Selenic acid is the inorganic compound with the formula . It is an

oxoacid An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

, and its structure is more accurately described as . It is a colorless compound. Although it has few uses, its derivative sodium selenate is used in the production of glass and animal feeds.Bernd E. Langner "Selenium and Selenium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. .

Structure and bonding

The molecule is tetrahedral, as predicted by

VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm the ...

. The Se–O bond length is 161 pm. In the solid state, it crystallizes in an orthorhombic structure.

Preparation

It is prepared by oxidising selenium compounds in lower oxidation states. One method involves the oxidation of

selenium dioxide Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium. Properties Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating seleniu ...

with

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%� ...

: : Unlike the production sulfuric acid by hydration of sulfur trioxide, the hydration of selenium trioxide is an impractical method. Instead, selenic acid may also be prepared by the oxidation of

selenous acid Selenous acid (or selenious acid) is the chemical compound with the formula . Structurally, it is more accurately described by . It is the principal oxoacid of selenium; the other being selenic acid. Formation and properties Selenous acid is a ...

() with halogens, such as

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

, or with

potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution. Potassium permanganate is widely used in the c ...

. Using chlorine or bromine as the oxidising agents 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. To obtain the anhydrous acid as a crystalline solid, the resulting solution is evaporated at temperatures below in a vacuum.Seppelt, K. “Selenoyl Difluoride” Inorganic Syntheses, 1980, volume XX, pp. 36-38. . The report describes the synthesis of selenic acid.

Reactions

Like sulfuric acid, selenic acid is a

strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions ...

that is

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...

and extremely soluble in water. Concentrated solutions are viscous. Crystalline mono- and di-

hydrates In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

are known. The monohydrate melts at 26 °C, and the dihydrate melts at −51.7 °C. Selenic acid is a stronger

oxidizer An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

than sulfuric acid, capable of liberating

from

chloride ion The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...

s, being reduced to

in the process: : It decomposes above 200 °C, liberating oxygen gas and being reduced to selenous acid: : Selenic acid reacts with barium salts to precipitate solid , 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. Treatment with fluorosulfuric acid gives selenoyl fluoride: : Hot, concentrated selenic acid reacts with

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

, forming a reddish-yellow solution of gold(III) selenate: :

Applications

Selenic acid is used as a specialized oxidizing agent.

References

{{selenium compounds Oxidizing acids Oxidizing agents Oxoacids Selenates