Bromine To Chlorine?

[chuckg]

Please help. Last June I installed a salt system on a bromine pool. Yes, I know that is strange but a friend had given me 150 lbs of bromine. The generator owners manual says it will generate either chlorine or bromine. The generator manufacturer told me that once a bromine pool, always a bromine pool. Last June I "seeded" the pool skimmer with bromine tablets for three weeks to make sure there was no residual chlorine and everything has worked beautifully since, until last month.

Last month the pool got cloudy, I had let the salt level get too low. Now salt is 3100, ph 7.6, alkalinity 40 and I cannot get any reading of chlorine or bromine. Now the manufacturer has told me that the residual bromine can be burned off over a period of 7 days with chlorine levels of 5-10 ppm, is this true. I woud prefer to have a chlorine pool but had been told in the past that I am stuck with bromine until I do a liner change.

Please help

Chuck

317-938-1019

[quantumchromodynamics]

Once the bromine atoms are in your pool, they can take a long time to dissipate or dilute out, so you're going to have a bromine pool for a while. The primary problem with bromine in an outdoor pool is that there is no known stabilizer to protect the bromine from the sunlight. The tablets are 1-bromo-3-chloro-5,5-dimethylhydantoin or 1, 3-Dibromo-5, 5-Dimethyl Hydantoin (DBDMH). They contain an available Bromine of 63.5%, and an available Chlorine of 28.2%, or just bromine. The dimethylhydantoin is a chemical similar to cyanuric acid. It forms a compound with bromine. It may provide some protection from the sun. It also makes the bromine more stable and less likely to dissipate.

An outdoor bromine pool is manageable, you just have to test more often and adjust your sanitizer more often. Your primary problem is that your salt system is not producing chlorine. The salt system produces chlorine and the chlorine oxidizes bromide ions into hypobromous acid. You need to figure out why the salt generator is not making chlorine. Until then, you need to add liquid chlorine (12% sodium hypochlorite) or regular, unscented Clorox bleach (6.00 sodium hypochlorite) to maintain a proper sanitizer level.

A 1-ppm concentration of chlorine has the exact amount of oxidizing capacity as 2.25 ppm of bromine as far as number of electrons the oxidizer can removed from other molecules. 1 molecule of hypochlorous acid can remove 2 electrons from another molecule. 1 molecule of hypobromous acid can remove 2 electrons from another molecule. The bromine atom just weighs 2.25 times as much as the chlorine atom.

Ozone (O3)……………….. Oxidation strength = 2.07. Oxidation capacity = 2 e-

Hydrogen peroxide (H2O2) Oxidation strength = 1.78. Oxidation capacity = 2 e-

Hypochlorous acid (HOCl) Oxidation strength = 1.49. Oxidation capacity = 2 e-

Hypobromous acid (HOBr) Oxidation strength = 1.33. Oxidation capacity = 2 e-

You can test for bromide ion concentration using an ion selective electrode if you really want to monitor your bromide ion concentration, although it's really not worthwhile. http://www.thermo.com/com/cda/product/deta...5,14992,00.html

You can begin a series of dilutions to begin to remove the bromine and dimethylhydantoin. You can drain water out of your pool until there is only 1 foot of water in the shallow end. Do not drain more than this or your liner will come loose.

[waterbear]

Sounds like you have a nascent algae bloom that is consuming the bromine/chlorine. Shock the pool with liquid chlorine or beach to about 15 ppm until the pool clears and the sanitizer level is not dropping overnight (to take the sun out of the equation). I suspect that one shocking could be enough but it depends on the organic load in the water.

As far as converting, just stop adding any bromine tablets or sodium bromide to the water and treat it like a chlorine pool. Eventually it will become one unless you have used a LOT of organic bromine. Remember that sodium bromide is the active ingredient is many mustard algae products and almost the entire line of United Chemical's products.

[billp]

"The generator manufacturer told me that once a bromine pool, always a bromine pool."

This is commonly believed but not entirely true. After bromine is added to a pool it is lost more rapidly than can be accounted for by normal water loss (backwash, splash/drag out). So unless you added a LOT of bromine it should not take terribly long for it to disappear. In the mean time you will lose most if not all of your active bromine to the sun during the day. If the chlorine generator is working (be sure to check that) you might try running the pump at night to maximize the efficiency of the system.

[chem geek]

According to Ben Powell at PoolSolutions, if the bromine is added via sodium bromide, then as long as it's not a very large quantity it will dissipate (probably outgas is my guess) perhaps over weeks, but if the bromine was added via tabs, then the DMH in the tabs prevents the bromine from dissipating. This sounds a lot like the relationship of CYA and chlorine so it's possible that there is an equilibrium between DMH and bromine though I've never seen any scientific literature on this equilibrium.

Also note that Ben says the dissipation requires shocking with chlorine which makes sense since that converts bromide to bromine so the more bromine the faster it will outgas or otherwise get removed. However, he also says it needs sunlight and that part doesn't make sense to me since it implies some sort of breakdown but there is nothing for bromine to break down into except bromide which can then be reactivated back into bromine.

[billp]

...it will dissipate (probably outgas is my guess)

Ben says the dissipation requires shocking with chlorine which makes sense since that converts bromide to bromine so the more bromine the faster it will outgas or otherwise get removed. However, he also says it needs sunlight and that part doesn't make sense to me since it implies some sort of breakdown but there is nothing for bromine to break down into except bromide which can then be reactivated back into bromine.

Outgassing: For the most part chlorine doesn't outgas (otherwise we wouldn't have the salt build-up when using chlorine) so I would think it even less likely that bromine would outgas since the molecular weight is much higher (80 vs 35).

As far as shocking with chlorine, when Cl is added to a pool containing Br it "activates" the Br, creating hypobromous acid and hypobromite ion, using up the Cl and turning it back to salt. The hypobromous acid is now the chemical doing the oxidizing and sanitizing. The only problem is in an outdoor pool it can not be protected from sunlight and will be destroyed just like chlorine in an unstabilized pool. So I don't think Ben's explanation above is correct.

I don't know where the bromide ion goes, perhaps it gets locked up in some compound which is filterable. All I know is in real life it "disappears" with time.

[chem geek]

Outgassing: For the most part chlorine doesn't outgas (otherwise we wouldn't have the salt build-up when using chlorine) so I would think it even less likely that bromine would outgas since the molecular weight is much higher (80 vs 35).

As far as shocking with chlorine, when Cl is added to a pool containing Br it "activates" the Br, creating hypobromous acid and hypobromite ion, using up the Cl and turning it back to salt. The hypobromous acid is now the chemical doing the oxidizing and sanitizing. The only problem is in an outdoor pool it can not be protected from sunlight and will be destroyed just like chlorine in an unstabilized pool. So I don't think Ben's explanation above is correct.

I don't know where the bromide ion goes, perhaps it gets locked up in some compound which is filterable. All I know is in real life it "disappears" with time.

Actually, if you look at the Henry's Law constants here, you'll see that it's around 930 for hypochlorous acid and 0.093 for molecular chlorine while for hypobromous acid it's widely varying from 1.8 to 6100 while for molecular bromine it's around 0.76. The lower the number, the greater the volatility from an equilibrium point of view (not the same as rate, but I can't find any numbers more directly related to volatility), but of course it depends on the concentration. For chlorine, the equilibrium at pool pH is such that it is hypochlorous acid that is what would outgas in a chlorine pool and you are right that it is fairly minimal, but that is due to having Cyanuric Acid (CYA) in the water that makes the hypochlorous acid concentration very low equivalent to around 0.1 ppm FC if there were no CYA in the water. Bromine doesn't get moderated by CYA and it too is mostly hypobromous acid as opposed to molecular bromine. So bottom line is that it could outgas quite a lot faster than chlorine so that could be the mechanism. Getting the bromine level up to 5 ppm would be 50 times higher than the normal hypochlorous acid equivalent level.

It could also be that bromine combines with more organics that then get filtered out (if they coagulate as with clarifiers, etc.), but that seems unlikely to me. It doesn't make much sense to be bromide that gets removed.

I can also tell you for sure that chlorine does in fact outgas, at least in a day or two when there is no CYA in the water, since adding tap water to aquariums back in high school meant putting them in buckets exposed to air (not even sunlight) at least overnight before adding them to the fish tanks. This would mostly dechlorinate the water. I'm not sure why we didn't just expose them to sunlight since that would have been faster, but we didn't. If there was 1 ppm FC with no CYA, that would be around 10 times the typical pool chlorine level equivalent with CYA (i.e. 0.1 ppm FC with no CYA equivalent in pools).

Richard

[quantumchromodynamics]

Outgassing: For the most part chlorine doesn't outgas (otherwise we wouldn't have the salt build-up when using chlorine) so I would think it even less likely that bromine would outgas since the molecular weight is much higher (80 vs 35).

The electrolytic cell will oxidize the chloride ions into chlorine gas and the bromide ions into elemental liquid bromine, which will then dissolve into the water to become hypochlorous acid and hypobromous acid. Bromide that is oxidized to bromine by electrolysis may be more easily volatilized than bromide that is oxidized by hypochlorous acid or other methods.

Bromine is an especially volatile liquid. As bromide ions are converted to bromine, the bromine will have a tendency to vaporize. Some of the chlorine gas that is created does not dissolve and enters the atmosphere. This causes a gradual loss of chlorine atoms.

Since the electrolytic cell can remove one electron at a time vs. two at a time for other oxidation methods, I think that there is more likelihood of creating the more volatile elemental bromine using the cell as an oxidizer. In addition, the chlorine gas created by the cell may be more likely to volatilize the bromide and cause it to dissipate.

Have you ever smelled chlorine or bromine compounds? Think about it; if you can smell it, then it must be outgassing. The reason chloride ions build up when using chlorine is that the chlorine atoms are being added faster than they are volatilized. If you are not adding new bromine atoms, then the bromine should eventually dissipate.

In a salt pool, the salt needs to be periodically replaced. Most of the lost chloride ions are lost to dilution, but some are lost to dissipation.

Let's also remember that when you are talking about the weight of bromine being 79.9, you're not talking about pounds, but grams per mole. The bromine atom has a mass of about 1.33 x 10^-23 grams.

One possible way to determine if there are any significant bromide ion levels present is to use silver nitrate in a test vial to create a precipitate of the halide ions (chloride, bromide) as silver halides. Silver chloride will be white, whereas silver bromide will be more of a pale yellow.

[billp]

I can also tell you for sure that chlorine does in fact outgas, at least in a day or two when there is no CYA in the water, since adding tap water to aquariums back in high school meant putting them in buckets exposed to air (not even sunlight) at least overnight before adding them to the fish tanks. This would mostly dechlorinate the water. I'm not sure why we didn't just expose them to sunlight since that would have been faster, but we didn't. If there was 1 ppm FC with no CYA, that would be around 10 times the typical pool chlorine level equivalent with CYA (i.e. 0.1 ppm FC with no CYA equivalent in pools).

If the chlorine without CYA were simply offgassing into the air then we would not be able to keep chlorine in indoor pools, which do not use CYA. My experience with indoor (commercial) pools just doesn't jive with this explanation. Also the salt buildup in an indoor pool seems to be similar to an outdoor pool, indicating the chlorine is reverting to salt, not offgassing.

The CYA-chlorine interaction is not a simple one of a hypochlorus molecule linking with a CYA molecule. It takes several CYA molecules to protect one hypochlorus molecule, indicating some kind of dynamic relationship which protects the hypochlorus molecule while leaving it available to do it's job.

We always left aquarium water out in the sun. Now I use RO water...

[chem geek]

If the chlorine without CYA were simply offgassing into the air then we would not be able to keep chlorine in indoor pools, which do not use CYA. My experience with indoor (commercial) pools just doesn't jive with this explanation. Also the salt buildup in an indoor pool seems to be similar to an outdoor pool, indicating the chlorine is reverting to salt, not offgassing.

The CYA-chlorine interaction is not a simple one of a hypochlorus molecule linking with a CYA molecule. It takes several CYA molecules to protect one hypochlorus molecule, indicating some kind of dynamic relationship which protects the hypochlorus molecule while leaving it available to do it's job.

We always left aquarium water out in the sun. Now I use RO water...

That's a good point about indoor pools, but they do tend to smell more like chlorine than pools with CYA, even when not in use (i.e. when not having chloramines from bather load) but of course there are differences in air circulation so it's not a fair comparison. As for salt buildup, it would be expected to be a lot less in an indoor pool since there is no sunlight (which ultimately breaks down chlorine into chloride -- hydrochloric acid and oxygen gas, actually) and the daily rate of chlorine usage (for any reason) is a lot less in a low-bather load situation so having similar salt buildup does not make sense. With a lower chlorine demand in an indoor pool and lower chlorine additions, there would be a slower buildup of salt unless the primary usage of chlorine was bather load (which is often the case for commercial/public pools, but not with residential pools). Also, if the indoor pool has a cover, which many do when not in use to reduce humidity, then that would obviously minimize any outgassing.

You are incorrect about the CYA-chlorine interaction. It is not any linking with multiple CYA molecules but chlorine literally replacing a hydrogen on a CYA molecule to form a combined compound and there are three such sites on the CYA molecule though the bulk of the combined compounds has just one hydrogen replaced and one hydrogen dissociated (i.e. it's HClCY^-). In fact, if two such sites get replaced, you get Dichlor, and if thress such sites get replaced you get Trichlor. See this paper that definitively determined the equilibrium constants between hypochlorous acid and Cyanuric Acid. The reason it is "available to do its job" is that it is a chemical equilibrium. As hypochlorous acid gets used up, more gets released from the combined chlorine-CYA compounds though technically it's going back and forth all the time since equilibrium implies equal reaction rates in both directions (so when one "side" gets low, then the equilibrium shifts and one reaction rate is faster than the other until a new balance is achieved with the same ratio of products to reactants). You can look at this as a somewhat weak binding, though in normal pool water 97% of the measured Free Chlorine (FC) is bound to CYA, 1.5% is hypochlorous acid and 1.5% is hypochlorite ion. You can see a comparison of the hypchlorous acid vs. pH traditional industry graph without CYA vs. the real graph with CYA here. The reaction rate for chlorine getting released from CYA has a half-life in one of the reaction paths of 0.25 seconds so is fast enough that the Free Chlorine (FC) tests essentially measure the sum of hypochlorous acid, hypochlorite ion, and mostly the chlorine bound to CYA (mostly HClCY^-).

Richard

[billp]

If the chlorine without CYA were simply offgassing... see #1 below

My experience with indoor (commercial) pools...

note my experience with indoor pools is commercial only. Most chlorine consumption is due to bather load.

You are incorrect about the CYA-chlorine interaction. See #2 below. It is not any linking with multiple CYA molecules but chlorine literally replacing a hydrogen on a CYA molecule to form a combined compound and there are three such sites on the CYA molecule though the bulk of the combined compounds has just one hydrogen replaced and one hydrogen dissociated (i.e. it's HClCY^-).

#1. Offgassing of chlorine. Now that I think of it you will never have offgassing of chlorine simply because chlorine gas in water immediately turns to HOCl and HCl. This is why a gas shooter can apply Cl gas to a pool, at the proper rate, and not gas the neighborhood. The "chlorine smell" often associated with pools is the fragrant aroma of chloramines.

#2. If the free chlorine-CYA interaction was simply a matter of each free chlorine hooking up with one CYA then we would see maximum UV protection at much lower levels of CYA. As it is a FC level of 2 to 3 ppm takes 30 to 40 ppm of CYA to get significant protection. Especially below 30 ppm the loss due to sunlight becomes very significant compared to a level over 40 ppm. (This is based on experience with commercial pools with constant monitoring.) So if maximum protection is provided when there are 4 or 5 molecules of CYA for each molecule of free chlorine then something else must be going on. Something to contemplate...

Another thing to contemplate is the interaction of the UV with the FC-CYA relationship. Something is going on there because when the UV is removed, the ORP increases. Something I have observed but have never seen explained anywhere.

[chem geek]

#1. This is not true because you are thinking that only chlorine gas can outgas where a hypochlorous acid itself (a neutral molecule) is in equilibrium both in water and as a gas. Think of water that is in equilibrium with liquid and with water vapor -- just as water can be in both liquid and gaseous form in equilibrium, the same is true for hypochlorous acid. Read the link I gave earlier on Henry's Law constants that explicitly lists hypochlorous acid. What you mostly smell in a clean pool with low bather load is a faint fresh Clorox-like scent that is hypochlorous acid. The chlorine gas is much too low in concentration to smell (due to pool pH and already low hypochlorous acid concentration). Now in your higher bather load situations you are right that what is mostly noticed is chloramines which are more volatile and don't combine with CYA so are more concentrated.

#2. This is not true because the UV protection increases with higher amounts of CYA -- there is no "maximum", but the largest UV protection occurs with initially smaller amounts of CYA and then increases with increasing CYA. With no CYA in the water, direct noontime sun cuts down chlorine by half in around 35 minutes. Even with 10 ppm CYA in the water, this is significantly reduced to losing half over several hours, but that's not enough to be useful (though better than no CYA at all). More CYA helps, but at a diminishing rate (see this link though there are other factors not shown that I'll write about). This effect is what is seen in practice. There is an additional effect from protection not related to chlorine being bound to CYA but from CYA itself absorbing UV and shielding lower depths. Finally, the chlorine bound to CYA may not be completely protected itself -- just as hypochlorous acid and hypochlorite ion break down from sunlight, it is possible that the chlorine bound to CYA does as well, but much more slowly, though I can find no specific numbers on this (the graph I linked to shows this, but this effect may not be as strong as shown in the graphs). The effect of maximum benefit between 30 and 40 ppm CYA is most likely due to the fact that most commercial/public pool operators don't see much benefit above 30-40 ppm CYA because the chlorine demand from bather load overwhelms the loss from sunlight. This is not the case in residential pools and additional protection of chlorine is seen at higher CYA levels, even if the FC level is raised proportionately, again possibly due to the CYA shielding effect. This has been seen in many, many residential pools and by experiments done by some homeowners (for example, this post) comparing 40 ppm CYA vs. 70 ppm CYA. In fact, most saltwater chlorine generator (SWG) manufacturers recommend a CYA level of 60-80 ppm (with 70-80 ppm preferred) precisely for this reason that it lets one lower the on-time of their SWG which not only extends its life but lowers the rate of pH rise due to carbon dioxide and chlorine outgassing.

Your concept of chemical molar (molecular) ratios is not the same as the chemical equilibrium that is actually going on. If there were truly a maximum, then that would be more like a binding effect that you are describing, like a metal sequestrant, but as I said the increased CYA level continues to show more "chlorine protection from sunlight" benefit in pools that don't have bather load swamping the chlorine demand. Also, look at the numerous links I already gave to you where the scientific studies on kill times show continued slowing down at higher CYA levels (at a constant FC level). This has also been validated through thousands of mostly residential pools at The PoolForum (and by commercial pools Ben Powell managed -- he started The PoolForum and PoolSolutions) and over 10,000 (so far) mostly residential pools at Trouble Free Pool. Also, did you read even the introductory section of the O'Brien paper I linked to? There is no multiple molecule binding effect -- it's a literal chemical substitution to produce a different compound in equilibrium -- this is scientific fact proven through multiple experiments and known chemistry.

Take a look at this post for graphs of ORP vs. FC and ORP vs. calculated hypochlorous acid (HOCl) level in real commercial/public pools and see which one you think is more correlated (in spite of a noisy portable ORP sensor). As for ORP and UV, does the effect you see only occur if the UV exposure is near the ORP sensor or does it also occur if the UV is only exposed to the bulk water and such water circulated to the ORP sensor? Was the UV stronger than sunlight or were you simulating sunlight on vs. off (such as by shading the sensor). Though there is a chemical equilibrium between chlorine attached to CYA (technically, this is a series of chemical compounds called chlorinated isocyanurates) and hypochlorous acid, with a strong UV the hypochlorous acid (and hypochlorite ion) will break down and it takes a short amount of time for the chlorine to release from CYA -- it takes about 0.25 seconds for half to be released so not very long and certainly fast enough such that the Free Chlorine (FC) tests all measure the total amount of chlorine including that which was bound to CYA. If the effect you saw only occurs when the UV is exposing the sensor itself and not just the water near the sensor, then there may be other effects of UV on the sensor itself. If the UV is very strong, then it can certainly deplete the chlorine faster than it is released, especially if this is being done near the sensor (as opposed to further away where there would be more time to recover). In practice, this depletion rate from the UV in sunlight is not nearly as strong as from a UV lamp though it is still probably measurable in ORP sensors (i.e. when the sun goes behind dark clouds), just not as dramatically.

Richard

[quantumchromodynamics]

The "chlorine smell" often associated with pools is the fragrant aroma of chloramines.

Monochloramine = NH2Cl

Dichloramine = NHCl2

Nitrogen Trichloride = NCl3

There are chlorine atoms in the chloramines that you say are outgassing. That means that you are admitting that chlorine is outgassing.

[quantumchromodynamics]

The "chlorine smell" often associated with pools is the fragrant aroma of chloramines.

Chloramines and trihalomethanes are especially volatile. Trichloromethane carries 3 chlorine atoms with it as it outgases.

You will also find that many of the pools that you service have very high levels of Dihydrogen monoxide. Dihydrogen monoxide volatilizes at very high rates during certain conditions.

[chem geek]

Very funny...dihydrogen monoxide (DHMO) is also discussed here.

I think Bill was referring to chlorine itself when he was talking about it not outgassing and he was just thinking of chlorine gas and didn't consider that hypochlorous acid itself could outgas. Clearly pools with higher bather loads have more chloramines so would outgas more chlorine, but I don't think outgassing of chlorine in pools with lighter bather loads is that strong -- at least not in pools with CYA since the effective chlorine level in such pools is on the order of 0.1 ppm. What is unclear is the actual rate of outgassing of bromine (hypobromous acid, actually), but I suspect it's lower than an equivalent pool with chlorine and no CYA, but higher than a pool with chlorine and CYA.

He is correct that chlorine gas from gas chlorinators does dissolve in water, but one can certainly turn it up too high and have chlorine gas escape before it fully dissolves and that's part of the art of what those people do -- if it's turned up to high then chlorine gas escapes while if it's turned down to low then the dissolving of chlorine gas makes the deep water too acidic and it also takes longer to add chlorine.

In fact, I suspect that chlorine gas outgassing happens with some saltwater chlorine generator (SWG) systems as well which may be part of the reason for the rise in pH (the other reason being the increased aeration which outgasses more carbon dioxide). If given enough time, chlorine gas very much dissolves in water forming hypochlorous acid and hydrochloric acid, but it does take time as it's a physical (diffusion) process.

[billp]

You will also find that many of the pools that you service have very high levels of Dihydrogen monoxide. Dihydrogen monoxide volatilizes at very high rates during certain conditions.

And what of the pools not included in the "many" you refer to above? Are you suggesting they don't have high levels of Dihydrogen monoxide? Why not??

[billp]

In fact, I suspect that chlorine gas outgassing happens with some saltwater chlorine generator (SWG) systems as well which may be part of the reason for the rise in pH (the other reason being the increased aeration which outgasses more carbon dioxide). If given enough time, chlorine gas very much dissolves in water forming hypochlorous acid and hydrochloric acid, but it does take time as it's a physical (diffusion) process.

Chlorine generators also make hydrogen gas which may contribute to pH rise.

"If given enough time..." Chlorine gas dissolves in water very rapidly.

[chem geek]

In fact, I suspect that chlorine gas outgassing happens with some saltwater chlorine generator (SWG) systems as well which may be part of the reason for the rise in pH (the other reason being the increased aeration which outgasses more carbon dioxide). If given enough time, chlorine gas very much dissolves in water forming hypochlorous acid and hydrochloric acid, but it does take time as it's a physical (diffusion) process.

Chlorine generators also make hydrogen gas which may contribute to pH rise.

"If given enough time..." Chlorine gas dissolves in water very rapidly.

Bill,

Definitely the hydrogen gas bubbles contribute to aeration of the water which can accelerate the rate of outgassing of carbon dioxide, but I ran calculations of how much volume of bubbles there are and the maximum amount of carbon dioxide (at equilibrium) they could carry out and it's not enough to explain the pH rise, though it certainly can contribute to it. I used to think that the pH rise was only from hydrogen gas outgassing, but between these calculations and also how lowering the TA helped but did not eliminate the pH rise in many pools I'm rather certain there are other things coming into play here. So there are other factors including the bubbles physically accelerating the outgassing near the surface (though unknown mechanisms). There is also a strong possibility of chlorine outgassing so I talk about that next.

I've talked to actual gas chlorinator people and most definitely they have to adjust their gas introduction rate (from nozzles at the end of hoses plaed at the bottom of a pool in the deep end) so that the chlorine gas does not escape from the pool. "rapidly" is a relative term. We're not talking about hours or days, but the chlorine gas does not dissolve in a few seconds but rather more like many seconds to perhaps a minute depending on conditions. Yes, the kinetics of chlorine gas already dissolved in water to hypochlorous acid is very fast (less than a second), but we're not talking about that. We're talking about chlorine in the gas phase separate from water and the diffusion of it into water and that is a physical process and not a chemical one. Since the hydrogen gas with its outgassing of carbon dioxide doesn't fully explain the pH rise and that lowering the TA level helps but does not fully solve the pH rise in quite a few SWG pools, the undissolved chlorine gas explanation was the next most logical thing to look at.

There also seemed to be a correlation between pools with a greater pH rise from SWGs and shorter runs from the SWG to the returns in the pool. However, a lot of this is rough anecdotal evidence so at this point it's just a strong hunch. If I run the actual numbers, the pH rise that is seen even at very low TA that would virtually eliminate carbon dioxide outgassing can be fully explained with a 10-30% chlorine outgassing (how much depends on the rate of pH rise).

Richard

[billp]

"If given enough time..." is an undefined statement. Since the discussion was about chlorine gas and chlorine generators I said "very rapidly" vs "enough time" assuming "enough" was more than several minutes. A gas applicator can inject as much (or more) chlorine in a pool in 5 or 10 minutes (approx) as a chlorine generator can make in a day. So in the chlorine generator discussion the rapid injection of Cl gas and possible saturation of a small area resulting in release to the surface is not an issue. The small amount of Cl gas being produced by a chlorine generator is in a well-mixed "closed" (not exposed to atmosphere) environment until it reaches the pool. That is enough time for all of the Cl gas to dissolve and form HOCl & HCl. This was demonstrated by Sophisticated Systems when they developed the Porpoise Cl Machine/Uniclor. It produces Cl gas which is injected through a venturi and it all dissolves before ever reaching the pool. These are the reasons I don't believe there is anyCl gas outgassing under normal conditions. Some Cl may be lost to chloramines but a well managed pool does not have significant chloramines (note: well managed).

I started looking at the links but there are soooo many, it takes time...

Re the UV-ORP interaction, the sensors/controllers have all been indoors so UV on the sensors is not an issue. It is just an interesting observation which indicates there is some kind of UV-CYA-Cl dynamic which affects ORP. The practical application is: make adjustments to the controller either in the daytime or at night, don't try both or you will drive yourself crazy.

[chem geek]

Mmmmm... well if chlorine gas is fully dissolving and if the increased aeration doesn't fully account for the pH rise, then we're still left with what is going on in quite a few SWG pools. The chemistry of the electrolysis is essentially identical to adding hypochlorite sources of chlorine so when accounting for chlorine usage it's pH neutral (the chemistry is described here. I could take a look at hypochlorous acid outgassing (chlorine gas dissolving in the water and hypochlorous acid going into the hydrogen bubbles), but the effect isn't as strong and my hunch is that wouldn't be enough to account for the pH rise. Certainly lowering the TA has helped a lot of SWG pools, but not completely in many cases. Back to the drawing board on that one, at least for a more complete answer (and possible solutions).

[billp]

...my hunch is that wouldn't be enough to account for the pH rise.

When trying to account for pH rise, are you taking into account the effect of plaster on pH? Even in well balanced water the pH in a plaster pool rises faster than in a fiberglassed pool, and the plaster pool will require significantly more acid. And a "cheap" plaster job (ie, low quality plaster) shows this effect more than a good quality plaster. At least in my experience...

[chem geek]

The plaster would tend to increase the pH in the first year or so, but we've seen plenty of pools with stable pH using hypochlorite before an SWG has been installed and then the pH rises much more after the SWG was put in place and this happens in vinyl and fiberglass pools as well. Lowering the TA helps somewhat, but for some pools it doesn't eliminate the problem. There's something else going on and we haven't quite put our finger on it. It's not in all SWG pools, but most have significantly more pH rise even after TA adjustment. Many have smaller pH rise with TA lowering and use of Borates, but they also have higher CYA levels (60-80 ppm) that let them turn down the SWG on-time so the SWG being on is definitely the primary source of the problem -- it's just a question of what output of the SWG are factors -- hydrogen gas bubbles with aeration/outgassing of carbon dioxide, chlorine gas dissolving to hypochlorite and possible outgassing. Clearly, the chlorine production itself raises the pH, but the consumption/usage of chlorine is acidic and the net result is the same as adding hypochlorite sources of chlorine (except for the extra salt).

[DEPontius]

Pardon me for jumping in on this thread, but I believe I'm having bromine/chlorine problems. Reading this thread has been helpful, as it seems to touch on those problems, so it seems a natural place to continue bromine/chlorine discussion.

For various purposes, this year I ended up buying my closing kit from a different source, and in all essence it's a bromine kit. But from what I can see, when all is done, it looks to me as if bromine, chlorine, and sunlight have all duked it out in my pool, and I've closed with only a trace of bromine/chlorine left as measured by an OTO kit.

I'm wondering where to go from here, because I think I'm potentially in real trouble in the Spring, if I understand my test kit readings.

Does OTO really read either bromine or chlorine?

Should I believe the low readings I see?

It has been suggested that I add polyquat 60 and more chlorine, as suggested in a "Pool Closing Procedure", either a thread here or a thread referenced from here. (troublefreepool.com ?) Is this a problem, adding these chemicals to a covered pool, since there can be no significant air exchange. I have a small pump that can probably do some circulation - is it necessary? - is it sufficient? My lines are already blown out, the equipment inside, the pressure valve disconnected and dangling inside the heater, open end down. It would be truly annoying to reconnect. The leaves are falling and there's quite a bit on the cover - it would be even more painful to get air exchange. I might almost prefer coping with soup in the Spring, at this point. I'd rather do something under the cover.

I normally run something like BBB during the summer, though this Summer I noticed my calcium hardness was low, so I ran calcium hypochlorite pucks in the skimmer, and added additional shock occasionally to keep the chlorine level high enough. I don't normally have algae problems, though there was a small bloom when we were away and the neighbors watched the pool, and it cleared up with a simple shocking.

I looked closer at the closing kit after following directions. The first chemical removed calcium (darn!) and iron. The second chemical was sodium bromide, and the third was a persulfate to activate the bromide. After all chemicals were added, I left it to circulate the recommended time, then ran shy of teflon tape while blowing out the lines. Full closing was delayed a few hours while I had lunch and got more tape. It was fair sun, not terribly strong, some shade.

Again...

Should I believe that OTO test that there's practically no bromine or chlorine left in the pool?

Can I add polyquat and more chlorine under the cover, as-is?

Do I need to circulate, and will a small auxilliary pump suffice?

Thanks for any suggestions.

[chem geek]

Should I believe that OTO test that there's practically no bromine or chlorine left in the pool?

Can I add polyquat and more chlorine under the cover, as-is?

Do I need to circulate, and will a small auxilliary pump suffice?

All of the sanitizer tests, OTO, DPD or FAS-DPD, will measure chlorine or bromine. The tests do not distinguish between these, though the units of measure for bromine are a factor of 2.25 higher than for chlorine so one needs to adjust for that depending on what test they think they are using (i.e. the units reported in that test).

Though the OTO test doesn't distinguish between Free Chlorine and Combined Chlorine very well and it can be hard to read various intensities of yellow, it does tell you if you have any sanitizer in the water. All of the chlorine tests can be fooled, however, by strong oxidizers. Persulfate (as opposed to monopersulfate) is strong enough to look like chlorine/bromine in the tests. Even monopersulfate can interfere, but usually only with the combined chlorine part of the test (so it could also show up in the OTO test). If your OTO test shows a very low level, then that does mean the sanitizer and oxidizer level is low.

PolyQuat is very viscous so would need circulation to mix well. You could try and do that manually, but it would be difficult. Pre-mixing in a bucket of water could help.

Yes, a small auxilliary pump would be OK, especially if left to run for an extended period of time, say 24 hours or so. That should mix the PolyQuat enough. The good news is that PolyQuat (unlike chlorine or bromine) isn't dangerous to the pool if it doesn't get well mixed. It's just not as effective if it doesn't get into all areas of the pool.

[DEPontius]

Would you advise adding (or avoiding) more chlorine, as well? I could premix with water, so the initial input isn't so concentrated. (same for the polyquat)