Potassium metabisulfite?

Stock solutions only last a few months. You will want to make it strong, like 5%, which is equilivant to 5 tablespoons per quart.

I make mine a lot stronger than that - more like 220g/l. The stuff is very reactive with air, so it needs to be stored in a nearly full, tightly sealed bottle to retain its potency.

Tom S

I make up a 10% solution - about a liter at a time, and store in brown chem bottles, tighly closed. Even at that, it appears that the solution loses strength, so I refresh if older than a month or so - and either sump or use the older for sanitizing solution.