All potassium metabisulfite grades should be stored in a cool, dark place.
When the storage temperature is maintained at a maximum of 25 °C and
the maximum relative humidity is 45%, the shelf life is six months.
http://worldaccount.basf.com/wa/NAF...umentum:eCommerce_sol_EU:09007bb280047673.pdf
I've got some 2 years old, still dry, and it will curl your nose hairs if you smell it.
It's cheap. I toss mine after 12 months regardless.
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