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

[jereece]

When I close my pool I do raise the chlorine concentration to about 8ppm. However I use bleach as my only source of chlorine. During the season, my pH never needs adjusting. Well that is once I get it right at opening. But my point is that bleach does not affect my pH over the course of the swimming season so I would not think it would affect the pH over the winter. The only thing I can think of is the water on top of my cover. I need to check into that.

Thanks for the information.

Jim

[jereece]

Here a new wrinkle in this problem. I tested the water on top of my winter cover and the pH is 7.9! So the water in my pool (under the winter cover) was 6.4 and the water on top of my winter cover is 7.9. The water on top of my cover is fairly clear even thought there are leaves under the water at the bottom of the cover.

After the discussions below, I was thinking the pH of the water on top of my cover would be low and hence leached into the pool water underneath. Obviously this is not the case. This problem began only a couple years ago. Prior to that, I never had this problem. I am still treating my pool water the same (bleach for chlorination), it's the same cover. I'm still perplexed as to why my pH would drop so low over the winter but during the summer be steady as a rock.

Could algae cause this? It seems like last year I had what appeared to be black algae on the bottom of my pool when I opened last year. It was a ring a couple feet wide around the circumference of the pool. Wiping with a Miracle Eraser easily removed it and I did not think about it again. I have not looked this year as I have not removed the cover yet. But can black algae cause the pH to drop? That's the only change I have noted over the past couple years.

I appreciate your thoughts.

Jim

[chem geek]

I am still perplexed. Even bacterial conversion of CYA to ammonia would tend to make the pH rise, not fall. The only thing that has the pH fall is when chlorine gets used up or consumed -- oxidation of ammonia or other organics is an acidic process. However, if you add a hypochlorite source of chlorine, then that exactly counteracts that and in fact has a small net pH rise due to the excess lye in bleach and chlorinating liquid. If Trichlor or Dichlor were being used, then a pH drop would be expected.

How about your TA level? Has that dropped some as well? An acid would drop both pH and TA.

[jereece]

Actually since my pool is AG I learned not to worry about alkalinity. Testing showed that as alkalinity increases the greater my pH would fluxuate. With alkalinity low, pH is stable as a rock. See this string of posts - http://www.poolforum.com/pf2/showthread.php?t=7870. I have not checked it in years but am confident my alkalinity is very low.

I added 2 boxes of Borax and pH has come up to normal.

This has me perplexed as well.

Jim