pH test accuracy with high chlorine levels?

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
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.



TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
Somewhere Taylor says the PH test is compensated up to a FC level of 15. There have been several reports of higher than actual readings with FC between 10 and 15, but it is difficult to get anything reliable to compare to so it isn't clear if those reports are accurate. There have been somewhat more reliable reports of dramatic PH reading errors with FC in the mid 20s and higher.

The PH sensor in my Total Control system agrees with the Taylor test at very low FC levels (below 2) and then I see small differences (Taylor .1 higher) with FC around 5 and larger differences (Taylor .2 or .3 higher) with FC around 10. I have no way of knowing which of them is right though.