Lowering Swimming Pool Alkalinity -- A Step by Step Guide


Caution: do not try to mix and match bits and pieces of this process. If you don't trust me enough to try the whole thing, just leave it alone for now. Mixing and matching can produce a variety of unsatisfactory results, including pool damage! As Yoda said, "do, or do not"!

Requirements, before you start
  1. A REAL reason to mess with this.
    Do not decide to lower your pH just because the numbers don't match somebody's pool book!

  2. Your VERY OWN reliable test for either CA (carbonate alkalinity) or TA (total alkalinity).
    Strips are not ideal for this. Get a Taylor or Lamotte drops-based TA test if at all possible. You may find one of these under another brand name at Walmart or Home Depot.
    --  Dealers WILL screw you up, especially on this. Do NOT try to do this via dealer testing!! --

  3. A way to aerate your pool.
    Possibilities include
    • a spa jet in your steps (VERY effective!),
    • a fountain connected to a jet, aiming a jet so that it 'riffles' the surface,
      filling the pool slowly with a nozzle set to force bubbles into the water,
    • a party of 10 year-old boys splashing around wildly!
  4. A way to lower the pH.
    Muriatic acid is best for your pool, but harder to handle due to fumes. Dry acid or dilute sulfuric acid are OK on vinyl or fiberglass pools, but less so on concrete or plaster pools. (Regardless of type, use gloves, eye protection, and rinse up any spills promptly.)

  5. A willingness to be persist and patient.
    Unless you have a spa type jet somewhere on your pool, this may NOT be a fast process. You must be patient; you can not hurry the process; it will come to a complete halt if you cover the pool with an gas tight cover (like an automatic cover). Mesh covers are fine. Depending on your ability to aerate, and how much CA you need to remove, this will take hours (with multiple spa jets) to weeks.

  6. Ability to keep your mouth shut around pool dealers!
    This process is SO contrary to everything in all published pool literature that it drives dealers nuts. Do NOT antagonize them with this; let me antagonize them for you!
    (Remember: you still need your dealer for pool parts, even if not for chemical help! Making him your enemy is not in YOUR best interests.)
  1. Test your ALK, and record it.

  2. Decide what your target ALK is.
    If your ALK is high, due to some past dosing error, target 120 ppm, unless you have some reason to go lower or higher. If you anticipate a continuing problem with high ALK, say due to high alkalinity fill water, target 100 ppm. (These are not magic numbers -- if you have a reason, and understand your options, other values can work as well or better!)

  3. Use small doses of acid
    to lower your pH to just ABOVE the lowest value on your testkit -- or 6.6, whichever is higher. Do NOT lower the pH to whatever the lowest value on your kit is! This is really important! In many cases, if you try to do so, you'll  actually end up with a much lower pH, which can damage your pool. Instead, make sure that your pH is at least a little higher than the lowest pH value your kit shows. Also, do NOT try to lower your pH all at once; for all sorts of reasons, it's important to do it gradually!

  4. Begin aerating.
  5. Test your pH AND your alkalinity.
    Continue adding acid to MAINTAIN your low pH until your alkalinity reaches your target value. Once it does,  continue aerating, but allow the pH to rise to the normal level. (If it doesn't rise on it's own after several days, you can add small amounts of borax -- NOT baking soda or soda ash -- to bring it up.)

  6. Once both pH and alkalinity are at target levels, stop aerating.

How it works

(You don't have to know this part, to use the method.)

The portion of your TA that you want to lower is the CA -- carbonate alkalinity -- which is composed of the bicarbonates (-HCO3) and carbonates (=CO3) in your water.

These two ions are part of the carbonic system in water, which actually has FOUR components:

Each of these components can be converted into any of the others; what determines how much of each there is of each, is the pH. At pH levels above 11, all of the 'carbonics' present are in the form of carbonates: there is NO bicarbonate and NO carbonic acid. At pH levels below 4, all the 'carbonics' present are in the form of carbonic acid + carbon dioxide, with no carbonates and no bicarbonates. You can monkey around with the alkalinity test results, simply by changing the pH . . . but not matter what you do, once you restore the pH, all the original alkalinity will reappear . . . unless you get rid of some of it, somehow. and physically remove it from your pool.

There are two . . . and ONLY two . . . ways to do this.

This process does the second: it converts some of your alkalinity into carbon dioxide, by lowering the pH, and gets it OUT of your pool, by aerating!

When you aerate your pool after a portion of your CA has been converted to carbonic acid (ie, by lowering the pH to 7.2 or lower) then gas laws come into effect. Contrary to what most people think, air has only TINY amounts of CO2. When you bubble air, with little CO2 through water with lots of CO2, the stuff follows the gas laws, and the CO2 levels try to equalize, so the concentration of CO2 in the bubbles is the same as the concentration in the water.

Now, you've actually done something; CO2 aka carbonic acid, has left the pool.

Because you've lost acid (carbonic acid, via carbon dioxide), the pH will rise, but this time, there's no carbonic acid to convert back to CA!

If your CA / TA level is not what you need to be, then you keep adding acid, so the pH will stay down:
. . . so the lost carbonic acid will keep being replaced with newly converted carbonic acid
. . . so the CO2 will keep gassing off
. . . so the CA will keep dropping.

When your CA gets where you want it, just stop adding acid, but keep aerating till the pH returns to the 'normal' level.


I wrote the original version of this page sometime before 2004 and posted on an earlier version of the PoolForum. After it crashed, I later recovered this page from Google's cache pages. When I did so, I also found the following post by "frayedends" which I hadn't seen before, but thought was interesting:

"This process by Ben is exactly correct. This is the same process we use in the Biotech industry to get the correct concentrations of CO2 in our media solutions. We basically add acid to lower pH and then mix the heck out of it until we have the correct gas concentrations. It makes so much sense that I would love to hear a pool store person try to argue with it. Great explanation Ben. Thanks!


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