Quote Originally Posted by mwsmith2
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
More chemistry geek stuff!
Chloroisocyanurates are a form of 'combined chlorine' that fall in a group known as chloramides. They will test as FC but do not have as high an "oxidantion potential" as FC. This means that they are not quite as good an oxidizer and sanitizer. Also , as more cyanuric acid is added to the water it shifts the equalibrium of the reaction to less FC and more chloroisocuanurates which is why you need to put more unstablized chlorine in when the stabilizer is higher...to shift the equilibrium point back and balance out the amount of FC and cloroisocyanurates in the water once again. This is why Ben's 'best guess' chart works. Once again, slightly short of a technical answer.