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Thread: Question about the phenol red pH test

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    stev32k's Avatar
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    Question about the phenol red pH test

    The TFP100 test kit does not say anything about using thiosulfate to kill the chlorine before checking the pH. All other test kits I've used in the past always had you adding a drop or two and I believe the indicator was always phenol red. I'm curious why the TFP kit does not do this?
    20' x 40' IG with vinyl liner volume approx. 35,000 gal.1.5 H.P. main pump, Polaris 280 cleaner W/ 3/4 H.P. booster pump
    Hayward sand filter, 3.14 sq ft, 62 gpm. Stenner 45 MPH10 chlorine feed pump. 1 micron final filter bags (home made and very effective)

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    JasonLion's Avatar
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    Re: Question about the phenol red pH test

    The PH reagent in the TF100 already contains the PH neutralizer, so you don't need to add it separately. The test is reliable up to FC levels around 10, and only off by a little with FC into the high teens.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    stev32k's Avatar
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    Re: Question about the phenol red pH test

    OK, thanks. I guess if I use another phenol red I need to use the thiosulfate. That's what I was wondering about.
    20' x 40' IG with vinyl liner volume approx. 35,000 gal.1.5 H.P. main pump, Polaris 280 cleaner W/ 3/4 H.P. booster pump
    Hayward sand filter, 3.14 sq ft, 62 gpm. Stenner 45 MPH10 chlorine feed pump. 1 micron final filter bags (home made and very effective)

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    AnnaK's Avatar
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    Re: Question about the phenol red pH test

    Oh %$#@! That translates to golldurnit, Tiz

    I ran out of the TF pH test reagent earlier this season and bought a bottle of phenol red at the pool store. Does this mean my pH measurements have been wrong all this time?

    If so, wrong which way, too high or too low? It's reading 7.5.
    — AnnaK —

    12,000 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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    JasonLion's Avatar
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    Re: Question about the phenol red pH test

    All of the PH test reagents manufactured by Taylor, which includes TFTestKits and HTH, have FC neutralizer built-in. Of the remaining brands, some include FC neutralization and others offer a separate reagent. When the FC level is high enough to exceed the amount of neutralization, the test will read far higher than actual. The indicator is converted to chlorphenol red, which reads a different, much lower, PH range. Even when there is enough neutralization, if the FC level is fairly high, the test can read a bit higher than actual, since the reaction of chlorine with the neutralizer will raise the PH.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    AnnaK's Avatar
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    Re: Question about the phenol red pH test

    The reagent bottle says Hydropool and Phenol Red. I bought it because I thought all phenol red pH test indicators are created equal. Equally?

    My FC runs between 3 and 6 and the pH reads at 7.5. TA is 70 and CYA might be at 40 now. The water is clear and sparkles and does not dry our skin. Something is right even if it isn't the pH

    I could do a pH reading comparison with one test using plain phenol red and to the other I could add a drop of the neutralizer from the TA test. In fact, I think I will.

    But this brings up another question - and I hope it's not perceived as a thread hijack.

    Why do we want to neutralize the chlorine when we do a pH test? Doesn't that produce an artificial pH result?

    Oh! Wait! I think I get it!

    In a normal pool the FC varies from day to day and within any given day. Even when we do our testing pretty much at the same time and from the same spot every day we should still anticipate a varying FC amount. By neutralizing the FC we are creating a standard base for measuring pH. And because pH is an enormously complicated entity we work within an acceptable pH range when we evaluate and treat our pool water.

    In my case, assuming the indicator I'm using does not have neutralizer, the reading of pH=7.5 might be higher than actual but since the water 'feel' is excellent and the TA buffer is holding steady, I guess I don't really have to sweat it. Right? Wrong?
    — AnnaK —

    12,000 gal AGP, Hayward sand filter, Pentair 2-speed pump, timer.
    Please visit our Pool Issues pages for information about step weights, managing the solar cover, and PoolSkim.

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    JasonLion's Avatar
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    Re: Question about the phenol red pH test

    You need to neutralize the chlorine because it will react with the phenol red indicator to create a different compound, chlorophenol red, which behaves differently. Chlorophenol red is also a PH indicator, but over a PH range from 5.2 to 6.6.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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