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Selenous acid - is the chemical compound with the formula H2SeO3. Structurally, it is more accurately described by (HO)2SeO. It is the principal oxoacid of selenium; the other being selenic acid.
S 1345 (OTTO) Selenous acid, 98% Cas 7783-00-8 - used in steel guns, wherein it changes the color of the steel from silver-grey to blue-grey.