CYA & FC Questions for Chemists

[stlmuelk]

I'm thinking about this too much and I'm sure a chemistry expert here will know the answers, hopefully in a dumbed-down fashion if at all possible .
Some of my (mis?)understandings and questions:
CYA and Chlorine (HOCl? OCl? Some other version?) forms a bond.
That bonded CYA+Cl molecule cannot be easily broken down by UV.
CYA+Cl has no (little?) disinfecting properties.
Do the Taylor/TF test kits measure this bonded molecule as FC?

 

[PoolStored]

Taylor tests all of it.

The deep end...

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

 

[stlmuelk]

Hmmm. It would make sense that it would, but all I can find in the world-wide web says things like this:

Chloroisocyanurates (CYA-bound chlorine) are not typically measured as Free Chlorine in the DPD-FAS test because they are not in an active, free state. These compounds are more stable and do not react as readily in the DPD-FAS test, ensuring the Free Chlorine measurement primarily reflects the active disinfecting chlorine.

(I searched here for "dpd-fas Chloroisocyanurate" and got no results)

 

[PoolStored]

FC and Active Chlorine, for all intents and purposes are the same but different definitions have been used in the past. Free chlorine, nowadays, not only includes hypochlorous acid, but also the chlorine bound to CYA which is technically available for disinfection as the hypochlorous acid gets used up. The FC portion of the DPD-FAS test measures both. Active chlorine typically refers to hypochlorous acid alone but a lot of people use the term interchangeably with free chlorine. For the pool user, you can just treat them the same.

 

[stlmuelk]

Thanks PoolStored. I found a resource that can back this up too:

Chloroisocyanurates
Chloroisocyanurates dissociate in water to give an equilibrium mixture with freechlorine and cyanuric acid. Because this equilibrium is a dynamic one, the DPD A reading for free chlorine also includes the reserve chlorine. As fast as the free chlorinereacts with the DPD indicator, more chloro-compound is decomposed, thus releasingall the bound chlorine.It is possible to analyze mixtures of chloroisocyanurates with free chlorine andchloramines by the normal DPD method, the diff erence being that the A reading willalso include the free chlorine available as reserve in the chloroisocyanurates.

source (pg 59)

This does suggest, to my reading, that a sufficient amount of powder is necessary to get an accurate reading.

 

[JoyfulNoise]

The chlorine atom is in equilibrium with the cynaurate. There is a constant bonding and unbonding occurring. So the chlorine is always available to form an active disinfectant (HOCl or OCl-). The equilibrium is fast and pH dependent. The DPD-FAS test is used a pH adjuster to make sure that the test Ian performed under acidic conditions because at low pH all of the chlorine is in the form of HOCl/OCl- and not bound to the cyanurate. This is why the test measures to total free available chlorine (HOCl + OCl- + CyaCl).

 

[stlmuelk]

The chlorine atom is in equilibrium with the cynaurate. There is a constant bonding and unbonding occurring. So the chlorine is always available to form an active disinfectant (HOCl or OCl-). The equilibrium is fast and pH dependent. The DPD-FAS test is used a pH adjuster to make sure that the test Ian performed under acidic conditions because at low pH all of the chlorine is in the form of HOCl/OCl- and not bound to the cyanurate. This is why the test measures to total free available chlorine (HOCl + OCl- + CyaCl).

Do you think if we had a high CYA (say 100) it would make a difference in the amount of DPD used, as far as accuracy goes? As in, would an amount that measures a 10 CYA sample accurately necessarily also measure 100 CYA accurately?

 

[blackout]

Unless I’m missing something, I think this is easily provable:

1. Confirm that there is CYA in the pool
2. Test the FC level
3. Dump a known amount of chlorine in the pool and circulate. Confirm via PoolMath the amount of FC added based on pool volume and chlorine bottle specs.
4. Test the FC level
5. Compare 4 vs. 2 to confirm they match 3

 

[JoyfulNoise]

Do you think if we had a high CYA (say 100) it would make a difference in the amount of DPD used, as far as accuracy goes? As in, would an amount that measures a 10 CYA sample accurately necessarily also measure 100 CYA accurately?

It’s doesn’t matter. The chemical equilibrium is such that once you are below a certain pH (it’s less than 5), 99% of the chlorine is in the form of HOCl. The test also doesn’t work the way you think it does. It doesn’t measure chlorine directly. It measures the fraction of the DPD that was oxidized by chlorine. So as long as you add enough DPD powder in excess of what is needed and the pH is in the right range, then the amount of oxidized DPD (pink color) that you measure correlates to the amount of chlorine in solution.

Unless I’m missing something, I think this is easily provable:

1. Confirm that there is CYA in the pool
2. Test the FC level
3. Dump a known amount of chlorine in the pool and circulate. Confirm via PoolMath the amount of FC added based on pool volume and chlorine bottle specs.
4. Test the FC level
5. Compare 4 vs. 2 to confirm they match 3

#3 is very hard to actually get unless you standardize the product you are adding before you add it.