Selenic acid

Structural formula
General
Surname	Selenic acid
Molecular formula	H 2 SeO 4
Brief description	colorless hexagonal prisms
External identifiers / databases
CAS number 7783-08-6 EC number 231-979-4 ECHA InfoCard 100,029,072 PubChem 1089 ChemSpider 1058 Wikidata Q413349
properties
Molar mass	144.97 g mol −1
Physical state	firmly
density	2.95 g cm −3
Melting point	58 ° C
boiling point	Decomposes above 260 ° C
pK s value	1.74 (pK s2 )
solubility	good in water
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary danger H and P phrases H: 331-301-373-410 P: 261-273-301 + 310-311-501
MAK	0.05 mg m −3
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Selenic acid is a strong (similar to sulfuric acid ) inorganic acid with the empirical formula H ₂ SeO ₄ . In aqueous solution it is largely dissociated and forms a clear, colorless liquid. Selenic acid was discovered by Eilhard Mitscherlich in Germany in 1827. Their salts are called selenates .

Extraction

Selenic acid can be represented in different ways.

One possibility is the oxidation of selenous acid (H ₂ SeO ₃ ) or selenium (IV) oxide with hydrogen peroxide (H ₂ O ₂ ), potassium permanganate (KMnO ₄ ) or chloric acid (HClO ₃ ):

${\displaystyle \mathrm {H_{2}SeO_{3}+H_{2}O_{2}\ \rightarrow \ H_{2}SeO_{4}+H_{2}O} }$

${\displaystyle {\ce {SeO2 + H2O2 -> H2SeO4}}}$

As a rule, however, elemental selenium is oxidized with chlorine or bromine water :

${\displaystyle \mathrm {Se+3\ Cl_{2}+4\ H_{2}O\ \rightarrow \ H_{2}SeO_{4}+6\ HCl} }$

${\displaystyle \mathrm {Se+3\ Br_{2}+4\ H_{2}O\ \rightarrow \ H_{2}SeO_{4}+6\ HBr} }$

properties

Pure selenic acid forms colorless crystals that melt at around 62 ° C. Above 260 ° C it decomposes to selenium dioxide and oxygen with elimination of water .

From a chemical point of view, selenic acid is very similar to sulfuric acid , but has a significantly more oxidizing effect (standard potential in acidic solution for reduction to selenic acid: + 1.15 V). It is a strong acid and has a hygroscopic effect. This means that, like concentrated sulfuric acid, it carbonizes organic substances by removing water. Like aqua regia, selenic acid is able to bring gold and platinum into solution:

${\displaystyle \mathrm {2\ Au+6\ H_{2}SeO_{4}\ \rightarrow } }$ ${\displaystyle \mathrm {Au_{2}(SeO_{4})_{3}+3\ H_{2}SeO_{3}+3\ H_{2}O} }$

Similar to aqua regia, a mixture of selenic acid and hydrochloric acid forms nascent chlorine.

${\displaystyle \mathrm {H_{2}SeO_{4}+2\ HCl\longrightarrow H_{2}SeO_{3}+2\ Cl_{nasc.}+\ H_{2}O} }$

This mixture can also dissolve gold with the formation of chloro complexes.

The salts of selenic acid (the selenates) often have a low solubility product in water, similar to sulphates .

safety instructions

Selenic acid is extremely irritating to the skin, respiratory tract and mucous membranes. It is able to destroy living tissue ( chemical burns ). Long-term exposure to the compound can cause severe physiological damage. Selenic acid is poisonous. In the event of poisoning with selenium compounds, permanent damage must be expected.

use

Cadmium selenate can be obtained by reacting cadmium oxide with concentrated selenic acid .

${\displaystyle \mathrm {CdO\ +H_{2}SeO_{4}\rightarrow CdSeO_{4}+H_{2}O} }$

Individual evidence

1. ↑ Entry on selenic acid. In: Römpp Online . Georg Thieme Verlag, accessed on July 26, 2014.
2. ↑ ^{a b c d e f g h} Entry on selenic acid in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .
3. ^ ^{A b} A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .
4. ↑ Not explicitly listed in Regulation (EC) No. 1272/2008 (CLP) , but with the indicated labeling it falls under the group entry selenium compounds with the exception of cadmium sulfoselenide, unless specifically listed in this appendix in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA ), accessed on August 25, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
5. ↑ G. Brauer (Ed.), Handbook of Preparative Inorganic Chemistry 2nd ed., Vol. 1, Academic Press 1963, pp. 432-3.
6. ↑ Dr. Heinrich Remy: Textbook of Inorganic Chemistry Volume I, Page 909, Leipzig 1970, Akademische Verlagsgesellschaft Geest & Portig K.-G.
7. ↑ E. Riedel, Christoph Janiak: Inorganic Chemistry . 8th edition. de Gruyter, 2011, ISBN 3-11-022566-2 , p. 458 .
8. ↑ C. Stålhandske: Structure of cadmium selenate monohydrate. In: Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 37, p. 2055, doi : 10.1107 / S0567740881007942 .