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Thread: sodium bromide + sodium persulfate system

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    sodium bromide + sodium persulfate system

    Hi,

    Some questions to the pool chemists on sodium bromide + sodium persulfate:

    In bromine spas, sodium persulfate, as opposed to potassium persulfate, is said to not re-activate used up bromine (organic bromides).

    On the other hand, the combination of sodium bromide + sodium persulfate is used in pool water desinfection, see patents http://www.google.com/patents/US6149821 and http://www.google.com/patents/US20040084383. The latter mentions that the combination needs some time ("aging"), up to some hours to fully develop the bromine from the added sodium bromide.

    Is the sodium persulfate just slower than the potassium type or does it really make a difference to generate bromine from "new" versus "used" bromide?

    So, would you have to re-add sodium bromide when re-adding sodium persulfate?

    Thx

  2. Back To Top    #2

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    Re: sodium bromide + sodium persulfate system

    Welcome to TFP!

    First of all, there is a difference between sodium persulfate aka sodium peroxydisulfate, Na2S2O8 and potassium monopersulfate aka potassium peroxymonosulfate, KHSO5 and the difference between these is not related to sodium vs potassium, but to the type of sulfate compound. Though persulfate has a high oxidation potential of +2.1V which is higher than monopersulfate with an oxidation potential of +1.85V, the reaction kinetics are slow against many chemical species including bromide ion. So yes, it is true that persulfate does not oxidize bromide to bromine, or does so very slowly, while monopersulfate does more quickly.

    Also, persulfate tends to be more irritating to the skin and is why when monopersulfate is used in the Nature2 system that provides silver ions, the small amount of persulfate contamination in the non-chlorine shock product does not irritate because it is activated and converted into other active chemical species (as described in this paper):

    S2O82- + Ag+ --> SO42- + SO4-• + Ag2+
    Persulfate Ion + Silver Ion --> Sulfate Ion + Sulfate Ion Radical + Divalent Silver Ion
    SO4-• + Ag+ --> SO42- + Ag2+
    Sulfate Ion Radical + Silver Ion --> Sulfate Ion + Divalent Silver Ion

    So forget "sodium" vs "potassium" as that has nothing to do with the difference. It is "persulfate" (or "peroxydisulfate") vs. "monopersulfate" (or "peroxymonosulfate") that is the difference. I would not use a persulfate product in a spa because 1) it is irritating, 2) it doesn't react quickly to oxidize many chemicals and 3) it too slowly oxidizes bromide. Use a monopersulfate product instead. It reacts more quickly to oxidize various chemicals and does so faster than its rate of oxidizing bromide to bromine so it will tend to get rid of bather contaminants before it creates more bromine. Note that chlorine reacts faster to oxidize bromide to bromine than it does to oxidize other bather contaminants, though for whatever reason it does tend to keep water clear when used weekly in a Nature2/MPS system so apparently it must react with some chemicals other than bromide ion to keep the water clear. Chlorine oxidizes bromide to bromine about 1500 times faster than monopersulfate (and about 30 times faster than ozone).

    The most common monopersulfate product is Dupont's Oxone which is found in most non-chlorine shock products. This is a triple salt of 2KHSO5•KHSO4•K2SO4. It significantly increases sulfates in the water so while that should not be a problem for acrylic or fiberglass spas, it is something that should not be overdone in pools or spas with plaster surfaces where sulfate levels above around 300 ppm may be damaging (see this thread).

    Finally, as a general rule, do not look at patents to learn anything about science. People write whatever they want in patents and they are not peer reviewed. What is written in a patent may not be technically correct at all.

    Why are you asking this question? What is it that you are trying to do?
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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  3. Back To Top    #3

    Re: sodium bromide + sodium persulfate system

    Hi chem geek,

    thanks for taking the time.

    >Finally, as a general rule, do not look at patents to learn anything about science. People write whatever they want in patents and they are not peer reviewed. What is written in a patent may not be technically correct at all.

    I am an electronics engineer by profession, and there I can more easily verify what is behind a patent. With pool chemistry, I still am interested but need obviously help.

    >Why are you asking this question? What is it that you are trying to do?

    Always wanted to understand if/why things work. We are pool newbies and have a 31m^3 vinyl pool. It was started with the pharmacist's mixture of copper sulfate in the prescribed dose. We did not want to use chlorine because of what we smell and feel at friends pools. Then I ran into the above balanced water patent and gave it a try.

    We run the pool since a month now and are very pleased. Except for a touch of turquoise taint, the water is clear, has no smell whatsoever and is relatively soft to the eyes so I can dive without getting red eyes. The weekly dosing is 2.8ppm sodium bromide and 4.2ppm sodium persulfate.

    It should be clear that I do not want to recommend this mix to anyone, for lack of more information. As long as we are happy with it, I will continue but also tell should anything go wrong.

  4. Back To Top    #4

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    Re: sodium bromide + sodium persulfate system

    You should be testing your bromine level. It doesn't sound like it's enough given your weekly dose so when there is no bromine and you only have copper ions in the water, the water is not disinfected at all against fecal bacteria or transmission of viruses. See this post for more technical info. Copper is primarily an algaecide, not a disinfectant, and if you are not careful with the copper level or if the pH gets too high, you can stain pool surfaces and turn blond hair greenish. That's why we don't recommend it.

    As for bromine, it is more expensive and can get used up more in sunlight than chlorine (with CYA). Is your pool exposed to sunlight all day?

    The problems you mention at your friends' pools are likely from them not properly managing those pools. People here on this forum who manage their pools according to the Pool School also report virtually no smell (in my pool, I have to churn the water near the surface to get the faintest detection of a clean bleach-like smell; others don't smell anything). Red eye comes either from improper pH or if one is in the water with eyes open for a while, then it's primarily from the salt level since human tears have 9000 ppm. Saltwater pools tend to be easier on the eyes with their 3000 ppm. Red eyes don't come from chlorine unless the level is very high (usually without CYA in the water) or, more likely, there is a buildup of chloramines.

    The active chlorine level that is the minimum to prevent algae growth is the equivalent of only 0.07 ppm FC with no CYA so is much, much lower than found in some commercial/public pools, especially indoor pools and it's lower than in bromine pools. This very low active chlorine minimizes the amount of volatile and irritating nitrogen trichloride while the regular dosing and maintaining of the chlorine level prevents any buildup of other chloramines (monochloramine, dichloramine, organic chloramines) while preventing algae growth and killing pathogens quickly.

    If you've been dosing with sodium bromide for a while (2.8 ppm * 4 = 11.2 ppm bromide), it may be too late for you to even try out the chlorine method in your pool. Once a bromine pool, always a bromine pool, or at least you're stuck with a bromine pool for quite some time. The reason is that when bromine gets used/consumed, most of it becomes bromide again. A small amount of bromine outgasses and some is removed in the filter as combined bromine on organic debris, but you pretty much have to do a substantial drain/refill to get rid of it. It's more or less a one-way commitment. I'm not sure why you are adding sodium bromide weekly. You normally establish a bromide bank large enough to produce a bromine level quickly enough from an added oxidizer and then just add a very small amount to maintain that bromide bank. See Using Bromine in a Spa (or Pool) for more info.

    Neither chlorine nor bromine will last a week in sunlight unless one doses to much higher levels and uses CYA in the water. That's why it's more common to see Trichlor tabs for chlorine and bromine tabs (BCDMH or DBDMH) for bromine since they are both slow-release, though have other problems with what they also add (CYA for chlorine and DMH for bromine). That's why you'll see most people on this forum initially establish a CYA level and then dose their pools regularly (usually every day or two; with a mostly opaque pool cover, twice a week) with bleach or chlorinating liquid or have an automated dosing system such as a saltwater chlorine generator or a peristaltic pump or The Liquidator.

    To be designated as a swimming pool disinfectant, a product must go through EPA DSS/TSS-12 efficacy testing and also be demonstrated to be safe. There are only three chemicals that have been approved (technically, it's products that are approved, with specific concentration instructions, but all products have one of these chemicals) and for pools these are chlorine, bromine and Baquacil/biguanide/PHMB. For spas, Nature2 with MPS (monopersulfate) has also been approved, but only for hot water temperatures (i.e. not for pools). Though you are using bromine, you are not doing so in the prescribed fashion where the bromine level is never below 2 ppm (or 0.8 ppm when using copper/silver or copper only ionization as with these products that are certified to NSF Standard 50).

    It is more usual for a residential swimming pool to develop algae than for people to get sick or ill effects, but it is not unheard of and we recently saw a case in this thread of Pseudomonas Aeruginosa that is more commonly seen in improperly treated hot tubs. On another forum, there were quite a few incidents of not only hot tub itch/rash, but also hot tub lung and one case of Legionnaire's Disease where the person nearly died. Some of these hot tubs used "alternative" methods of disinfection not approved by the EPA; others used chlorine or bromine but in insufficient amounts so their levels were too low or zero; still others had a longer term buildup of CYA making the chlorine less effective.
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  5. Back To Top    #5

    Re: sodium bromide + sodium persulfate system

    First, thank you for putting together all this information.

    I get a bromine level of around 1ppm, which should be ok according to
    "NSF Standard 50 require a minimum of 0.4 ppm chlorine or 0.8 ppm bromine"
    But now I wonder if copper ionization is already given by the copper sulfate, or do you need electrolysis?

    I am not sure about fluctuations though and since the pool is exposed to sunlight, the bromine level could drop further. I might increase the dose. What else could you recommend now with my bromine pool?

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    Re: sodium bromide + sodium persulfate system

    The copper sulfate is the same as the copper ionization -- just two different ways of getting copper ions into the pool.

    I think you're just going to have to monitor your bromine levels over time to figure out the normal depletion rate. The usual size of a bromide bank in a spa is 30 ppm which is roughly 1/2 ounce of sodium bromide per 100 gallons. So I wouldn't add so much sodium bromide on a regular basis -- just add enough to make up for expected bromide/bromine losses from water dilution plus a little more for bromine outgassing or organic bromamine removal in the filter.

    You are lucky that you don't find the persulfate irritating. Many people do which is why monopersulfate is generally used instead for non-chlorine shock.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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