CYA and chlorine relation question.

[morzh]

This is the part I am not sure I fully grasp: if CYA holds the chlorine in reserve not allowing it to react with anything, then this allowed higher level of chlorine is the same as the lower without CYA? Why have higher level when it acts as lower level? And any excess where CYA is no longer binding (say 2-3 ppm) it and which is free to oxydize and dissipate, how is it any less harsh on the skin and clothing than that same amount of Chlorine, 2-3ppm, without CYA?


That is, in short, if we use the anount of FC that is on top of the CYA-bound one, why do we need the bound part?

 

[Patrick_B]

The short answer is because it not only buffers it, but Cya protects some from sunlight. Even though it's higher, it buffered, and this makes all the difference. You have to overcome the bound part to have enough is all.

 

[JoyfulNoise]

Other's will likely explain the chemistry to you but let me try with an analogy you might understand being an electrical engineer and one who enjoys rebuilding old analog radios.

The CYA in the pool water is analogous to a capacitor in an electrical circuit. When small changes occur to the hypochlorous acid concentration (i.e., it gets used up by disinfection or oxidation of organics), the chlorine bound to the CYA gets released and becomes hypochlorous acid. There are six different chemical species that are chlorinated cyanurates, but I believe only two or three of them exist in any significant quantity in normal pool water conditions. As hypochlorous acid is used up, the reaction equilibrium shifts and the chlorine bound to the cyanurate ion gets released.

Now for the practical explanation in terms of linear-system control theory - the chlorinated CYA in the pool water acts as a continuous distribution system for chlorine with it's own closed-loop feedback (i.e., the chemical equilibrium mentioned above). You only need a very small amount of hypochlorous acid in water (something like 0.04% I think) to act as a disinfectant. So yes, you could go with 0ppm CYA and 1ppm FC BUT you will have an impossible time trying to dose and control such a small level of FC. As well, the pool water is not a homogenous system and contains stagnant spots so you can not assume that if you add 0.5ppm FC to your water that it will instantly mix and distribute nor will the hypochlorous acid be consumed in a homogenous manner. Finally, in sunlight, the chlorine in the pool water only lasts about 20mins while chlorine in water with 50ppm CYA lasts several hours. So if you had no CYA in the water you would have to spend your entire day by the poolside testing, dosing and mixing to make sure the water stayed properly sanitized. That's an impossible task.

Hope that helps. Other more technical explanations will likely follow.

 

[JVTrain]

Hope that helps. Other more technical explanations will likely follow.

I hope not. That was plenty and informative!

 

[morzh]

So let me translate it back to the chemical from electrical: the chlorinated cyanurate will release some hypochlorous acid as needed, so as it gets spent there will be more released. OK.
Why then do we keep talking of minimum FC needed when stabilized and if below, FC being ineffective? Someone before explained it like this today in a thread I started to write this for!

 

[JoyfulNoise]

So let me translate it back to the chemical from electrical: the chlorinated cyanurate will release some hypochlorous acid as needed, so as it gets spent there will be more released. OK.
Why then do we keep talking of minimum FC needed when stabilized and if below, FC being ineffective? Someone before explained it like this today in a thread I started to write this for!

Read the first chart in this post and this post by chemgeek.

You have to think about it in terms of a chemical equilibrium between many different species (hypochlorous acid, hypochlorite, chlorinated cyanurates, etc). Once the FC level drops below a certain concentration, there is not enough hypochlorous acid available to properly disinfect. Disinfection is defined by the EPA and CDC as the concentration and kill times required to inactivate pathogens and other biologicals by a disinfecting agent (hypochlorous acid). SO when the concentration drops below a certain threshold (see that first post), you begin to allow algae to grow at a rate faster than it is killed. If it goes even lower, then bacteria and viruses are able to survive.

Remember, when you measure the FC level with your test kit, it is measuring all forms of chlorine (except for the combined chloramines) - the hypochlorous acid, hypochlorite and chlorinated cyanurates all together. It's analogous to the TA, or Total Alkalinity, measurement which measures ALL forms of alkalinity in the water. We intentionally over-chlorinate our water when we follow the FC/CYA ratios described in the TFP Recommended Levels as that ensures that, for most of the day, there's a sufficient amount of hypochlorous acid available to meet the pools disinfection needs.

 

[chem geek]

So let me translate it back to the chemical from electrical: the chlorinated cyanurate will release some hypochlorous acid as needed, so as it gets spent there will be more released. OK.
Why then do we keep talking of minimum FC needed when stabilized and if below, FC being ineffective?

It is not that below a certain FC/CYA ratio which is proportional to the active chlorine (hypochlorous acid) level that it is completely ineffective. It is less and less effective in that the concentration is below the level at which the rate of chlorine killing of algae is faster than algae's growth rate. The FC/CYA ratio level that is in the Chlorine / CYA Chart in the Pool School is set somewhat above this threshold where the chlorine kill rate of algae equals algae's best case growth rate aka generation time or the time it takes algae to double in population. Under ideal conditions with plenty of algae nutrients, ideal levels of sunshine, and ideal temperature, algae doubles in population every 3 to 8 hours. The concentration of active chlorine needs to be at a level where it is able to kill half the algae faster than this timeframe, otherwise algae grows faster than chlorine can kill it so will have net growth, increased chlorine consumption, and eventually visible algae (or dull/cloudy water first, then green).

I think you are stuck on the fact that if there is chlorine in reserve and it is able to be released quickly, then why does the active chlorine level matter. The reason is that the chlorine bound to CYA does not kill algae and it can be instantly released but will not affect the kill rate since that is solely dependent on the active unbound chlorine concentration. An analogy might be helpful. Consider soldiers fighting an enemy and where only the soldiers on the front-line have weapons to kill the enemy. The rate of killing the enemy has only to do with the number of soldiers on the front line. It doesn't matter how many soldiers you have in reserve who can replace any killed front-line soldier even if such replacement is instantaneous. The reserve just tells you how long you can continue to fight, not the RATE at which you are killing the enemy.

Chlorine bound to CYA (and to some extent hypochlorite ion) is like the soldiers in reserve. Hypochlorous acid is like the front-line soldiers with weapons.

Also as was noted in the previous post, the FC test measures all the chlorine because as the hypochlorous acid is used oxidizing the dye or reacting with FAS reagent, more is released from CYA and this is faster than the time of the test. About half of the FC is released from CYA every 0.5 seconds if the hypochlorous acid were to instantly disappear. So what you are really measuring in the chlorine tests is mostly the chlorine reserve or capacity, not its rate of kill or effectiveness. The FC/CYA ratio is a much better proxy for determining the chlorine effectiveness or kill rate against algae, bacteria, viruses, etc. though obviously actual kill rates vary depending on the species. Generally speaking bacteria are easiest to kill, then viruses, then algae, and then protozoan oocysts and bacterial spores and biofilms.

 

[JoyfulNoise]

^^^^ ^^^^

I like the soldiers-on-the-frontline analogy soooo much better!


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[morzh]

I think you are stuck on the fact that if there is chlorine in reserve and it is able to be released quickly, then why does the active chlorine level matter. The reason is that the chlorine bound to CYA does not kill algae and it can be instantly released but will not affect the kill rate .....

Chem Geek,

this one is a good analogy, with soldiers. And, no, this is not what I was stuck at, it is the mechanism of the release and the rate of it. The way I saw most explanations was "chlorine in reserve is bound so it does not kill". Hence my question, why we need reserve that does not kill. As long as the reserve releases the acid at the rate to compensate the lost one (to the Sun or to fighting algae), I understand. Then there is an optimal rate of release and it should depend on FC/CYA ratio.
Or, using the reserve analogy, how quickly the logistics can transport the new soldiers to the frontline.

 

[chem geek]

You can ignore the rate of release since that is much, much faster than needed. Half the FC is released every 0.25 seconds if the chlorine were to get used up that fast. The fact is that the chlorine isn't used up that fast killing anything. The generation (doubling) time for bacteria in ideal conditions is 15-60 minutes which is far far longer than the time it takes for chlorine to be released from CYA. Chlorine does kill bacteria quickly, but the rate of release from CYA is not relevant to this since it is plenty fast enough. And chlorine bound to CYA won't even enter cells. It's hypochlorous acid that enters cells because it looks very much like water.

The FC/CYA ratio has nothing to do with any "optimal rate of release". The FC/CYA ratio tells you how much active (unbound) chlorine there is (i.e. its concentration) so the effectiveness of rate of kill of the chlorine (for a given species). The rate of release of chlorine from CYA is proportional to FC (since 97%-99% of FC is bound to CYA), but again is irrelevant since it is so fast compared to the hypochlorous acid loss rate.

Basically, for practical purposes, you can consider the new soldiers INSTANTLY replacing soldiers on the frontline. You can see that this rate of replacement if fast enough really has nothing to do with the rate of kill of the enemy. The number of front line soldiers is what determines that. Having a very small percentage of them used up and waiting for replacements does not change the kill rate significantly. Only if the rate of replacement were SLOW would it be an issue. That is not the situation with chlorine and CYA. The rate of release of chlorine from CYA is much faster than the needed rate of kill for pathogens and algae.

So again, all that really matters is the actual concentration of hypchlorous acid in the water. The amount of chlorine in reserve is really irrelevant and only tells you if you will run out of chlorine if there is a large demand. The amount of chlorine bound to CYA and its fast rate of release do NOT affect the kill rate. Only the concentration of the active killing chemical, hypochlorous acid, matters. Only that, nothing else.

Neither the FC level alone nor the rate of release of chlorine from CYA have anything to do with why pools with chlorine get algae. The reason pools with chlorine get algae has nothing at all to do with the rate of release of chlorine from CYA. It could be released instantaneously and it wouldn't help (and yes, if it were released very slowly then that would be a problem, but that is not the case). The reason pools with chlorine get algae is because the algae is able to grow faster than the rate that hypochlorous acid is able to kill it. The concentration of hypochlorous acid is proportional to the FC/CYA ratio so if your CYA is climbing and you don't proportionally raise the FC level, then algae is able to grow faster than chlorine can kill it.

 

[morzh]

Thanks, Chem geek, it starts making sense for me now.

 

[wisco]

The rate of release of chlorine from CYA is proportional to FC (since 97%-99% of FC is bound to CYA), but again is irrelevant since it is so fast compared to the hypochlorous acid loss rate.

So what drives the release of CL from the CYA? Is it some kind of equilibrium reaction where consuming the HClO puts pressure on the CYA to give up the CL atoms?

 

[morzh]

And while we are at that, what's the ideal CYA/FC numbers?
Also, what is the speed at which Cl oxydizes CYA? (and does it?) I understand it might depend on the ratio, but say at optimal ratio, or at 100ppm of CYA and 8ppm FC?

 

[JoyfulNoise]

And while we are at that, what's the ideal CYA/FC numbers?
Also, what is the speed at which Cl oxydizes CYA? (and does it?) I understand it might depend on the ratio, but say at optimal ratio, or at 100ppm of CYA and 8ppm FC?

What do you mean by "what's the deal"? The FC/CYA numbers are designed to be a constant ratio. If you look over those posts I commented on earlier you will see links to more detailed FC/CYA chart and how it is derived. Basically the ratio of FC to CYA is fixed and related to the level of hypochlorous acid needed in the water for proper disinfection. There is also a mathematical formula that allows you to calculate the concentration of hypochlorous acid if you know the FC and CYA values.

As for chlorine oxidizing CYA, yes it does but at an exceptionally slow rate at normal pool temperatures and pH values. The amount oxidized is somewhere around 2-5ppm per month. If you have very high FC (well above shock level) and very high pH (above any normal pH that you could swim in), the rate of oxidation increases. But for all practical purposes, you can ignore the oxidation rate of CYA.


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[lefty51]

Even though I REALLY do understand chem geeks' explanation, the capacitor and soldiers explanations would be so much easier for me to use. Great posting

 

[chem geek]

So what drives the release of CL from the CYA? Is it some kind of equilibrium reaction where consuming the HClO puts pressure on the CYA to give up the CL atoms?

Yes, when the hypochlorous acid concentration drops from breakdown from sunlight or from combining with or oxidizing ammonia or a nitrogenous organic compound, then more is released from being bound to CYA via chemical equilibrium. However, there is no "trigger" for this or any intelligence. In reality, chlorine is going from being unbound to bound back and forth over time so all that happens is that if one direction gets lower then the back and forth fills it back up. I think that this post and the further links it gives will provide you with more details if you are interested.

 

[morzh]

Chem Geek,

Thanks for good explanation.
I appreciate good analogies. By my early training I am a physicist and it was always easier to me to explain the process than to write formulae. Plus analogies are used in physics extensively to explain invisible processes using visible (like electro-mechanical ones). At some point I might bring myself to read up on it at more theoretical level, but these explanations are great and help understanding a lot.

Cheers!

 

[chem geek]

And while we are at that, what's the ideal CYA/FC numbers?
Also, what is the speed at which Cl oxydizes CYA? (and does it?) I understand it might depend on the ratio, but say at optimal ratio, or at 100ppm of CYA and 8ppm FC?

The Chlorine / CYA Chart in the Pool School has the ideal FC/CYA ratio for killing green and black algae faster than they can grow in both non-SWG and SWG pools. The link JoyfulNoise gave earlier to this chart shows more detail. These levels are high enough for worst-case conditions of ideal algae nutrient (phosphate and nitrate) levels, sunlight, and temperature. One can certainly not get algae growth at lower FC/CYA levels if their pool is low in algae nutrients or if the temperature is cold or if there is no sunlight. This FC/CYA ratio is roughly 7.5% for non-SWG pools and 5% for SWG pools. These charts have some safety leeway in them given some inaccuracy of test measurements, but there's not a lot of leeway for worst-case pools. Note that yellow/mustard algae requires higher FC/CYA levels in worst-case pools -- roughly a 15% ratio -- and this can get costly so instead the recommendation is to completely eradicate the algae if possible and if that doesn't happen then use last-resort methods such as algaecide or phosphate remover.

The technical derivation of why the FC/CYA ratio works as being proportional to the active chlorine (hypochlorous acid) level is in this post. Further information on why the active chlorine (hypochlorous acid) level is what matters in terms of disinfection and oxidation is in the "Chlorine/CYA Relationship" section in the first post of the thread Certified Pool Operator (CPO) training -- What is not taught.

Chlorine does oxidize CYA and this is described in the second half of the first post in the thread Degradation of Cyanuric Acid (CYA). At normal pool FC/CYA ratios, it is a slow process usually around 2-3 ppm CYA per month which uses 5-7.5 ppm FC per month (0.2 ppm FC per day), though some report higher losses up to 10 ppm (I discuss this in the degradation thread). In hot spas, it's faster at around 5 ppm CYA per month using 12.5 ppm FC per month (0.4 ppm FC per day). The oxidation will go faster at higher FC/CYA levels and at higher pH, both conditions generally occurring during a SLAM. However, a SLAM doesn't usually last long enough to make this a viable way of lowering the CYA level but people do report seeing a CYA drop especially at yellow/mustard SLAM levels or if doing a regular SLAM for an extended period of time.

 

[BuckeyeChris]

ok, so adding my own half-understanding/half question:

With 0 CYA: if you start your day at 0 FC and add CH to get to a FC of 5. Say every hour you lose 1 PPM to oxidizing whatever. After 5 hours, you reach 0 ppm and now things can multiply freely. (Question: assuming a constant addition of bacteria every hour, does the kill rate/usage rate stay constant at 5ppm down to 1ppm? Or does the oxidation rate change depending on the PPM level in this 0 CYA scenario)


With 50 CYA: you start at 4 and add CH to get to 9. Same amount of active (or, above saturation?) FC (5ppm) as the above scenario. So 5 hours later, your FC is at 4 and now the stuff can start growing, but not totally freely. The remaining 4ppm of CH will still get used up killing off things, but because the active concentration is lower than it was when FC was above 4ppm, it can't keep up with algae growth, and can only slow it a bit. (if this is true, then the "front line of soldiers" at 4ppm has fewer soldiers than the front line of soldiers at 9ppm. Right?)

But then, adding the sun shielding effect, all the CH that was held in suspension with CYA was not degraded by the sun until it was released to active status, so the overall loss rate due to sun exposure is lower. plus, I think CG mentioned that the CYA might also have a sunscreen affect that prevents UV from penetrating deeper into the water, which would save the active/unbound FC from sun degradation as well, which is why you get better degradation rates at higher CYA in a not-totally-linear ratio.

sorry, rambling.

 

[BuckeyeChris]

reading that degradation thread, I remember now that for a pool with 0 CYA a FC of 5 would be extremely high. Probably more akin to high shock levels in the teens/20's of a pool with CYA.