Selenic acid
| Selenic acid | |
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Selenic(VI) acid |
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Other names
Selenic acid |
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| Identifiers | |
| CAS number | 7783-08-6  |
| PubChem | 1089 |
| ChemSpider | 1058  |
| KEGG | C05697 |
| ChEBI | CHEBI:18170 |
| RTECS number | VS6575000 |
| Jmol-3D images | Image 1 |
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| 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 |
| Main hazards | Corrosive, highly toxic |
| NFPA 704 |
 0
3
2
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| Related compounds | |
| Other anions | selenious acid hydroselenic acid hydrogen selenide |
| Other cations | sodium selenate |
| Related compounds | Sulfuric acid Selenium dioxide Selenium trioxide Telluric acid |
 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| 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]
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Preparation [edit]
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 [edit]
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
Applications [edit]
Used as a reagent for alkaloids and as an oxidizing agent.
References [edit]
- ^ a b c d Don M. Yost (2007). Systematic Inorganic Chemistry. READ BOOKS. pp. 343–346. ISBN 1-4067-7302-6.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ Lenher, V.; C. H. Kao (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.
- ^ 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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