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Thread: Testing pH with high chlorine levels

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    Join Date
    Jan 2015
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    Testing pH with high chlorine levels

    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?

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    Re: Testing pH with high chlorine levels

    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.
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    Richard320's Avatar
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    Re: Testing pH with high chlorine levels

    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
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    Re: Testing pH with high chlorine levels

    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.

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