Sodium metabisulfite is a white, slightly yellowish crystaline powder with a sulfur dioxide odor (rotten eggs), dissolving easily in water
When mixed with water or a strong acid solution, sodium metabisulfite releases sulfur dioxide (SO2), a pungent, unpleasant smelling gas that can also cause breathing difficulties in some people. Released sulfur dioxide however makes the water a strong reducing agent.
Sodium metabisulfite is commonly used to precipitate dissolved gold out of aqua regia and gold chloride solutions. There are multiple other uses such as in pharmaceuticals and cleaning processes.