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Thread: Confused about Salt Residual

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    phalcon51's Avatar
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    Confused about Salt Residual

    I'm still trying to figure out what happens to the salt in my pool. My understanding so far is that the salt is broken down by the SWCG into its more basic constituents, one of them being chlorine gas which is dissolved into the water. After the chlorine is used to sanitize the water by oxidizing bacteria and other pathogens I think I understand that it somehow reverts back to salt again. What I'm confused about is what type of salt; the same sodium chloride that I poured in to begin with or something else? Is the residual salt used over and over to generate chlorine? What happens to the salt eventually to lower the salt level in the water? Is there some form of salt left over that builds up over time that isn't used to liberate chlorine? If so, do I eventually have to do a partial drain and refill to remove the salt residual?

    Thanks for clearing this up for me,

    Gary
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    Mod Squad JohnT's Avatar
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    Re: Confused about Salt Residual

    The salt is used over and over. After the chlorine sanitizes the water it converts back to the same salt it was when you put it in the pool. Eventually some of the salt gets splashed out or otherwise removed from the pool in the water and it has to be replaced.
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    Re: Confused about Salt Residual

    Someone is sure to point you to a better description, but basically: when you put salt into the water it becomes dissolved, meaning it is no longer salt. It is broken up into its Na (sodium) and Cl (chloride) elements. The SWCG puts energy into the Cl to make it effective as a cleaning agent. As it cleans, it loses that energy and must be "recharged" by the SWCG to be effective again.
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    Re: Confused about Salt Residual

    The basic bare bones answer.
    You've got the basic premise correct, and yes it reverts back to salt (Sodium Chloride) in solution and repeats the process all over again. The salt gets removed by splashing out, back washing, or by draining the pool level (when you get lots of rain and need to lower the level).
    There isn't salt left over that builds up. You never have to refill or partial drain to remove the salt residual. There isn't one to worry about.

    There's some good reading in pool school: pool-school/salt_water_chlorine_generators
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    Re: Confused about Salt Residual

    salt in your pool is exactly like salt in the ocean. It doesn't go anywhere, only when the water goes with it... There isn't anyone anywhere adding salt to the oceans, as far as I know.
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    Re: Confused about Salt Residual

    I wouldn't say exactly like the ocean. The ocean has all kinds of other ions in it, effectively the salts are things like sodium chloride, calcium chloride, magnesium chloride etc... But your point still stands, the salt doesn't go anywhere, as water evaporates, the salt stays when it rains (you add water to your pool) it brings it back to delusion.

    SWG's don't break salt into constituent parts so much as converts salt water (NaCl + H2O) into Sodium Hydrochlorite (Bleach) Hydrogen gas and Sodium Hydroxide? (I think, sorry I'm not a chemist).

    When bleach combines with contaminates its converts to salt, water and whatever (depending on what its combined with) in the case of bleach added via SWG back into salt, like the kind you added.
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    Re: Confused about Salt Residual

    Guess what's left in your pool after liquid chlorine is added and used up? Yup...mainly salt and whatever other inert ingredients are left. Same with stabilized chlorine in Trichlor or Cal-hypo, other than they add CYA and CH, respectively.
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    Re: Confused about Salt Residual

    Quote Originally Posted by Puffin
    SWG's don't break salt into constituent parts so much as converts salt water (NaCl + H2O) into Sodium Hydrochlorite (Bleach) Hydrogen gas and Sodium Hydroxide? (I think, sorry I'm not a chemist).
    The sodium is irrelevant. One could use potassium chloride or magnesium chloride and have the SWG still produce chlorine. MajorDIYer had a good description. When salt is added to water, it dissolves into separate charged particles called ions (see this video). It is the chloride in salt that is relevant. The chloride ion is boosted in energy by the SWG to become chlorine (technically combining with water to become hypochlorous acid and related chemicals, but that's an unimportant subtlety for this discussion). When chlorine reacts with a pathogen to kill it or oxidizes bather waste or gets broken down by sunlight, it loses its energy and becomes chloride (salt) again.
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    Re: Confused about Salt Residual

    Thanks Chemgeek. Like I said, I'm not a chemist, lol just a nerd remembers enough from chemistry class to be dangerous.
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    Re: Confused about Salt Residual

    Thanks for the explanations everybody - it makes a lot more sense now. I thought the chlorine was somehow used up doing it's job, depleting the salt and leaving a basically useless residual that would build up in the water over time. I had no idea the chloride was basically recycled and used over and over. Chemistry is amazing stuff I'm getting a real education on this site.

    Gary
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    Re: Confused about Salt Residual

    First reply. You all have it right salt dissolves into sodium (Na+) and chloride (Cl-) and energy makes chlorine (Cl2) from chloride (Cl-). The chlorine dissolves in the water and makes HOCl and OCl-. After the HOCl is finished being an oxidizer, it becomes chloride (Cl-) again. The loss of salt (chloride) comes from splash out or drag out. The chloride loss can also come from HOCl combining with an organic contaminant or ammonia in the water. We call this combined chlorine but they can also be trihalomethanes (THMs), disinfection byproducts (DBPs) and others. Whenever HOCl combines with anything in the water, the chloride will not revert back. I also would not say exactly like the ocean. Sea water has about 35,000 ppm TDS and SWG water about 3,500 ppm TDS mostly chloride. Tears in humans are about 4,00 ppm TDS mostly chloride.

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    Re: Confused about Salt Residual

    Quote Originally Posted by Pb2Au
    Whenever HOCl combines with anything in the water, the chloride will not revert back.
    That varies actually, most things chlorine combines with subsequently get further broken down by additional chlorine and the chloride ion gets returned to the water. It would only be the rare cases where the combined chlorine is volatile and evaporates out of the pool that the chloride ion would be lost.
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    Re: Confused about Salt Residual

    Wow. That is great. I am sure glad to know that chloroform, dibromochloroform, haloacetonitriles, cyanogen halides, halopicrins, haloacetic acids, chloral hydrates and haloacetones are all destroyed by additinal chlorine. Also the 186 nitrogenous componds that can combine with chlorine that are in sweat and urine.

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    Mod Squad Bama Rambler's Avatar
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    Re: Confused about Salt Residual

    I don't know what y'all do down there in your pools but I'm not coming to swim with you!

    P.S. When you figure out that lead to gold thing, let me know, I'd like to get in on that!
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    Re: Confused about Salt Residual

    Quote Originally Posted by Pb2Au
    Wow. That is great. I am sure glad to know that chloroform, dibromochloroform, haloacetonitriles, cyanogen halides, halopicrins, haloacetic acids, chloral hydrates and haloacetones are all destroyed by additinal chlorine. Also the 186 nitrogenous componds that can combine with chlorine that are in sweat and urine.
    Pb2Au,

    Welcome to TFP!

    That's not what Jason is saying. He is saying that the chlorinated organics (and volatile inorganic chloramines) that are not fully oxidized are a very small fraction of the total amount of chlorine that is consumed so is normally not even measurable in our test kits. It's not that they are all destroyed with additional chlorine but that the quantity created of these compounds is very small.

    For example, in a pool where the FC is roughly 10% of the CYA level so the hypochlorous acid level is roughly 0.05 ppm, then for ammonia the oxidation is mostly complete with the amount of nitrogen trichloride being around 2 ppb (that's 0.002 ppm) that will eventually outgas since it is fairly volatile (it doesn't build up to a higher level because it reacts with dichloramine to produce nitrogen gas). There is more produced from urea, but even there it's still low at < 10 ppb (that's 0.01 ppm) and these amounts are low because the active chlorine level is low. As for the trihalomethanes, the largest amount is chloroform where the quantity varies but is usually in the range of 50-100 ppb (when fill water has around 30 ppb) as a steady-state in most pools but is produced at a low rate, mostly because the active chlorine level is so low.

    The vast majority of chlorine loss in residential pools is from sunlight as is obvious by comparing the loss during the day (usually 1.5-2.5 ppm) vs. at night (usually < 1 ppm; often < 0.5 ppm) and also by comparing an uncovered pool vs. one with an opaque cover during the day. The next largest loss is from oxidation of organics where the largest component of sweat and urine (by far) is urea. Urea very slowly combines with chlorine to form N-chlorourea (probably mostly monochlorourea) and is mostly oxidized to nitrogen gas and carbon dioxide gas with the chlorine becoming chloride ion. Even Ernest "Chip" Blatchley's experiments (see this paper) at very high chlorine and urea concentrations have most of the chlorine consumption result in chloride ion, though nitrogen trichloride gets up to 0.45 ppm in the more extreme experiments. However, the nitrogen trichloride level is proportional to the hypochlorous acid concentration which in our pools is about 1/70th the concentration used in some of his experiments at around 7 ppm FC with no CYA.

    In residential pools with low bather load, there is probably more loss of chlorine from oxidation of organics that are not from bather load, such as from pollen, leaves, and pool surfaces and components themselves. There is also some outgassing of chlorine itself, though that is fairly minimal.

    Again, it's not the number of different chlorinated organic compounds that matters, but their quantity. As described in this WHO document, the proportion of nitrogenous compounds (those that would most likely react with chlorine) are the following concentrations and relative percentages:

    ...................................... Sweat ........................................ Urine
    Compound ........ mg/L .. % of total Nitrogen ...... mg/L .. % of total Nitrogen
    Urea ..................... 680 ............ 68 ..................... 10,240 .......... 84
    Ammonia ............. 180 ............ 18 .......................... 560 ............ 5
    Amino acids ........... 45 .............. 5 .......................... 280 ............ 2
    Creatinine ................ 7 .............. 1 .......................... 640 ............ 5
    Other compounds .. 80 .............. 8 .......................... 500 ............ 4
    ------------------------------------------------------------------
    Total nitrogen ...... 992 ........... 100 .................... 12,220 ......... 100

    The chlorine bound to organics would mostly register as Combined Chlorine (CC), but that is almost always <= 0.2 ppm in most outdoor residential pools and most of what does register as CC is likely to be monochlorourea since it is very slow to form (so urea builds up in pools) and slow to oxidize so builds up as an intermediate (and is not volatile). In outdoor pools, the UV from sunlight creates hydroxyl radicals from the breakdown of chlorine that likely help to more readily oxidize both urea and N-chlorourea intermediates as well as other organic compounds.

    Quote Originally Posted by Pb2Au
    The chloride loss can also come from HOCl combining with an organic contaminant or ammonia in the water. We call this combined chlorine but they can also be trihalomethanes (THMs), disinfection byproducts (DBPs) and others. Whenever HOCl combines with anything in the water, the chloride will not revert back.
    I would qualify this to say that whenever HOCl combines with anything in the water AND that compound does not get further oxidized, then the chloride will not revert back. Chlorine combining with ammonia to form monochloramine is not permanently lost. More chlorine combines with monochloramine to form dichloramine and again to form nitrogen trichloride that then reacts with dichloramine and water to form nitrogen gas, hypochlorous acid, hydrogen ion and chloride ion (see this post for more details). Urea is more complex, but also results in chloramines that go through a similar substitution followed by oxidation process.

    The bottom line is that most chlorine reverts back to chloride. It is water dilution (splash-out, carry-out, backwashing, rain overflow) that is the primary source of chloride (salt) reduction. I can tell you from my own measurements in my pool over several seasons that the salt level rises over the summer swim season in accordance with the chlorine and acid additions with nothing lost beyond the small amount removed from dilution. I have a mostly opaque pool cover so increase from evaporation/refill is minimal and I have an oversized cartridge filter that is only cleaned once a year so there are no backwashing losses. The pool is used almost every day so there is some splash-out and carry-out, but that's about it for dilution until the winter when I use winter rains to dilute the water. Salt (chloride) measurements at the start and end of the summer season confirm that there is minimal chlorine/chloride loss from volatile compounds or permanently combined chlorine (even if not measured as CC in that test). In fact, one of the main reasons I use winter rains for water dilution is to keep the salt levels in check.
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    Re: Confused about Salt Residual

    Thank you. I knew joining TFP had its rewards.

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    Re: Confused about Salt Residual

    Please explain why I have to add a bag of salt to the pool several times per year, if the pool is rarely used. Is it the sunlight, wind, and temperature? You imply that the salt stays in the water, no matter what. However, the more water that evaporates, it seems, the more I must add water AND add salt (and acid). The water evaporates into the air, but where is the salt going?
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    Re: Confused about Salt Residual

    alanpaul, welcome to TFP.

    You don't lose salt to evaporation, it stays in the water. You only lose salt when water is removed from the pool, either by splash out, backwashing, or a leak.

    Is it possible that you actually have a leak?
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    Re: Confused about Salt Residual

    Welcome to TFP We especially see this in pools equiped with an auto fill device that may be masking a leak in the pool or in area where there is excess water replacement via rain.
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    Re: Confused about Salt Residual

    alanpaul.

    What kind of filter do you have? DE and Sand filters require periodic backwash cycles. The backwash cycle dumps both water and salt.

    Any possibility of a slight leak in your pool? Slow leaks may not be noticed, especially where there is an automatic fill line, or if you're normally topping your pool off with a garden hose, due to evaporation. It may not be all evaporating...

    In either case, the salt does not evaporate with the water and is usually lost by water splash out, bathing suit drag out, leaks, or backwash cycles.
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