Does Rain affect TA level?

[frustratedpoolmom]

Question.... Does Rain affect TA level?

We added almost an inch of rainwater to the pool yesterday from the rain....the TA went from 150 to 160.....PH stayed the same.

 

[chem geek]

No, it shouldn't. If anything, the water would get more diluted and the TA would drop some. I suspect measurement error. A difference of one drop (10 ppm) in the TA test isn't significant. It's certainly not anything to worry about. Your TA level is on the high side, however, so IF you experience rising pH in your pool you could consider lowering it. On the other hand, if you use an acidic source of chlorine such as Trichlor tabs/pucks, then a high TA is appropriate.

 

[frustratedpoolmom]

Thanks for the quick answer, I didn't think so but I was kinda wonderin....

The PH and TA have been steady at those since I opened, a couple small drain/refills to lower the CYA raised the TA a tiny bit. I'll keep an eye on it and try the aeration if necessary. The kids have been active a few times and it hasn't dropped. Does the PH have to be a 7.0 for aeration to work?

 

[chem geek]

No, the pH doesn't have to be low for the aeration to work, BUT at lower pH the aeration works MUCH faster. So at normal pH if you don't see much of a rise in pH from aeration then clearly you won't be adding much acid and the TA won't drop very much after doing so. It sounds like your situation is pretty stable so there's nothing to worry about or adjust at this point. If you do want to lower the TA, then yes, you would lower the pH to make the process go quickly. Technically, in a pool with rising pH, you end up lowering your TA over time anyway -- it just takes longer. The total amount of acid you cumulatively have to add is the same in both cases -- it's just done a lot more in a short time with the acid/aeration procedure to lower TA so that after that you don't have to be frequently adding acid. It's a "pay me now or pay me later" sort of situation. But again, it sounds like you are not in that situation.

 

[Rangeball]

I've found the best time to lower alk via aeration is right before a rain. Rain works great for aeration

 

[KurtV]

Richard,
I understand that the pH of non-"acid rain" is about 5.5 while "acid rain" has a pH of something in the low 4s. Wouldn't either one lower pH and alkalinity?

 

[frustratedpoolmom]

reduce ta through aeration

I've found the best time to lower alk via aeration is right before a rain. Rain works great for aeration

I feel silly for asking but seriously? And does the solar cover need to be off for that kind of aeration? And if you are making fun of me your in trouble mister.... :-D

 

[chem geek]

Acid rain is extraordinarily weak as an acid. Every change of 1.0 in pH is a factor of 10 in the amount of hydrogen introduced because pH is a logarthmic scale. Muriatic Acid has a pH of -1 (yes, the pH scale can go negative -- this is an exceptionally strong acid). So even acid rain with a pH of 4.0 would have 1/100,000th the amount of excess hydrogen as Muriatic Acid. That means it would take 100,000 times as much acid rain by volume as Muriatic Acid to have the same effect on a pool's pH. So let's say that a massive storm had an incredible 1 foot of rain added to your pool. Let's say this is a 16x32 pool is 512 cubic feet of rain which is 3830 gallons. So with a pH of even 4.0, this is equivalent to 3830/100000 = 0.0383 gallons or 0.6 cups of Muriatic Acid. In a 18,000 gallon pool, this would be a pH drop of about 0.1 if the starting pH were 7.5 and the TA were 100.

As for TA, the above amount of pH change from the acid would lower the TA by about 2 so not even measurable. However, the TA of rainwater is very low since it is in equilibrium with carbon dioxide in the air so the amount of carbonates is quite low (especially since the pH is low). Essentially, it is like adding carbonate-free water to the pool. So the dilution of the pool water by 18000/(18000+3830)=82% would reduce the TA to roughly 82% of it's original value. In practice with continuous dilution (with spillover) the reduction is less, but could be 10 or so and measurable. But again, this is ONE FOOT (12") of rain.

[EDIT] By the way, the apparent inconsistent treatment in how I did the calculations above is due to the fact that for pH there is more hydrogen ion in the acid rain (pH 4.0) than in the pool water (pH 7.5) so this is treated as if one were adding more of a substance (hydrogen) to the pool water and I ignored the dilution effect since it wouldn't change the result by much. For the TA, the amount of carbonates in the pool water (around 90 ppm) is far higher than that in the acid rain (less than 10 ppm) so I treated that as a pure dilution. [END-EDIT]

Richard

 

[ktdave]

WOW! Richard, you never cease to amaze me with your wizardry! Were you an alchemist in another life?

 

[chem geek]

Ever heard of Merlin?

 

[ktdave]

I seriuosly thought that's what you'd say!

Keep it flowin'!

 

[Rangeball]

Re: reduce ta through aeration

I've found the best time to lower alk via aeration is right before a rain. Rain works great for aeration

I feel silly for asking but seriously? And does the solar cover need to be off for that kind of aeration? And if you are making fun of me your in trouble mister.... :-D

Yes it works, well for me anyway. The rain drops act just like a fountain or other form of aeration.

I imagine the cover needs to be off for it to work