pH, TA, and aerarion. (Was: Can I predict how much acid...)

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pisceanwoman

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Aug 17, 2007
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Rockville, MD
I'm attempting to follow step 1 of waterbear's fabulous how-to on adding borates to pool, but I cannot get the TA down to the target range of 70-80. I did drop initially from about 140 down to 120, but i'm stuck there.

It took me a day to realize that I had to add more acid to "fuel" the aeration (and again, thank you, waterbear, for the amazing explanation on the carbonate ions and how the aeration process lowers TA in TA--What is it really?. This same process occurs in our lungs to maintain our acid-base balance in our bodies - when acid levels get too high, our lungs "blow off" CO2 to bring pH up in the blood.

So, now, I'm wondering, to get from 120 to 70-80, how much acid will be required, and how often will I need to resupply the acid?? I have to work today, so I can't be checking chemistry all day, but the weekend's coming (woo-hoo!)
 
You will need in the neighborhood of 6-7 qts (1 1/2-1 3/4 gallons) more acid to get your TA down. You don't want to add that all at once, however. Add enough to lower your pH to about 7.0 BUT DON"T GO ANY LOWER because you can damage the liner and then aerate until the pH rises to above 7.4. (My suggestion is to add a quart of acid at a time diluted in a 5 gal bucket of pool water, brush the pool, wait about 30-60 minutes and test your pH. Repeat this until the pH is at or very near 7.0 .) Once the pH has risen add more acid to drop the pH back to 7.0 and then aerate. Test your TA each time you drop the pH. When the TA is where you want it aerate until the pH is around 7.5-7.6 and you are done. Then you can start adding the acid and borax.

The nice thing about lowering TA is that if you are patient you just have to drop your pH and wait. The pH will eventually rise by itself. The aeration just speeds the process up. So if you don't have time to do it all at once just drop your pH to 7. You can start aerating when you can pay more attention to monitoring the pH as it rises.
 
excellent - thanks! i've obviously been over-aerating - my neighbors will be happy to have me turn off the fountain overnight!!

i'll get right on it...and, i'm happy to realize that with aeration, i won't have to add baking soda to bring the pH back up to 7.5 before getting to the borate/acid additions - it seemed odd to me that i have to do so much adding of acid and then alkali!

thanks again!
janaki
 
Baking soda is used to raise TA! IF you chemically raise the pH after adding acid you will cause the TA to rise also!

The ONLY way to raise pH without raising the TA is by outgassing CO2 by aeration.
 
thank you for that clarification! i guess that was obvious to everyone, but understanding the henderson-hasselbach equation was alway my weak area in chemistry! in fact, i always preferred O-chem to general chemistry!!

i guess i hadn't realized the usefulness of active aeration!! and i'm certainly learning fast how efficiently it raises the pH.
 
OK, another question then....

Is there a point where one can't raise the pH with aeration? i.e. is there a lower limit to dropping the TA?

I managed to get the TA down to 80, and the pH was at 7.2, so i kept aerating, and now the TA is 70 and the pH just won't budge above 7.2 after aerating for 24 hours....

is it time to add a little soda ash or borax to get up to 7.4 (my goal is to proceed with adding borates as described in: So you want to add borates to your pool
 
Yes, you eventually reach a point where the TA is low enough that PH doesn't want to rise any more. It varies from pool to pool, usually somewhere around 40 to 60. As you approach that point the rate of PH increase from aeration slows noticeably. Give it another day or two and PH will probably be where you want it. If you are in a hurry a little borax to raise PH won't hurt, but I would wait.
 

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