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A solution of sodium arsenite, Na3AsO3, is not oxidised to arsenate on exposure to air, but a solution of sodium sulphite, Na2SO3, is oxidised to sulphate on exposure. When a solution containing both arsenite and sulphite is exposed to air, both salts are oxidised:
Na2SO3 + Na3AsO3 + O2 = Na2SO4 +Na3AsO4.
This induced oxidation may be explained on Clausius's or on Bach's theories. The Na2SO3 is called an inductor, the O2 molecule the actor, and the Na3AsO3 (which is not oxidised by itself) the acceptor:
- Na2SO3 + O2 +H2O = Na2SO4 + H2O2,
Na3AsO3 + H2O2 = Na3AsO4 + H2O. - Na2SO3 + O2 = Na2SO5,
Na3AsO3 + Na2SO5 = Na2SO4 + Na3AsO4.
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