Forgot to mention this in all my worry about measuring CYA... and I've developed the habit of checking pH first, FC second, then TA, & so forth. For some reason, while I was doing all my testing yesterday, I did the FC first. The vial turned clear, I added the CC reagent, vial turned clear again. I got my results, and simply set the vial aside, rather than immediately dumping the water like usual. I did the TA test. When I finished with that one, I looked at the chlorine vial, and the contents had turned pink again.
Is this normal? I am very careful to make sure the vial turns absolutely clear on the FC and CC tests, and almost always compare it to a sheet of paper to ensure any hint of the pink is gone. Why will it turn pink again after sitting for a few minutes? Are all my test results now questionnable?