selenic acid

Selenic acid
IUPAC name	Selenic(VI) acid
Other names	Selenic acid
Identifiers
CAS number	7783-08-6
RTECS number	VS6575000
SMILES	OSe(O)(=O)O
ChemSpider ID	1058
Properties
Molecular formula	H2SeO4
Molar mass	144.9734 g/mol
Appearance	Colorless deliquescent crystals
Density	2.95 g/cm3, solid
Melting point	58 °C (331 K)
Boiling point	260 °C (533 K) (decomposes)
Solubility in water	130 g/100 mL (30 °C)
Acidity (pKa)	similar to H2SO4
Refractive index (nD)	1.5174 (D-line, 20 °C)
Structure
Molecular shape	tetrahedral at Se
Hazards
R-phrases	23/25-33-50/53
S-phrases	20/21-28-45-60-61
NFPA 704	0 3 2
Related compounds
Other anions	selenate
Other cations	selenyl ion, SeO2+2
Related compounds	Sulfuric acid Selenium dioxide Selenium trioxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Selenic acid is the chemical compound with the formula H₂SeO₄. It is an oxoacid of selenium, and its structure is more accurately described as (HO)₂SeO₂.

As predicted by VSEPR theory, the selenium center is tetrahedral, with a Se–O bond length of 161 pm.^[1] In the solid state, it crystallizes in an orthorhombic structure.^[2]

Preparation

Due to the instability of selenium trioxide, it is impractical to synthesize selenic acid by dissolving selenium trioxide in water, unlike sulfuric acid, which can be synthesized by dissolving sulfur trioxide in water.^[1] Instead, it must be prepared by oxidising selenium compounds in lower oxidation states.

One method of preparing selenic acid is by the oxidation of selenium dioxide with hydrogen peroxide:

SeO₂ + H₂O₂ → H₂SeO₄

To obtain the anhydrous acid as a crystalline solid, the resulting solution is evaporated at temperatures below 140 °C (413 K; 284 °F) in a vacuum.^[3]

Selenic acid may also be prepared by the oxidation of selenous acid (H₂SeO₃) with halogens, such as chlorine or bromine, or with potassium permanganate.^[4] However, using chlorine or bromine as the oxidising agent also produces hydrochloric or hydrobromic acid as a side-product, which needs to be removed from the solution since they can reduce the selenic acid to selenous acid.^[5]

Another method of preparing selenic acid is by the oxidation of elemental selenium in water suspension by chlorine:^[4]

Se + 4 H₂O + 3 Cl₂ → H₂SeO₄ + 6 HCl

Chemistry

Like sulfuric acid, selenic acid is a strong acid that is hygroscopic and extremely soluble in water. Concentrated solutions are viscous. Crystalline mono- and di-hydrates are known.^[4] The monohydrate melts at 26°C, and the dihydrate melts at −51.7°C.^[1]

Selenic acid is a stronger oxidiser than sulfuric acid,^[4] capable of liberating chlorine from chloride ions, being reduced to selenous acid in the process:

H₂SeO₄ + 2 H⁺ + 2 Cl⁻ → H₂SeO₃ + H₂O + Cl₂

It decomposes above 200°C, liberating oxygen gas and being reduced to selenous acid:^[4]

2 H₂SeO₄ → 2 H₂SeO₃ + O₂

Selenic acid reacts with barium salts to precipitate BaSeO₄, analogous to the sulfate. In general, selenate salts resemble sulfate salts, but are more soluble. Many selenate salts have the same crystal structure as the corresponding sulfate salts.^[1]

Treatment of selenic acid with fluorosulfuric acid gives the dioxydifluoride (b.p. −8.4 °C):^[3]

H₂SeO₄ + 2 HO₃SF → SeO₂F₂ + 2 H₂SO₄

Hot, concentrated selenic acid is capable of dissolving gold, forming a reddish-yellow solution of gold(III) selenate:^[6]

2 Au + 6 H₂SeO₄ → Au₂(SeO₄)₃ + 3 H₂SeO₃ + 3 H₂O

References