Relating to or containing arsenic in a lower oxidation state, particularly As(III) rather than As(V).
From arsenic with -ous suffix, which typically indicates a lower oxidation state (compare arsenous vs. arsenic). Used in chemistry since the 1800s.
The -ous vs. -ic suffixes in chemistry encode oxidation states—ferrous (Fe2+) vs. ferric (Fe3+), arsenous vs. arsenic—turning oxidation state differences into spelling differences so chemists can literally read a compound's chemistry from its name.
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