rising ph levels

[chem geek]

I am really confused now. Are you saying that my pH should not be changing very much? I use bleach for chlorination and muriatic acid to bring the pH down. I have a 19,000 gallon pool and add about 3/4 gallon bleach a day which raises the chlorine concentration about 2ppm. My pH increases about .2 to .3 units per week, so I have to add about a quart of Muriatic acid each week to bring it back down. I have been doing this for 5 years and it's been pretty consistent. This is pretty common from what I have read on this board. So am I missing something? Should my pH not be rising like it is? Below are my numbers.

FC=4
CC=0
pH = 7.5
TA = 120
CYA = 30
CH = 10
Temp = 83

I'm sorry I confused you. Yes, I am saying that your pH should not be changing very much, but what you are seeing may be what "aquarium" said, namely that your higher TA is causing more outgassing of CO2 and that causes pH to rise. However, if that were true, you should be seeing your alkalinity drop (after you add acid), albeit slowly (I tell you how much, later in this post).

Some of the pH rise comes from adding so much bleach. It looks like you are adding about 2 ppm of chlorine per day, right? Unfortunately, I have not verified if extra base is added or kept in bleach to make it more stable -- I know that this is true for 12.5% liquid chlorine and if I use the same proportions for your (presumably 6%) bleach I calculate you would get a rise in pH of about 0.1 per week due to this extra base in the bleach.

Of course, you are seeing more than that. If "aquarium" is correct and this extra pH rise is due to your outgassing carbon dioxide from your pool to the air, then my calculations show that with a weekly rise of 0.3 in pH and adding 3 cups of acid to compensate (to get back to 7.5) you should be seeing a drop in TA of 3.4 ppm per week which probably wouldn't show up in the tests that often and instead would look more like a drop of 10 every 3-4 weeks. Are you seeing that? Or put another way, do you ever need to add sodium bicarbonate (baking soda; bicarbonate of soda) to maintain your 120 TA?

If you are seeing the slow drop in TA, then "aquarium" hit the nail on the head and the solution to your problem may very well be to drop your TA. Of course, if it has been dropping already you could just wait longer and eventually your pool will stabilize, or you can accelerate the drop using Ben's method. The explanation is that every pool has a "natural" rate of outgassing CO2 based on aeration and usage and while every pool will drift up in pH with this process, this can be slowed quite a lot by maintaining a lower TA level. Ironically, this will make your pool more susceptible to pH swings from other sources (i.e. the pH moves more with the same amount of acid/base), but that's the catch-22 tradeoff with TA. However, you could safely go to a TA of 100 which could very well solve your problem and still give you decent buffer capacity.

By the way, do you have any aeration features in your pool such as water slides, fountains, etc.? Do you have kids or others who splash a lot in the pool? There are three factors that increase the outgassing of CO2 and make this upward pH drift worse: high TA, low pH, and aeration. You have the first, not the second, and I don't know about the third.

If you are not seeing a slow drop in TA over time, then perhaps there is more base in your 6% bleach then I would have guessed.

Also, your CH can't be 10 -- is that some sort of mistake? It's probably around 200 isn't it?

Anyone else have any other ideas as to the cause of the increasing pH?

Richard