I Don't Understand CYA

[Kleve13]

I'm having a hard time with CYA. I understand the basics; it's sunscreen for your FC so it doesn't burn off so quickly. What I don't get is why more is not better.

If CYA protects FC like sunscreen does, then ff you have a higher CYA, then FC burns off more slowly, leaving the FC in the pool to fight organic matter. Sure, you would need more chlorine to make it effective, but once you got to the higher level, you wouldn't need to add as much daily as it wouldn't burn off.

So if my CYA is 40 and I lose 3ppm to sunlight per day, I need to add 3ppm each day to get it back to 5. If my CYA is 100 and I only lose .5ppm to sunlight, then I only need to add .5ppm each day to get 12. Sure I used more bleach to get it up to 12 initially, but now I only add .5 every day to keep it there.

I'm sure this isn't how it works, but I can't get my brain around why not. Can someone explain better how CYA works?

Thanks

 

[woodyp]

One thing I can offer up is this. IF..you get an algae problem at that 100 level of cya, it's a whole heckuva lot harder to knock out and the reason we don't recommend levels that high.

 

[PAGirl]

The way I understand it is that the higher the CYA, the more chlorine you need for it to be effective. In turn, the higher the chlorine level, the more loss, as it is a percentage, not a fixed number. So at a CYA OF 30, you need to add less chlorine to get to your target, at 100, you need three times more. Not sure if I stated my thoughts in a coherent way, lol.

 

[Isaac-1]

The sunscreen analogy is a bit over simplified, CYA buffers chlorine and the higher the level of CYA the higher the FC needs to be to effectively fight the nasty things in the water. Which is all fine and good until a certain point when other side issues start to pop up,, lets just use one example here of a side issue.

The FC/CYA chart shows that at a CYA level of 90 ppm the target FC should be 10 ppm, however at FC levels 10 ppm the pH test becomes invalid so if you were to maintain accepted FC levels you would have no way to measure pH.

 

[JoyfulNoise]

Unfortunately the link to @chem geek's technical post about the FC/CYA Chemistry is not working (help admins!!).

[EDIT]

I found the link working HERE

[END-EDIT]

It's helpful to remember that there are different chlorine compounds in pool water - chlorine gas (from and SWG perhaps), hypochlorite (perhaps from bleach or cal-hypo, etc), hypochlorous acid (what chlorine turns into when it reacts with water) and chlorinated cyanurates (the chemical compounds formed by chlorine reacting with CYA).

The sunscreen analogy is only one function of CYA and if you only consider that aspect of it then it makes sense that more is better. It is true that Cl binds itself to CYA to form chlorinated cyanurates (there are 6 different types of these) and those chemical species do not react with UV light. Thus the chlorine will not be degraded by sunlight.

However, there is another more important aspect of CYA, i.e., it acts a buffer for chlorine in water. Understanding buffer chemistry is not easy but if we access the charts in Richard's post you can see an amazing Excel Spreadsheet graph showing how the concentration of hypochlorous acid (the good oxidizing & disinfecting chemical species of chlorine in water) and hypochlorite (the conjugate base of hypochlorous acid and a weaker version of chlorine) vary with pH and different levels of CYA concentration. The short answer is the variation of hypochlorous acid with CYA at a fixed pH IS NOT LINEAR. Therefore, as you increase the CYA concentration, you decrease the amount of hypochlorous acid available for disinfection in a non-linear way. So it's takes proportionally higher and higher levels of FC as you raise the CYA to get the proper amount of hypochlorous acid in the water. And this is critical - hypochlorous acid is what you want in water, all the other chemical species of chlorine either do not disinfect and oxidize or only do so very weakly.

So, in the end, you have competing properties - you want lots of CYA to protect your chlorine from UV and to hold a reserve of chlorine in water BUT you want as little CYA as possible so as to maximize the amount of hypochlorous acid in water (or, at least, have an appropriate amount of hypochlorous acid so that you meet or exceed the minimum threshold kill times, which are concentration dependent, for pathogens in water as established by the EPA).

 

[Donldson]

One error you are making is the assumption that 100% of FC loss is due to UV. The average pool loses 1-2 FC a day due to the constant fight against organics that find their way in to the pool. Even overnight when sunlight doesn't factor in to it at all a 1 ppm loss of FC is considered normal, as per the OCLT rules. So in your example you will never be at a point of only losing 0.5 FC a day, you will still lose at least 1-2 due to chlorine doing what it is supposed to do. So say you do drop from 3 FC loss a day to 2 FC loss a day. Is that enough to offset the downsides, such as the inability to measure pH correctly or the incredible amount of chlorine you will need in the event of a SLAM?

EDIT: I see you might be getting those numbers from an old post by JasonLion. As such I might be wrong here , but would like to hear from some of the experts because I really think that my thinking on this is sound.

 

[Patrick_B]

Unfortunately the link to @chem geek's technical post about the FC/CYA Chemistry is not working (help admins!!).

It's helpful to remember that there are different chlorine compounds in pool water - chlorine gas (from and SWG perhaps), hypochlorite (perhaps from bleach or cal-hypo, etc), hypochlorous acid (what chlorine turns into when it reacts with water) and chlorinated cyanurates (the chemical compounds formed by chlorine reacting with CYA).

The sunscreen analogy is only one function of CYA and if you only consider that aspect of it then it makes sense that more is better. It is true that Cl binds itself to CYA to form chlorinated cyanurates (there are 6 different types of these) and those chemical species do not react with UV light. Thus the chlorine will not be degraded by sunlight.

However, there is another more important aspect of CYA, i.e., it acts a buffer for chlorine in water. Understanding buffer chemistry is not easy but if we could access those charts in Richard's post you'd see an amazing Excel Spreadsheet graph showing how the concentration of hypochlorous acid (the good oxidizing & disinfecting chemical species of chlorine in water) and hypochlorite (the conjugate base of hypochlorous acid and a weaker version of chlorine) vary with pH and different levels of CYA concentration. The short answer is the variation of hypochlorous acid with CYA at a fixed pH IS NOT LINEAR. Therefore, as you increase the CYA concentration, you decrease the amount of hypochlorous acid available for disinfection in a non-linear way. So it's takes proportionally higher and higher levels of FC as you raise the CYA to get the proper amount of hypochlorous acid in the water. And this is critical - hypochlorous acid is what you want in water, all the other chemical species of chlorine either do not disinfect and oxidize or only do so very weakly.

So, in the end, you have competing properties - you want lots of CYA to protect your chlorine from UV and to hold a reserve of chlorine in water BUT you want as little CYA as possible so as to maximize the amount of hypochlorous acid in water (or, at least, have an appropriate amount of hypochlorous acid so that you meet or exceed the minimum threshold kill times, which are concentration dependent, for pathogens in water as established by the EPA).

What link are you talking about?

 

[JoyfulNoise]

What link are you talking about?

I'm mobile right now so I can't look it up. If you go in the site and look at the FC/CYA Relationship thread, I believe Richard points to a thread started about the detailed chemistry. I've read that thread many times as it is a fascinating exploration of pool water chemistry.

Sorry, I should have copied the link in my original post even if it was broken.


Sent from my iPhone using Tapatalk

 

[JoyfulNoise]

What link are you talking about?

Found the link and it is working now....probably just my Safari browser acting weird.

 

[Kleve13]

Thank you all. Very helpful. Yes, I think I was stuck too much on the sunscreen analogy. It makes much more sense now.

 

[jblizzle]

Found the link and it is working now....probably just my Safari browser acting weird.

There have been some comments from the new Admin that Safari is not a very good browser. Just an FYI.

 

[JoyfulNoise]

There have been some comments from the new Admin that Safari is not a very good browser. Just an FYI.

Thanks.

The wife's MacBook is the laptop in the house that is up high enough where I can use it but the ankle-bitters can't get to. So I check TFP a lot with it.

Other than that, I'm typically a PC/Chrome user.


Sent from my iPhone using Tapatalk

 

[chem geek]

The simplest explanation is that the main way that CYA protects chlorine from breaking down from the UV in sunlight is that the vast majority of chlorine (around 98% or so) is bound to CYA and that this chlorine bound to CYA is for practical purposes not a disinfectant nor an oxidizer. It is essentially held in reserve. Only the unbound chlorine is "active" and of that about half (the hypochlorous acid) is the primary disinfectant. As the unbound chlorine is used/consumed, it is released from being bound to CYA and this happens very quickly (fast enough so that a chlorine test kit measures both the unbound and bound chlorine when it measures FC).

Roughly speaking, the amount of active chlorine is proportional to the FC/CYA ratio (detailed technical explanation for why this is true is in this post in the thread others have linked to above).

Now interestingly if the entire protection of chlorine from breakdown from sunlight were only from binding chlorine to CYA, then the chlorine loss should be constant at all FC and CYA levels with the same FC/CYA ratio. It turns out that higher CYA levels even with proportionately higher FC levels have a lower chlorine loss. I speculate that this is from a direct CYA (or chlorine bound to CYA) UV shielding effect, but we haven't found core science to support this observed effect (the UV spectroscopy that is published doesn't cover the UV of sunlight in the 300-380 nm wavelength range).

So you might ask why not run at high CYA levels with proportionately higher FC levels? The main reason was pointed out by others in that it makes it more difficult to SLAM a pool IF any problems occur because the SLAM level is an FC that is 40% of the CYA level so at higher CYA that's a pretty darn high FC so a lot of chlorine to be added to the pool to maintain the SLAM level. Nevertheless, in saltwater chlorine generator pools, we normally recommend 80 ppm CYA to reduce chlorine losses from sunlight and such pools, due to the automation, tend to not get into problems. You can certainly run a manually dosed pool the same way and some people do in particularly hot sunny areas, but for most pools a CYA of around 50 ppm balances the loss from sunlight against the more difficult SLAM if one is ever needed.

 

[JoyfulNoise]

This Master's Thesis link from NC State University has an interesting set of tables and graph's showing a bucket test of chlorinated water samples with varying concentrations of CYA left out in the sun. It at least demonstrates the effect of CYA on FC in sunlight (though I'm not sure how well they controlled the water buckets for airborne contamination).

I'm also curious about how the various isomers of CYA react to UV. I believe it is true that CYA forms tautomers upon dissolution in water and that the relative concentration of each isomer present is pH dependent. I found the first page of an interesting study ("Absorption Spectra and Tautomerism of Cyanuric Acid, Melamine and Some Related Compounds", J. Am. Chem. Soc., 1947, 69 (4), pp 801–803) from the late 1940's on this showing that the different isomer have different UV absorption properties, but I am not able to access the PDF without paying a lot of money If you have any further reading on this subject that is publicly available, I'd appreciate it.

 

[chem geek]

This is a Deep End discussion. Nevertheless, I've bought that paper as well as this one (I've spent well over a thousand dollars on papers over the past decade) and neither one answers the question because the paper you refer to only shows the UV spectrum up to around 240 nm and is also scaled too high since we're looking at molar absorptivity of between 1 and 50 that would explain what we see. The paper I linked to does go up to 2500 Angstroms (250 nm) and shows the log of absorptivity but has it stop at around 50 (absolute, not log) at 250 nm so it seems unlikely it's absorbing in the 300-380 range, but we have no other explanation. Also, this paper shows the UV spectra for the different CYA species including those bound with chlorine which shows that the deprotonated single chlorine bound to CYA HClCy^- has 1330 molar absorptivity at 230 nm so perhaps that is the species doing the absorbing, but again we don't have definitive UV data in the proper range to be sure.

It makes some sense that the chlorine bond to nitrogen in CYA may be absorbing in the UV range and it may even degrade itself though not nearly as readily as the bond to oxygen in hypochlorite ion or hypochlorous acid (technical details in this post). Dichloramine (NHCl₂), for example, breaks down in the UV of sunlight and that has a chlorine nitrogen bond (see this paper where Figure 1 shows UV absorption measured up to 300 nm and it's at least 220 M^-1cm^-1), but of course the devil is in the details of the quantum states.

 

[JoyfulNoise]

This is a Deep End discussion.

Oh snap! I forgot to check the header, I thought we were in the Deep End My bad!

But :bowdown::bowdown::bowdown::bowdown: thank you, thank you, thank you for those links. I will read them with great pleasure (and, like you, I may be forced to pony up some dough for the ones behind the pay walls....some days I miss my old grad school days...)