Cyanuric Acid and Free Chlorine Realtionship

[UaVaj]

can someone link (no luck googling) or explain the relation between free chlorine and cyanuric acid.

FC is the sanitizer.
for sake of easy comparison. let give FC has a sanitizing power of 100.



control group.
with CYA at zero. FC will obviously have its full sanitizing power of 100.


experimental group.
with CYA at 30ppm. FC will bond to the CYA (hence the stabilizing).
questions. what happens to FC's sanitizing power?
questions. why is a higher FC ppm needed for shock?

 

[zea3]

I have copied this post from chem geek that appears in the Chlorine CYA relationship thread.

The science behind the chlorine / CYA relationship has been known definitively since at least 1974 when in this paper the equilibrium constants between chlorine bound to CYA and chlorine that is unbound were definitively determined. What makes the chemistry complex is that there are many different chemical equilibria involved, but one can understand the principle by just looking at the most dominant chemical species at pool pH which are the following:

HClCY^- + H₂O <<<---> HOCl + HCY^-
"Chlorine bound to CYA" + Water <<<---> "Active Chlorine" + Cyanurate Ion (a form of CYA)

When CYA is present, most of the chlorine is bound to it -- that is, the chemical equilibrium shown above is way to the left. Chlorine bound to CYA is barely reactive and for practical purposes can be ignored except as a reserve or reservoir of chlorine. It is hypochlorous acid, shown on the right, that is the active chlorine that disinfects, kills algae, and oxidizes bather waste (see the "Chlorine/CYA Relationship" section in this post for more details).

So the first thing you need to get out of your mind is the idea that CYA acts like a "sun shield" to protect chlorine. That's what many in the industry call it, but it is terribly misleading since it makes it sound like CYA and chlorine are kept separate which is not true. CYA binds to the chlorine to produce a new compound that, for practical purposes, does not react to disinfect, prevent algae, or oxidize bather waste. When CYA is present, most of the FC is inactive.

In order to prevent algae growth, one needs a minimum active chlorine level. So the question is how does one determine that level given that all one knows are the pool water chemistry parameters we measure (FC, CC, pH, TA, CH, CYA, temp)? The answer is derived technically in this post where due to the chemical equilibrium I showed above, the active chlorine level is proportional to the FC/CYA ratio. So 3 ppm FC with 30 ppm CYA has the same active chlorine level as 6 ppm FC with 60 ppm CYA and as 10 ppm FC with 100 ppm CYA.

As for the actual active chlorine level itself, it is very low since most of the chlorine is bound to CYA. At a pH of 7.5 and 77ºF (to be conservative), an FC that is at the minimum FC we recommend for manually dosed pools which is around 7.5% of the CYA level has the same active chlorine level as a pool with only 0.06 ppm FC and no CYA. That is not a typo. Fortunately, it takes a low active chlorine level to prevent algae growth.

However, to kill off already established algae, one needs a significantly higher chlorine level since it gets used up locally quickly and must penetrate algae clumps which takes longer to do so in order to get ahead of algae growth (reproduction), one needs a higher chlorine level. Though technically something like 20% would probably be enough to kill algae faster than it grows even in a bloom, it would take longer to kill off so we use an FC that is 40% of the CYA level as the shock level for clearing a pool of existing algae. This FC/CYA ratio has the same active chlorine as an FC of 0.6 ppm with no CYA.

So a pool with 32 ppm FC and 80 ppm CYA has an active chlorine level the same as only 0.6 ppm FC with no CYA. So these high numbers are just that, high numbers, and the only thing actually "high" is the amount of chlorine in reserve.

Another way to think about this is considering soldiers fighting a war. The front-line soldiers have weapons for killing the enemy (though more technically a closer analogy would be that they sacrifice themselves in hand-to-hand combat with an enemy and only kill an enemy soldier when they themselves get killed). Whenever a front-line soldier gets killed, a replacement is made from soldiers in reserve. You can see that the rate of killing the enemy is related only to the number of soldiers on the front-line and that the number of soldiers in reserve is irrelevant in terms of the rate of kill. The reserve just tells you how long you can continue to fight.

Does that make sense? If not, let me know what is still not clear and I'll see if there's another explanation that might help clarify.

As for your worry about your kids swimming in a pool with high FC, again the FC number by itself is irrelevant towards chlorine's actual effects when CYA is present. My wife has personally experienced this difference when she has swum in an indoor commercial pool with 1-2 ppm FC and no CYA and her swimsuits would degrade (elasticity gets shot) in just one winter season of swimming and her skin was flakier and hair frizzier. In our own outdoor residential pool with 3-6 ppm FC and 40 ppm CYA, her swimsuits would last for 7 years and the effects on her skin and hair were substantially less noticeable.

 

[chem geek]

Free Chlorine (FC) as measured in chlorine tests (OTO, DPD, FAS-DPD) effectively measures the sum of a variety of chlorine compounds including hypochlorous acid (HOCl), hypochlorite ion (OCl^-), and six different chemical species of chlorine bound to Cyanuric Acid (CYA). For practical purposes, only the HOCl has fast disinfection power.

Even with no CYA in the water, all of the measured chlorine is not HOCl and the amount of HOCl vs. OCl^- is a function of pH.

At a pH of 7.5, 30 ppm CYA will lower the HOCl concentration by roughly a factor of 35 compared to no CYA (at usual FC and CYA levels; it changes somewhat at higher FC/CYA ratios). However, you need to think of have multiple ppm FC with no CYA as essentially being over-chlorinating and being too strong in its strength. Most pathogens are killed quickly by chlorine and the table in this post shows the time in minutes for a 3-log reduction (99.9% kill) with 0.1 ppm FC with no CYA. That level of disinfection is achieved with an FC that is 11% of the CYA level so roughly 3 ppm FC with 30 ppm CYA.

When we Shock Level and Maintain (SLAM) a pool it is usually to get rid of algae that has already grown enough to be clumped or where there is other abnormal chlorine demand. It takes a higher level of chlorine than normal algae kill levels because clumped algae has the outer layers using up chlorine while algae on the inside can continue to grow so it takes higher concentrations of chlorine to more rapidly penetrate the clumps to kill off all the algae faster than it can reproduce. It's also set at a high enough level to clear the pool more quickly. Note that shocking a pool is not necessary if one properly maintains the FC level relative to the CYA level since algae is continuously killed faster than it can reproduce.

 

[UaVaj]

found this HOCL level breakdown.
https://www.troublefreepool.com/~richardfalk/pool/HOCl.htm

this is exactly the info I am looking for.



can you experts validate this. thanks.

 

[jblizzle]

Chem Geek created it ... what more validation do you need?

That shows the "active chlorine" level as a function of FC and CYA. And makes the buffering effect of the CYA very obvious.

 

[Patrick_B]

found this HOCL level breakdown.
https://www.troublefreepool.com/~richardfalk/pool/HOCl.htm

this is exactly the info I am looking for.



can you experts validate this. thanks.

Richard is the expert, and he has validated it. It has been studied and validated for decades.

 

[chem geek]

The equilibrium constants between chlorine and cyanuric acid were definitively determined in 1974 as described in this paper. I wrote a spreadsheet using those constants called Pool Equations. You can see a validation of how this chemical relationship predicts kill times for bacteria, viruses, and protozoan oocysts, predicts oxidation rates, and correlates to ORP by looking at the links in the "Chlorine/CYA Relationship" section of this post. You can also see a derivation of why the FC/CYA ratio is a reasonable approximation at pool pH (half of that ratio would be hypochlorous acid near pH 7.5) in this post. I believe you already found the thread Pool Water Chemistry.

Note that the color coding in the table you linked to does not reflect the minimum active chlorine level needed to prevent algae growth. That minimum is incorporated into the Pool School Chlorine / CYA Chart which is based on my more detailed chart where the rough algae inhibition levels were determined by Ben Powell at The PoolForum.

 

[UaVaj]

so chem geek is richard falk. nice to put the two together. he is definitely an expert. always have to read his post 2 times to start to sink in. very detailed info.



question.
the state board health department minimum FC is 1 (with CYA 0). that would mean minimum active HOCL is 0.484.

to maintain the same level of active HOCL.
CYA of 30. FC would need to somewhere between 10-20 (HOCL 0.213-0.883)

Chlorine / CYA Chart suggests 2-4. what is amiss?

 

[JasonLion]

There are two important values, the active chlorine level, and the total chlorine level. The department of health has FC at a minimum of 1 to maintain the total chlorine level, not the active chlorine level. The department of health does not acknowledge the chlorine/CYA relationship, so they don't break it down this way, but the implications are clear if you look back at the way they arrived at their standards.

You need a certain total amount of chlorine to allow for things that use up chlorine, for example many sweaty bathers entering the pool at the same time. While you need a minimum active level to kill germs and break down contaminates. When CYA is very low, or zero, the total level is the important one, because there is so much less chlorine, it is far easier to use it all up. But when CYA is higher, there is always lots of extra chlorine held in reserve by the CYA, so running out is not really a concern, and the active level becomes the dominant issue.

 

[eqbob]

There are two important values, the active chlorine level, and the total chlorine level. The department of health has FC at a minimum of 1 to maintain the total chlorine level, not the active chlorine level. The department of health does not acknowledge the chlorine/CYA relationship, so they don't break it down this way, but the implications are clear if you look back at the way they arrived at their standards.

You need a certain total amount of chlorine to allow for things that use up chlorine, for example many sweaty bathers entering the pool at the same time. While you need a minimum active level to kill germs and break down contaminates. When CYA is very low, or zero, the total level is the important one, because there is so much less chlorine, it is far easier to use it all up. But when CYA is higher, there is always lots of extra chlorine held in reserve by the CYA, so running out is not really a concern, and the active level becomes the dominant issue.

I realize asking a new question on a thread is somewhat frowned upon, but the is highly related so I'm not sure of protocol on this particular forum... Apologies if not the preferred method....

You state that there is always lots of extra chlorine held in reserve. Easy to see why and how with what's been posted and discussed over time. The question I don't understand is what is the mechanism by which CYA releases it from being bound? It must over time. It can't keep an ever increasing amount indefinite amount bound and new FC gets added or produced frequently. It's not like CYA 'knows' when there are a ton load of sweaty bathers and says release and go fight off the bad stuff.. Does the CYA process bind and then release over time according to some known process and then re-bind to 'new' FC? thats the only thing that would really seem to make sense, but that aspect of the process never seems to be discussed. If it gets bound and that's obviously factual, how does it get 'unbound' and in what timeframe?

 

[chem geek]

It's a related question so is fine for the same thread.

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.

So yes, it sort of works as you described in that the HOCl is consumed so more chlorine replenishes it by the reaction moving to the right and then when you add more chlorine that HOCl then forces the reaction more to the left. So some of the chlorine that is now present in the water is from what you added and that which was consumed is gone, usually converted to chloride ion (salt) if the chlorine broke down from the UV in sunlight or oxidized ammonia or an organic compound.

 

[eqbob]

Now that makes a lot of sense. Thank you. I neglected to consider the whole chemical equilibrium aspect of it and that equation lays it out nicely as to what happens. So the statement of more CYA binds more and has it available to fight 'bad stuff' makes a lot more sense when looked at in conjunction with this equation. I think of all the TFP methodology, the WHY of the CYA/Chlorine relationship is the hardest to grasp.

 

[UaVaj]

clearly the MATH does not add up?



I.
per the HOCL chart
CYA 0 and FC 1.0 = HOCL 0.484

to maintain this "same" level of HOCL with CYA 30.
FC would have to be at ~15.

here is the check.
per the FC Chart
CYA 30 and FC 15.8 = HOCL 0.500


II.
at CYA 30. Chlorine / CYA Chart for non swg suggests FC 2 to 4 = HOCL 0.027 to ~0.060.



are you simply stating that? "HOCL Chart" and "FC Chart" is the true HOCL calculation, while "Chlorine / CYA Chart" is the bare minimum?

 

[chem geek]

What you wrote is correct and actually makes perfect sense since CYA significantly moderates chlorine's strength. The way you need to look at it is that not using CYA in the water has one over-chlorinate the water, at least from an active chlorine (hypochlorous acid) point of view. My wife has personal experience with this difference where her use of an indoor commercial community center pool for the 5-month winter season each year had us changing her swimsuits every year since their elasticity got shot whereas in our own outdoor residential pool with the same brands of swimsuits over a 7-month summer season they would last for 7 years. Also, the effects on her skin and hair were hugely different between the two pools. The indoor pool had 1-2 ppm FC with no CYA while our outdoor pool had 3-6 ppm FC with 30-40 ppm CYA. That's a difference in active chlorine level of roughly a factor of 5-30 that explains the difference in experience.

I already linked in this earlier post in this thread to point you to the "Chlorine/CYA Relationship" section in this post that shows this significant moderation of chlorine effect in terms of disinfection rates, oxidation rates, and ORP. Again, the way to look at it is not that CYA reduces the active chlorine level by too much, though certainly too low an FC/CYA ratio can do that, but rather that not using CYA has one have the active chlorine level too high unless one maintains a very low FC and that's hard to maintain in a pool (i.e. 0.1 ppm FC or so). So CYA should be seen as a chlorine buffer holding most chlorine in reserve as a reservoir of chlorine able to be used anywhere in the pool as needed while moderating chlorine's otherwise too high strength.

In Europe, this over-chlorination is well understood since they are more focussed on significantly lowering chlorinated disinfection by-products in commercial/public swimming pools so the DIN 19643 standard specifies 0.3 to 0.6 ppm FC with no CYA when ozone is not used or 0.2 to 0.5 ppm FC with no CYA when ozone is used (as a supplement in the circulation path). In the U.S., however, the minimum FC is 1.0 ppm However, the overly stringent EPA DIS/TSS-12 laboratory test requirement requires effectively a 6-log kill (99.9999% reduction) in E.coli in 30 seconds and uses a reference sodium hypochlorite standard set at 0.6 ppm FC with no CYA. The main reason for the 1 ppm FC minimum in the state codes is to ensure that the chlorine does not run out, but such EPA standards and state codes do not take into account the FC/CYA relationship. So the FC allowed can range from 1 ppm to 10 ppm in Florida yet the CYA can range from 0 ppm to 100 ppm. These combinations range in active chlorine (HOCl) level by a factor of 1290 which is nuts.

The HOCl and FC charts are just calculations for the active chlorine (HOCl) level. They do not say what the minimum should be. The "Chlorine / CYA Chart" says what the minimum active chlorine level should be in order to prevent green and black algae growth. This is also high enough for a fast kill of most pathogens as I showed in a link in an earlier post because bacteria are generally much, much easier to kill than algae.