High pH, but low TA...how is that possible?

[TreeFiter]

So I came across a few pools that had high (at least on the high side) pH, but relatively low TA (40). If I'm not mistaken, the pH will only rise if the TA is high enough to shift the equilibrium in that direction. At lower TA, the pH should either remain stable, or fall when an acid is introduced. The TA might be sacrificed when the acid is introduced, but in that situation, the pH and TA would both be on the low side. It seems to me that some other alkaline material would have to be added to raise the pH that wouldn't be part of the Carbonate/Bicarbonate reactions involved in buffering pH. I mean if we added Sodium carbonate to raise the pH, the TA would rise with it. And if the pH increase were the result of aeration, wouldn't the rate of off-gassing decrease as the pH increases, ultimately limiting the pH rise and the TA loss?

 

[chem geek]

As you note, pH can rise from other sources besides carbon dioxide outgassing. Plaster curing or degradation can introduce calcium hydroxide into the water that raises CH, TA and pH or if curing properly by exchanging hydroxide for carbonate then the CH and TA do not change but the pH rises (i.e. this looks the same as carbon dioxide outgassing). Also, if one has an SWCG pool, then undissolved chlorine gas outgassing will have the pH and TA rise. If there is aeration, then there can also be hypochlorous acid outgassing causing both pH and TA to rise. Lower quality chlorinating liquid or bleach may have more excess lye in it leading to a more noticeable pH and TA rise.

 

[TreeFiter]

The pools I noticed this in, were liner pools, so it wouldn't have to do with plaster curing or degrading. As far as I know, none of them have had liquid chlorine added recently, and they weren't aerated any more than what I would consider normal. Its also entirely possible that I'm getting bad readings from the test strips, but I tested a few times to verify what I was seeing, and the same bottle of test strips seemed to be giving typical results everywhere else.

 

[chem geek]

I suspect it's the test strips though keep in mind that if the pH were already up for whatever reason, it's not going to fall back down without acid even if the TA is low. This is because the equilibrium pH with air is still rather high except at very, very low TA. Specifically, the following gives the equilibrium pH at various TA. Fortunately, the pH rise slows down a lot before getting to equilibrium.

TA ... pHeq
20 ... 7.75
40 ... 8.04
60 ... 8.21
80 ... 8.33
100 . 8.42
120 . 8.49

 

[TreeFiter]

So based on those numbers, its very possible to see what I saw, especially if the pool has been aerated well. Would heavy rain create enough aeration to create a situation like this? One of the pools was at a pH of about 7.8 and TA of 40.

 

[chem geek]

We've seen pools that rise in pH due to heavy rains, probably because of the increased aeration as you point out.