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Thread: TA PPm Calculation

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    TA PPm Calculation

    Does anyone know/understand how the "poolcalculator" figures out TA concentrations? I am having trouble duplicating the TA result generated by the poolcalculator. Given a volume of 20,000 gallons (US) and 0 ppm TA, the calculator generates a result of 563 oz (35 lb) of baking soda required to reach 120 ppm. However, (35 lb) / (20,000 gal x 8.35 lb/gal) x 1,000,000 = 210 ppm. Does anyone know how the calculator arrives at 120 ppm?

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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: TA PPm Calculation

    Welcome to TFP!

    By convention, TA is measured as ppm of CaCO3, even though that isn't what you are adding to raise TA. The difference between Ca and Na are ignored, as they have no real impact on the chemistry.

    To adjust the molecular weight you multiply by the weight of CaCO3 (84 ish), divide by the molecular weight of NaHCO3 (100 ish) and then there is another factor of 2 (which I always get wrong, but roughly speaking that is why they call it bi-carbonate).
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    Re: TA PPm Calculation

    Ah, I knew there was a molar ratio involved! So, TA is really a measure of the carbonate ion concentration, that's why you divide by 2 at the end. Thanks!

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    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: TA PPm Calculation

    Yes, carbonate ions can accept 2 protons so count twice towards TA. So you need half as much carbonate as bicarbonate in molar units.
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