Stability of Sodium Metabisulfite

[Futur]

During my attempt at filtering I've noticed some obscure issues with my quantity of SMB on hand when using it to remove O2 from a mixture of starstan.

In multiple experiments I required almost 4 grams of SMB to reduce the O2 from approximately ~7ppm O2 in a litre of water. Whereas previously I'd only used a fraction of that quantity for the same effect. I ran some side by side experiments with newly purchased SMB vs my old SMB and the new SMB performed as expected, however the older SMB had variation - some worked as described and some didn't (from the same container). This was with tap water, RO water and starsan/tapwater mix.

I believe the SMB I have stored has deteriorated and isn't scavenging O2 as it should be, proven with an O2 meter. There is some smaller clumps in the container (plastic with screw lid) and it's a bit over 6 months old.

Has anyone experienced anything like this? This potentially could impact many of you if what I've observed is replicated elsewhere.

[wobdee]

I've read it has a shelf life of 6 months so maybe your experiments are confirming this. Thanks for the info.

[Cavpilot2000]

I don't have a DO meter, but I (probably like most of us) am keenly interested in any further experimentation to confirm or deny the 6 month thing (or any information related to reduced effectiveness over time).
I am now thinking it was a waste to but the pound bag, since it will probably still be around long after it has stopped being effective.

[lupulus]

Seems extremely unlikely.
I suggest measuring it with a sulfite test kit or at least strips.

[Owenbräu]

My SMB is a year old and when I test my wort still measures out the same as when I bought it.

[Bilsch]

[Futur]

From the dow corning website:
Solid sodium metabisulfite has a typical shelf life of 4 to 6 months under cool, dry storage conditions.

SMB reacts with air and reduces to SO2 as well as being hygroscopic in the solid form. So depending on the storage conditions the ability of your SMB to reduce O2 will be reduced based on these factors.

[Futur]

[Bilsch]

I find your results on the age of your meta interesting as I have very old lab samples myself, circa 1990's, that seem to perform as well as new. Possibly atmospheric conditions and the quality of the container have the largest effect on stability. FYI the humidity level where I am is rarely over 20%.