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Thread: Why does pH rise with salt systems?

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    Why does pH rise with salt systems?

    I've been told that pH will rise on pools with salt systems, but I haven't really seen this in my own experience. I'm no chemist, but from what I understand, most of the things that find their way into pools are acidic (rain, urine, sweat, decaying plants and animals). It also appears that salt has no affect on pH, and the system is generating sanitizer in the form of hypochlorous acid. So what causes it to rise?
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    Mod Squad jblizzle's Avatar
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    Re: Why does pH rise with salt systems?

    There are detailed explanations on this somewhere ... I dont feel like searching

    Basically, inside the cell there is gas generated that results in the PH rising (like aerating). At least that is how my simple mind pictures it.

    If you have a nice low TA, and low pH fill water, then you might be lucky and have stable pH.
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    Re: Why does pH rise with salt systems?

    Normal swimming pool TA levels cause the water to become over-saturated with CO2, essentially like carbonated soda. When CO2 out gasses the PH goes up, CO2 is carbonic acid in the water, when it goes away that causes an increase in PH. In a standard pool with no SWG or water features, CO2 outgassing happens quite slowly and can be ignored. But anything that aerates the water creates opportunities for CO2 outgassing. SWGs produce small hydrogen bubbles that are effectively aeration, and encourage CO2 outgassing.

    The rate at which CO2 out gasses is a function primarily of the PH and TA levels, along with the total amount of aeration and some other smaller factors. High TA, low PH, and more aeration causes more CO2 outgassing, and thus faster PH increases. In a typical trichlor tablet pool this whole effect is drowned out by the acidic trichlor tablets constantly lowering the PH. But in a liquid chlorine or SWG pool there aren't any constant acid additions to drown out the PH increases.

    With TA around 60 and PH around 7.8 you can usually slow the rate of CO2 outgassing down enough so it isn't significant (though additional aeration can make even those levels insufficient). At higher TA or lower PH you tend to see the PH drifting up. When the TA is fairly high, say 150+, the rate of PH increase can be rather dramatic and very annoying. In more extreme cases, the addition of borates to the water along with lowering TA down to around 50 will usually be enough to stop the PH drift.
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    Re: Why does pH rise with salt systems?

    One other possible factor for SWG pools is that the chlorine gas that is produced in the salt cell may not get fully dissolved into the water and can outgas. When the chlorine dissolves in the water, it is acidic and counteracts the alkalinity produced at the other plate in the cell so if chlorine gas leaves the water before dissolving then this counteracting effect doesn't happen. So short pipe runs from the SWG to the first return can make the problem worse, especially if the return is pointed upwards. However, we haven't "proven" that this effect is really happening. It's just reasonable speculation and is consistent with the chemistry and would explain why some SWG pools still rise in pH in spite of the TA being significantly lowered. For sure, the carbon dioxide outgassing effect Jason refers to is dominant in most situations which is why lowering the TA usually helps at least somewhat.
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    Re: Why does pH rise with salt systems?

    So if my pH is already on the high side, I might not see it creep upwards? Does it ever reach a "maximum" pH?
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    Re: Why does pH rise with salt systems?

    As the pH rises, the rate of rise will slow down. See this table for how "over-carbonated" pool water is with respect to being in equilibrium with air.

    If you get to the 0.0 point in the table (or even close like 1.0) and the pH still rises, then that is from something other than carbon dioxide outgassing. So the chlorine gas outgassing can still occur and is not dependent on pH since it is more a matter of how quickly the chlorine gas is able to dissolve into the water and is limited more by physical processes (diffusion rates) rather than chemical ones. So if there is any chlorine gas outgassing, there will never be a "maximum pH".

    The following table shows the equilibrium pH at various carbonate alkalinity aka adjusted alkalinity where you subtract out the CYA contribution from Total Alkalinity (TA). I also show the pH where there is twice as much carbon dioxide in the water as the equilibrium amount relative to air -- the pH usually rises quite slowly if it gets to this point. Note also that the rate of rise is proportional to the square of the TA and I don't show that effect in the table (since the table is not about the rate of pH rise, but the amount out-of-equilibrium).

    CarbAlk ... pHeq ... pH2x
    .... 10 ...... 7.55 ... 7.26
    .... 20 ...... 7.85 ... 7.55
    .... 30 ...... 8.02 ... 7.73
    .... 40 ...... 8.14 ... 7.85
    .... 50 ...... 8.23 ... 7.94
    .... 60 ...... 8.30 ... 8.02
    .... 70 ...... 8.37 ... 8.08
    .... 80 ...... 8.42 ... 8.14
    .... 90 ...... 8.47 ... 8.19
    .. 100 ...... 8.51 ... 8.23
    .. 120 ...... 8.58 ... 8.31
    .. 140 ...... 8.64 ... 8.37
    .. 160 ...... 8.69 ... 8.42
    .. 180 ...... 8.73 ... 8.47
    .. 200 ...... 8.77 ... 8.51
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    Re: Why does pH rise with salt systems?

    Awesome!! Thanks so much for the info.
    TreeFiter

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    Re: Why does pH rise with salt systems?

    Why does pH rise with salt systems? Why? It just does! It does no good to ask why. You might as well ask why a dropped piece of bread will land butter side down! Or why a child needs to pee seconds after you passed the last gas station for 120 miles.

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    Re: Why does pH rise with salt systems?

    Understanding the process can allow you to take steps to minimize the effects. As I said in my original post, I haven't really seen it in my experience. A lot of the guys I work with are going through cases of pH minus every week, but I hardly use the stuff. So I was curious what could be responsible for the difference. So I'm thinking that I stumbled across the right balance of higher pH and lower TA without knowing it.

    Asking why is the most important question you can ask. Otherwise you can keep treating symptoms, but never cure the disease. By asking why, I found out that offgassing plays a big role in the process, which means that if I can minimize agitation by adjusting return jets, or limiting the use of water features. I also found out about a relationship between pH and TA that I didn't previously know about. When my customers are paying for a clean healthy pool, I don't think there is any excuse for not asking why.
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    Re: Why does pH rise with salt systems?

    It has always been my understanding that one of the bi-products of a SWG is NaOH which will cause the pH to rise. Am I wrong in my understanding of this?
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    Re: Why does pH rise with salt systems?

    Quote Originally Posted by TreeFiter
    Asking why is the most important question you can ask. , I don't think there is any excuse for not asking why.
    I think Richard is a fan of "Joe Dirt" and was para-phrasing a quote from the movie? At least that's how I took it, not that you asked an invalid question.

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    Re: Why does pH rise with salt systems?

    Quote Originally Posted by Mizu
    It has always been my understanding that one of the bi-products of a SWG is NaOH which will cause the pH to rise. Am I wrong in my understanding of this?
    You need to look at the products from both plates. The chlorine gas when dissolved in water produces acid and this counteracts some of the hydroxide from the other plate. You end up with hypochlorous acid and hydroxide, but those combined are the same as hypochlorite. So it's the same as adding chlorinating liquid or bleach to the pool.

    So when you add the chlorine, the pH rises, but chlorine usage/consumption is acidic. So the net result at a steady FC level is no change in pH. Technical details about all this are in this post.

    The rise in pH comes from carbon dioxide outgassing from increased aeration which is why lowering the TA usually helps. There may also be some rise from chlorine gas outgassing if it doesn't get completely dissolved into the water.
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    Re: Why does pH rise with salt systems?

    Thanks for clearing that up, Chem Geek!
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    Re: Why does pH rise with salt systems?

    All I know is that if I keep the TA at 90 my pH changes very little over the season; I might have to add acid maybe twice a summer.
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