Selenium

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Other names/abbreviations:

Se

Element no:	34
Element group:	16 (IUPAC) VI A (American labelling) VI B (European labelling) Chalcogens
Element type:	Nonmetals
CAS-Number:	7782-49-2
EINECS-Number:	231-957-4

Chemical properties

Reaction of selenium with acids
Selenium does not react with dilute non-oxidizing acids.

Selene(VI) as selenates is oxidized to selene(VI) by strong hydrochloric acid

SeO₄^2- (aq) + 2 H⁺ (aq) + 2 Cl^- (aq) H₂SeO₃ (aq) + Cl₂ (aq) + H₂O (l)

Reaction of selenium with metals/metal ions
Cd reacts with Se when heated rigorously [5].

Cd (s) + Se (s) CdSe (s)


Selene(IV) as selenious acid is precipitated by silver ions in weak nitric acid. The precipitate is dissolved in stronger acids

H₂SeO₃ (aq) + 2 Ag⁺ (aq) Ag₂SeO₃ (s) [white] + 2 H⁺ (aq)


Selene(IV) as selenious acid is not precipitated by barium ions.


Selene(VI) as selenic acid is precipitated by barium ions.

SeO₄^2- (aq) + Ba²⁺ (aq) BaSeO₄ (s) [white]


Reaction of selenium with permanganate
Selene(IV) as selenious acid is oxidized to selenic acid by permanganate.

5 H₂SeO₃ (aq) + 2 MnO₄^- (aq) 4 H⁺ (aq) + 5 SeO₄^2- (aq) + 2 Mn²⁺ (aq) + 3 H₂O (l)


Reaction of selenium with peroxide
Selene(IV) as selenious acid is oxidized to selenic acid by hydrogen peroxide.

H₂SeO₃ (aq) + H₂O₂ (aq) 2 H⁺ (aq) + SeO₄^2- (aq) + H₂O (l)


Reaction of selenium with sulfide
Selene(IV) as selenious acid is precipitated by hydrogen sulfide in 0.4 M hydrochloric acid. The yellow precipitate is usually SeS₂, but it can be a mixture of Se and S or all three components

H₂SeO₃ (aq) + 2 H₂S (aq) SeS₂ (s) [yellow] + 3 H₂O (l) (+ Se(s) + S(s))

Selene(VI) as selenic acid is not precipitated by sulfide.


Reaction of selenium with sulfur dioxide
Selene(IV) as selenious acid is reduced by sulfur dioxide in hydrochloric acid.

H₂SeO₃ (aq) + 2 SO₂ (aq) + H₂O (l) Se(s) [red] + 2 SO₄^2- (aq) + 4 H⁺ (aq)

Selene(VI) as selenic acid is not precipitated by sulfur dioxide


Solubility
Water: Insoluble (20 °C) [4]

Quantitative analysis

Method 3500-Se H Atomic Absorption Spectrometric Method [2]. A portion of the sample is digested in a combination of acids. The digest is atomized in a graphite tube and resulting absorption of light is measured at 196.0 nm.

Method limit of detection in water = 0.004 mg/L
Method limit of detection in soil = 0.40 mg/kg

Safety

Symbol: T
R-phrases: 23/25-33
S-phrases: 20/21-28-45

Physical properties

Appearance
	Physical state @ 20°C:	Solid [4]
	Color:	Grey to black [4]
	Odor:	Odourless [4]

Bulk properties
	Molecular weight (g/mol):	78.96 [1]
	Melting point (°C):	217 [1]
	Boiling point (°C):	685 [1]
	Density (g/cm3):	Hexagonal: 4.189 [1] alpha-monoclinic: 4.389 [1] Vitreous: 4.285 [1]

Thermodynamic properties
	Δ H°atomization (kJ/mol):	206.7 [1]
	ΔfH°gas (kJ/mol):	227.1 (monoatomic) [3] 146.0 (Se2) [3]
	ΔfH°solid (kJ/mol):	0.0 (monoatomic) [3]
	ΔH°fusion (kJ/mol):	6.69 [3]
	S°gas (J/K·mol):	176.7 (monoatomic) [3] 252.0 (Se2) [3]
	S°solid (J/K·mol):	42.4 [3]
	ΔfG°gas (kJ/mol):	187.0 (monoatomic) [3] 96.2 (Se2) [3]
	Cp (gas) (J/K·mol):	20.8 (monoatomic, 25 °C) [3] 35.4 (Se2, 25 °C) [3]
	Cp (solid) (J/K·mol):	25.363 (monoatomic, 25 °C) [3]
	Critical temperature (°C):	1493 [3]

Electronic properties
	Electron configuration:	1s2-2s2-2p6-3s2-3p6-3d10-4s2-4p4
	Atomic radius (Å):	1.19 (2-coordinate covalent radius) [1] 1.40 (12-coordinate) [1]
	Ionic radius (Å):	Se(-II): 1.98 [1] Se(IV): 0.50 [1] Se(VI): 0.42 [1]

Conductivity
	Electrical resisitvity (μΩ·cm):	1016 (25 °C) [1] 1.6·1011 (400 °C) [1]

References

1: Greenwood,N.N., Earnshaw,A. Chemistry of the elements
2nd edition (1997) Edited by Greenwood,N.N., Earnshaw,A. pp. 1-1340, Butterworth-Heinemann. Oxford. Great Britain

2: Standard Methods for the Analysis of Water and Wastewater, APHA, 1992, 18th edition

3: CRC Handbook of Chemistry and Physics
75th edition (1994) Edited by Lide,D.R. , CRC Press Inc. Boca Raton, USA

4: Merck. ChemDAT The Merck Chemical Database Ver. 1.1.5

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© Michael Pilgaard
Created: October 6, 2008
Last update: October 14, 2008