rising ph levels

[chem geek]

Rising pH
Are you using the all bleach method? If you are, that may be why it is climbing.
Anything that you add to the pool, can change the pH depending on the pH of what you are adding.

Bleach, sodium hypochlorite, has a pH of around 10 or 11, very alkalotic, so the more you add, the higher your pH will go.

Though you are correct that bleach is alkaline (the 12% stuff from pool stores has an even higher pH of 13), chlorine usage is an acidic process so the net effect of chlorine addition and usage is a very small rise in pH due to the extra base that is added to (or kept in) the chlorine bleach (to keep it more stable). When chlorine is broken down by sunlight (UV) or consumed by ammonia to breakpoint or oxidizing organics, the pH goes down by almost the same amount as it goes up when you added it.

[EDIT]
By "chlorine usage" I mean when chlorine gets "used up", not your usage (addition) of chlorine. Sorry for the confusion.
[END-EDIT]

After reading several posts about pH rise, I am coming to the conclusion that it is most often due to outgassing of CO2 which raises pH (and when you add acid to compensate, the alkalinity is lowered and you get back to the original pH -- so the net change of CO2 outgassing plus adding acid is that the alkalinity drops). Aeration of pool water is the most common cause for this. It looks like many people live with a lower alkalinity of around 80 in such environments. Also, keeping your pH closer to 7.5 helps since lower pH accelerates the outgassing process.

Richard

[wolfcue]

Thanks to everyone for the help.

[azimmer1]

chem geek/Richard
By what mechanism is adding bleach an acidic process. I am not doubting you, just curious as to why

Andy

[chem geek]

chem geek/Richard
By what mechanism is adding bleach an acidic process. I am not doubting you, just curious as to why

Andy

I'll do this in words and in symbols. Adding bleach is a basic process; it is the using up of bleach (chlorine) that is an acidic process so the net result is almost neutral. When I said "chlorine usage" I didn't mean your using chlorine (i.e. adding it) -- I meant when chlorine gets used up by "doing its thing" or "breaking down". Sorry for the confusion I caused.

Adding Chlorine
NaOCl + H2O --> Na+ + HOCl + OH- (+ extra base Na+ + OH-)
HOCl <--> H+ + OCl-
Sodium Hypochlorite (liquid chlorine or bleach) combines with water to produce sodium ions (part of regular table salt) plus disinfecting chlorine plus hydroxyl ion. The hydroxyl ion makes this a basic reaction that raises pH, but because the disinfecting chlorine is a weak acid this overall reaction raises the pH by less than a strong base would. Note that there is a small amount of extra base in the form of Sodium Hydroxide (lye or caustic soda) that comes with Sodium Hypochlorite and is there to help preserve it, but this amount is rather small.

Using Up Of Chlorine
Breakdown of Chlorine by Sunlight (UV)
2HOCl --> O2(g) + 2H+ + 2Cl-
2OCl- --> O2(g) + 2Cl-
Chlorine breaks down in the presence of ultraviolet radiation, such as found in sunlight, and forms oxygen gas and chloride ion (and hydrogen ion, if starting with HOCl hypochlorite). Because a hydrogen ion is produced, this is an acidic process, but since disinfecting chlorine is a weak acid, only some of it breaks down in a way that lowers pH as shown above (i.e. only HOCl produces H+; OCl- does not).

Net Chlorine To Breakpoint (Ammonia "Oxidation")
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
OCl- + H+ --> HOCl
The disinfecting form of chlorine (HOCl) combines with ammonia through a series of reactions (that I have not shown) with the net result being the production of nitrogen gas (which is why it is important to keep your cover off and have good circulation when shocking) plus hydrogen ion and chloride ion. Though by itself this would be a strong acid reaction, there is also OCl- present that will combine with hydrogen ion to form more HOCl since the ratio of HOCl to OCl- will remain constant (and is about 50/50 at pH 7.5). So the net reaction is acidic, but not strongly so.

Overall combination of adding chlorine and having it used up
The net reactions are as follows if you combine the ones I showed above.
2NaOCl --> 2Na+ + 2Cl- + O2(g)
3NaOCl + 2NH3 --> 3Na+ + 3Cl- + N2(g) + 3H2O
So the overall net reaction of adding sodium hypochlorite to your pool and having it used up in its most typical ways is simply to produce salt (yes, sodium chloride or table salt, dissolved in water, of course) and either oxygen or nitrogen gas (and water).

Other things that could happen
If you do not have enough chlorine in your pool relative to your bather load (ammonia demand), then the chlorine may not completely oxidize ammonia and instead you will get chloramines (first, monochloramine). This reaction is basic. However, sunlight may break down monochloramine which will result in the rest of the breakpoint process which overall is acidic (so it's the same as I showed above overall).

It is also possible for chlorine to combine with organic compounds to form chlorinated organics that are hard to breakdown. When people talk about the health problems with chlorine, it is usually about some of these chlorinated organics (Disinfection By-Products, DBPs) known as Tri-Halo-Methanes(THMs) such as chloroform. Also, some chloramines such as nitrogen trichloride (NCl3) not only smell bad, but can cause health problems (especially in indoor pools with poor air circulation). In an outdoor pool exposed to sunlight and with a good residual of chlorine you typically don't get these "bad" compounds. If you are really concerned and have money to burn, you can use a constant maintenance level of non-chlorine shock (monopersulfate, MPS) to oxidize organics before chlorine gets a chance, but this is probably overkill for an outdoor pool (though may be a good idea for an indoor pool).

Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reaction:
2H2O + 2Cl- --> Cl2(g) + H2(g) + 2OH-
Cl2(g) + OH- --> HOCl + Cl-
----------------------------------------------
2H2O + Cl- --> HOCl + OH- + H2(g)

Note that the products of HOCl and OH- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is "used up".

If you have a salt pool and don't use CYA (this isn't normal) then you could also outgas chlorine in the same way that CO2 is outgassed. This is more likely if you are aerating the water (e.g. have water features, slides, fountains, jets pointed up, lots of splashing, ...). This process is strongly basic and greatly increases the pH (HOCl + Cl- --> Cl2(g) + OH-). The reason this would tend to only happen in a salt pool without CYA is that a high concentration of both chloride ion (Cl-) and disinfecting chlorine (HOCl) are needed and it occurs more readily at lower pH.

I know, I know...more than you wanted to know. I hope it helps and that you made it this far...

Richard

[jereece]

Adding bleach is a basic process; it is the using up of bleach (chlorine) that is an acidic process so the net result is almost neutral.

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

[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

[jereece]

Thanks for your very interesing post. I want to learn more about this, because not too long ago I made a similar post about pH increasing and was told to increase my TA from 80 to 100 - 125. Here is a link to that post: http://www.poolforum.com/pf2/showthread.php?t=3715.

I do not have any water features that would increase aeration and I normally try to keep my return flow so that it does not splash. I have read on this forum that splashing can decrease TA, so I have always tried to keep return flow from splashing very much.

I believe I do see TA slowly decreasing. I have not taken notes on this, but my recollection is that it slowly decreases over time. I accounted this to the process you describe being accelerated when I add acid. Normally when my pH gets to 7.8 I will add acid to get it back down to 7.4 or so. But even when my TA was 80 (earlier this year), I still saw pH rising at about the same rate. After increasing the TA, I have not noticed pH rising slower.

So, are you saying that my TA will continue to decrease until it gets to a stable concentration and then pretty much stay there. And at that point, my pH should only rise about 0.1 units per week? If that's true, that would be great. I will pay more attention to my TA and see if I can notice where it gets to a stable point and if my pH increase slows.

Very interesting info, but it seems to go against what I have read so far which is to keep my TA around 125.

Thanks again for the interesting info.

Jim

[aquarium]

If "aquarium" is correct and this extra pH rise is due to your outgassing carbon dioxide from your pool to the air,

Not really what I said, and I'm wrong anyway.

I keep what is called a high-tech planted aquarium. I inject CO2 from a pressurized cylinder into the aquarium water so the plants will grow well. I've been down the road of -very- high tech automation and don't want to go there again with the pool. But I am familiar with the alkalinity-CO2 relationship.

In Austin, from where we recently moved, the water comes out of the tap at 10 pH and about 80ppm alkalinity. They intentionally drive all CO2 out of the water to -raise- pH and -lower- acidity so the water will be gentle on the pipes. Let that same water just sit out for a few hours and it will take in CO2 from the atmosphere and settle in at a pH of about 7.6.

I should have known that if I lowered my pool water to 80ppm alkalinity that it should behave similarly, and so it has. The pH has now risen to 7.6 and so far is holding there. But it's too soon to tell if that will hold. It seems to move in increments.

I wasn't suggesting that outgassing is caused by pH, I'm suggesting that the alkalinity of the water will -set- the pH due to the natural interaction between the water and air as CO2 in both reaches a balance point.

Tom