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.