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Thread: The Myth of Adding Acid

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    The Myth of Adding Acid

    The myth: If muriatic acid is poured in a concentrated area, variously referred to as a column, slug, well or cloud, the alkalinity will be drastically reduced, but the pH will not drop as much as it otherwise would. And that if acid is added by walking it around a pool and evenly distributing it throughout the water, the pH will be preferentially lowered, with only a minor decrease of alkalinity.

    The plain and simple fact of the matter is that a given amount (or “dose”) of acid added to a fixed volume of water (the pool) will result in an identical reduction of both pH and alkalinity. Every time. No matter how it is added. That’s the rule, that’s science, and it can easily be demonstrated at poolside by anyone with a test kit. The only real chemical difference between the two addition methods is the time required for the acid to blend throughout the entire pool.

    Now – what really happens when acid is added in a concentrated fashion to a pool? Since acid is noticeably heavier than water, the acid sinks to the bottom, and flows to the lowest part of the pool. If that lowest part happens to include an operating main drain or an operating suction-side cleaner, the concentrated acid dose winds up in the circulation system.

    In fact, we have performed experiments adding acid in this manner and have verified that this method can indeed create conditions where the pH of the water at the bottom of the pool is less than 2.5.

    Although the movement of the water will eventually dilute and blend the acid into the rest of the pool, the initial contact of concentrated acid can potentially etch the plaster it touches, and attack the circulation system it flows through. Even adding several smaller “slugs” of concentrated acid will result in this puddling of acid on the bottom – try adding dye to your acid and watch it. Its fun – as long as you are not worried about uniformly etching the bottom of the pool!

    Of course, etching the plaster and eating the components of the circulation system is generally considered a bad thing… yet the proposed treatment process (“slugging” the acid) recommends this potentially damaging treatment technique, all for the sake of a theoretical, unfounded attempt at chemistry manipulation. Even if it worked (which it doesn’t), would it be worth the risk?

    What is the right way? The best way to manually add acid to a pool is to pre-dilute the acid and to add it by “walking” it around the perimeter of the pool, pouring it evenly, close to the surface, and slowly enough to minimize splashing. When added in this fashion, the acid blends throughout the pool water faster, and the pool is protected from low pH.

    In pools treated with the acid dilution and distribution method, the lowest pH levels measured next to the plaster were in the 7.1-7.2 range. (Repeat – once the acid addition was blended throughout the water, the end-result pH and total alkalinity reduction was identical, no matter how the acid was added.)

    It is time to get this nonsense laid to rest, and stick to what is scientifically sound and that we all know works. And quit risking damage to pools in our care.



    The picture above depicts two quarts of “dyed” acid being added to the deep-end of a pool and with the equipment shut off. The acid puddle remained that way for more than 30 minutes.

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    Re: The Myth of Adding Acid

    Another good post from OnBalance!!!

    I would just point out that TFP generally recommends slowly pouring the acid from a few inches above the water directly in front of a return jet in the deepest part of the pool and then letting the pool circulate for an hour or so. The return jet will help to mix the acid into the water and lower the likelihood of it pooling on the floor and causing problems. A quick run of the brush along the bottom will also ensure the acid is mixed.

    One reason for this recommendation instead of that below, is that it requires less handling of the acid (no pouring and mixing and walking around) so there is a lower chance of getting it on your clothes, skin, or deck.

    Quote Originally Posted by onBalance
    What is the right way? The best way to manually add acid to a pool is to pre-dilute the acid and to add it by “walking” it around the perimeter of the pool, pouring it evenly, close to the surface, and slowly enough to minimize splashing. When added in this fashion, the acid blends throughout the pool water faster, and the pool is protected from low pH.
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    Re: The Myth of Adding Acid

    I agree that slowly adding acid over a return line (with filter pump running) is another appropriate way of adding acid.

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    Re: The Myth of Adding Acid

    Though the TA drops identically based solely on the amount (and concentration) of acid that is added and not on how it is added, the rise in pH might vary depending on how the acid is added. That is, the net drop in pH from adding acid might be somewhat less when the acid is added in a more concentrated way since the locally low pH will outgas carbon dioxide more quickly. This may be what led to the myth in the first place since such acceleration of carbon dioxide outgassing means that one will use less or perhaps no "pH Up" products afterwards (which, of course, counteracts the lowering of TA in the first place).

    Of course, that's no reason to slug the acid. If one wants to lower the TA, then one should lower the pH of the pool overall, but to still safe levels near 7.0, and then aerate the water significantly.
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    Re: The Myth of Adding Acid

    Richard, you have a valid point and I agree with the potential increase in speed of carbon dioxide out-gassing, but since acid is heavier than water and sinks to the bottom of a pool, I am not sure in practice that there would be much of a difference in the rate of out-gassing and pH rise. The acid will blend and distribute into the water at the bottom of the pool and may not affect the surface water much. Even when diluting the acid a little bit and trickling it around the pool (or over a return line), the acid is still somewhat concentrated and sinks to the bottom, and it would also create high amounts (or pockets) of carbon dioxide where it is added.

    Since the solubility of carbon dioxide is around 1600 ppm, and the TA is generally less than 200 ppm, the solubility level won't be exceeded when only one quart of acid (or sometimes two) is generally added at a time. Of course, if one considers adding enough acid to dissolve and etch the plaster surface, then perhaps the solubility of CO2 could be exceeded if enough acid is added. But that would be a lot of acid!

    Personally, I don't have any idea how someone came up with that ridiculous myth. I think any kind of water movement in the pool is going to negate a difference in CO2 out-gassing between the two methods of addition.

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    Re: The Myth of Adding Acid

    Quote Originally Posted by onBalance
    Since the solubility of carbon dioxide is around 1600 ppm
    The solubility of carbon dioxide in an uncovered swimming pool is determined by the Henry's Law Constant for CO2 at the pool water temperature, and by the partial pressure of carbon dioxide in the atmosphere. I am getting a solubility in the 0.53 to 0.57 ppm range.

    In pool water the concentration of CO2 will usually be in the 1 to 10 ppm range depending on the TA, pH etc. because it is in equilibrium with bicarbonate and carbonate. Because it is above its inherent solubility, it constantly off gasses, which causes pH rise. The off-gassing is typically fairly slow as long as the concentration of CO2 does not get too high.

    The concentration of CO2 increases as the pH is lowered and/or the carbonate alkalinity is increased. In areas of lower pH, the CO2 levels will increase and off-gas faster.

    How did you determine the solubility of carbon dioxide to be 1,600 ppm?

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    Re: The Myth of Adding Acid

    In terms of "equilibrium with the atmosphere," you are correct that only about .5 ppm of carbon dioxide (CO2) will remain in the water (given enough time), and depending on temperature, bicarbonate alkalinity, etc.
    In terms of "solubility in water," as much as 1600 ppm of CO2 (at 68 degrees F) can remain dissolved in water without forming bubbles of CO2 and immediately leaving the water within seconds or minutes. Yes, the pH would be very low and somewhere around 4.0. (I don't remember exactly how low).

    I brought up the Solubility issue because of the "old bogus claim" that dumping acid into the deep end of a pool lowers the pH enough (below 4.5), or that so much CO2 is formed, that CO2 immediately (within seconds) bubbles out of the water. My point being that CO2 is fairly soluble and would not (in normal circumstances) form bubbles and bubble out when acid is added in any manner. Another point being that when a high concentration (but below the solubility level) of dissolved CO2 is at the bottom of the pool and hasn't yet blended throughout the pool water and particularly at the water surface, there is no additional loss of CO2 taking place. When the content of CO2 at the water surface finally exceeds the "equilibrium" level, then that is when CO2 begins to out-gas into the atmosphere, and eventually (usually several days or more) may achieve the numbers you quoted.

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    Re: The Myth of Adding Acid

    Ok, thanks for explaining. I think that we can all agree that the slug method has no benefit and can only cause problems.

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