Raising pH through Aeration.

[TreeFiter]

Is there a way to calculate how much you can raise your pH by aerating the water based on how high your TA is? Once in a while I run into a situation where the TA is on the high side and the pH is low for whatever reason. It would be nice to be able to estimate how much the pH can come up by aerating rather than put more chemicals in a pool.

 

[JamesW]

There are multiple factors that determine how much the pH will rise with aeration. Things such as starting pH, temperature, TA, cyanuric, amount of aeration etc.

Usually the amount of carbon dioxide in the water and water temperature are the most important factors.

 

[chem geek]

I'd start by looking at this table. That tells you how over-carbonated the water is with respect to air. In addition to that, the rate of outgassing appears to be proportional to the square of the TA level and that's not shown in the table. Without any natural aeration sources (waterfalls, spillovers, fountains), I'd figure that getting from the red section to the orange section will be fairly easy and worth it; getting from orange to green will take longer and may not be worth it; and being already in the green will take effort so probably not worth it (assuming the pH seems to be fairly stable already).

The problem is figuring out how quickly the pH will rise with a given amount of aeration. As James noted, the pH buffering in the pool is a factor and you can get a rough idea of that amount of buffering (so resistance to pH change) by carbonates, cyanurates, and borates in this post, though it is really just saying that you'll have a harder time getting the pH up when the CYA is higher and when there are borates.

 

[JamesW]

Note that the important part of the TA is the carbonate alkalinity. So, you would subtract the other forms of alkalinity such as cyanurate and borate.

 

[TreeFiter]

Thank you both very much.

JamesW, are you saying that I should subtract CYA in ppm from TA in ppm and use that number to determine how over carbonated the water is? So a typical example might be a pool with a CYA of 50 and TA at 120 (My pools don't use borates). The Carbonate Alkalinity would be 120-50=70. Then I could take that number to the chart chemgeek provided and see how readily the pool will off-gas CO2. So based on the chart, for the situation I just described, anything with a pH of 7.3 or higher wouldn't increase due to aeration, unless TA was increased first. Does that sound about right?

I'm not sure I understand how cyanurate adds to alkalinity. How is cyanurate different than CYA?

I'm still trying to connect the dots here. Is there a way to predict the potential change in pH when CO2 is lost? In other words if you move up a given column on the table, what happens to the pH?

I feel fairly comfortable with the idea that for a given pH and TA, I can get a sense for how readily the pH will climb. In the red, it would be hard to prevent it from climbing, orange, should climb if aerated, and in green, probably won't climb without some serious effort. I'd still like to get a sense for just how much the pH could potentially climb. So if I'm in the red, and I aerate, the TA will drop (potentially to where the orange and green meet), but where will the pH rise to?

 

[JamesW]

chemgeek's chart assumes 30 ppm cyanuric acid. How much the cyanuric acid contributes to the TA depends on the pH. Typically, you would subtract about 1/3 of the cyanuric acid from the TA to get the carbonate alkalinity.

In any case, I find that there is a simple rule that works pretty well:

If the pH is constantly too high, then the TA is too high. If the pH is constantly too low, then the TA is too low. When the TA is right, the pH will stay right.