Selenic acid

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Selenic acid
Structural formula of selenic acid Space-filling model of selenic acid
Identifiers
CAS number 7783-08-6 N
PubChem 1089
ChemSpider 1058 YesY
KEGG C05697 YesY
ChEBI CHEBI:18170 YesY
RTECS number VS6575000
Jmol-3D images Image 1
Properties
Molecular formula H
2
SeO
4
Molar mass 144.9734 g/mol
Appearance Colorless deliquescent crystals
Density 2.95 g/cm3, solid
Melting point 58 °C (136 °F; 331 K)
Boiling point 260 °C (500 °F; 533 K) (decomposes)
Solubility in water 130 g/100 mL (30 °C)
Acidity (pKa) similar to H
2
SO
4
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
Main hazards Corrosive, highly toxic
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard OX: Oxidizer. E.g., potassium perchlorateNFPA 704 four-colored diamond
Related compounds
Other anions selenious acid
hydrogen selenide
Other cations sodium selenate
Related compounds Sulfuric acid
Selenium dioxide
Selenium trioxide
Telluric acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Selenic acid is the chemical compound with the formula H
2
SeO
4
. It is an oxoacid of selenium, and its structure is more accurately described as (HO)
2
SeO
2
.

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:

SeO
2
+ H
2
O
2
H
2
SeO
4

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 (H
2
SeO
3
) 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 H
2
O
+ 3 Cl
2
H
2
SeO
4
+ 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:

H
2
SeO
4
+ 2 H+
+ 2 Cl
H
2
SeO
3
+ H
2
O
+ Cl
2

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

2 H
2
SeO
4
→ 2 H
2
SeO
3
+ O
2

Selenic acid reacts with barium salts to precipitate BaSeO
4
, 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

H
2
SeO
4
+ 2 HO
3
SF
SeO
2
F
2
+ 2 H
2
SO
4

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

2 Au + 6 H
2
SeO
4
Au
2
(SeO
4
)
3
+ 3 H
2
SeO
3
+ 3 H
2
O

Applications

Selenic acid is used as a reagent for alkaloids and as an oxidizing agent.

References

  1. ^ a b c d Don M. Yost (2007). Systematic Inorganic Chemistry. READ BOOKS. pp. 343–346. ISBN 1-4067-7302-6. 
  2. ^ Mathias S. Wickleder (2007). Francesco A. Devillanova, ed. Handbook of chalcogen chemistry: new perspectives in sulfur, selenium and tellurium. Royal Society of Chemistry. p. 353. ISBN 0-85404-366-7. 
  3. ^ a b Seppelt, K. “Selenoyl difluoride” Inorganic Syntheses, 1980, volume XX, pp. 36-38. ISBN 0-471-07715-1. The report describes the synthesis of selenic acid.
  4. ^ a b c d e Anil Kumar De (2003). A Text Book of Inorganic Chemistry. New Age International. pp. 543–545. ISBN 81-224-1384-6. 
  5. ^ Lenher, V.; Kao, C. H. (June 1925). "The preparation of selenic acid and of certain selenates". Journal of the American Chemical Society 47 (6): 1521–1522. doi:10.1021/ja01683a005. 
  6. ^ Lenher, V. (April 1902). "Action of selenic acid on gold". Journal of the American Chemical Society 24 (4): 354–355. doi:10.1021/ja02018a005. 







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