According to Taylor:
High chlorine or bromine level may cause interference; to prevent, add thiosulfate per instructions prior to testing.
I've actually added a drop of 6% bleach to the 44 ml sample that should have been equivalent to around 70 ppm FC and though it initially turned purple as expected due to the high pH of the chlorine and the interference, it quickly reverted back to its original correct pH color. There is some thiosulfate already in the pH test and apparently it handles rather high FC levels. Quite frankly, I was surprised as I had thought that it might only handle FC up to around 10 ppm, but apparently it does better than that. Also, I've tried adding thiosulfate drops to the pH test and not noticed any shift in pH. I'm going to write to Taylor about this to see what they think.
Richard