Selenious Acid: Monohydrated selenium dioxide; selenous acid. H₂SeO₃; mol. wt. 128.98. Se 61.22%, SeO₂ 86.03%.

Prepared by dissolving selenium dioxide in hot water and cooling: Berzelius, Acad. Handl. Stockholm 39, 13 (1818).

Deliquescent hexagonal prisms; (Ref.) d¹⁵₄ 3.004; v.p. _15° 2 mm. Hg; v.p. _35° 4.5 mm. Hg; v.p. _40.3° 7 mm. Hg; K₁ = 0.0024; K₂ = 4.8 X 10^–9.

Gives off water upon heating and selenium oxide sublimes. Oxidized to selenic acid by strong oxidizing agents such as ozone, hydrogen dioxide, chlorine. Reduced to selenium by most reducing agents including hydriodic acid, sulfurous acid, sodium hyposulfite, hydroxylamine salts, hydrazine salts, hypophosphorous acid, phosphorous acid. 90 parts dissolve in 100 parts water at 0°, 400 parts at 90°; very soluble in alcohol; insoluble in ammonia.

Use: As a reagent for alkaloids.