Trying to understand TFP method and actual FC numbers

[swamprat69]

I understand that listed numbers for minimum FC is actually 7.5% of CYA. At this minimum FC level you must have a certain amount of FC that is not bound up in CYA and available for disinfection. What is the formula for figuring the actual FC level available for disinfection and what are the actual FC levels avialable for disinfection at minimum, target and slam goals.

 

[mknauss]

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

I believe the start of this thread will provide the information you area looking for.

 

[JoyfulNoise]

Try to think of it this way - there really is no such thing as "FC" ... it's a made up concept. What is in the water are the following chemical species -

Chlorine bound to CYA (reserve chlorine)
Hypochlorous Acid (oxidizer and disinfectant)
Hypochlorite anion (mild oxidizer and lousy disinfectant)
Combined chlorine (chloramines and other organic chloride compounds some of which can be tested for)

The first three items add up to FC and the last one is CC. Added altogether and you get TC. You can only test for FC and CC.

When CYA is in the water at a concentration greater than 30ppm or more, about 96% of the chlorine you add to a pool will be bound to CYA. It does no oxidizing or sanitizing while it is bound and it is completely unnoticed by the swimmer. The remaining 4% or so will split up into hypochlorous acid (HOCl) and hypochlorite anion (OCl^-). At a pH of 7.5, that split is 50/50. So, in a pool with an FC to CYA ratio of about 7.5%, you will get about 100 parts per BILLION (ppb) of HOCl + OCl^-. The active chlorine species that does all the work is hypochlorous acid (HOCl). So in a clean and clear TFP swimming pool at normal FC levels and typical pH, you have anywhere from 0.04 to 0.08 ppm HOCl. That's all you need to keep a pool sanitized against bacterial growth and it will keep algae from growing faster than it is killed. In a green pool with large amounts of algae, you typically need over 0.1ppm (100ppb) HOCl to kill algae and generally higher levels than that to overwhelm an algae bloom). If you are SLAM'ing a swimming pool, the HOCl levels can go as high as a few ppm but that is incredibly harsh and will bleach out clothing pretty quickly. You can swim at those levels but I doubt most people would really want to because the reaction to chlorine at those levels can vary widely. Some people won't notice it and others will get stinging eye and very smelly chlorine odor when toweling off.

 

[swamprat69]

Although I can understand formulas and math easily, thank you for the laymans explanation! Only one more question... what triggers the release of HOCL/OCL from the chlorine that is bound up in the CYA and is there a specific balance point/ratio that is maintained?

 

[JoyfulNoise]

Although I can understand formulas and math easily, thank you for the laymans explanation! Only one more question... what triggers the release of HOCL/OCL from the chlorine that is bound up in the CYA and is there a specific balance point/ratio that is maintained?

Chemical equilibrium reactions are just that - an equilibrium. When you look at a chemical equation, in reality, the arrows point both ways. Statistically speaking the forward and reverse reactions are occurring simultaneously at a rate dictated by temperature and the reaction rate constant. Those parameters are what eventually become part of the equilibrium constant for the reaction.

So the chlorine atom that attaches to the CYA is constantly coming on and off. One study pegged the reaction rate (the time interval for the chlorine to become unbound and react with something) at about one attempt every 250ms or so. As that chlorine gets used up, another takes it’s place. The same is true with hypochlorite hydrolysis to form hypochlorous acid - it’s a constant back and forth that, on average and at a pH of 7.5, results in about a 50/50 split.

Chemistry gets very complicated on at the quantum mechanical level