Thread now 16 years old and still proving its worth! I too found some, still sealed jug so clearly brand new (if many years old). Used the same test: Cautious open and small pour on discrete concrete - definite reaction 
Friend's pool's pH (found to have drifted high) now being corrected, ready for a little baking soda stabilization.
Friend's pool's pH (found to have drifted high) now being corrected, ready for a little baking soda stabilization.