Please clarify - question about shock - combined chlorine

[AmirB]

Can someone clarify how does shock removes combined chlorine. will combined chlorines be reduced if the pool is kept at the proper or slightly elevated levels over time? does shock accelerates the breakdown? If the combined chlorine is the byproduct of chlorine use then we should always have combined chlorine in the pool and it should accumulate in time if it is not constantly being removed. Does chlorine use due to sunlight creates combined chlorine also.
Thanks

Amir

 

[frustratedpoolmom]

Here's an earlier post on the subject: It may help understand? Duraleigh posted it on another thread...

What does Shock mean?
Quote:
Insufficient chlorination of ammonaceous or ammoniacal waters leads to the formation of a mix of chloramines (monochloramine, dichloramine, and even nitrogen trichloride) which irritate the skin and eyes and impart an unpleasant odor to the water.

At sufficiently high concentrations of active chlorine (hypochlorite, hypochlorous acid, and molecular chlorine), a phenomenon known as breakpoint chlorination occurs. In breakpoint chlorination, ammonaceous and ammoniacal materials are completely oxidized to dinitrogen and the active chlorine is simultaneously reduced to chloride. In order to accomplish breakpoint chlorination, swimming pools are shock-treated. Over the winter, a large amount of organic matter has accumulated in most swimming pools. At the beginning of the swimming pool season, the chlorine demand is too high for normal amounts of chlorine to completely destroy all the organic matter. Consequently, significant concentrations of chloramines and other partial oxidation products are present. In shock-treatment, a large amount of active chlorine is added so as to completely oxidize all of these products.

 

[chem geek]

Basically the breakdown (oxidation) of Combined Chlorine (CC) occurs continuously whenever there is Free Chlorine (FC) available in the water. Some CC also degrade on their own from the UV in sunlight while others are volatile and may outgas from the pool. There are different kinds of CC and some break down from chlorine more easily than others so some may build up over time. It is unusual to find CC in a properly managed outdoor pool (with good air circulation and sunlight exposure) unless there is a high bather load or large increase in organic matter (e.g. pollen).

The breakdown of chlorine by sunlight does not produce CC. It simply turns chlorine into chloride (salt).

You are correct that shock levels of chlorine accelerate the breakdown of CC.

Richard

 

[randytsuch]

Basically the breakdown (oxidation) of Combined Chlorine (CC) occurs continuously whenever there is Free Chlorine (FC) available in the water. Some CC also degrade on their own from the UV in sunlight while others are volatile and may outgas from the pool. There are different kinds of CC and some break down from chlorine more easily than others so some may build up over time. It is unusual to find CC in a properly managed outdoor pool (with good air circulation and sunlight exposure) unless there is a high bather load or large increase in organic matter (e.g. pollen).

The breakdown of chlorine by sunlight does not produce CC. It simply turns chlorine into chloride (salt).

You are correct that shock levels of chlorine accelerate the breakdown of CC.

Richard

So, if I keep my pool covered most of the time, is it more susceptible to some level of CC?
Basically, I only uncover it on one, or both days of the weekend, depending on what we are doing.

Randy

 

[frustratedpoolmom]

I keep my solar cover on alot too, I had a trace of CC's and I figured it was from that....

 

[chem geek]

My pool is covered most of the time and I rarely see more than 0.2 ppm CC, but you are right that a pool that is covered has a somewhat greater chance of accumulating CC since the CC will only break down from the chlorine without additional assistance of outgassing or sunlight breakdown. Of course, you aren't covering the pool while you are using it and having the pool open and in use is when the precursors to CC get into the pool so it sort of works out to be OK.

 

[AmirB]

Thank you all for the reply. Richard - that's what I was looking for. Thanks again.
Amir

 

[randytsuch]

Richard and FPM,
Thanks for the responses.
I saw a post one time where it said that it was bad to leave your pool covered all the time, and I did not understand why.
Now, it makes sense.

BTW, (knock on wood), after shocking my pool a month ago, CC has been basically 0, and it looks great, thanks to this place and the guru's here.

Amir
Sorry to hijack your thread.

Randy

 

[duraleigh]

Great post, Richard. Very timely.

It's really great having some one around that can explain these things rather than my block and copy from an educational website....your post makes much more sense to me.