Please explain relationship of CYA and sun; chlorine loss

[contrite&reformed]

I've hunted around on the site and have not found an explanation of the exact nature of the relationship between the CYA in the pool and the loss of chlorine levels when the sun is out. I read that if my CYA is at 50, chlorine won't be lost to the sun.

And I see this on one of the PoolSchool pages:

"If your CYA level is too low, most of your chlorine will be lost to sunlight, instead of fighting algae"

Can someone tell me exactly how the chlorine is lost to the sunlight? A chemcial reaction? If so, what?

And second question:

If my CYA is at 50, is the only other possible explanation for chlorine loss that organics in the pool (algae or other) are consuming it in a chemical reaction?

And third (and follow-up) question:

If organics are consuming chlorine in a chemical reaction, would I necessarily see the results of this in the form of some kind of precipitate on the bottom of the pool? Is there any way that large amounts of chlorine would be consumed, but there would be no evidence of precipitate on the bottom of the pool when the water is clear?

Thank you in advance for shedding light on this whole subject!

 

[jblizzle]

I will not go into the details, since I do not know them off hand

The higher the CYA, the more FC that is protected from the sun.
At a CYA of 50ppm, you may lose about 50% of your FC to the sun everyday.
At a CYA of 0ppm, you will lose all your FC to the sun in a few hours.

There is always some FC loss to the sun ... following the recommended levels for CYA and FC, you can expect to have to add 2-4ppm of FC everyday.

You can certainly lose a lot of FC to something in the water and not see anything ... one example would be ammonia in the water.

 

[contrite&reformed]

Thank you for your quick response. I'm actually hoping for the details.

I don't think ammonia is an issue here--no CC.

I'm curious: Where did you find the information regarding 50% loss of FC to the sun every day? And are you saying 50% of whatever amount happens to be in the pool?
Thanks again!

 

[Divin Dave]

basically what happens is that the UV from the sun breaks chlorine down very quicly.

Im borrowing the following from previous posts on this subject.
2OCl- --> O2 + 2Cl-

Liquid Chlorine produces Hypochlorite ions, which do the work of destroying organics. However, they're relatively unstable, and bombardment by UV light splits them into O2 (Oxygen molecules) and a Chlorine ion...which then, yes, outgasses.

There is much more technical detail in FC breakdown and use of CYA as a buffer in this post. This should hopefully do it for you on the technical details
Pool Water Chemistry

 

[contrite&reformed]

Thank you for this part! I'll look it over carefully.

 

[contrite&reformed]

Reply to jblizzie,

Ah, the link provided by Divin' Dave below provided an answer to my question regarding your post about the percentage of chorline loss and the CYA:

"A rough rule of thumb that applies is that at a pH of 7.5, the effective chlorine level is reduced by a factor about equal to the ppm of the CYA. So, a CYA of 30 ppm reduces the disinfecting chlorine (HOCl) level to about 1/30th of what it would be with no CYA."

So, as I read this, at a pH of 7.5 and a CYA level of 50ppm, the disinfecting chlorine level would be reduced to 1/50 of the level with no CYA, not allow the sun to reduce the chlorine by 50%.

 

[jblizzle]

You are mixing 2 different pieces of information. I stand by my statement as a rough ball park figure.

 

[Richard320]

Scroll down to Chlorine/CYA Relationship in this thread

 

[contrite&reformed]

Thank you!

 

[JoyfulNoise]

See this Thread for a detailed discussion of chlorine chemistry in water

 

[contrite&reformed]

Thank you!