Testing pH with high chlorine levels

[colcha]

It is often said that testing pH when FC levels are above 10ppm is likely to give unreliable results.

Can this issue be overcome by first adding Thiosulfate to the test solution to neutralise the chlorine?

 

[chem geek]

Unfortunately no because thiosulfate solutions themselves are high in pH and their effect will vary depending on the TA of the water sample. If your high FC is also associated with high CYA levels, you may be able to measure the pH by doing so quickly since the reaction of chlorine with the phenol red dye to form chlorophenol red is not instantaneous.

The Taylor pH tests have a special mixture of different chlorine neutralizers that in combination have a fairly pH neutral effect so give an accurate reading until the FC gets high. How high depends on the TA and on the active chlorine level (FC/CYA) and how long you wait. There's not a simple answer.

We usually recommend lowering the pH before one does a SLAM or elevates the chlorine level significantly for any reason.

 

[Richard320]

Pictures of what happens! http://www.troublefreepool.com/threads/58834-Accurate-pH-test-during-shock-levels-with-R-007

 

[colcha]

Thanks for the information Chem Geek & Richard320. Some really interesting points there. I hadn't realised that the Phenol Red sold by Taylor already contains chlorine neutralisers.