Breakpoint Chlorination - "Point of no return"

[Adam_FL]

A number of State Health Department publications mention a "point of no return" when attempting to reach breakpoint chlorination.

The Indiana State Department of Health published a guide that states:

"To achieve the breakpoint, the free chlorine (FC) added to the water must be about ten times the amount of combined chlorine (CC). This is an “all or nothing” process. Not adding enough chlorine to reach breakpoint will make the problem even worse as the result is the formation of more chloramines and re-dissolving of chloramines back into the pool water. Continual “shocking” but not reaching breakpoint will result in the pool reaching a point of no return. Partial or complete draining of the pool water and refilling with fresh water may be the only remedy at this point."

https://www.in.gov/health/files/How_To_Shock_The_Pool.pdf

Is this true?

If this is true, what is the chemistry surrounding the "point of no return"?

 

[JamesW]

Complete nonsense.

 

[Adam_FL]

Thanks for the quick reply.

 

[JamesW]

They don’t even get the basic science right.

Specifically, breakpoint chlorination is the point at which enough free chlorine is added to break the molecular bonds; specifically the combined chlorine molecules, ammonia or nitrogen compounds.

Wrong.

Chlorine atoms take two electrons as they go from chlorine to chloride.

This is called “oxidation”.

The chlorine oxidizes the nitrogen in ammonia.

The chlorine is not breaking molecular bonds.

When the chlorine oxidizes the nitrogen, the nitrogen no longer bonds to hydrogen to form ammonia because the nitrogen is in the wrong oxidation state to create ammonia.

The nitrogen mostly loses 3 electrons to become nitrogen gas.

It takes a ratio of chlorine to ammonia atoms of 7.6 to 1 to reach breakpoint, other contaminants (i.e. bacteria, algae) are also present that must be oxidized, so 10 times the amount of combined chlorine must be added.

Chlorine atoms take 2 electrons and nitrogen loses 3 electrons, so the ratio is 1.5 chlorine atoms to 1 nitrogen atom.

Chlorine can replace 1, 2 or 3 of the hydrogen atoms in ammonia to form monochloramine, dichloramine or trichloramine and these can be relatively stable.

So, the process can get “stuck” if you don’t add enough chlorine to overcome the tendency to form stable molecules rather than oxidize.

But, there is no “point of no return” because you can always add more chlorine to continue the process.

For high use indoor pools, the buildup of chloramines can be a serious problem because ammonia and other nitrogen and carbon compounds are continually being introduced faster than the chlorine can oxidize the nitrogen and you get persistent chloramines unless you use a dangerously high amount of chlorine.

UV light is especially helpful because it knocks electrons lose while the chlorine is pulling on the electrons and this eliminates the chloramines.

For low use outdoor pools, the natural sunlight is enough to help prevent any persistent or problematic chloramines.

For a high use indoor pool, you need to use a UV system to eliminate chloramines.

 

[Adam_FL]

Thanks again for the detailed explanation. Your guidance is much appreciated.

 

[JamesW]

The idea of a “point of no return” really does not have a scientific basis unless they are talking about a situation where bather waste is continually being added and it is accumulating enough that it becomes impractical to try to break it down with chlorine alone.

You can get to a point where it makes more sense to just drain and refill rather than try to deal with all of the accumulated disinfection byproducts that can accumulate in a pool or hot tub.

 

[JamesW]

If an indoor pool facility has inadequate air exchange with outdoor fresh air, it will be necessary to add air circulation fans with doors and windows open to keep the air above the pool water level moving to prevent re-dissolving of nitrogen (by product of breakpoint chlorination) leading to more chloramine formation.

This is not accurate.

Any nitrogen gas that forms is not going to redissolve and form chloramine because the nitrogen is not in the correct oxidation state to create ammonia.

The main reason for ventilation is to get rid of the nitrogen trichloride, which is a dangerous toxic chlorinated gas.

Small amounts of it might redissolve but it should not be much and it will not increase the chloramine level to any higher than it ever was.