Breakpoint Chlorination

[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