TA high, crazy scaling in just 1 day

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
chem geek
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Location: San Rafael, California

Postby chem geek » Thu 20 Sep, 2007 23:13

Dumping acid in one place in an attempt to lower the Total Alkalinity (TA) is known as the slug method or the acid column method and is debunked in this link. Lowering the pH via addition of acid will ALWAYS lower the TA no matter how you add the acid. pH and TA move together when acid or base is added. It's not that dumping acid in won't remove any carbon dioxide at all -- it's just that it is far more efficient to combine aeration with the approach and to lower the entire body of pool water to the lower pH so that the entire surface of the pool participates in the outgassing.

The only way to lower TA is to drive carbon dioxide out of the pool and that only happens to any significant degree through aeration. It also happens faster at lower pH and higher TA levels as shown in this chart showing relative outgas rates as a function of pH and TA. This post describes the fastest way to lower TA by the combination of aeration at low pH with frequent acid addition to keep the pH low. One can, of course, just let the aeration make the pH go up higher and add the acid less frequently, but the process will take a little longer as the fastest outgassing (removal) of carbon dioxide (and carbonates) from the pool occurs at lower pH.

On the other hand, I agree with you that if you dump a gallon of acid into a 15,000 gallon pool that starts out even at a pH of 7.8 with a TA of 120 (and CYA of 30), you will end up with a pH of 6.67 and a TA of 87 and will much more rapidly outgas carbon dioxide which will make the pH rise with no change in TA. So you may end up with a pH of 7.2 to 7.5 with a TA of 87 after such a large acid addition and giving it some time for the outgassing (a pool cover will inhibit that pH rise, for example). So in that respect, I agree with you, that a large acid addition will end up lowering the TA (and be too low in pH, especially for vinyl, as well), but I disagree with the mechanism. It has little to do with adding it in one place or distributing it around the pool. It mostly has to do with lowering the pool's overall pH so low as to drive off carbon dioxide.

Also, in pools with very high TA, they are already outgassing a lot of carbon dioxide and having the pH rise even at moderate pH, so in that situation just adding acid to get the pH down to around 7.2 or so and then letting it rise back up again will work to lower TA. When the TA is lower, however, then getting it even lower is much easier to do by forcing the pH lower to 7.0 and, most especially, through adding additional aeration. An air compressor works really well as do waterfalls, fountains and other aerating water features, but even pointing up the returns helps. Those pools with an SWG already have lots of aeration due to the hydrogen gas bubbles produces in the SWG cell (you can see them at night if you run the pump and SWG with a pool light on) and is the reason most SWG pools tend to rise a lot in pH. Having a lower TA helps reduce this problem in such pools as does using 30-50 ppm Borates as an additional pH buffer and algaecide (which lets one lower the SWG output since less chlorine is consumed preventing algae growth).


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