Ph Effects On Chlorine

[Hillbilly Hot Tub]

I have not seen this mentioned in the forum very much and think it is very important for spa owners to know. This information is taken from the National swimming pool foundations CPO handbook where they reference RW Lowry for the facts

(I can not seem to get these to space correctly, HOCl is listed first, then PH then OCl)

Active HOCl PH Less Active OCl

97% 6.0 3%

91% 6.5 9%

76% 7.0 24%

66% 7.2 34%

50% 7.5 50%

33% 7.8 67%

24% 8.0 76%

9% 8.5 91%

Hypochlorous acid (HOCl) is the active killing form of chlorine. Hypochlorous acis is constantly dissociating into and reforming from hypochlorite ion (OCl-) and free hydrogen. Hypochlorous acid and hypochlorite ion are in equilibrium with each other, which means a hydrogen ion attaches to make HOCL and detaches to make OCl- many times a second. If some HOCl molecules are consumed, in an instant OCl- ions attach to H+ to maintain the same ratio of HOCl and OCl- ions. As the PH goes down, the water is more acidic, there are more H+ ions in the water and the more HOCl is formed. As HOCl is used, OCl- immediately converts back to HOCl to maintain the equilibrium according to the PH. If a pool contained 3 ppm of chlorine at a 7.5 PH, there would be 1.5 ppm HOCl and 1.5 ppm of OCl-. If 1 ppm is consumed, the free chlorine would drop to 2 ppm, still safe killing chlorine. If the PH rises to 7.8 and the free chlorine is at 2ppm, the HOCl would be .66, not so safe killing chlorine and the OCl- would be 1.34, the shift is instantaneous.

It is very important for chlorine effciancy to keep your PH in check. As you can see, if you keep your PH on the higher side your chlorine is much less effective.

Bromine is not effected as much by PH

HOBr (active) PH OBr (less active)

100% 5.0 0%

100% 6.0 0%

98% 7.0 2%

96% 7.2 4%

94% 7.5 6%

87% 7.8 13%

83% 8.0 17%

Because PH can change very rapidly in a hot tub, this is another reason bromine is "better" choice in hot water environments. I have better in qoutes because your comfort is key to what system you choose.

[chem geek]

The CPO book and Lowry are completely and totally wrong when they show the concentration of hypochlorous acid vs. pH when there is Cyanuric Acid (CYA) in the water. The chart you show is only valid when CYA is zero (and only at 77F, not spa temps). I show a comparison of the traditional graph with no CYA vs. the correct graph at a typical FC that is around 10% of the CYA level in this post. The log graph with CYA is the same as Figure 14.5 in the paper linked to on this web page except that I have consolidated all the chlorinated cyanurate species (i.e. chlorine combined with CYA) into one blue "Cl-CYA" curve for simplicity. Note that these graphs are at 77F temperature.

At a pH of 7.5 with no CYA, roughly half of the chlorine is hypochlorous acid and half is hypochlorite ion. Actually, at 104F spa temperatures, it's 42% hypochlorous acid and 58% hypochlorite ion. However, with 3.5 ppm FC and 30 ppm CYA, the hypochlorous acid concentration is around 6.6% of the FC amount and the hypochlorite ion concentration is around 9.2% of the FC amount. The other 84.2% is in the form of chlorine bound to CYA in a series of compounds called chorinated cyanurates.

At a pH of 8.0 with no CYA and 104F spa temperatures, the hypochlorous acid is 18% while hypochlorite ion is 82%. With 3.5 ppm FC and 30 ppm CYA, the hypochlorous acid concentration is around 5.0% of the FC while hypochlorite ion is around 22.3%. The other 72.7% is chlorine bound to CYA.

So going from a pH of 7.5 to 8.0 with no CYA has the active chlorine, hypochlorous acid, go from 42% to 18% of the FC, or a reduction of 57% of the original amount. With CYA in the water, however, the active chlorine goes from 6.6% to 5.0%, or a reduction of 24% of the original amount. Of course, the active chlorine amount with no CYA in the water is so much higher to begin with that a drop of 57% isn't a big deal. In absolute terms, at a pH of 8.0 with 3.5 ppm FC and no CYA, there is 0.65 ppm active chlorine while at a pH of 7.5 with 3.5 ppm FC and 30 ppm CYA there is 0.23 ppm active chlorine. So the drop in active chlorine, as dramatic as it sounds, is really moot. The fact is that chlorine with no CYA in the water is way, way too strong to begin with.

The bottom line is that CYA acts as an active chlorine (hypochlorous acid) buffer, resisting changes in its concentration including that caused by changes in pH. So while it is true that there is less active chlorine at higher pH, it is not true that it drops as dramatically as shown in the traditional industry graph that does not account for CYA in the water. Bromine doesn't get affected by typical spa pH as much as chlorine, but chlorine with CYA doesn't get affected as much as chlorine with no CYA.

By the way, I've written to NSPF that does the CPO courses as well as to APSP that does the TECH courses yet neither has changed their courses to reflect the truth of what happens with active chlorine when CYA is in the water in spite of even meeting several of the key people in person. They won't even add the basic chemical facts of how much CYA or CH increases from Trichlor, Dichlor and Cal-Hypo using the simple rules that relate these to FC added and are independent of concentration of product or size of pool or spa. I've also commented on pending pool standards including APSP-11 and CDC MAHC. I have been trying to get this industry to start telling the truth, but have made absolutely zero progress as far as I can tell. Apparently, since I'm just a homeowner with a pool and don't work in the industry and am not in academia, I have no credibility even though I'm only referring to existing sources and chemical facts that no one has denied. If anyone has some suggestions as to a better approach, I'd like to hear it. For example, HHH, you said you were going to talk to a chemist or technical person at one of the courses -- did you get a chance to do that and what did they say? I'd love to communicate with them if they need more info.

Richard

[Hillbilly Hot Tub]

This was at 86 degrees, not 77, but yes....with no CYA in the water. They are actually passing laws in many states that CYA can not be used in indoor pools or in hot tubs because of the issue with CYA getting high and making chlorine less effective.

As far as 1 or 2 people changing the minds of the ruling "pool and spa gods", good luck. It takes a lot to get them to budge and by law we in the industry must follow their rules or be struck down by a cryto ball !!!

I will PM you with some other info....

[chem geek]

This was at 86 degrees, not 77, but yes....with no CYA in the water. They are actually passing laws in many states that CYA can not be used in indoor pools or in hot tubs because of the issue with CYA getting high and making chlorine less effective.

As far as 1 or 2 people changing the minds of the ruling "pool and spa gods", good luck. It takes a lot to get them to budge and by law we in the industry must follow their rules or be struck down by a cryto ball !!!

I will PM you with some other info....

I've seen the laws saying no CYA in indoor pools or hot tubs, but it's ridiculous since they should just set a low CYA limit, such as 20 or 30 ppm, and not say no CYA at all. With no CYA in the water at all, the active chlorine concentration is WAY too high and there is no way one can easily manage to have only 0.1 or 0.2 ppm FC which is all that is needed for very fast sanitation. It's also completely inconsistent with outdoor pool in terms of active chlorine concentration. My wife goes through swimsuits degrading every winter season because the indoor community center pool has no CYA and around 2 ppm FC compared to our own outdoor pool with 3-4 pm FC and 30 ppm CYA that is technically equivalent to around 0.1 ppm FC with no CYA. The indoor pool effectively has 20 times the concentration of active chlorine so having her swimsuits, skin and hair get oxidized 20 times faster is not a surprise. There are also likely side effects to having 20 times the rate and amount of very irritating nitrogen trichloride from ammonia oxidation by chlorine.

Thanks for your support. Just bringing up these questions in your CPO classes would be helpful and referring them to the definitive 1974 technical paper (especially if they are chemists) would also be helpful.

Richard

[Hillbilly Hot Tub]

Maybe you could get a local pool and spa store to sign you up as an "part time employee" so you could attend some of these classes. Maybe that would help get your foot in the door.

I would help you out, but being on the oppisite side of the country, I think your travel would be a bit costly

[Elbyron]

Because PH can change very rapidly in a hot tub, this is another reason bromine is "better" choice in hot water environments. I have better in qoutes because your comfort is key to what system you choose.

Anyone who's researched the differences between chlorine and bromine systems has probably already heard that bromine is less sensitive to pH. But did you know that bromine is also better at handling ammonia?

Chlorine preparations will react with ammonia to produce monochloramine (or sometimes dichloramine or trichloramine), which have some disinfecting ability but not as much as hypochlorous acid and far lower than that of hypobromous acid. But bromine systems will react with ammonia to produce a mixture of mono-, di- and tribromo amines. These decompose quickly and are good at disinfecting: somewhat better than that of BCDMH, and far better than that of chloramine.

In my research I also discovered that the volatility of hypobromous acid is about tenfold less than that of hypochlorous acid. So bromine is more stable - which I assume just means that you don't need to keep adding as much.

Bromine has a lower oxidizability than chlorine, meaning that it won't corrode metals as easily.

In some rare conditions, such as with the presence of humus acid, chloroform or bromoform can be produced. If you watch crime dramas you've probably heard of the former. Both are known carcinogens (cause cancer) but the main difference is that bromoform is quickly decomposed in water, while cholorform is not.

Some say that the bromine smell is harder to shower off than the chlorine smell. But in terms of comfort, I think most people would find bromine to be better... I'm not a vendor or anything, just a homeowner who's done lots of research.

[chem geek]

I mostly agree. When bromine combines with ammonia to form mostly monobromamine, this is still a sanitizer whereas when chlorine combines with ammonia to from chloramine it is not a good sanitizer (about 30-50 times less effective, not accounting for the CYA effect on chlorine).

The volatility of hypobromous acid vs. hypochlorous acid isn't definitive. As shown in the Henry's Law constants <a href="http://www.mpch-mainz.mpg.de/~sander/res/henry.html" target="_blank">here</a>, hypochlorous acid has a Henry's Law constant of 260 to 930 depending on the scientific source. Hypobromous acid has a Henry's Law constant of 1.8 to 6100 depending on the scientific source. Lower numbers are more volatile. I agree that it's most likely that bromine is less volatile than chlorine and am not sure of why there are some sources with low Henry's constants for hypobromous acid. I would trust Blatchley's numbers of 930 for chlorine and >1900 for bromine.

Bromine has a lower oxidation potential than chlorine, but chlorine in the presence of Cyanuric Acid (CYA) has very low effective concentration, on the order of 0.2 to 0.6 ppm FC with no CYA. So the difference isn't as strong as may first appear since the bromine concentration is not moderated. This lower effective concentration also reduces its effective volatility though bromine is still probably somewhat less volatile.

If one has an ozonator, then bromine can get oxidized to bromate which is a suspected carcinogen. Both bromine and chlorine can form various disinfection by-products, but I wouldn't necessarily say one is that much better than the other in this regard, especially at the low effective chlorine levels when CYA is present. [EDIT] Actually, more recent research is showing that the brominated trihalomethanes (THMs) are worse than the chlorinated THM (i.e. chloroform) in terms of genotoxicity and carcinogenic potential. [END-EDIT]

Both bromine and chlorine are fine sanitizers. Bromine breaks down in sunlight (as does chlorine), but is not protected by combining with CYA so is not efficient to use in outdoor pools. It's also more expensive. So for indoor or covered spas, it makes sense as a nice alternative. It is also more convenient with the use of bromine tabs in a floating feeder. If some people didn't find the smell objectionable, it would probably be the main choice for spas. It should also be noted that most people who use chlorine in residential spas end up adding it after the soak so don't really soak in any sanitizer at all so it's not really a fair comparison (i.e. comparing soaking in bromine vs. soaking in nothing, but adding chlorine after the soak).

Richard

[Hillbilly Hot Tub]

People keep talking about the smell. I have found that Brilliance chlorine free bromine (chunks, not tablets) and the salt systems that produce bromine do not have that strong smell. it really does not smell much at all. I still think the smell comes from mixing bromine and chlorine tegether, maybe also because you are not forming chlorimines since there is no chlorine....just my 2 cents

Also, its great reading all of this information and reasonable debates!

[chem geek]

Thanks. I keep forgetting that you've used the DBDMH bromine-only tablets and haven't found an objectionable smell. Most people use the BCDMH bromine-chlorine combo tablets. On the other hand, bromine does have a unique smell of its own and some people using the 2-step method, which is just sodium bromide plus shocking to make only bromine, have still commented on the smell. So there are probably some people who don't like the smell -- it's different than chlorine, but with lower volatility it may not be as strong (as you point out).

It's certainly easy enough for people to try both methods and see which they prefer.

[Hillbilly Hot Tub]

It does have a slight smell, just nothing like traditional tablets or the one step. It gets stronger as time goes on, about 3 months, but it is due for a water change by then. Bromine is more drying to my skin than chlorine seems to be, if I keep the chlorine level low and suppliment with N2 and MPS.