Cya

[PaP dude]

I need some help (from chem geek) concerning CYA.

I have read many post regarding many things about CYA and have even had Richard show me charts and graphs. (which I found very helpful) What I really need to know is HOW. I got into a conversation with someone about the effects of CYA on the concentration of hypochlorous acid with changing p.H. but I couldn't explain how though. I more interested to know what is happening at a molecular level.

Any chemical equations, charts, graphs, anything and everything about CYA would be helpful. Just lay it out.

Thank You

[chem geek]

I forget if I linked you to my primary post that describes the chlorine/CYA relationship here. Since you aren't the first one to ask this question, I posted a general response in this post that is hopefully what you are looking for. Let me know if it answers your question in the way you want.

As for what happens when the pH changes, because CYA acts as a hypochlorous acid buffer, it resists changes to the concentration of hypochlorous acid. So if the pH changes, the hypochlorous acid concentration is much more stable in the presence of CYA due to its buffering effect (to compensate, the hypochlorite ion concentration swings much more wildly with pH changes when CYA is present). For comparison, going from a pH of 7.5 to 8.0 when no CYA is present causes the hypochlorous acid concentration to drop by 53% (roughly cut in half) while at 3 ppm FC and 30 ppm CYA going from a pH of 7.5 to 8.0 causes the hypochlorous acid concentration to drop by only 14%. Now, realistically, the drop of 53% isn’t as big a deal as it seems since the absolute hypochlorous acid concentration is much higher. Even if the FC without CYA were only 1 ppm, a change in pH from 7.5 to 8.0 has the hypochlorous acid concentration drop from 0.48 ppm to 0.23 ppm. With 3 ppm FC and 30 ppm CYA, it drops from 0.042 ppm to 0.036 ppm. So even though the drop is much larger in percentage terms without CYA, the absolute concentration is still vastly higher when there is no CYA (it's actually too high in terms of being far more than needed for disinfection or oxidation).

Interestingly, the opposite effect also occurs. When you add chlorine to a pool with CYA, the pH will rise more than it normally would (or will drop less than it normally would, depending on the pH of the added chlorine). The opposite happens when the chlorine level drops. So the pH is actually less stable from chlorine concentration swings when CYA is present even though CYA itself is a buffer against pH changes.

Richard

[PaP dude]

This information is just what I was looking for. I had a feeling that this was going to be complex, but this information is good. Thanks again.

Richard C. (Yes, my name is also Richard...not to be confused with chem geek )