CYA and Lifetime of Chlorine

[chem geek]

I looked up the absorption coefficient of light in water and I knew that blue penetrated more deeply (which is why water looks blue -- the longer wavelengths get absorbed at shallower depths and they also scatter more). The absorption rate may be seen in a graph at this link where the absorption in the 235-240 nm range that is the peak area where hypochlorous acid (and hypochlorite ion) breaks down from UV is 0.0005 cm^-1 = 0.05% per cm = 1.5% per foot so there is very little absorption of UV from the water in pool depths. Even at 8 feet the light intensity is exp(-0.015*8)=88.7% so only an 11% loss. For red light the coefficient is 0.01 cm^-1 = 30% per foot so at 8 feet the light intensity is exp(-0.30*8)=9.1% so nearly 91% of red light is absorbed.

[EDIT] It is true that the shorter wavelengths near 200 nm are similar to the red wavelengths in that water strongly absorbs such wavelengths. So the real calculation is to take the spectra of sunlight and multiply it by the absorption (actually, the photolysis) spectra of hypochlorous acid and hypochlorite ion vs. wavelength and multiply that by the absorption by water vs. wavelength to see the net effect. I can't find the photolytic absorption and breakdown of chlorine vs. wavelength anywhere, so if anyone can find that then this would be helpful. [END-EDIT]

The bigger problem with the CYA absorption theory is that the absorption spectrum in the link in the first post in this thread shows that the CYA absorption is at lower wavelengths and doesn't appear to be near 235-240 nm. [EDIT] However, later calculations in posts below show that it only takes a relatively low absorption to have a significant effect and that chlorine itself shields lower depths essentially through sacrificial breakdown. [END-EDIT]

Richard