Want to understand more about CYA

[Mikesuze]

Ok. I get the correlation numerically between CYA, CC, and FC. I get it the the CYA keeps some chlorine from burning off in the sun.

My questions arehow does CYA do that and does it ever 'release' it? Or, does it always 'protect' the same chorine so that the remainder you add is free to work and/or burn off by the sun?

Then my question is : does the sun screened chlorine also sanitize the pool, or just the FC. And, if it doesn't, then WHY protect it with CYA? Why not have little to no CYA and keep the pool sanitized with less chorine?

 

[chem geek]

There is a chemical equilibrium between chlorine bound to CYA and chlorine unbound to it as follows ("CY" is the core cyanuric acid ring):

HClCY^- + H₂O <<<---> HOCl + H₂CY^-
"Chlorine bound to CYA" + Water <<<---> Hypochlorous Acid + Cyanuric Acid (cyanurate ion)

There is a constant back and forth between the chemicals on the left and those on the right (i.e. it is not "locked up"), but the balance is more to the left so that most of the chlorine is bound to CYA (but because of the constant back-and-forth, it's not the same chlorine atom always being bound all the time). When the chlorine is bound to CYA it does not break down in sunlight (or it breaks down more slowly) while that which is unbound on the right breaks down quickly (at pH 7.5 about half is broken down in noontime sun every hour if no CYA is present). So you need CYA in the water to slow down this rate of breakdown. You could just have around 0.1 ppm FC with no CYA and lose around the same amount or less chlorine as with our recommendations, but it is not practical to be able to maintain, let alone measure, that low an FC, even if one used automatic feeding systems.

With chemical equilibria, the higher concentrations on one side of a chemical equation force movement to the other side so for the above if you increase the CYA then it forces more chlorine to be bound to it. To keep the same amount of active chlorine (hypochlorous acid), you need to add more chlorine to keep the FC/CYA ratio constant.

The reaction rate is quite fast where if all the active chlorine where instantly consumed then half of the chlorine bound to CYA will be released every 0.25 seconds. However, the chlorine bound to CYA is itself essentially not reactive in terms of disinfection and only very slightly reactive (1/150th) in terms of oxidation.

What you measure as FC is the sum of both the chlorine bound to CYA and the chlorine unbound to it. The reason is that the FC test consumes the unbound chlorine (that's what turns the dye pink/red and what gets used up by the FAS R-0871 drops) and the chlorine bound to CYA is released quickly all during the time of the test. So you are not measuring only the active chlorine level with the FC test, but rather are measuring the chlorine reserve or capacity. The active chlorine level which is what determines the level of disinfection and oxidation rates is proportional to the FC/CYA ratio. That is why the Chlorine / CYA Chart in the Pool School keeps that FC/CYA ratio constant.

 

[Mikesuze]

Thanks. If you're not an educator outside TFP, you should be.

 

[chem geek]

Thanks for the compliment. I'm not an educator, but here at TFP we sort of get trained to be one by responding to many posts and learning how things need to be explained to be understood.