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Thread: pH Keeps Creeping Up

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    pH Keeps Creeping Up

    I refilled my hot tub at the beginning of February with 380 gallons of municipal water (San Diego). I gave it a first boost of dichlor to really sanitize everything - about 2.25 tbsp. The next day, my measurements were as follows: FC = 10.0, CC =0.5, pH = 7.6, TA = 120, CH = 190, temp = 98F. So far, so good. A couple days later, I added 0.7 oz of dry acid to get the pH and TA down a bit, but two days later I was measuring pH at 7.9 and TA at 110. I added another 0.6 oz acid and another 1 oz a day or so later, and a few days later measured pH at 7.6 and TA at 90. Eight days later, the pH is back up to 8.0 while TA remains 90. So, another 1 oz of dry acid brings the pH back down to 7.3. A week later, pH is back up to 8.0 and TA is down to 70! I added yet another 1oz of dry acid and a week later (today) pH is back up to 7.8 and TA is reading 60. Altogether, I have added 5.1 oz of dry acid, which seems like a lot. What am I doing wrong to keep the pH at 7.4?

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    Mod Squad jblizzle's Avatar
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    Re: pH Keeps Creeping Up

    Aeration raises pH. Spas have a lot of aeration. Rising pH is pretty normal.

    Why do you want the pH at 7.4? Why not just keep it closer to 7.8? As the TA lowers, the pH rise should slow down.
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    JasonLion's Avatar
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    Re: pH Keeps Creeping Up

    Another way of saying that is you are keeping your TA too high, and that is causing PH to rise. For a hot tub you want the highest TA where PH is stable, which is typically TA around 50 give or take.
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    Re: pH Keeps Creeping Up

    Ok, that's good to know. I'm not actually keeping the TA too high - it started at 120 "out of the tap" and I have progressively brought it down to 60 with doses of dry acid. So perhaps now the pH will remain more stable if I bring it down from 7.8. The manufacturer of the spa says keep pH between 7.2 and 7.6, so I guess my "ideal" target is 7.4. I'm not averse to keeping it closer to 7.8 if that's where it wants to stay, but it appears to continually creep back up toward 8.0, even with all the acid additions, so I'm just wondering what might be causing that. My last batch of water had a TA that stabilized at 80 and maintained a pH of 7.4 without any of this upward creep, so something seems different. Manufacturer recommended range for TA is 40-120.

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    Re: pH Keeps Creeping Up

    The manufacturer says to keep the pH from 7.2 to 7.6 because they assume your TA will be higher and possibly your CH so are preventing scaling and also they don't understand water chemistry with regard to the chlorine/CYA relationship so they think that the disinfectant capability is reduced a lot at higher pH, but when CYA is present that is not true. There should be nothing wrong with you having the pH get to 7.8 unless you've got metal ions (e.g. iron or copper) that could stain.
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    Re: pH Keeps Creeping Up

    Ok, I guess I'll stop fighting the constant pull to 8.0. My latest numbers are TA = 60, pH = 7.8, CH = 200, CYA ~ 30. Just weird that it seems to be pulled to 8.0 this time...only thing different is my CYA is closer to 30 than 150 with the last batch of water. Oh, and the N2 metal ion system is gone.

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    Re: pH Keeps Creeping Up

    It's the carbonate alkalinity that determines the rate of carbon dioxide outgassing. With TA 60 and CYA 30 and pH 7.8, the carbonate alkalinity is 49 ppm. With a CYA of 150 ppm, the TA would have to be 102 ppm to have the same carbonate alkalinity. So if your TA was lower than that before, then you would need to lower your current TA lower than 60 to be equivalent.
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    Re: pH Keeps Creeping Up

    Ah...that's interesting. As always, thank you for your analysis!

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    Re: pH Keeps Creeping Up

    I now have the TA at 50 and the (latest) pH at 7.4. Perhaps it will stabilize there - we'll have to see. So far, (other than dichlor and bleach) all I have added to the water is dry acid. Given my water parameters, do you think I should add some borax to this tub? If so, when? Only if the pH keeps creeping up even with the TA at 50?

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    Re: pH Keeps Creeping Up

    You can add 50 ppm Borates most easily by adding boric acid you can get at DudaDiesel or at The Chemistry Store. It will most certainly slow down the rate of pH rise. We usually recommend that for spas when using hypochlorite sources of chlorine and having the TA be low at 50 ppm. It provides additional pH buffering that will not contribute to rising pH. See Using Chlorine In a Spa.
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