Why does pH drop only in the winter

[jereece]

I have a 27' AG pool. I have been using bleach for about 5 years. Each year I normally have to adjust the pH up a little using borax. However last year and again this year, the pH has dropped out the bottom of my test kit over the winter. I live in NC.

At startup last year it took 2-3 boxes of borax to get the pH back up but then it ran perfect all year with no further adjustments. As normal, I check the chlorine concentration (under the cover) about every 2 months and adding a gallon of bleach if necessary. Once again however the pH has fallen at or below 6.4, the lowest reading on my test kit. I mixed up a box of borax in some water and pumped it in and am using a submersible pump for mixing.

Why all of a sudden does the pH drop over the winter. I don't recall my pH dropping so much until last year.

As always, thanks for the help.

Jim

[chem geek]

'Tis a mystery to me. Is there anything that is getting into the pool over the winter such as leaves (tannic acid)? Something acidic is getting into the pool. Is your rain particularly acidic?

[CarlD]

I believe you've had far more precipitation in NC this winter. I ALWAYS see my pH drop over the winter, which I attribute to acid rain.

[aylad]

My pH tends to drop over wintertime, too--I think it's because I'm not so attentive to the leaves that blow in from my neighbor's yard, in addition to the added rain, as Carl suggested.

Janet

[jereece]

I am using a cover over the pool. Leaves do get on the cover. The cover however is a few years old and I suspect rainwater migrate some from the cover to the pool and vice versa. So I guess this is a possibility. And yes rainfall has been more than normal this year, but this was a problem last year as well. I have not checked the pH of our rain but will do that next time it rains. I will also check the pH of the water that is on top of my cover.

One other question. Since this is an A/G pool I do not worry about alkalinity. I used to think that alkalinity stabilized my pH but through testing I concluded it does not...at least for vinyl liner pools. See this string of posts - http://www.poolforum.com/pf2/showthread.php?t=7870. My question is, would increasing the alkalinity at the time I close the pool help keep pH from dropping or should I just keep a closer eye on pH and adjust ever month or so?

Thanks again for the help.

Jim

[chem geek]

TA is a source of rising pH in its own right. A higher TA means there is more over-carbonation of the pool water and when that carbon dioxide outgasses from the pool, the pH rises. In your case with a pool cover, this effect is almost eliminated so you may not find that the pH rises at high TA levels.

A higher TA also buffers the pH more which means for the same acid or base getting added to the water, the pH change is less. Between this effect and the carbon dioxide outgassing effect described above, the latter is normally more dominant at high TA levels. In your case, with a pool cover, the former effect is the dominant one so a higher TA should have your pH move less. However, when you have to adjust the pH back to "normal", you will still need to add as much acid or base to compensate. That is, a higher TA reduces the rate of pH change, but does not change the amount of acid or base needed to bring the pH back to where you want.

Your significant pH drop is still a mystery, however. Something acidic is getting into your pool (or an acidic chemical process is occurring) and I don't know what that is. When chlorine gets used up, the pH will drop, but I assume you didn't close your pool by adding a lot of chlorine and lowering the pH at that time (normally you just let the pH go up when chlorine gets added).

Richard