rising ph levels

[chem geek]

My comments below in bold.

Thanks for the additional info. I have a few comments / questions.

1. I just purchased enough bleach (found it on sale) to last me the rest of this season. I normally close early to mid September. And the only local pool store I have had never heard of using bleach let alone selling it. When I asked from Sodium Hypochlorite, they looked at me like I was an alien. When I told them it was bleach, they still looked confused. So I can't find the higher strength stuff, but I have used at least 3 different brand names of bleach this year and seem to get similar results.
If they sell it at all, it's called liquid chlorine or chlorinating liquid.

2. I forgot to mention this in my past post, but my CA is in fact 10ppm. I have very, very soft water. Using my Taylor test kit, it only takes one drop to get a color change. I have asked if this is a problem on this board before and even Ben has said for vinyl pools low CA is not a concern. I thought about using some Calcium Hypochlorite for a while to increase it, but I can get bleach cheaper so I have just stuck with bleach.
OK, you've got a vinyl pool and that's true you don't need extra calcium, but it does change my calculations in subtle ways, but not too much.

3. I have not really noticed needing to add acid any less frequent at 120 TA than at 80. However I have not really been keeping records either. So it may be but I just have not taken notice. I still add acid about once a week.
What I was asking was the total amount of acid you add per week, but it sounds like that hasn't changed very much so that means that the outgassing of carbon dioxide is probably not very high and not the source of this rising pH problem.

4. Now for a radical question. If having high TA causes an imbalance in the water and if having low TA causes the pH to want to increase less, why not just go with little or no TA? I understand that pH would swing more, but from what I hear you saying it will tend to reach equilibrium and maybe the pH would increase 0.1 units per week. That would not be so bad. So why would I not just have very little or no TA so my pool would be at equilibrium?
If you had a plaster/grout pool then you would need the TA along with CH to prevent corrosion, but since you have a vinyl pool you don't need TA nor CH for that purpose. However, if you had little or no TA, then your pool's pH would swing HUGELY and you'd have to be adding acid every day if not more frequently!

If we assume, based on your numbers, that the additional "hidden" source of increasing pH is equivalent to roughly 1 ounce weight of caustic soda or lye per day, then every day you add chlorine to your pool your pH goes up 0.15 and the chlorine then gets used up during the day and your pH drops back down resulting in a net increase of 0.04 per day. This is at a TA of 120. At a TA of 80, adding chlorine has your pH go up by 0.23 and then the chlorine gets used up and your pH has a net gain of 0.06. At a TA of 10, adding chlorine would make your pH would go up almost 1.1 units! And yes, as the chlorine got used up your pH would drop back down for a net of 0.26, but that's a huge swing and is why you don't want very low TA in your system. Even adding acid every day wouldn't help the situation with a TA of 10. You'd have to add a very small amount of add acid almost every hour to maintain a reasonable pH! If you were willing to add acid every day along with your chlorine (not mixing them, of course), then you could go with a TA as low as 60 which would give a 0.30 increase when adding chlorine and a net gain of 0.07 which you would neutralize with about 3.5 fluid ounces of Muriatic acid per day. I wouldn't go any lower than this 60 TA, but I really don't see much benefit of that since it's more convenient to have the higher TA (say, 100) and add acid less frequently, but it's really up to you.

Thanks again for the interesting posts.

Jim

[jereece]

...every day you add chlorine to your pool your pH goes up 0.15 and the chlorine then gets used up during the day and your pH drops back down resulting in a net increase of 0.04 per day. This is at a TA of 120.

Then something is definetly adding more to pH than the bleach. In 5 years, I have never seen this small of a net increase in pH. I have only used generic brand bleach (5.25 and 6%), but have used a number of different sources. All seem to have similar results. So I don't understand what it could be. I only add bleach and muriatic acid, then maybe adjust alkalinity once around mid-summer with baking soda.

Thanks again for the info.

Jim

[chem geek]

Then something is definetly adding more to pH than the bleach. In 5 years, I have never seen this small of a net increase in pH. I have only used generic brand bleach (5.25 and 6%), but have used a number of different sources. All seem to have similar results. So I don't understand what it could be. I only add bleach and muriatic acid, then maybe adjust alkalinity once around mid-summer with baking soda.

Thanks again for the info.

Jim

.04/day is about .3/week and that seemed to be around what you were seeing. In your post #13 you said that the rise in pH was ".2 to .3 units per week" so aren't we consistent?

Does anyone out there know the pH of bleach? For 12.5% liquid chlorine, the pH of just sodium hypochlorite in water would be 10.6 but the actual pH is about 13 because of sodium hydroxide (caustic soda or lye) that is added since concentrated chlorine breaks down faster at the lower pH. So maybe bleach is more basic than I would have thought and that would explain a strong tendency towards increasing pH for anyone on the BBB system.

Richard

[jereece]

.04/day is about .3/week and that seemed to be around what you were seeing. In your post #13 you said that the rise in pH was ".2 to .3 units per week" so aren't we consistent?

You are correct. I misread and thought you said .04 per week. Sorry, you are correct. That is about right.

The MSDS for 5.25% bleach says the pH is 11.4.

I think what I need to do is check my pH, add bleach and let circulate for an hour, check the pH again then the next day when chlorine concentration has decreased check pH again. It may be difficult seeing any change however with my test kit.

[chem geek]

The MSDS for 5.25% bleach says the pH is 11.4.

Well, if the 5.25% means the weight percentage of sodium hypochlorite then the natural pH would be 10.5 and the amount of base added to make it more stable (if any) is much less than what is in 12.5% liquid chlorine. So that isn't what is going on. Too bad since that would have been an easy answer.

Richard

[CToon]

Since I also suffer from the increase of ph syndrome , I've taken an interest in this ..I dont have the ph swings that some have reported , but I do need to add muriatic acid maybe every 10 days (varies , I havent kept track)

My pool
ph 7.5 normally
ALK 110
vinyl liner

Thinking of the possible cause/contributors , I wonder how much an effect DE is having behind the scene. I dont know the ph of diatomaceous earth , but since its a "chalk" , wouldnt it be alkaline ? and following that couldnt leaching be a contributor ?

just a thought

[chem geek]

Since I also suffer from the increase of ph syndrome , I've taken an interest in this ..I dont have the ph swings that some have reported , but I do need to add muriatic acid maybe every 10 days (varies , I havent kept track)

My pool
ph 7.5 normally
ALK 110
vinyl liner

Thinking of the possible cause/contributors , I wonder how much an effect DE is having behind the scene. I dont know the ph of diatomaceous earth , but since its a "chalk" , wouldnt it be alkaline ? and following that couldnt leaching be a contributor ?

just a thought

That's a possibility, but we'd have to see if there's a trend with the folks who see the consistent pH increase that isn't explained by other known factors like outgassing of carbon dioxide (which one can tell via the alkalinity drop over time, assuming pH is adjusted by adding acid). Are all of these people using DE filters?

Another possibility is the outgassing of chlorine gas, but this would not normally happen fast enough to affect pH except in salt pools -- either SWG pools or pools that have added salt. The concentration of salt (chloride ion) and the concentration of disinfecting chlorine (HOCl) determine the maximum rate of outgassing, though there are the same physical aeration issues as with carbon dioxide except that chlorine escapes more readily since there is virtually no chlorine in the air at all (so it's much further out of equilibrium) However, I don't have any evidence that this is really occurring in pools and it's very difficult to distinguish the difference between chlorine usage from breakdown by sunlight vs. outgassing except for the pH difference (the former is acidic, the latter is basic) and with so many things that can affect pH, well it's a mess to figure out!

You should also think about anything else added to the pool -- any other chemicals at all, including things to tie up metals and prevent staining. I know that non-chlorine shock (MPS) is very acidic, for example.

Then of course, there's the fact that people swim in the pool! I didn't think the pH of sweat was basic and I've seen reports of this rise in pH from people who don't use their pools much, so I don't think that's it.

At some point we may just give up on trying to explain this and just "live with it" and deal with it, but I'm not ready to quit yet!

Richard

[jereece]

I use a cartridge filter and it's 3 years old, so I don't think my filter could be a contributor. I only use bleach, muriatic acid and baking soda. Nothing else. My swimmer load is very light this year, but I have not seen much differnce based on swimmer load either.

[chem geek]

I use a cartridge filter and it's 3 years old, so I don't think my filter could be a contributor. I only use bleach, muriatic acid and baking soda. Nothing else. My swimmer load is very light this year, but I have not seen much differnce based on swimmer load either.

Well we have a mystery then. I'll double check my calculations since I could have made a mistake somewhere though I've used the same calculations elsewhere where they've been more consistent. Also, as has been pointed out in other threads (Saturation Index in The China Shop), the science can only go so far because the pool is not a closed system. Still, it would be nice to know the source of this base to cause pH to rise -- it must be annoying to have to add chemicals so frequently. Might be time to look at investing in a pool cover

[EDIT]I verified the calculations and don't see anything wrong with them. Because you haven't seen any change in the amount of acid you have to add when you were at a TA of 80 vs. a TA of 120 and because you don't see a drop in alkalinity over time, I think that rules out the outgassing of carbon dioxide as the source of the problem. So we are still left with what is adding base to your pool. Both you and "CToon" both have vinyl pools, I wonder if that has anything to do with the problem or if the low CH is a factor. Anyone else out there with a mysterious rising pH? Do you have plaster pools or vinyl pools and if vinyl pools, what's your CH?[END-EDIT]

Richard