rising ph levels

[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

[chem geek]

Jim (jereece),

There's an experiment you can try that will at least isolate whether the bulk of the pH rise is due to the chlorine (or chlorine chemistry). If you double the level of chlorine in your pool which will probably double your chlorine usage, then if the pH increase per day and the amount of acid you have to add to compensate both roughly double, then the source of the pH rise is either in the chlorine (the MSDS is wrong) or in the chlorine chemistry (there is a breakdown or outgassing of chlorine as a basic or neutral process rather than being acidic, so the net overall isn't as neutral as originally thought).

The only processes I know that increase pH from chlorine are 1) when the liquid chlorine or bleach is added to the pool, 2) if the chlorine outgasses from the pool (which is a VERY basic process, but unlikely to occur enough in your pool), 3) when chlorine combines with ammonia or organics to form chloramines and chlorinated organics. The processes that are acidic and exactly counteract the basic (1) and (3) are 1) breakdown of chlorine by light, 2) breakdown of chlorine into chlorites and chlorates by temperature, 3) breakpoint of chloramines or oxidation of chlorinated organics.

If your "double concentration" chlorine test doesn't double the rate of pH increase nor the amount of acid you have to add (which you'll have to do more frequently to keep pH in check -- probably twice a week instead of once a week), then the source of the extra base is elsewhere.

I'll completely understand if you're not willing to try this experiment. It'll cost you twice the chlorine and possibly twice the acid plus is more maintenance, but you'll only have to do this for a few weeks (maybe just one week, if the results are immediately obvious). By the way, the double concentration of chlorine (8 ppm vs. 4 ppm) is no where near being hazardous. It results in 0.18 ppm HOCl which would be the same as adding about 0.4 ppm of chlorine in your pool if it didn't have any CYA in it.

Richard

[jereece]

First I want to run some lab tests. I'll try to do this weekend, but no guarantees. I was a lab chemist and water treatment operator for about 10 years (20 years ago), so I have some experience working with chlorination. I have a few lab tests that I want to perform. I will post a summary when I get them done. Not sure if it will tell me anything or not, but something I want to try.

Thanks,
Jim

[chem geek]

First I want to run some lab tests.

Cool. I'm on vacation starting this Thursday and through next week so I look forward to your results upon my return (and am keeping fingers crossed that you find out something definitive).

[jereece]

Chem Geek - I posted the results and conclusions to my testing here. Please review and let me know what you think.

Thanks,
Jim