What am I Missing? CYA and FC

[JM7]

I've read volumes about the relationship between CYA and FC. However, I must be missing something. If FC is what actually sanitizes the water, and the actual FC level is, let's say 3ppm, why isn't that enough FC even if the CYA is extremely high at say, 300 ppm? Isn't there still 3ppm FC that is still available to do it's job?

 

[mknauss]

Welcome to the forum.
A good article to start with CYA Chlorine Relationship - Further Reading

Then, you can dive into Pool Water Chemistry

 

[mgtfp]

Most of the chlorine is bound to CYA where it is protected from UV, but has bugger all sanitizing effect. It is HOCl that is the sanitizing form of FC. Unfortunately, the chlorine bound to CYA still shows up in the FC test. That's why FC has to be adjusted to the CYA level to essentially have same amounts of HOCl. As HOCl gets used up, more gets released from the reservoir of chlorine bound to CYA.

Same ratios of FC/CYA yield same amounts of HOCl.

 

[Rocket J Squirrel]

Most of the chlorine is bound to CYA where it is protected from UV, but has bugger all sanitizing effect.

You know we Yanks don't generally understand "bugger all", eh mate? Not even sure we want to.

 

[mgtfp]

You know we Yanks don't generally understand "bugger all", eh mate? Not even sure we want to.

Isn't that what the Queen would have said back in the old days?

Maybe I should rephrase that as: "... but has Buckley's to kill any algae or critters". That surely must make it clear to everyone.

 

[maggiesfan]

Isn't that what the Queen would have said back in the old days?

Maybe I should rephrase that as: "... but has Buckley's to kill any algae or critters". That surely must make it clear to everyone.

It does to me, but then I’m a Pom

 

[JoyfulNoise]

As amused to above, Free chlorine (FC) is a measure of three components - hypochlorous acid (HOCl), hypochlorite anion (OCl-), and chlorinated cyanurates (chlorine atom bound to a molecule of CYA).

HOCl = powerful oxidizer and a fast sanitizer; it does all the heavy lifting.

OCl- = middling oxidizer, slow sanitizer; it reacts with UV light and gets turned into oxygen gas and chloride salt

HCyCl = neither a sanitizer nor an oxidizer but the chlorine bound to it can be released quickly and while it is bound to the CYA molecule it is fully protected from UV.

HOCl and OCl- exist in an equilibrium (like a see-saw in a kids park) that is pH specific. At a pH of 7.5, the concentrations of HOCl and OCl- are equivalent. When either of those chemicals react with something (oxidize or sanitize), they get used up and turn into water and salt (Cl-). When that happens, the chlorine from the CYA is released because it too is in equilibrium and tries to maintain a constant concentration balance. Roughly 95% of the chlorine in a pool is locked up by CYA and held in reserve when the pH is 7.5 and your FC/CYA ratio is 7.5%. The chlorine is there and ready to go, it’s just waiting in reserve.

Unfortunately the only thing you can measure with the test kits that are available to pools is the FC value. There is no way to measure any of the quantities separately. This is why TFP recommends always maintaining an FC/CYA ratio rather than a specific FC value. Doing so ensures that your water is always sanitary with enough disinfectant to handle most bather loads.

 

[mgtfp]

As amused to above, Free chlorine (FC) is a measure of three components - hypochlorous acid (HOCl), hypochlorite anion (OCl-), and chlorinated cyanurates (chlorine atom bound to a molecule of CYA).

HOCl = powerful oxidizer and a fast sanitizer; it does all the heavy lifting.

OCl- = middling oxidizer, slow sanitizer; it reacts with UV light and gets turned into oxygen gas and chloride salt

HCyCl = neither a sanitizer nor an oxidizer but the chlorine bound to it can be released quickly and while it is bound to the CYA molecule it is fully protected from UV.

HOCl and OCl- exist in an equilibrium (like a see-saw in a kids park) that is pH specific. At a pH of 7.5, the concentrations of HOCl and OCl- are equivalent. When either of those chemicals react with something (oxidize or sanitize), they get used up and turn into water and salt (Cl-). When that happens, the chlorine from the CYA is released because it too is in equilibrium and tries to maintain a constant concentration balance. Roughly 95% of the chlorine in a pool is locked up by CYA and held in reserve when the pH is 7.5 and your FC/CYA ratio is 7.5%. The chlorine is there and ready to go, it’s just waiting in reserve.

Unfortunately the only thing you can measure with the test kits that are available to pools is the FC value. There is no way to measure any of the quantities separately. This is why TFP recommends always maintaining an FC/CYA ratio rather than a specific FC value. Doing so ensures that your water is always sanitary with enough disinfectant to handle most bather loads.

Thanks for adding in the details into my rough outline.

Especially for making clear for the non-Australian or -POM audience that

"has bugger all sanitizing effect"
or
" has Buckley's to kill any algae or critters"

equals
"neither a sanitizer nor an oxidizer"

 

[Newdude]

You know we Yanks