pH vs. Aeration vs. Total Alkalinity

[ultimate_ed]

I've been mostly lurking and absorbing info around here for the last several months while my pool was being built and since it has been up (and down) and up again.

I've gotten myself a TF100 kit and am in the process of getting my chemistry sorted out. Water looks great so far.

However, I've got two things I'm going to be fighting, based on what I've read. First, my fill water comes in at a pH of 8.4+ and a high TA to go with it. In addition, by design, between my spa spillway and rock waterfall, my pool gets a lot of aeration - which raises pH.

So, I see that I'm probably going to be buy stock in a muratic acid plant to keep my pH in line. But I'm curious about a couple of points.

Adding muratic acid will lower the pH and TA.

Aeration raises pH but doesn't affect TA

This tells me that if I keep adding acid to counteract the aeration, my TA will keep dropping. So far, my TA and pH are still on the high side, but eventually I should get to a point where my pH is good, but my TA is very low. Is this something that I should be concerned about? I haven't come across anything here that suggests a way to raise TA that doesn't also raise pH.

On a related note - I've read in the school that aeration raises pH. How does that work? And, assuming I ran my waterfall constantly and added no acid, what is the limit of how far aeration alone would raise the pH of my water?

 

[yeggim]

Baking soda will raise TA with a minimal rise in PH. A pound of baking soda will raise TA in 10,000 gallons of water by 7 and PH by 0.01. That's assuming your TA isn't near zero and your PH is near normal. When TA and PH is super low, baking soda will have a greater effect on PH.

 

[chem geek]

This tells me that if I keep adding acid to counteract the aeration, my TA will keep dropping. So far, my TA and pH are still on the high side, but eventually I should get to a point where my pH is good, but my TA is very low. Is this something that I should be concerned about?

No, you do not need to be concerned about a low TA so long as you are compensating for it in other ways. Your pool has a rock waterfall so I presume it might be a plaster pool in which case you may need to raise the Calcium Hardness (CH) level and/or have a higher pH target (e.g. 7.8) to have the saturation index not get too negative. This will protect your plaster surfaces from dissolving. You're a long ways away from that, but if you find you need the TA to get below 70 ppm, then you can see. Also, if the TA has to get very low for your pH to become more stable you can also consider using 50 ppm Borates. Again, something as potential for the future, but not to worry about now.

On a related note - I've read in the school that aeration raises pH. How does that work? And, assuming I ran my waterfall constantly and added no acid, what is the limit of how far aeration alone would raise the pH of my water?

Aeration increases the rate of carbon dioxide outgassing. Lower pH also increases that rate so the combination really accelerates this. When carbon dioxide is in the water, it forms a certain amount of carbonic acid. So when you are removing carbon dioxide by aerating you are removing an acid from the water. Just as adding an acid lowers the pH, removing one raises the pH. As for why the TA remains unchanged, that's more technical.

Aeration of the water will raise the pH theoretically until the amount of carbon dioxide in the water is in equilibrium with the air. This pH depends on the TA level. This post contains a table of carbonate alkalinity (which is TA less the CYA contribution) vs. equilibrium pH. At roughly 200 ppm TA (ignoring the CYA), the pH could theoretically rise to 8.77, but in practice you'd see it at 8.5 or less. You can see that pools are intentionally way over-carbonated. We are just fortunate that the rate of outgassing is relatively slow and that even aeration has a limited effect of speeding it up. If you want to run your waterfall all the time, then you'll likely live a life of acid addition, though can at least get the amount down. You might consider having a timer for your waterfall so that it mostly runs when you are around to enjoy it and not when you're at work (for example) and not in the middle of the night.

 

[ultimate_ed]

Thanks to you both. I actually do have the waterfall on a program so that it runs for 30 minutes a day. However, we also run it whenever we're out using the pool and my wife, who works from home, also likes to turn it on for her self during the day.

I need to put together a sig file with all my vitals, but yes it is a plaster pool with a SWCG. I'm working on getting my CYA up so I can keep the chlorine stable and also getting my pH and TA down.

I've read the write up on borates and I'll probably be joining that club next year.