Baking soda - shelf life and how to store it?

I bought a 50lb bag of baking soda as we use it both for cooking and for laundry.   I had a couple open boxes of baking soda that were open to the air, and they seemed still good after three years - in terms of making pancakes rise.  So I figured if I get a lot, and store it airtight, it should last a long time.

Google is not helpful with this question, as most answers say baking soda goes bad 6 months after opening, which I think is true of baking powder but not soda.

So I'm wondering:

How long can baking soda keep if stored properly?  By keep, I mean retain most of its ability to react with acid, which is how it works for baking and laundry.

I have a suspicion that it could keep indefinitely if stored in an airtight container.  But is airtight even necessary?  How quickly will it go bad if exposed to air?  What about humidity?  Anyone know?

I agree, 6 months is baking powder.
I have always stored mine airtight. I don't know that humidity bothers it, but I'd think if anything was going to bother it, that's what it would be. I have stored it for... oooh... trying to think how old some of this is... at least 10 years, with no degradation.

My opinion is it's not going to go bad in many years, but if it was, the culprit would be humidity.

:D

My chemist brother says it will keep indefinitely if stored airtight where there's nothing for it to react to.