Chlorine/pH/CYA Relationships

[milynor]

WOW! Taking me back to chem101! Still don't understand it all, but I'm working on it. Is the chlorine "used up" faster with lower stabilizer levels with normal PH (7.5)?

[chem geek]

Is the chlorine "used up" faster with lower stabilizer levels with normal PH (7.5)?

Yes, the chlorine is "used up" by sunlight (UltraViolet radiation) faster with low or zero stabilizer levels. The graphs in this thread don't show the breakdown rates of chlorine with and without stabilizer. They just show how stabilizer reduces the amount of the disinfecting/oxidizing form of chlorine and it is true that this form is relatively unstable and breaks down in sunlight at a rate such that half of it is "used up" or destroyed in about 30 minutes.

The chlorine that is combined with stabilizer (Cl-CYA) is more resistant to breakdown from sunlight (UV) such that half of it is "used up" in about 8-1/2 hours (at infinite CYA levels). At normal CYA levels of around 30 ppm, the half-life of chlorine in sunlight is around 6 hours (since some of the chlorine is in the form that breaks down in 30 minutes while most is in the form that breaks down in 8-1/2 hours). Obviously, losing half of your chlorine in 6 hours is better than losing half in 30 minutes which is the main reason to use stabilizer. This graph shows how stabilizer extends the life of chlorine in sunlight and shows how most of the benefit comes from relatively small amounts of stabilizer.

What the graphs in this thread show is that the normal amounts of stabilizer that are used, even 30 ppm CYA, combine with almost all of the chlorine to form the more stable compounds (and these last longer in sunlight), but that means that the disinfecting/oxidizing chlorine that remains is in very small amounts. Fortunately, it does not take very much disinfecting/oxidizing chlorine to be effective -- around 0.05 ppm seems to work well at preventing the growth of typical pathogens and algae (at least green algae).

The effect of pH on this stabilization of chlorine is not particularly large in the normal range of pH in pools (7.0 - 8.0) and the effect on pH on the amount of disinfecting/oxidizing chlorine is less pronounced when CYA is present than when no CYA is used. In a sense, CYA acts as a "buffer" of disinfecting/oxidizing chlorine, resisting changes in this chlorine level due to variation in pH (similar to the way baking soda acts as a buffer resisting changes in pH due to variation in the amount of acid or base added to the water).

Some people describe stabilizer as somehow "shielding" chlorine from sunlight with the implication that chlorine's effectiveness is only reduced a small amount, but this is simply not true. Stabilizer works by combining with chlorine to from a completely different chemical compound that is much more resistent to destruction from sunlight, but is also virtually ineffective to disinfect or oxidize.

If you have any suggestions for a better graph or explanation to make these concepts clear, please let me know. And thank you for taking the plunge to try to understand these concepts. I'm sure that any difficulty you may have is due to my inability to present the ideas clearly which is why I welcome any suggestions for improvement.

Thanks,
Richard

[milynor]

Thanks Geek! That would make sense as to why too much CYA will disable your chlorine.