Quote Originally Posted by gonefishin
What is it with "unprotected" chlorine that allows it to be used up by a warm sunny day?

Also...

What is the interaction between CYA and chlorine that seems to protect and stabilize the chlorine in water?
WARNING: CHEMISTRY GEEK STUFF AHEAD!


Well, Cl is an unstable compound, and it happens to be very unstable in sunlight because its absorption maxima is around 290 nm. That's the UVB range that gives you sunburn. So, if you could erect a huge UV shield over your pool, you wouldn't need stabilizer. That's neither practical nor cost-effective though!

However, when you mix Cl with cyanuric acid you end up with chloroisocyanurates, a compound that the absorption maxima is well below 290 nm. This is great for us because wavelengths below 290 nm are absorbed by the atmosphere. This also happens to be why the sky is blue.

So that's the slightly techical short answer of why cya works.

Michael