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Thread: A Theory About Rising pH in SWG Pools

  1. #21
    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Pat,

    You do a very good job explaining the chemistry in simpler terms for people to understand. Thank you for doing that. There are a couple of corrections to what you wrote in your thread, but they don't detract from the main points too much except that it is not true that the production AND consumption are an alkaline (basic) process because they are not -- the net of the two (production and consumption) are neutral in pH.

    The products from electrolysis are only Hydrogen (H2) gas and Chlorine (Cl2) gas. There is no sodium (Na+) produced nor consumed by this process. There are also no hydroxide ions (OH-) directly produced from the process, but because hydrogen ions (H+) are consumed and due to the balance of the two with water, the net effect is as if hydroxide ions are created (from water).

    2H+ + 2e- --> H2(g)
    2Cl- --> Cl2(g) + 2e-
    Cl2(g) + H2O --> HOCl + H+ + Cl-
    H2O --> H+ + OH-
    ----------------------------------------
    Cl- + 2H2O --> H2(g) + HOCl + OH-

    Now compare the above with introducing liquid chlorine into the water:

    NaOCl + H2O --> Na+ + HOCl + OH-

    You can see that both process result in the same "HOCl + OH-" and therefore have the same slightly alkaline (basic) effect on pH. The effect is not strong because HOCl is a weak acid that partly counteracts the OH- (so some product is OCl- + H2O).

    If we just stopped here, then one would conclude that both the SWCG process and adding liquid chlorine cause a rise in pH and though that is true, it is not the whole story. The chlorine doesn't just keep growing and growing in concentration in the pool. Instead, it gets used up. There are several processes that use up chlorine and I list some of these in this post, but the net result is that these processes do the following:

    2HOCl --> O2(g) + 2H+ + 2Cl-
    2OCl- --> O2(g) + Cl-
    2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
    H+ + OCl- --> HOCl

    Note that the processes that consume the chlorine (breakdown from sunlight and oxidation of ammonia and nitrogenous organics) produce hydrogen ion (from HOCl or no pH change from OCl-) that exactly counteracts what was formed during the production of the chlorine. The net result of producing chlorine from a salt cell and then having it consumed is as follows:

    2Cl- + 4H2O --> 2H2(g) + 2HOCl + 2OH-
    2HOCl --> O2(g) + 2H+ + 2Cl-
    ----------------------------------------------
    2H2O --> 2H2(g) + O2(g)

    6Cl- + 12H2O --> 6H2(g) + 6HOCl + 6OH-
    4NH3 + 6HOCl -->2N2(g) + 6H+ + 6Cl- + 6H2O
    ----------------------------------------------
    4NH3 --> 6H2(g) + 2N2(g) -or equivalently- 2NH3 --> 3H2(g) + N2(g)

    So you can see that the normal process of producing chlorine through an SWCG results in no net change in pH. Instead, Hydrogen and Oxygen gases are produced (when chlorine is broken down by sunlight) or Hydrogen and Nitrogen gases are produced (when chlorine is consumed by ammonia or nitrogenous organics) and I believe other organics can also produce carbon dioxide since the oxidation is similar to burning.

    With liquid chlorine, the net result is somewhat similar except that the net result is the production of salt (sodium Na+ and chloride Cl-) along with oxygen gas from the breakdown of chlorine by sunlight or nitrogen gas when chlorine is consumed by ammonia and nitrogenous organics. So using liquid chlorine increases TDS (specifically salt) over time while an SWCG does not (the chloride ion consumed in production and turned into chlorine simply goes back into chloride ion during chlorine consumption).

    Richard
    Last edited by chem geek; 08-20-2006 at 02:22 PM.

  2. #22
    PatL34 is offline Lifetime Member Widget Weaver PatL34 2 stars PatL34 2 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Quote Originally Posted by chem geek
    Pat,

    You do a very good job explaining the chemistry in simpler terms for people to understand. Thank you for doing that. There are a couple of corrections to what you wrote in your thread, but they don't detract from the main points too much except that it is not true that the production AND consumption are an alkaline (basic) process because they are not -- the net of the two (production and consumption) are neutral in pH.

    The statement above corroborates what SWCG/liner pool owners have been saying in that they see very little change in pH over close to a week. I would have to say that what SWCG/plaster pool owners like me are seeing is a continuous leaching of Ca(OH)² from the plaster, and hence continuous addition of acid to neutralize this. This appears to clear up an anomaly about the steady pH in SWCG/liner pools. Thanks.

    The products from electrolysis are only Hydrogen (H2) gas and Chlorine (Cl2) gas. There is no sodium (Na+) produced nor consumed by this process. There are also no hydroxide ions (OH-) directly produced from the process, but because hydrogen ions (H+) are consumed and due to the balance of the two with water, the net effect is as if hydroxide ions are created (from water).

    2H+ + 2e- --> H2(g)
    2Cl- --> Cl2(g) + 2e-
    Cl2(g) + H2O --> HOCl + H+ + Cl-
    H2O --> H+ + OH-
    ----------------------------------------
    Cl- + 2H2O --> H2(g) + HOCl + OH-

    Now compare the above with introducing liquid chlorine into the water:

    NaOCl + H2O --> Na+ + HOCl + OH-

    You can see that both process result in the same "HOCl + OH-" and therefore have the same slightly alkaline (basic) effect on pH. The effect is not strong because HOCl is a weak acid that partly counteracts the OH- (so some product is OCl- + H2O).

    If we just stopped here, then one would conclude that both the SWCG process and adding liquid chlorine cause a rise in pH and though that is true, it is not the whole story. The chlorine doesn't just keep growing and growing in concentration in the pool. Instead, it gets used up. There are several processes that use up chlorine and I list some of these in this post, but the net result is that these processes do the following:

    2HOCl --> O2(g) + 2H+ + 2Cl-
    2OCl- --> O2(g) + Cl-
    2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
    H+ + OCl- --> HOCl

    Note that the processes that consume the chlorine (breakdown from sunlight and oxidation of ammonia and nitrogenous organics) produce hydrogen ion (from HOCl or no pH change from OCl-) that exactly counteracts what was formed during the production of the chlorine. The net result of producing chlorine from a salt cell and then having it consumed is as follows:

    2Cl- + 4H2O --> 2H2(g) + 2HOCl + 2OH-
    2HOCl --> O2(g) + 2H+ + 2Cl-
    ----------------------------------------------
    2H2O --> 2H2(g) + O2(g)

    6Cl- + 12H2O --> 6H2(g) + 6HOCl + 6OH-
    4NH3 + 6HOCl -->2N2(g) + 6H+ + 6Cl- + 6H2O
    ----------------------------------------------
    4NH3 --> 6H2(g) + 2N2(g) -or equivalently- 2NH3 --> 3H2(g) + N2(g)

    So you can see that the normal process of producing chlorine through an SWCG results in no net change in pH. Instead, Hydrogen and Oxygen gases are produced (when chlorine is broken down by sunlight) or Hydrogen and Nitrogen gases are produced (when chlorine is consumed by ammonia or nitrogenous organics) and I believe other organics can also produce carbon dioxide since the oxidation is similar to burning.

    With liquid chlorine, the net result is somewhat similar except that the net result is the production of salt (sodium Na+ and chloride Cl-) along with oxygen gas from the breakdown of chlorine by sunlight or nitrogen gas when chlorine is consumed by ammonia and nitrogenous organics. So using liquid chlorine increases TDS (specifically salt) over time while an SWCG does not (the chloride ion consumed in production and turned into chlorine simply goes back into chloride ion during chlorine consumption).

    Richard
    I will revise my writeup accordingly to reflect what we discussed.

    Pat
    20,000 Gallon IG Diamond Brite pool, 1.5 HP Sta-Rite pump, Hayward Microclear DE3600 filter, Favco solar panels, Poolpilot DIG-220 with SC-48 cell.

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  3. #23
    KurtV is offline Registered+ Widget Weaver KurtV 0
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    Default Re: A Theory About Rising pH in SWG Pools

    Pat,
    Are you (and others) seeing constantly rising calcium hardness? I am not, yet I still have the huge acid demand.

  4. #24
    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Kurt makes a good point. We have not gotten to the bottom of this significant rise in pH some users experience, though we are making progress. As far as I can figure out so far, the following are possible sources of pH rise:

    1) High TA and/or Low pH and/or high aeration. This refers to my CO2 outgassing chart though this chart is very qualitative due to variance in aeration. This process has been proven through several users successfully eliminating their pH rise by lowering their alkalinity to 80 or even lower (or their pH to 7.2 in one case). This process can be distinguished from the others by the continual drop in TA over time (after adding acid to restore pH). NOTE: It's techically carbonate alkalinity that drives the CO2 outgassing, not TA, so adjustments should be made for CYA levels or Borates (Boric Acid; sodium tetraborate) that are used. My chart assumes no borates and 30 ppm CYA.

    2) Curing of plaster (technically, the concrete in plaster/gunite). When enough Calcium Hydroxide has been produced from the curing process to raise the Calcium Hardness level by 10 ppm (which is probably the minimum detectable amount in test kits), this causes a rise (with a TA of 100) of 0.91 in pH so that 43 ounces of Muriatic Acid are needed (for a 16,000 gallon pool) to restore back to a 7.5 pH. So, the rise in CH will be relatively slow, though if you see it rise about 10 ppm after cumulatively adding 5-6 cups of acid over time (per 16,000 gallons), then this may be the cause of the pH rise. I also would expect that this curing process wouldn't last (in significant amounts) much more than 1 year at the most and that most of the curing would occur in the first 3-6 months. Don't forget that backwashing DE and sand filters as well as splash-out will dilute the CH (and everything else) over time so may hide the rise in CH (at a CH of 300, a loss of 10 is a 3.3% dilution or 333 gallons per 10,000 gallons in a pool). On the other hand, evaporation and refill will tend to increase CH unless the fill water has no CH.

    3) SWCG systems that do not fully dissolve their generated chlorine gas. For every 1 ppm of chlorine that is generated and does not dissolve, this causes a 0.15 rise in pH which requires about 8 ounces (1 cup) of acid to restore pH (in a 16,000 gallon pool). My hunch is that after #2 above for newer plaster/gunite pools, that this is the main reason for rising pH in SWCG pools, but I have no proof of this (yet). One experiment to try is to use a lower power setting and run for a longer period of time (to make up for the lower power setting in terms of total chlorine produced in a day). In theory, the lower power setting should produce smaller bubbles and amounts of chlorine per volume of water flow and should have more chlorine dissolve in the water vs. outgassing to cause a rise in pH.

    4) Other factors. These include fill water that is high in pH. Bather sweat is probably not very alkaline and rain is normally acidic so these are not likely to increase the pH.

    Kurt's data showed that at least part of the pH rise was due to the outgassing of CO2 since his TA kept dropping. Kurt, if you are willing, continue to let it drop to about 50 (see this post for an example of where that worked for someone else. That still provides some buffering, but would virtually slow to a crawl the CO2 outgassing (at a pH of 7.5 or above). Someone who has an SWCG system and experiences rising pH can also try the experiment I mention in #3 above to lower your power setting and increase your time on your SWCG. Let us know what happens if you do this! Oh yes, and Kurt, how old is your pool if it's plaster/gunite?

    Kurt, if you do conclude that your specific source of rising pH is the outgassing of CO2, then you might consider adding 50 ppm Borates (sodium tetraborate) to act as a substitute buffer replacing part of your carbonate buffer system. That would let you run with much, much lower TA readings. Before we get to that point, however, we need to see if you can get to a low TA level where the pH rise slows down significantly.

    Richard
    Last edited by chem geek; 08-22-2006 at 05:43 PM.

  5. #25
    PatL34 is offline Lifetime Member Widget Weaver PatL34 2 stars PatL34 2 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Kurt,

    As I am still curing my Diamond Brite plaster, my last CH reading was 270 ppm.

    I filled the pool entirely with softened water, to help the curing, and tried as much as I could to keep the pH to between 7.2 - 7.6 using roughly a pint of acid a day.

    During the first month I kept the SWCG off line, and used trichlor and bleach to maintain about 5 - 7 ppm Cl. Once I got to about 90 ppm CYA, I started using bleach only. I have not added any baking soda, letting the TA build up of its own accord.

    As of now The PoolPilot SWCG is on at a power level of 1 and 50% output. So far no hint of algae, and I can only ascribe this to the plasterer who really knew what he was doing, and to what I am doing.

    I will be taking a full set of figures today, and let you know what I have.

    Pat
    Last edited by PatL34; 08-21-2006 at 01:06 PM.
    20,000 Gallon IG Diamond Brite pool, 1.5 HP Sta-Rite pump, Hayward Microclear DE3600 filter, Favco solar panels, Poolpilot DIG-220 with SC-48 cell.

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  6. #26
    chem geek is offline PF Supporter Whibble Konker chem geek 4 stars chem geek 4 stars chem geek 4 stars chem geek 4 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Quote Originally Posted by PatL34
    I have not added any baking soda, letting the TA build up of its own accord.
    Normally the curing would produce Calcium Hydroxide and would not raise the TA. The TA would only rise if Calcium Carbonate was released into the pool. After curing, this would be considered "corrosion" of the pool surface. I'm not sure if it is normal for this to occur during the curing process. Did your pool contractor say that it was normal not only for CH to rise, but for TA to rise during curing or do you know if that's true from any other source?

    It would be great, after we have this all figured out, to put together a simple flow chart that people can follow that would explain the normal processes they can expect in their pools. New plaster/gunite pool in first year -- rising CH (and TA?) and especially pH. TA > 80 and/or pH < 7.4 and/or lots of aeration -- rise in pH. Etc. Some would just be explanatory items while others could have solutions.

    Richard

  7. #27
    PatL34 is offline Lifetime Member Widget Weaver PatL34 2 stars PatL34 2 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Richard,

    I should have added baking soda to bring the TA up to 100 ppm, so you are correct again. Bear in mind that this information came from most of their customers, presumably plasterers. (See the Diamond Brite installation manual in the PDF below.)

    I do know that the CH will rise when curing, as this was my experience the first time i used Diamond Brite, but am not sure about the TA going up. I do think that by keeping the Ca(CO3)² and pH down it would help more in the curing process. Just a theory of mine.

    I agree that a flow chart would be nice, but feel many poolowners are more interested in swimming, rather than the mechanics of plaster curing. So it would be available to the dedicated few like ourselves, but can we tell new pool owners having plaster pools under construction what the experiences we have been having.

    Pat

    http://sgm.cc/download/installationm...amondBrite.pdf
    20,000 Gallon IG Diamond Brite pool, 1.5 HP Sta-Rite pump, Hayward Microclear DE3600 filter, Favco solar panels, Poolpilot DIG-220 with SC-48 cell.

    + SWCG OPERATION thread here: http://www.poolforum.com/pf2/showthread.php?t=1226
    + SWCG Running Costs post here: http://www.poolforum.com/pf2/showthread.php?t=316
    + Effective Stabilizer addition post here: http://www.poolforum.com/pf2/showthread.php?p=6645

  8. #28
    KurtV is offline Registered+ Widget Weaver KurtV 0
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    Default Re: A Theory About Rising pH in SWG Pools

    Quote Originally Posted by chem geek
    ...

    2) Curing of plaster (technically, the concrete in plaster/gunite). When enough Calcium Hydroxide has been produced from the curing process to raise the Calcium Hardness level by 10 ppm (which is probably the minimum detectable amount in test kits), this causes a rise (with a TA of 100) of 0.91 in pH so that 43 ounces of Muriatic Acid are needed to restore back to a 7.5 pH. So, the rise in CH will be relatively slow, though if you see it rise about 10 ppm after cumulatively adding 5-6 cups of acid over time, then this may be the cause of the pH rise. I also would expect that this curing process wouldn't last (in significant amounts) much more than 1 year at the most and that most of the curing would occur in the first 3-6 months. Don't forget that backwashing DE and sand filters as well as splash-out will dilute the CH (and everything else) over time so may hide the rise in CH (at a CH of 300, a loss of 10 is a 3.3% dilution or 333 gallons per 10,000 gallons in a pool). On the other hand, evaporation and refill will tend to increase CH unless the fill water has no CH.

    This is why I asked Pat whether he was continuing to see calcium hardness increase. I have not see any increase at all in the 8 or 9 months I've had my Diamond Brite finish installed. None. In fact, because my fill water has a CH of 50 ppm, I occasionally have to add calcium to replace that lost to splash out and backwashing. This can't be the source of my pH rise.

    ...

    4) Other factors. These include fill water that is high in pH. Bather sweat is probably not very alkaline and rain is normally acidic so these are not likely to increase the pH.
    My fill water has a pH of about 8.0 but as I add little of that I probably isn't a significant contributor.

    Kurt's data showed that at least part of the pH rise was due to the outgassing of CO2 since his TA kept dropping. Kurt, if you are willing, continue to let it drop to about 50 (see this post for an example of where that worked for someone else. That still provides some buffering, but would virtually slow to a crawl the CO2 outgassing (at a pH of 7.5 or above). Someone who has an SWCG system and experiences rising pH can also try the experiment I mention in #3 above to lower your power setting and increase your time on your SWCG. Let us know what happens if you do this! Oh yes, and Kurt, how old is your pool if it's plaster/gunite?
    I suspect you're right about a significant portion of my pH rise coming from CO2 outgassing. I've been letting the alkalinity drop for several weeks after reading your other posts on the subject but haven't seen any benefit from it so far. I'll let it continue down to 50 ppm; I'll probably be there within a week.

    Kurt, if you do conclude that your specific source of rising pH is the outgassing of CO2, then you might consider adding 50 ppm Borates (sodium tetraborate) to act as a substitute buffer replacing part of your carbonate buffer system. That would let you run with much, much lower TA readings. Before we get to that point, however, we need to see if you can get to a low TA level where the pH rise slows down significantly.
    I've considered this also since reading Evan's posts on the subject. I'll probably wait until later this fall when the pool is unswimmably cool to start messing around with that.

    Richard
    I'll update my data this evening. I might actually be starting to see a bit of slackening in the rate of pH rise.

    Is there starting to be some consensus here that SWGs themselves have gotten an undseserved reputation as the source for constant pH rise even if we can figure out what the real culprit is?

    Pat,
    You might be interested to know that I finally took the SWG plunge. I'm hoping my AquaRite will arrive in the next couple of days.

  9. #29
    PatL34 is offline Lifetime Member Widget Weaver PatL34 2 stars PatL34 2 stars
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    Default Re: A Theory About Rising pH in SWG Pools

    Quote Originally Posted by KurtV
    I'll update my data this evening. I might actually be starting to see a bit of slackening in the rate of pH rise.

    Is there starting to be some consensus here that SWGs themselves have gotten an undseserved reputation as the source for constant pH rise even if we can figure out what the real culprit is?

    I figure that those with plaster pools will always see a rise in pH with or without a SWCG. It will probably occur with vinyl pools, but so slowly, it will be hard to distinguish. It is looking more like the nature of the beast, or as one of my teachers said way way back, an "inherent feature of the system". I hope Richard will come up with an answer that breaks that hypothesis.

    Pat,
    You might be interested to know that I finally took the SWG plunge. I'm hoping my AquaRite will arrive in the next couple of days.
    Way to go Kurt.

    You are going to love every minute it is running. Guaranteed!!


    Pat
    20,000 Gallon IG Diamond Brite pool, 1.5 HP Sta-Rite pump, Hayward Microclear DE3600 filter, Favco solar panels, Poolpilot DIG-220 with SC-48 cell.

    + SWCG OPERATION thread here: http://www.poolforum.com/pf2/showthread.php?t=1226
    + SWCG Running Costs post here: http://www.poolforum.com/pf2/showthread.php?t=316
    + Effective Stabilizer addition post here: http://www.poolforum.com/pf2/showthread.php?p=6645

  10. #30
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    Default Re: A Theory About Rising pH in SWG Pools

    Quote Originally Posted by chem geek
    Just wondering. Do you have any water features (waterfalls, etc.) or would you say that your pool doesn't have much aeration? What brand of SWG system are you using? Do you use a pool cover and is it opaque or a clear "solar" cover?

    Since you aren't seeing the same sort of pH rise that others see in SWG pools, I'm just trying to figure out what's different about your pool.

    Richard
    I have a waterfall that's on 12 hours a day, so lots of aeration! I have an Aquarite SWG. No pool cover of any kind, lots and lots of UV on the pool - we're on Day 52 of air temps at 100 degrees or higher.

    Also, although I WAS just on my 3rd gallon of muriatic, I've now broken the seal on the 4th gallon.
    Sandy
    15,600 gallon, screened 15x30 IG plaster sport pool with 6x8 tanning area, Aquarite SWCG, Hayward cartridge filter, Polaris 280 cleaner

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