Aeration, pH and Lowering TA

[RaviePoolGirl]

I am quite curious after reading a few posts about the effects of Aeration, pH and lowering TA.

I used to have a saltwater aquarium and there could be such a range in pH just by having the aquarium close to a fresh air source, i.e. an open window. Fresh air takes the pH up and when windows are closed during the night the pH trends back down. (Along with a few other influencing factors as well)

So I have been reading that to lower Alkalinity, reduce the pH low to 7.0-7.2 and aerate the water to bring the pH back up. So my question is "Once you end aeration at a pH of 7.6 or so, wont the pH just go back down to the value from the Muriatic Acid addition? :?

 

[linen]

So I have been reading that to lower Alkalinity, reduce the pH low to 7.0-7.2 and aerate the water to bring the pH back up. So my question is "Once you end aeration at a pH of 7.6 or so, wont the pH just go back down to the value from the Muriatic Acid addition?

Yep you got it. Then you repeat and repeat until your TA is down where you want it. It usually takes several cycles of lower ph and aeration. At that point you stop bringing the pH down so low and hopefully your pH will not move much. Did you read: http://www.troublefreepool.com/pool-school/lowering total alkalinity

 

[RaviePoolGirl]

Ok, I did read that thread, but read it again and went to the paper that was published to find the answer. Aeriation blows off the CO2 and CO2 does not reenter the water (essentially).

Thanks :-D

 

[chem geek]

Yes, that's right because the water is over-carbonated with respect to air. There is more carbon dioxide in the water than the equilibrium amount there would be when exposed to air. A lower pH makes that difference greater and essentially increases the rate at which the carbon dioxide outgasses. Aeration also speeds this up and that includes wind. The thing to keep in mind is that the outgassing of carbon dioxide raises the pH with no change in TA while the addition of acid lowers both pH and TA. So combining aeration (especially at low pH) with acid addition has the net effect of lowering the TA.

As for our aquarium, the pH trending back down when the windows were closed meant that there were some net acidic chemical processes going on in the water. If you didn't have fish, plants, etc. in the water, then the pH would rise with the breeze from the day but would not go back down at night. For typical TA levels in water near 80, the equilibrium pH where the amount of carbon dioxide in the water is in balance with that in the air is at around 8.4. In practice, the rate of outgassing slows down before then so that things slow down considerably around 8.0 to 8.2 even with a lot of aeration.