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Thread: What exactly is TA

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    What exactly is TA

    I have extremely high TA fill water @ 340. I'm wondering what exactly is TA? And what chemical reaction takes place for TA to raise pH. I'm having trouble understanding why if you add acid why it lowers pH fast but TA very slowly?
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    Re: What exactly is TA

    Yep, I find that confusing too, so some clarification from the experts would be much appreciated - in language that we can understand please
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    JasonLion's Avatar
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    Re: What exactly is TA

    TA can be thought of as a measurement of how much acid it takes to lower the PH by a fixed amount. Most of TA in swimming pools comes from a bicarbonate buffering system that resists PH changes. As you add acid it uses up the buffer, so PH changes significantly, and the amount of buffer present goes down more slowly.

    The increase in PH due to high TA plus aeration results from the outgassing of CO2. A bicarbonate buffer will always be party formed by dissolved CO2. If the CO2 outgases, the PH goes up.
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    Re: What exactly is TA

    Total alkalinity (TA) indicates the water's ability to buffer pH changes. Buffering means you need to use a larger quantity of a chemical to change the pH. At low TA levels, the pH tends to swing around wildly. At high TA levels, the pH tends to drift up slowly, or even quickly in extreme cases.

    The rule of thumb is to focus on pH over TA. Think of pH as the main channel knob that allows you to change channels and TA as the fine-tuning knob that brings the selected channel into sharper focus.

    For on-going pool water management, if your pH is in-range and is relatively stable, then I would leave TA where it is. OTOH, if pH swings around a lot between tests, then you should consider adjusting TA (Increase TA if pH is dropping quickly; Lower TA if pH is increasing quickly).
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    Re: What exactly is TA

    Thanks,
    Now I see in pool math that my TA of 340 takes 39oz of acid to make the same pH change that 9.2oz of acid would do @ a TA of 70. That's over four times the acid requirement. So am I correct in assuming that my high TA is doing no other harm other then increased acid usage for the time being and that it should eventually come down just from the acid usage to maintain proper pH?
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    Richard320's Avatar
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    Re: What exactly is TA

    When chemicals dissolve in water, they're no longer the same. The compounds split up into ions, positively and negatively charged and they bounce around and join with other ions. Like a square dance, moving around, constantly changing partners.

    Now at this square dance.... The positive ions are boys, the negative ions are girls. They arrive as a couple, one positive, one negative. Neutral. Balanced. pH is neutral, 7.0. They split up and dance with other people.

    If some of the boys leave to sneak a beer or do donuts in the parking lot, the dance floor starts to become girl-heavy, more negative ions than positive, so pH goes down. If some girls get mad and leave, the crowd gets boy heavy, so pH goes up.

    If the dance is really crowded, say 200 couples, you'd hardly notice if two girls left. pH hardly changes. But if there's only ten couples and two girls leave in a huff, the pH changes dramatically.

    And so it is with alkalinity. TA is the size of the crowd. High alkalinity means that a few ions can leave and there isn't much change to the pH. Low alkalinity means a slight change in the number of positive or negative ions really throws things off. Too high alkalinity means there is no room to dance, so the boys take off, driving the pH up.
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    Re: What exactly is TA

    Quote Originally Posted by tcm View Post
    Thanks,
    Now I see in pool math that my TA of 340 takes 39oz of acid to make the same pH change that 9.2oz of acid would do @ a TA of 70. That's over four times the acid requirement. So am I correct in assuming that my high TA is doing no other harm other then increased acid usage for the time being and that it should eventually come down just from the acid usage to maintain proper pH?
    The only other harm that high TA may cause is if the CH and/or pH are also high since then the Calcite Saturation Index (CSI) can be high which means you can get calcium carbonate scaling.

    If the CH is very low, then you won't get scaling and the only negative result to the high TA is what you described, namely a tendency for the pH to rise (since the pool is significantly over-carbonated which is in part what TA represents) and that it takes more acid to more the pH down compared to lower TA levels.
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    Re: What exactly is TA

    Thanks chem geek, But now that brings up another question. My CH right now is only 40, when I plug my test numbers into the pool math calculator my CSI is -.03 which is balanced. Does this mean that the high TA is counteracting the Low CH from eating away at my plaster?
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    Re: What exactly is TA

    Yes, and no. Technically, yes balanced. However, you don't want CH that low with a plaster pool. The PH can change quite easily, and with CH that low you will very rapidly run into problems. Besides, you will want to lower the TA regardless, which is going to require a higher CH level anyway.
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    Re: What exactly is TA

    Good I was worried about the low CH, I am at the mercy of PB for the first two weeks. But he only comes to check the pool every two or three days, so I have taken it upon myself to monitor pH daily and adjust as needed with acid. What he doesn't know won't hurt me right. If I didn't the pH would be over 8.2 in a day and half. He must think he is doing a great job when every time he comes the pH is between 7.5 and 7.7. I will just wait until my TA get a little more in range and then ease the CH up.
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    Re: What exactly is TA

    That might not be such a great idea if they are doing a bicarbonate startup. For a bicarbonate startup you use very high TA level and PH right around 8.2 to give you a better quality plaster surface. If they are specifically doing that, lowering PH can cause problems. On the other hand if they are doing a normal startup then it is critical to keep the PH down with acid. The high TA suggests they might be doing a bicarbonate startup, even though they are fairly rare.
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    Re: What exactly is TA

    Hmmm, now I must ask him. But I'm pretty sure he is not doing a bicarb start-up do to the only things he added were Sequestrant, Acid, Clarifier, algecide, and trichlor pucks. But I am going to ask him just to be sure.
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