Selenous Acid - Formation and Properties

Formation and Properties

Selenous acid is analogous to sulfurous acid, but it is more readily isolated. Selenous acid is easily formed upon the addition of selenium dioxide to water. As a crystalline solid, the compound can be seen as pyramidal molecules that are interconnected with hydrogen bonds. In solution it is a diprotic acid:

H2SeO3 H+ + HSeO−
3 (pKa = 2.62)
HSeO−
3 H+ + SeO2−
3 (pKa = 8.32)

It is moderately oxidizing in nature, but kinetically slow. In 1 M H+:

H2SeO3 + 4 H+ + 4 -
e Se + 3 H2O (Eo = +0.74 V)

In 1 M OH−:

SeO2−
3 + 4 -
e + 3 H2O Se + 6 OH− (Eo = −0.37 V)

It is used in organic synthesis for the synthesis of 1,2-diketones (e.g. glyoxal).

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