Alkalinity and pH - I am confused but happy

[chem geek]

Apparently, you had some sort of source of acid in your pool since that's the only way the pH could have gone down (from 7.5 to 7.2). At the lower TA, any source of acid or base will make the pH change more so by lowering the TA you reduced the carbon dioxide outgassing, but also made your pool more susceptible to whatever is going on with it in terms of acid/base addition. So I really don't have any definite explanation, especially for why the pH went down or stayed low for so long. I can only guess it's the "noise" from acid/base external sources. At the lower TA, you see this up/down swing more, but I can't explain the source of such swings.

Just so you know, the tables show the TA, but are at a specific CYA level of 30 ppm. It isn't really the TA, but rather the carbonate alkalinity, sometimes called "adjusted TA", that is important, but so people wouldn't have to calculate that I just used the real TA in the table with a specific 30 ppm CYA level. Your pool, though, sounded like it was near 30 ppm CYA (I think you said 25 ppm).

As for acid sources for your pool, the using up of chlorine is acidic so one needs to compare pH at the same FC level. If one shocks the pool with hypochlorite, for example, then the pH goes up at first when the FC goes up and then comes back down as the FC drops. If you use other sources of chlorine such as Trichlor or Dichlor, then these are acidic. If you use a non-chlorine shock, then that is acidic. The addition of CYA directly is acidic. Rain can be acidic, though you need quite a lot of it to more the pH (but at lower TA it's easier to move). Certain organic matter falling into the pool may be acidic.

Last, but not least, is that the rise in pH is very much a function of aeration so raindrops aerate the pool so can make the pH go up -- so whether rain causes pH to go down from acid rain or up from aeration depends on the rain's pH. Aeration also varies with wind, if it chops up the water. And, of course, can be from more swimming, splashing, waterfalls, spillovers, fountains, etc.

The bottom line, though, is that lower TA has your pool not have the rising pH and regular acid addition problems and that's what is really important.

Richard

[jereece]

Thanks Richard.

Just as a follow-up, my swimmer load has always been just a couple family members, I don't use any kind of fountains, it has not rained in a month, I have not used Trichlor or Dichlor in years. I always take my readings in the late afternoon (shade). I do get a few leaves every now and then, but nothing abnormal.

Is it possible that the decrease in pH after I added borax was due to my new liner? As I stated in my original post, I put a new liner in this spring filled, with fresh water and added CYA to 25ppm. I know the CYA caused the initial pH drop, but after I added the borax could the liner (being plastic) have some kind of ion exchange going on that would cause the pH to decrease until a balance was reached? I used to work in a chemical treatment plant and know that ion exchange resin is made of plastic and even earth can have some ion exchange properties. I was just wondering if something from the liner could have caused this until it reached equilibrium.

Thanks again for your insight.

Jim

[chem geek]

I really don't know, but many chemicals exposed to new environments (i.e. water) will leach out especially initially before reaching more of an equilibrium. We know, of course, that plaster works that way, but that's a specific chemical process of curing. With vinyl, I'm sure it's not so dramatic, but something like that could be happening.

I would say that if many people with new vinyl pools find initial pH dropping or not rising as quickly and then later find it stable or rising faster, then you very well could be right. Otherwise, this is just one more mystery without an explanation.

Richard