When mixed with water, sodium metabisulfite releases sulfur dioxide (SO2), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. For this reason, sodium metabisulfite has fallen from common use in recent times, with agents such as hydrogen peroxide becoming more popular for effective and odorless sterilization of equipment. Released sulfur dioxide however makes the water a strong reducing agent.
sodium metabisulfite gives sulfur dioxide with hydrochloric acid:
Na2S2O5 + 2HCl → 2NaCl + H2O + 2SO2
on heating it releases sulfur dioxide, leaving sodium oxide behind:
Na2S2O5 → Na2O + 2SO2
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