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Thread: Chlorine at 10 ppm, Testing pH Question

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    Join Date
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    Chlorine at 10 ppm, Testing pH Question

    If the chlorine is at 10 ppm, will it affect the reading in the pH test. I did Google this and found an old post stating that Taylor's "Phenol Red" test seems to be compensated for up to 15 ppm of chlorine, is this correct?

    If not, would adding 7 drops of Thiosulfate from the Taylor's alkalinity test help? My understanding from the same post is it would not help and could skew the reading, the same post does not state why it will not help.

    Thank you to those that might be able to answer the question.
    10,000 gal plaster pool,3/4 hp WhisperFlo pump, Sta-Rite Great White GW9500 pool cleaner, Hayward 300 lb Sand Filter
    Use Magenta Stuff for Iron and Silica control.
    Balance: pH 7.7 Cl 7 -8 Alk 70-80 CH 325 CyA 30 in winter - 50 in summer NaCl 1010 TDS 1200

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    JasonLion's Avatar
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    Re: Chlorine at 10 ppm, Testing pH Question

    The Taylor PH works quite well at an FC level of 10. It might read a little high, less than 0.1 higher than actual, in that situation, but that is generally an acceptable error. Taylor says in various places that the PH test is good up to an FC level of 10, and that it is good up to an FC level of 15. In practice, it seems to get further and further off as the FC level goes up until it suddenly goes wildly wrong at an FC level in the very high teens or low 20s. As long as you are aware that it can be a little higher than actual in the FC at 10 to 15 range it is very usable.

    Using Thiosulfate is not a good idea. The thiosulfate will neutralize the chlorine, but it will also change the PH. That PH change might be quite significant, or it might not be, depending on several other factors, so it is difficult to compensate for.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: Chlorine at 10 ppm, Testing pH Question

    JasonLion,

    Thank you for the quick reply and great news, that puts my mind at rest very quickly.

    Aloha.
    10,000 gal plaster pool,3/4 hp WhisperFlo pump, Sta-Rite Great White GW9500 pool cleaner, Hayward 300 lb Sand Filter
    Use Magenta Stuff for Iron and Silica control.
    Balance: pH 7.7 Cl 7 -8 Alk 70-80 CH 325 CyA 30 in winter - 50 in summer NaCl 1010 TDS 1200

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