I had this going on another thread, but there were two issues going on simultaneously and to avoid further confusion I wanted to post my latest results on this topic now that I've tried new reagents.
Basically, I'm having trouble getting the titration to "finish" properly when performing the FAS-DPD test for chlorine when the chlorine levels are high (I'll be more specific in a moment). When checking the FC level after shocking with liquid chlorine I have noticed that after a certain point the sample would go from pink to a "pale brown/salmon-ish" color, and would just remain like that no matter how many more drops I added.
Purchasing new reagents (both the powder and titrating solution) didn't change anything
Tonight I checked the FC level with the old reagents and it titrated just fine...at 15 ppm (30 drops). I added a gallon of chlorine, which should have raised my FC to ~25 ppm. After a couple of hours with the pump running, I took another sample and attempted to titrate. I expected to put in around 50 drops. At about 43-44 drops, the sample turned its "salmon" color and never changed. So I tried the new reagents and got the exact same result.
So in the past couple of days I've had successful titrations at 15 ppm FC and 13 ppm FC, and unsuccessful titrations (permanent salmon color, never going colorless) with expected FC levels of ~33 ppm and ~25 ppm.
My experience seems to agree with what the Taylor people say (this test is good up to 20 ppm FC), but I know that everyone agrees that the test should work for FC levels up to 50 ppm. Before I first shocked a couple of days ago, my other levels were:
CC 0.5 or less
CH normally ~350, haven't measured in a few months
So I guess I'm stumped. The good news is that 99% of the time I'm checking FC levels in the 4-8 ppm range so the test should be no trouble as usual. If anyone can think of anything else I should try in order to solve this mystery, please let me know.