Selenium dioxide

Selenium dioxide
CAS number 7446-08-4 YesY
PubChem 24007
ChemSpider 22440 YesY
UNII 9N3UK29E57 YesY
UN number 3283
RTECS number VS8575000
Jmol-3D images Image 1
Molecular formula SeO2
Molar mass 110.96 g/mol
Appearance White, yellowish white[1], or red[1] crystalline solid
Density 3.95 g/cm3, solid
Melting point

340 ºC (sealed tube)

Boiling point

315 °C subl.

Solubility in water 38.4 g/100 mL (20 °C)
39.5 g/100 ml (25 °C)
82.5 g/100 mL (65 °C)
Solubility soluble in benzene
Solubility in ethanol 6.7 g/100 mL (15 °C)
Solubility in acetone 4.4 g/100 mL (15 °C)
Solubility in acetic acid 1.11 g/100 mL (14 °C)
Vapor pressure 1.65 kPa (70 ºC)
Acidity (pKa) 2.62; 8.32
Crystal structure see text
trigonal (Se)
EU Index 034-002-00-8
EU classification Toxic (T)
Dangerous for the environment (N)
R-phrases R23/25, R33, R50/53
S-phrases (S1/2), S20/21, S28, S45, S60, S61
NFPA 704
NFPA 704.svg
Flash point Non-flammable
Related compounds
Other anions Selenium disulfide
Other cations Sulfur dioxide
Tellurium dioxide
Related selenium oxides Selenium trioxide
Related compounds Selenous acid
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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



SeO2 is a polar molecule, with the dipole moment pointed from the midpoint of the two oxygen atoms to the selenium atom. Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating selenium 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. The vapour has an odour resembling horseradish sauce and can burn the nose and throat on inhalation. 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.[2] 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 terms selenous acid and selenium dioxide are used interchangeably. It reacts with base to form selenite salts containing the SeO2−
anion. For example, reaction with sodium hydroxide produces sodium selenite:

SeO2 + 2 NaOH → Na2SeO3 + H2O


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

3 Se + 4 HNO3 + H2O → 3 H2SeO3 + 4 NO
2 H2O2 + Se → SeO2 + 2 H2O
H2SeO3 SeO2 + H2O


The natural form of selenium dioxide, downeyite, is a very rare mineral. It is found in only a very few burning coal dumps.[3]


Organic synthesis

SeO2 is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO2 gives glyoxal[4] and the oxidation of cyclohexanone gives cyclohexane-1,2-dione.[5]. The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off.[5] This type of reaction is called a Riley oxidation. It is also renowned as a reagent for "allylic" oxidation,[6] 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. ^ a b
  2. ^ Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, ISBN 0-12-352651-5 
  3. ^ American Mineralogist 62: 316–320. 1977. 
  4. ^ Ronzio, A. R.; Waugh, T. D. (1955), "Glyoxal Bisulfite", Org. Synth., ; Coll. Vol. 3: 438 
  5. ^ a b Hach, C. C. Banks, C. V.; Diehl, H. (1963), "1,2-Cyclohexanedione Dioxime", Org. Synth., ; Coll. Vol. 4: 229 
  6. ^ Coxon, J. M.; Dansted, E.; Hartshorn, M. P. (1988), "Allylic Oxidation with Hydrogen Peroxide–Selenium Dioxide: trans-Pinocarveol", Org. Synth., ; Coll. Vol. 6: 946 

External links

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Look at other dictionaries:

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