Selenic acid

Selenic acid is the chemical compound with the formula H₂SeO₄. This oxoacid is more descriptively written (HO)₂SeO₂, which emphasises that Se is attached to four oxygen atoms. As predicted by VSEPR theory, the selenium center is tetrahedral.

Preparation and reactions

Selenic acid is prepared by 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<140 °C in vacuum.

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. Unlike sulfuric acid, H₂SeO₄ is a sufficiently strong oxidizer to liberate chlorine from chloride ions, being reduced to selenious acid (selenic [IV] acid).

Selenic acid reacts with barium salts to precipitate BaSeO₄, analogous to the sulfate. Calcium selenate is however more soluble than the sulfate.

Treatment of selenic acid with fluorosulfuric acid gives the dioxydifluoride (b.p. -8.4C):

H₂SeO₄ + 2 HO₃SF → SeO₂F₂ + 2 H₂SO₄

Hot selenic acid is capable of dissolving gold.