Cautions with potassium metabisulfite powder

I keep forgetting to post this, but if anyone buys this product (available at NB) for dechlorinating water or stunning wild yeast in cider, grape must, etc., be aware that this off gasses big time if allowed to absorb moisture. I think it generates sulfur dioxide gas. I’m not sure if the Campden tablets do this at all because I have no experience with them. I thought it would be safer to take it out of the bag it came in and store it in airtight dry glass jars (as I do with all of my brewing salts). However, the potassium metabisulfite powder still somehow absorbs moisture and off gasses even in the jar. The problem then is that the gas builds so that when I open the jar, it knocks you hard. So I just open the jar outside and let it air out a bit before using it. I ended up going with the powder since the amount needed to dechlorinate tap water is very small, and I’m measuring it on a gram scale. My advice is to leave it in a Ziplock bag so it can breath and not be allowed to build up gas in a glass jar.

Thanks for the heads up