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Thread: pH color changes with time

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    Richard320's Avatar
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    pH color changes with time

    This doesn't happen every time, but often enough that it makes me wonder what's going on.

    Yesterday, FC = 9, pH=7.5+ a little. Added 2 cups MA, pH should have gone down to 7.2

    Today I scooped up my sample, and checked pH. Perfect match to 7.2. I would have expected it to be just a little above 7.2, knowing the way my pool runs. FC= 8. Go to empty the two tests out and pH is now 7.5. Not even 2 minutes has elapsed, all I did was the FAS-DPD test.

    I could see things changing if it sat in the sun for an hour or something, but not that fast. And as I said, it has happened before a few times. When it does happen, I treat it as the later reading, and so far, it has reacted the way it should. The reagent is fresh, no more than a couple months old.

    Does this happen to anyone else? Does anyone (chemgeek??) have a clue why it is so?
    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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    JasonLion's Avatar
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    Re: pH color changes with time

    Various things that can shift the PH happen far far more quickly in the vial than they do in the pool. CO2 and chlorine both outgas far more quickly and the contents of the vial are much more sensitive to sunlight.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    tlwilson's Avatar
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    Re: pH color changes with time

    It happens to me too, especially when I put water in the sample bottle. If I dip the testing tube in the water and rinse it really well, then dip it down into the water the length of my arm, the test is pretty consistent. When I use the sample bottle, the first time I get 7.2 to 7.4 and then subsequent tests I get 7.6 to 7.8. I'm getting tired of testing twice and using double the reagant every time I test pH, so I've started dipping directly out of the pool.
    24,000 Gal In Ground pool
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    Vette's Avatar
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    Re: pH color changes with time

    I actually find it easier to use the bottles!
    11,700 gallon 21' round 54" deep all-resin AGP - June 2012
    Waterway 1HP 2-speed pump / 100 sq ft cartridge filter
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    I use and recommend the TF-100 for testing

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    JasonLion's Avatar
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    Re: pH color changes with time

    This is the reason it is important to do the PH and FC tests first when using a bottle of water from the pool. PH and FC will both tend to start to drift as soon as the water is removed from the pool. You want to test them right away before the drift becomes significant.
    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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    linen's Avatar
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    Re: pH color changes with time

    If I may hijack Richard320's thread just slightly...this brings up a question I have had.

    When using the K-1000, it is recommended to read the ph first when doing the tests together. Is this because the OTO test needs a little time to stabilize (react with any CC that is present), is it because ph can start changing from it's real value if it sits (quicker than the OTO test), or something else?
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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