CYA FC relationship question

[pabeader]

I hope I've let this one marinate long enough to make sense when I type it out.

Since the CYA acts like a buffer for the CL, why even use it? I'm going to be adding 2-3ppm everyday anyway. So what is the difference if I'm adding it at 6ppm or 1 ppm?

From all my reading what I have garnered is that for a given CYA level a certain amount of CL is bound up, leaving 2-3 ppm to oxidize the organic material in the water. So with or without CYA i still have 2-3 ppm doing the work. It's not like the bound up CL is going to suddenly be released to help with the oxidation.

 

[chem geek]

Your understanding of how this works is incorrect. Most chlorine loss is from sunlight, not from bather load or chlorine oxidizing things or disinfection. One person-hour in your 5000 gallon pool only requires 0.21 ppm FC of chlorine to oxidize the bather waste. The amount of chlorine used in disinfection and algae prevention is negligible assuming you are at least at the minimum FC/CYA ratio or higher and that algae did not already get started forming clumps.

Since most chlorine loss is from sunlight, you need something to protect the chlorine from such loss. If you were to not use any CYA at all, then at a pH near 7.5 about half the chlorine would be lost in noontime sunlight every hour in an average depth pool. That is a far, far, faster rate of chlorine loss than when the pool has CYA in it since without any CYA 8 hours or direct noontime sun would deplete 25 ppm FC down to 0.1 ppm FC. CYA is not just a buffer for hypochlorous acid, but also protects its loss from sunlight -- that is the part you are missing.

With the minimum FC/CYA ratio, the amount of unbound chlorine is only 0.06 ppm FC so the daily loss from it is around 0.24 ppm FC. The rest of the chlorine loss is from chlorine oxidation of CYA (about 0.2 ppm FC per day) and mostly from chlorine lost from sunlight from the chlorine bound to CYA. The chlorine bound to CYA is broken down much more slowly than the unbound chlorine especially considering that there is much more chlorine bound to CYA than unbound. The loss rate of chlorine bound to CYA varies but is on the order of 3-7% per hour (compare that to 50% per hour for unbound chlorine). Furthermore, the CYA (or possibly the chlorine bound to CYA) shields lower depths from the UV in sunlight in a non-linear way which is why higher CYA levels even with proportionally higher FC levels lose less absolute FC per day.

 

[BuckeyeChris]

It's not like the bound up CL is going to suddenly be released to help with the oxidation.

Here is where i try to spit out what I've read here, and eventually other smarter posters come clean up after me.....

That is what happens, the bound fc will vet released from cya as the equilibrium continues to adjust. The concentration of unbound cl will continue to decrease as the ratio gets smaller and smaller, and it will no longer outpace algae growth, but you will still have unbound cl doing some amount of work.

Plus, there is a bit of increased uv shielding witnessed at higher cya levels. Protecting the unbound cl.

 

[pabeader]

Nope, it hadn't marinated long enough. Thanks for the new info ChemGeek and Buckeye. I didn't really ask the question I am trying to get out of my brain. I can tell because the answers are just a little off the mark. Not very far, though. I think if I let the new info simmer with the old, I'll do a better job.

Why did I not know that the CL gets released from bondage? I don't think I've read that in any of the thousands of posts I've read since I started here. But then again, I haven't spent near enough time in the Deep End.

Yay!! more reading to do!

- - - Updated - - -

and another question. why doesn't the CYA hide the CL from the FC test? I will happily accept a short answer to this one.

 

[chem geek]

BuckeyeChris, thanks for filling in that other missing piece I neglected to cover. The chlorine bound to CYA is not permanently bound. It's in equilibrium with the unbound chlorine such that roughly speaking the amount unbound is proportional to the FC/CYA ratio while the amount bound is close to the FC level (FC less the small unbound amount). As the FC drops, the bound amount drops but so does the unbound amount -- each dropping proportionally.

Specifically, at 6 ppm FC with 40 ppm CYA there is 5.86 ppm chlorine bound to CYA and 0.15 ppm chlorine unbound to CYA. At 3 ppm FC with 40 ppm CYA there is 2.94 ppm chlorine bound to CYA and 0.06 ppm chlorine unbound to CYA. So it's not exactly proportional, but it's close. The FC/CYA ratio rule approximately holds but not exactly at all ratios and it falls apart as one get towards SLAM levels.

As for why the CYA doesn't hide the chlorine from the CYA test, it is again due to the equilibrium between the bound and unbound chlorine and the fact that if all the unbound chlorine were to instantly disappear and continue to be consumed as fast as it appeared, half the chlorine would be released from CYA every 0.25 seconds so far faster than the timeframe of the chlorine tests. The unbound chlorine reacts with the DPD dye and more chlorine is then released from being bound to CYA and this happens quickly. This has been discussed in numerous threads, not all in The Deep End. See Cyanuric Acid and Free Chlorine Relationship, Chlorine/CYA relationship, Want to understand more about CYA, CYA and the chlorine relation question, and for more technical detail there is Questions about Chlorine and CYA, and FC - Concept of "Reserve".

 

[pabeader]

Thanks CG for the followup. turns out, that was the answer i was looking for in the first place. guess i should practice what i preach, and try a search once in a while.

 

[eqbob]

I found the equilibrium relationship ChemGeek,posted in the free chlorine thread revered above to be very very useful in understanding things. I have it bookmarked and read it often.

There is a chemical equilibrium between chlorine bound to CYA and hypochlorous acid unbound from it. Though there are multiple simultaneous chemical reactions due to the multiple chemical species involved, the most dominant reaction relevant to what we are talking about is the following:

HClCY- + H2O <<<---> H2CY- + HOCl
"Chlorine Bound to CYA" + Water <<<---> "CYA Ion" + Hypochlorous Acid

The reaction goes back and forth though at equilibrium at usual pool levels of FC and CYA it is mostly towards the left as I indicate with the "<<<" arrows.

If you were to instantaneously eliminate the hypochlorous acid (HOCl) on the right hand side, 94% of the bound chlorine would be released in 1 second to replenish it (half released every 0.25 seconds -- technically for the above reaction it's 4 seconds but there is another pathway through related reactions that is 0.25 seconds). So no "knowledge" is needed -- with chemical equilibria if species are consumed the reaction rebalances to replenish it. Technically, species are going back and forth from left-to-right and right-to-left very quickly and at equilibrium the forward and reverse reaction rates are equal. Chemical equilibrium does not mean that reactions are not occurring, but rather only that the concentrations of the chemicals are not changing so the forward and reverse reaction rates are equal. Roughly speaking, an amount of chlorine equivalent to the FC level goes back and forth every 1/3rd second. For more details, see this post, this post and this post.