We've had the solar cover on for about 4 weeks steady. Ours is cut into five sections to facilitate moving it on and off. During the day I remove one section, exposing 1/5 of the water surface to sun and air.

Water chemistry has held steady during the cover-on phase and chlorine consumption has been as expected. No change in TA. But there has been a slow downward drift in pH. The pH has been at 7.5 for the entire season, rock steady, but has been slowly moving downward over the past 4 weeks and measured 7.2 this morning.

Why does the use of the solar cover seem to result in a pH drop?

A solar cover blocks CO2 outgassing, so some PH increase you experience without the cover is no longer happening. You must have some other process that is slowly lowering the PH, which used to be more or less balanced out by the slow CO2 outgassing.

I believe Anna uses a mixture of some Trichlor with some bleach / chlorinating liquid (added separately, of course). The Trichlor will be acidic so will lower the pH if there is no carbon dioxide outgassing.

We've had very limited use of the pool during the past month. When the dogs go in regularly it's very splashy since they do what amounts to belly flops 10 - 20 times per day, which certainly produces a lot of aeration. In the absence of this activity I can see where there's been much less outgassing.

Yes, I do use trichlor tabs, about seven 3" pucks every 8 weeks - less than 1 puck per week, approximately, or, over the period where I've observed the pH drift down, a total of maybe 2 pucks. You're saying that this minimal use is enough to have dropped the pH from 7.5 to 7.2 in a month? The CYA increase is not measurable. I read it at between 40 and 50, depends on lighting.


(Edited to correct math)

2 3" 8-ounce Trichlor pucks in 11,000 gallons would raise the FC by 9.1 ppm and the CYA by 5.5 ppm, but would drop the pH from 7.5 to 7.14 if the TA were 100 ppm. Trichlor is very acidic and by itself lowers the pH from 7.5 to 7.34, but the consumption of chlorine then further lowers it even more.

So your pH drop is completely explained by the Trichlor since the pool aeration has been virtually eliminated.

Thank you!

I have noticed what seems to be a solar cover reduction in ph/TA as well. Our area is alkaline, and ph/TA drop has not been an issue until the solar cover went on. I was adding water from evaporation loss weekly and now am not, which is to be expected. Now I am adding a little borax each week to keep ph and TA up. It is not a major drop, and I wanted to add borates anyways, but just did not expect this.
My ph was consistently 7.5 and TA around 80 until I put on the clear solar cover. Now the ph keeps drifting down in a day or two to 7.2 and the TA goes to around 70. I leave the solar cover on all the time except when swimming daily, as we have been having unusually "cool" (for TX) and cloudy weather, mostly low 80s with only occasional low 90s, but 60s at night (I am a wimp- I like my water warm). I use cal-hypo (for now until I run out of the bucket the pool store sold me) then will go to bleach. I would say that I am using BBB, except that until the solar cover went on, I was just using chlorine, did not need any other B's.
Mine is a new pool and I have been working the CYA levels up gradually during this time- now up to 40. My CH is 110 right now, up from a start of 70, and I realize the cal-hypo is bringing it up, but am thinking I am still at a good level to finish the bucket I have. I keep my chlorine levels stable at the recommended FC of 3-5 for CYA and have sparkly clean CC=0 water all the time.
Any thoughts on my solar cover "acidity"?

An addition of 10 ppm CYA would lower the pH from 7.5 to 7.3, but the TA would stay the same. So I can't explain the degree of drop you are seeing unless there are some other acidic sources somewhere.

Ok, fair enough on the CYA. I did add about that much over a week period, so that would explain the pH drop. I will see if it keeps up now that the CYA has stabilized. I still think the solar blanket is contributing somehow, but not sure how.

Your experience reflects mine with regard to the pH drop. My use of trichlor is very minimal, so low in fact that I still need to add liquid chlorine to maintain 5 ppm; I just don't have to add chlorine on a daily basis. The trichlor has trickled in throughout the summer and the pH has stayed at 7.5 during the entire time. Only when the solar cover went on for an extended period did the pH begin to drop, and the change happened relatively quickly - within a few days. Since nothing else changed I have to relate the pH drop to the solar cover rather than to the trichlor.

The breakdown of chlorine by sunlight is an acidic process. Putting on a solar cover would dramatically slow the breakdown of chlorine resulting in a relative increase of PH, all other things being equal.

Increase of ph? How confusing. But, the results are the same- lower ph and TA since the solar cover went on full time, whatever may be causing it.

So how important is keeping pH around 7.6? Is there a chemical reason, or is it a best practice type of thing? The reason I ask is that TA is 80*, yet the pH "insists" on being 7.2. I add borax, and the pH will go up for about a day, then drop back down to 7.2 and stay there. I have added about 10 lbs of borax in the last 2 weeks-2 lbs every few days. The TA was about 75 at the beginning of the borax addition period. The lower pH started right after I put the clear bubble-type solar cover on full time in early Sept. It stayed at ph 7.5 and TA 80 during the summer without the cover or a fountain (pool built in mid-August, but 100 degree plus temps ended almost immediately, and has been strangely mild since).
My numbers:
FC=3 (I keep it in the 3-5 region, adding chlorine every other day as needed in the form of 65% cal-hypo- will go to bleach when the cal-hypo is used up)
CC=0
pH=7.2
TA=80*
CH=120
CYA=45
water temp 83 degrees
The water is clean and beautiful, and I have not had a CC reading in weeks. Nothing else added except we had one rain of 1.5 inches, and my water level was a little low at that point, so I only had to drain about .5 inches. Evaporation is slow with the solar cover, and chlorine use is normal.
Do I really need to worry about trying to raise the pH, or should I just be glad it stays where it does and leave well enough alone?
(*Edited from 100 due to mistest of TA first time- dozed off while counting drops I guess.)

It is important to keep the PH between 7.2 and 7.8. Aiming for 7.6 is a nice goal within that range, but hardly essential. If your PH is actually stable at 7.2 then that is fine. If it tends to drift down below 7.2 there could be problems.

Well, now my pH has stabilized at 7.5 but the TA has been on a slow march upward for the last week to 110. I had a little cloudiness and green tinge start a few days ago as well as massive amounts of pollen due to a mowing incident, so have been shocking, brushing, vacuuming, and backwashing for about 4 days now. The last borax I added was 9/22 and I have been bringing the FC up to 14/16 each evening for the last three days using 65% cal-hypo. Nothing else added into the pool other than swimmers and whatever drops out of the sky. The FC drops to 8-10 by the next afternoon (24 hours) but we have bright steady sun and warm air temps, so that does not surprise me. The green is gone (other than remaining pollen) and the cloudy is about cleared up. And, lesson learned, I am opening the solar cover a few hours each day in the strong sun to let the pool breathe. Anything else I need to do? Should I do some TA rise prevention, or let it ride? We will be probably be swimming for a couple more weeks, but the weather here is unpredictable, so that is plus or minus 2 weeks.