Breakpoint Chlorination

[chem geek]

Yet another update in this continuing saga. This thread seems to indicate that perhaps the "ppm Chlorine" definition is based on the Chlorine gas (Cl2) equivalent where chlorine gas is 100%. I know that this is what is used for what is termed "available chlorine" when comparing different chlorine sources, but I did not know if that is what is also used for measuring chlorine from the test kits. This seems doubful and strange, but this wouldn't be the first time an unusual convention is used in the pool industry. I'll keep y'all posted. [EDIT] So far, most every source I can find says that ppm chlorine has the "part" in part per million be the weight of chlorine and the context has always been HOCl + OCl- (CYA is not talked about so Cl-CYA is included as if it were HOCl/OCl-). [END-EDIT]

[SECOND-EDIT] I found the answer! It's at this website where it says "Although Cl2 does not exist in potable waters, for historical reasons FAC, CAC and total chlorine are still expressed as mg/L as Cl2" so I need to correct my spreadsheet. This will somewhat effect all of my graphs, tables, etc. calculating ppm HOCl from FC (the FC numbers will go up by 35% holding everything else constant). The points and principles won't change, but some of the specific numbers will. I'm sure Michael will be very relieved to know that the proper adjustment to his algorithm in his program is quite minor and can possibly be ignored. [END-SECOND-EDIT]

Richard

[chem geek]

This is a continuation from this post where the comments in bold are from waterbear (Evan)

I don't have a salt cell, but even at full power and running 24/7 you are right that the increase in chlorine levels attained in the pool will be much slower than dumping liquid chlorine into the pool. However, if one is starting from an existing free chlorine level then whatever chloramines that would have been formed have already done so.
If the organic load is higher than the chlorine available then more chloramine would form as the chlorine was increased, correct? From my understanding it is only after all the ammonia and organics present have combined that breakpoint can be reached.
Free chlorine combines with ammonia very, very quickly to form chloramines. It is the next steps to breakpoint that are slow (and free chlorine oxidizing organics can also be slow). So from a chemistry point of view, I don't think the slow increase in FC from a salt cell would be an issue unless the FC were at or near zero for some reason -- perhaps if there were a major ammonia (i.e. urine) accident that overwhelmed much of the free chlorine in the pool, remembering that you need about 10 times as much FC to achieve breakpoint as there is ammonia.

My understanding of the chemistry is that incomplete breakpoint (from a slow rise in chlorine) would favor the formation nitrogen trichloride (even at higher pH) and if the organics are from complex sources such as algae and not just ammonia (which is often the real world case when we 'shock') then it would cause the formation of other disinfection by products such as organochloramines, which are difficult to break down. Also reaction would not go to endpoint, which would be the release of nitrogen gas as the ammonia is broken down but cause the formation of nitrates in the water, which are a souce of food for algae.

Urine is about 2.5% urea by weight and urea is about 50% nitrogen (ammonia) so 2 cups of urine produces around 5 grams of ammonia and that requires about 50 grams of chlorine for breakpoint. A 10,000 gallon pool is about 38,000 liters fo 50 grams of chlorine in 38,000 liters is 1.3 ppm. So depending on size of pool, FC level, and size and number of accidents, one could use up all the FC in a pool. And certainly one uses up the FC in a local area rather quickly.

So if you need to superchlorinate because of an "accident", then I agree that quickly administering a large dose of chlorine is wise so that breakpoint is more easily achieved. If instead it's just a small amount of measured combined chlorine that has accumulated, possibly from slow combining with organics (not ammonia), then a slow rise in chlorine would probably be fine. Of course, the issue of the life of the salt cell is real and adding liquid chlorine is easy and relatively inexpensive.

[EDIT] In the presence of CYA, the breakpoint chlorination process is slowed down considerably since CYA reduces the disinfecting chlorine concentration. A normal breakpoint at an FC of 2.0 ppm with no CYA takes around 30 minutes to effectively complete so with 30 ppm CYA this would take about 11 hours if there was no sunlight. I don't know how much faster breakpoint goes with sunlight. [END-EDIT]
Once again this seems to indicate that the slow rise in FC caused by superchlorinating with the cell is not an effective way to reach breakpoint.

Richard

It is true that since the formation of monochloramine (chlorine plus ammonia) is a very fast reaction that this will occur before any formation of dichloramine or trichloramine and that these occur roughly in sequence because each one is created from the previous one. You are also correct that if the chlorine demand from ammonia and organics is larger than the amount of chlorine, then all of the chlorine would get used up and a combination of monochloramines (and chlorinated organics) and leftover ammonia (and organics) would result.

It is not at all necessary to form anything beyond monochloramine to get to breakpoint, assuming you have enough chlorine available. The following reaction can "break" monochloramine directly:

2NH2Cl + HOCl --> N2(g) + 3H+ + 3Cl- + H2O

Monochloramine and dichloramine are the dominant species between a pH of 4.5 to 8.5 though monochloramine is most dominant above a pH of 8.0 while trichloramine is dominant below a pH of 4.5. So, the ideal pH for breakpoint chlorination would be between 8.0 and 8.5. However, normal pool pH above 7.0 and especially around 7.5 will mostly form monochloramines with some much smaller amount of dichloramines.

At lower pH, there is more HOCl present (relative to OCl-) though there is less NH3 present (relative to NH4+). Since the reaction time for the conversion of free chlorine to monochloramine is slower at lower pH, it would appear that the NH3 concentration is the dominant factor. At a pH of 7 and 25C temperature with 2x10^(-4) moles/liter HOCl (actually 14.2 ppm Cl2) and 1x10^(-3) moles/liter NH3, it takes 0.2 seconds to form monochloramine.

What is not clear to me is whether the formation of di- and tri-chloramines at lower pH is due to the slower reaction rate for the formation of monochloramine or specifically to the higher hydrogen ion concentration. I'll continue to research this to see if I can figure this out since clearly the presence of CYA significantly reduces HOCl concentration and would slow down reactions involving that species, so could significantly affect how breakpoint chlorination works (or doesn't). The CYA would not affect the stoichiometry, so there can still be sufficient chlorine to achieve breakpoint, but the slowdown in the reactions could produce different products. I have also not found literature on the photolytic breakpoint to explain whether sunlight can also breakdown combined chlorine, though the reference I give later in this post does refer to monochloramine being prone to decomposition by light and heat.

So the bottom line at this point is that we don't know what's going on. It's one thing to have a pure chlorine and ammonia mix, but quite another when CYA is present, or when sunlight is present, etc.

[EDIT] Here's an interesting link that summarizes both existing literature on breakpoint chlorination and gives some real-world measurements, albeit with no CYA (since it's for sewage treatment). I'll see if I can work with the rate and equilibrium constants to come up with some meaningful conclusions. [END-EDIT]

Richard

[waterbear]

[EDIT] Here's an interesting link that summarizes both existing literature on breakpoint chlorination and gives some real-world measurements, albeit with no CYA (since it's for sewage treatment). I'll see if I can work with the rate and equilibrium constants to come up with some meaningful conclusions. [END-EDIT]

Richard

Richard, Funny you should post that link. That is one of the sources that I was looking over before posting my comments! I am continuing to research this also. There really does not seem to be any definative answers from what I have been able to find and there are a lot of 'real world' variables that need to be taken into consideration. My main interstest would be if a slow rise in chlorine over a, say 24 hour period, would be as effective as a fast addtion of chlorine all at once to reach breakpoint since this the main difference between shocking with a SWG and shocking with the manual addition of chlorine. It might be interesting to also try and determine what difference shocking at night vs. shocking during the day might produce since from what I have been able to research does indicate that sunlight is effective in breaking down the chlorine-ammonia bond (at least for the lower, non-organic chloramines). It does seem that most of the research on the subject is from the field of sewage treatment and not specifially pools.

Edit: I was able to find some info that indicates that incomplete oxidation of organic ammonia compounds by chlorine will lead to the formation of trihalomethanes (chloroform seems to be the most commen one formed in pools) which are very difficult to break down by normal means. Not sure what effect sunlight would have on them.

[chem geek]

Evan,

Here is another interesting source of information giving rate constants for a variety of reactions. I verified that these constants are consistent with those in the earlier link (note the constants are in "per hour" rather than "per second"). Unfortunately, some of the data in this table is incomplete and some seems just plain wrong as there is no path to breakpoint that occurs in a reasonable time. The original paper I read on CYA equilibria was in a book that also had breakpoint data that pretty much had breakpoint occurring in around 30 minutes or so for 0.1 - 1.0 ppm ranges of chlorine and ammonia. If you find anything that makes sense, let me know. At least I've got logical reaction times even with CYA for monochloramine (seconds) and dichloramine (minutes) formation. I just need the rest of the breakpoint times so I can figure out dominant species.

As for the THMs, these appear to occur when organics are present and happen more when the pH is [EDIT] higher [END-EDIT]. It isn't so much an incomplete breakpoint that makes them happen as just having chlorine available to combine with such organics -- if there's enough chlorine, then THMs can form. The newer approach to water treatment using chloramines avoids the THMs because the chlorine is all consumed with the fast reaction to form chloramines and doesn't combine with organics. That's not what we do in pools as we usually have enough chlorine for continual breakpoint (hopefully).

At higher pH, including those of typical pools, there are [EDIT] more [END-EDIT] THMs formed, but [EDIT] fewer [END-EDIT] VOC (general volatile organic compounds) so there doesn't seem to be any way around getting something unless you use KMPS non-chlorine shock to get the organics oxidized before chlorine can combine with them. Fortunately, the rate of production for these nasties is low, especially with the use of CYA that makes effective chlorine levels very low, so whatever gets produced gets swept away by a light breeze quite readily. The same cannot be said for indoor pools that have no CYA and therefore high chlorine levels and generally poor air circulation. This is why I propose either using KMPS in indoor pools (as a preventative weekly oxidizer) and/or use a small amount of CYA (< 10 ppm) to keep the chlorine levels lower (while still being able to maintain a large FC residual). This use of CYA would also make swimsuits last longer which I know my wife would appreciate (the rubber in her suits deteriorates in the winter when she uses an indoor pool -- never had a problem when using our outdoor pool in the summer).

Richard