Probably a dumb question but...

[snazzy]

Can someone please explain the chemistry behind FC and CC? (or just tell me where I am wrong here in the below assumptions). If you start with a TC of 10, where FC = 6, and CC = 4. Then you add 1 ppm bleach then (assuming pitch black/no sunlight effect and no time, yet, to work on killing algae) your new TC should be 11. I think FC can be anywhere from 6-11 and CC can be anywhere from 0-4. Is that correct or will your CC only go down 1 ppm because that is how much bleach you added (is there a correlation between what you add and how much CC can go down?) Just been buggin me.

Thanks!

 

[jblizzle]

It does not work like that. The bleach adds FC. CC are bad and the result of things breaking down in the pool. The sun breaks down both the FC and the CC.

Ideally the CC should be zero. When you add chlorine you are adding FC.

Not sure if that helps our not ... certainly not going into the chemistry details that other posts do.

 

[woodyp]

FC can be much, much higher. I really don't know how high CC can go. Two different animals. See the chlorine/cya chart in pool school to get an idea.

 

[snazzy]

I should have been more clear that this is a hypothetical question. Any chem. teachers out there?

 

[TreeFiter]

I think what snazzy is trying to ask is what actually happens when you shock a pool to get rid of CCs. Are they destroyed? Are they converted back into FC? What is happening chemically? This is a really good question, which I also have wondered about.

 

[anonapersona]

Yes, CC are destroyed and consume some more chlorine as they go.

We don't really care about TC but that some tests only tell you TC. We know that some of TC is actually CC which is not useful and in fact is a problem needing to be addressed.

I think that in the example question, first CC of 4 is awfully high, but if CC was 4 and FC was 6 so TC was 10. Overnight, assuming a lot of organics in the pool to explain the huge CC level, FC might drop to 2, CC might go up to 6, so TC might be 8 by morning. Then with sun and wind, some of the CC should be carried away. TC would be falling overnight, as the FC was consumed by both the organics and the CC. CC could go up or down, probably depending on how much FC there was to complete the chemical processes. Kinda like products of combustion, need to keep the flame burning bright to burn them up completely.

 

[JamesW]

Most CC will be monochloramine (NH₂Cl). Monochloramine results when chlorine displaces a hydrogen in an ammonia molecule.

HOCl + NH₃ --> H₂O + NH₂Cl

Ammonia is broken down when chlorine oxidizes the nitrogen in the ammonia from the -3 state to the 0 state, where the nitrogen combines with another nitrogen to become nitrogen gas N₂.

It takes 1.5 chlorine to oxidize 1 nitrogen. Since monochloramine already has 1 chlorine, it only takes another 0.5 chlorine to complete the reaction.

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

For example, if you had 7 FC and 3 CC, you have a total of 10 TC. 3 CC takes 4.5 total chlorine to oxidize the nitrogen to nitrogen gas. Therefore, you would be left with 5.5 FC after the ammonia is oxidized.

There will be some dichloramine and trichloramine formed in the process, but the basic idea is that it takes 1.5 times the CC to fully oxidize the ammonia (assuming that all of the ammonia is in the form of CC). Typically, you need to go to shock level to get the process to happen at a reasonable rate.

Note 1) Most ammonia will be in the form of the ammonium ion. The net reaction can be shown as:

2NH₄⁺ + 3HOCl --> N₂(g) + 5H⁺ +3Cl^- + 3H₂O

Note 2) Some of the nitrogen is oxidized to nitrate, and can increase the overall amount of chlorine needed to oxidize all of the ammonia.

Note 3) It's not all ammonia. There are many contaminants in swimming pool water. Swimmers contribute many contaminants, such as hair, lotion, saliva, skin, cosmetics, hair care products, sweat, urine etc. Disinfection byproducts other than chloramine, such as trihalomethane, can occur

The process is speeded up when UV light hits the monochloramine.

3NH₂Cl + UV --> N₂ + NH₄⁺ + 2H⁺ + 3Cl

Dichloramine and trichloramine are also degraded by UV light. This is why indoor pools can benefit from UV systems. UV can also help provide protection from chlorine resistant organisms, such as Cryptosporidium.

 

[snazzy]

Perfect! Thank you for answering ;-)