rising ph levels

[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

[chem geek]

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

Tom,

The problem is that in a pool the extra carbonate (alkalinity) that is put into the pool as a buffer puts the pool way out of balance with respect to CO2 in the air. At standard NSPI middle ranges of pH 7.5, TA 100, CYA 40, CH 300, there is almost 10 times as much dissolved CO2 in the pool water as there should be for equilibrium with air. Yes, that means that if you were to fully aerate your pool, you would lose some of the carbonate in the pool and the pH would rise. My calculations show that full aeration and achieving equilibrium would happen after about 14% of the total carbonate (no TA change since TA is also a function of pH) in the pool were outgassed and the pH rises to 8.4

So there is always this natural tendency for pH to rise with all pools that aren't running at 8+ pH (Ben wrote about having pools operate at high pH, but that's another discussion...). So with this pool situation, we simply can't set a balance point that is at equilibrium because we won't be running the pool at 8+ pH. So we instead have to figure out the sweet spot between the two competing factors -- low TA large pH swing vs. high TA lower pH swing but increasing pH "push". However, my gut tells me that there's some other significant source of base being added to this pool and that's what we need to figure out.

[EDIT]The graph CO2.png (which I think you've seen before in some of my other posts) shows the relative outgas rates of CO2 at various levels of alkalinity and pH. The relative scale is 0 at equilibrium, 1 when there is twice as much dissolved CO2 in the water as in the air, 2 when there is three times as much dissolved CO2 in water vs. air, etc. So a relative number of 20 outgasses twice as fast as one of 10. The "Limit" line is somewhat arbritrary but seems to be close to where many people complain of an upward pH drift that seems excessive.[END-EDIT]

Richard

[chem geek]

My comments in bold in your quoted post below.

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.
Thank you so much for including this link. The advice that was given to you was correct for the problem you described at that time because a large pH shift at the lower TA of 80 will get reduced (somewhat) at higher TA because higher TA buffers the water to make the pH move less when there is a "push" to make the pH move (in your case, from base to increase pH). TA can be confusing because having a low TA makes your pool more easily change in pH, but a high TA makes your pool more likely to rise in pH from outgassing carbon dioxide. With more info, a "balance" can be obtained though it will still result in a rise in pH, but will hopefully lower the amount of acid you need to add each week.

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.
What you read on this forum is correct. Splashing, combined with adding acid, will decrease TA. When you splash, you can increase the transfer of carbon dioxide from the pool water droplets into the air and this process raises pH without changing alkalinity. Then, when you later add acid to compensate to restore the pH, you lower both pH and alkalinity. So it is the combination of splashing and adding acid that lowers alkalinity, but for practical purposes, you can think of splashing as lowering alkalinity which is what you've heard on this forum.

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.
In the post you linked to (above), you mention adjusting from a pH of 8.0 down to 7.4 once a week and that's a larger change than you are dealing with now which is from 7.8 to 7.4 (or 7.7 to 7.4 since you said a 0.2-0.3 pH change per week) so it does look like the higher TA is helping to reduce the pH bounce somewhat (which is expected). On the other hand, it may also be starting to cause an additional pH increase with outgassing -- this is why the change in alkalinity over time is important to note as this is how to answer what is going on.

Do you notice needing to add more acid per week now than you did when the TA was at 80?

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.
Well, sort of, but not quite. At the recommended pH near 7.5 and TA near 80-120, a pool is always going to be out-of-balance and will want to outgas carbon dioxide. So there will always be some pressure for pH to increase. The idea is to find your pool's ideal sweet spot where the outgas effect is relatively small (TA is low enough) while at the same time you have sufficient buffer to minimize your changes in pH (TA is high enough). That's two competing factors. At the sweet spot, the amount of added acid each week is smaller but without too much of an annoying pH swing.

Very interesting info, but it seems to go against what I have read so far which is to keep my TA around 125.
With the additional info you provided that earlier on you had a TA of 80 and had a large increase in pH, then the advice is not inconsistent. You were clearly in a bad situation with the TA of 80 and you are in a somewhat better situation now with a TA of 125, but perhaps you might be in an even better situation with a different TA (maybe 100 or 110?) but we really haven't figured out what that should be yet.

Thanks again for the interesting info.

Jim

Jim,

Since you still had a very high tendency to increase pH even when the TA was low, I don't think the main problem you are having is outgassing of carbon dioxide. Yes, by setting the TA as high as you have you may have started to introduce a little more extra pH increase as would be seen by a more rapid (but still quite slow) drop in alkalinity and a small increase in the amount of acid you need to add. So perhaps your TA should be dropped a little bit (which will happen slowly over time if you don't add any bicarbonate of soda), but that will not solve the core problem you are having, namely that something is adding a lot of extra base to your pool.

Unless there are other chemicals you are adding to your pool besides chlorine and acid, this extra base being added to your pool must either be coming from the chlorine you are using or from something entering into your pool. One thing you could try, just for a few weeks, is to use a different source of bleach. Even though it's not part of the BBB system, if you go to the pool store and get the more concentrated 12% liquid chlorine, then at least I'm more certain of the amount of base that is in it. Just remember that you only need to add about half as much as you were before. At least this is something to try.

If anyone else has any ideas as to the possible source of this pH increase that isn't significantly affected by TA, please chime in.

Richard

[jereece]

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.

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.

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.

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?

Thanks again for the interesting posts.

Jim

[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