Selenium

4 Se(s) + S₂O₆F₂(aq) [Se₄]²⁺ [SO₃F]²⁻(aq) [yellow] [1]

[Se₄]²⁺(aq) + 4 Se(s) [Se₈]²⁺(aq) [green] [1]

Se₈(s) + 6 AsF₅(s) [Se₄]²⁺(aq) + [AsF₆]²⁻(aq) [yellow] [1]

Se₈(s) + 5 SbF₅(s) [Se₈]²⁺(aq) + [Sb₂F₁₁]²⁻(aq) [green] [1]

7½ Se(s) + ½ SeCl₄(aq) + 2 AlCl₃(s) [Se₈]²⁺(aq) + [AlCl₄]₂⁻(aq) [green-black] [1]

Selenium does not react with dilute non-oxidizing acids.

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

SeO₄²⁻(aq) + 2 H⁺(aq) + 2 Cl⁻(aq) H₂SeO₃(aq) + Cl₂(aq) + H₂O(l)

Selenium burns in air, forming selenium(IV) oxide, SeO₂.

Se₈(s) + 8 O₂(g) 8 SeO₂(s)

Selenium reacts with fluorine, F₂. At 0 °C, selenium(IV) fluoride is formed, and if burned, selenium(VI) fluoride is formed:

Se₈(s) + 15 F₂(g) 8 SeF4(s) [colourless]
Se₈(s) + 24 F₂(g) 8 SeF6(l) [orange]


Depending on the conditions, selenium reacts with chlorine, Cl₂ and bromine, Br₂, forming the corresponding Se(I) or Se(IV) halides:

Se₈(s) + 4 Cl₂(g) 4 Se₂Cl2(s) [orange]
Se₈(s) + 16 Cl₂(g) 8 SeCl4(s)

Se₈(s) + 4 Br₂(g) 4 Se₂Br2(s) [orange]
Se₈(s) + 16 Br₂(g) 8 SeBr4(s)


Selenium reacts with iodine, I₂, forming Se(IV) iodide:

Se₈(s) + 16 I₂(g) 8 SeI4(s)


Selene(IV), as selenious acid, is oxidized to selene(VI) by chlorine, ₂:

H₂SeO₃(aq) + Cl₂(aq) + H₂O(l) SeO₄²⁻(aq) + 2 H⁺(aq) + 2 Cl⁻(aq)

Selenium burns in air, forming selenium(IV) oxide, SeO₂.

Se₈(s) + 8 O₂(g) 8 SeO₂(s)

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.

SeO₄²⁻(aq) + Ba²⁺(aq) BaSeO₄(s) [white]

Selene(IV) as selenious acid is oxidized to selenic acid by permanganate.

5 H₂SeO₃(aq) + 2 MnO₄⁻(aq) 4 H⁺(aq) + 5 SeO₄²⁻(aq) + 2 Mn²⁺(aq) + 3 H₂O(l)

Selene(IV) as selenious acid is oxidized to selenic acid by hydrogen peroxide.

H₂SeO₃(aq) + H₂O₂(aq) 2 H⁺(aq) + SeO₄²⁻(aq) + H₂O(l)

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.

Selene(IV) as selenious acid is reduced by sulfur dioxide in hydrochloric acid.

H₂SeO₃(aq) + 2 SO₂(aq) + H₂O(l) Se(s) [red] + 2 SO₄²⁻(aq) + 4 H⁺(aq)

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

Selenium(IV) oxide reacts with water forming selenious acid.

SeO₂(s) + H₂O(l) H₂SeO₃(aq)

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