium dioxide

Other names Selenium(IV) oxide
Selenium dioxide
Molecular formula SeO2
Molar mass 110.96 g/mol
Appearance White crystalline solid
CAS number [7446-08-4]
Density and phase 3.95 g/cm3, solid
Solubility in water 3.950 g/100 ml (25 °C)
Boiling point 315 °C sublimes
Acidity (pKa) 2.62 (H2SeO3 HSeO3 + H+)
8.32 (HSeO3 SeO32− + H+)
Crystal structure chain structure
EU classification Toxic (T)
Dangerous for
the environment (N)
NFPA 704
R-phrases R23/25, R33, R50/53
S-phrases (S1/2), S20/21, S28
S45, S60, S61
U.S. Permissible
Exposure Limit
5 ppm
IDLH (NIOSH) 100 ppm
Flash point non-flammable
RTECS number WS4550000
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Other cations Sulfur dioxide
Tellurium dioxide
Related compounds Selenium trioxide
Selenous acid
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium.



Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating Se and oxygen atoms. Each Se atom, which is pyramidal, bears a terminal oxide group. The relative stereochemistry at Se alternates along the polymer chain (syndiotactic). The solid sublimes readily. Gaseous selenium dioxide adopts a bent structure very similar to that of sulfur dioxide. Dissolution of SeO2 in selenium oxydichloride give the trimer [Se(O)O]3.[1] Whereas SO2 tends to be molecular and SeO2 is a one-dimensional chain, TeO2 is a cross-linked polymer.

SeO2 is considered an acidic oxide: it dissolves in water to form selenous (selenious) acid. Often the term selenous (selenious) acid and selenium dioxide are used interchangeably. It reacts with base to form selenite SeO32-:

SeO2 + 2 NaOH → Na2SeO3 + H2O


Selenium dioxide is prepared by oxidation of selenium by burning in air or by reaction with hydrogen peroxide, but perhaps the most convenient preparation is by the dehydration of selenous acid.

3Se + 4HNO3 + H2O → 3H2SeO3 + 4NO
H2SeO3 SeO2 + H2O


Organic synthesis

SeO2 is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO2 gives glyoxal[2] and the oxidation of cyclohexanone gives cyclohexane-1,2-dione.[3]

It is also renown as a reagent for "allylic" oxidation,[4] a reaction that entails the conversion

R2C=CR'-CHR"2 + [O] → R2C=CR'-C(OH)R"2

(where R, R', R" are alkyl or aryl).

As a colorant

Selenium dioxide imparts a red colour to glass: it is used in small quantities to counteract the blue colour due to cobalt impurities and so to create (apparently) colourless glass. In larger quantities, it gives a deep ruby red colour.

Selenium dioxide is the active ingredient in some cold-blueing solutions.

It is also used as a toner in photographic developing.


  1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  2. ^ Ronzio, A. R.; Waugh, T. D. (1955). "Glyoxal Bisulfite". Org. Synth.; Coll. Vol. 3: 438. 
  3. ^ Hach, C. C. Banks, C. V.; Diehl, H. (1963). "1,2-Cyclohexanedione Dioxime". Org. Synth.; Coll. Vol. 4: 229. 
  4. ^ Coxon, J. M.; Dansted, E.; Hartshorn, M. P. (1988). "Allylic Oxidation with Hydrogen Peroxide–Selenium Dioxide: trans-Pinocarveol". Org. Synth.; Coll. Vol. 6: 946. 

Further reading

  • Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th Edn.) New York:Wiley-Interscience. ISBN 0-471-19957-5.
  • Lide, D. R. (Ed.) (2002). CRC Handbook of Chemistry and Physics (83rd Edn.). Boca Raton (FL):CRC Press. ISBN 0-8493-0483-0. 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Selenium_dioxide". A list of authors is available in Wikipedia.
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