I found the following on this or another site and saved it . Thanks to the original author.......
"Each reagent bottle has a Lot # printed
on the bottle. The last four digits are the month and year of
manufacture. Example: Lot # 28A0102. This is a pH reagent manufactured
in January of 2002. Pond Care Wide Range pH, Ammonia, High Range pH,
Nitrate, Phosphate, Copper, calcium and GH
all last for three years.
Nitrite and KH
will last for four years. Freshwater pH(low range) and
Pond Care Salt Level will last for five years. I would not trust these
kits after they have expired."