I bought the 1/4# of DPD powder a couple years ago and keep it in the fridge along with a bunch of other bulk refills I purchased. For the DPD powder, I may open the 1/4# container once a year to refill smaller containers that I've accumulated. Keep it cold and dry. I posted somewhere that I'm running a test to determine how degraded DPD powder affects test accuracy. Final check will be done soon at end-of-season. From years of doing color photo processing as a hobby I found the fridge is the best place to keep chemicals. Color processing is extremely sensitive to chemical changes and have used chemicals, papers, and films over 2 years old with no noticable changes. Paper and film in the freezer, chemicals in the fridge.
Al
Edit: here's the thread. Stay tuned for more info.
http://www.poolforum.com/pf2/showthread.php?t=4659
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