Cya

[stryker709]

I am running a chlorine generator and went into a few local hot tub stores to find test strips that measure CYA since I have read a lot on here about getting the CYA up to 20 to 30 parts per million. The guy at the first store was in the business for around 6 months and had not even heard of the concept of chlorine generators. The girl at the second had about the same experience and knew little about CYA. The girl at the third store had about 15 years experience and told me that CYA was only needed in swimming pools due to being exposed to uv rays that would drop the chlorine. She said it is pointless for a hot tub that does not see much direct sunlight. Any thoughts on this or possibly an explanation of why CYA would be important in a hot tub?

[stryker709]

I think I may have found the answer to this from chemgeeks post about how without the Cya the chlorine will be too strong at levels above 2ppm. Still, any other comments on this subject would be appreciated.

[chem geek]

First of all, do NOT get test strips, especially not for CYA. They are not very accurate. Get yourself a good test kit, the Taylor K-2006 you can get at a good online price here. Since you can't get that in Canada and can only get the Taylor K-2006 expensively through their exclusive master distributor Lowry & Associates (available through this retailer), you might see if you can find the Palintest Pooltester SP315C which is similar to the Taylor K-2005 so has everything you need except that the chlorine test is DPD (compare intensity of pink/red against a standard) instead of FAS-DPD (count the drops until the sample goes from pink/red to clear).

What exactly are you looking for? If it is the definitive technical paper that determined the equilibrium constants between chlorine, CYA and the chlorine attached to CYA (the chlorinated isocyanurates), then that is here. If you want to see a comparison of the traditional industry chart showing active chlorine (hypochlorous acid) concentration vs. pH compared to the true graph when CYA is present, then that is in this post. CYA protects chlorine from breakdown from sunlight not only by shielding CYA directly to protect lower depths of water, but it mostly combines with chlorine and these combined substances are not effective sanitizers or oxidizers (i.e. the bulk of the sanitation and oxidation is borne by the small amount of hypochlorous acid that remains unbound). This is demonstrated through many different technical papers including the following:

Killing of bacteria: here, here, here, here

Inactivation of viruses: here

Inactivation of protozoan oocysts: here, here

Inhibition of algae growth: this paper claimed no correlation, but real pools say otherwise; Sommerfeld never wrote back to me when I questioned this

Oxidation of ammonia and organics: here

There is also the correlation with ORP (see this post). I've also gone through field study data where the industry makes claims that only Free Chlorine (FC) matters in "real pools" yet I saw that bacteria are killed so easily that you can't even draw that conclusion from such studies and that hypochlorous acid (HOCl) is at least as good a predictor though they never looked at that correlation nor the FC/CYA ratio as a proxy (see this thread).

Though hardly scientific, my wife personally experienced this difference in terms of the rate of degradation of her swimsuits (elasticity gets shot) where they would barely last one winter season of swimming in an indoor community pool that did not use CYA and had 1-2 ppm FC (typically) compared to our own outdoor pool that usually had an FC of around 10% of the CYA level for an effective chlorine level that was equivalent to 0.1 ppm FC with no CYA (in terms of hypochlorous acid concentration). This 10-20 times difference in active chlorine concentration may have caused 10-20 times the rate of degradation of the swimsuits in the indoor pool over 5 months with no CYA compared to our pool with CYA where the swimsuits did not degrade and could be used over multiple 7-month seasons. Again, not scientific, but consistent with the chemical facts.

In a cool pool closer to 77F (say, 80-85F), at a pH of 7.5 and an FC of 3 ppm with a CYA of 30 ppm, then 97.2% of the chlorine is bound to CYA in multiple compounds called chlorinated isocyanurates. 1.4% of the chlorine is hypochlorous acid and 1.4% is hypochlorite ion and this is the same as occurs with 0.08 ppm FC with no CYA. At hotter spa temperatures (say, 100-104F), 84% of the chlorine is bound to CYA, 6.6% is hypochlorous acid and 9.4% is hypochlorite ion and this is the same as occurs with 0.5 ppm FC with no CYA.

Richard

[stryker709]

Wow, thanks Richard. That is an insane amount of info. So it seems there is much more to the Cya than just protecting the chlorine from sunlight. I kinda figured as much from what I have read on this site. While not as scientific, I actually found your wife's evidence the most practical. I'm curious if there is a constant based on the Cya levels that can be used to determine the relationship between how effective the chlorine is as a sanitizer and how damaging it is to swimwear, covers, skin and such. For example lets say at 20 ppm Cya, a free chlorine level of 3ppm will be as effective as 2ppm free chlorine without Cya as a sanitizer but with the harmful effects of .5ppm free chlorine without Cya. Not sure if I am even close to understanding how this works, but hopefully I am getting my question across here.

[chem geek]

At lower pool temperatures, the rough rule-of-thumb near a pH of 7.5 is that the equivalent chlorine level with no CYA is the FC/CYA ratio. So if the FC is around 10% of the CYA level, then that's roughly equivalent to 0.1 ppm FC with no CYA. At higher spa temperatures, it's around 5*FC/CYA so in this example of an FC that is 10% of the CYA level that would be roughly equivalent to 0.5 ppm FC with no CYA.

Regardless of temperature, however, the amount of active chlorine concentration when CYA is present is proportional to the FC/CYA ratio which means that 3 ppm FC with 30 ppm CYA has the same active chlorine concentration as 10 ppm FC with 100 ppm CYA, or that 3 ppm FC with 100 ppm CYA is around 1/3rd (actually 3/10 = 0.3) the amount of active chlorine, pretty much independent of temperature.

This is why continued use of Dichlor-only has the chlorine become less and less effective over time. At 4 ppm FC per day chlorine usage, roughly equivalent to 35 minutes of one person soaking at 104F with no ozonator, the CYA level rises by over 100 ppm every month. So compared to the 30 ppm CYA level I'm recommending, after one month the chlorine is around 3 times lower in concentration, after 2 months it is around 7 times lower, after 3 months it is around 10 times lower. These lower concentrations have oxidation of bather waste take that much longer which is part of what can cause cloudy water (algae growth is also possible, though that is mostly for pools, not spas). The disinfection rate is also lowered by that same amount (which is what those studies I linked to showed).

In your example of 3 ppm with 20 ppm CYA, at spa temperatures this has the same active chlorine (hypochlorous acid) as about 0.7 ppm FC with no CYA so is roughly similar to 0.7 ppm FC with no CYA in terms of sanitizing power and oxidizing power. Fortunately, it takes a rather low active chlorine level to kill bacteria, inactivate viruses, etc. The problem is that without CYA in the water, it is very hard to consistently maintain, say, 0.5 ppm FC. If you could, then you wouldn't really need the CYA which acts as an active chlorine buffer. By having CYA in the water, you can have a reservoir of chlorine (as FC) so that you don't run out (i.e. get to zero) while it's active strength is lower so is better for skin, swimsuits, hot tub covers, outgassing, etc. The lower active chlorine concentration is also better when you add chlorine after a soak since it theoretically reduces the creation of some irritating and volatile disinfection by-products such as nitrogen trichloride.

You need to stop thinking of FC as meaning anything in terms of chlorine strength when there is CYA in the water. FC is simply a measurement of the total reservoir of chlorine available and NOT a measure of its active strength. The FC/CYA ratio (or scaled up version for spas) is the relevant number for chlorine's true strength.

Richard

[kbaumann]

I need some clarification on “reservoir” in this statement:

Richard stated, “…By having CYA in the water, you can have a reservoir of chlorine (as FC) so that you don't run out…”

So what is taking place? As in this example used above:

Richard stated, “In your example of 3 ppm with 20 ppm CYA, at spa temperatures this has the same active chlorine (hypochlorous acid) as about 0.7 ppm FC with no CYA so is roughly similar to 0.7 ppm FC with no CYA in terms of sanitizing power and oxidizing power…”

The water has “a strength of” 0.7 ppm FC when it has “a reading of” 3 ppm FC with 20 ppm CYA present… as the pathogens are killed by this 0.7 FC and CC is produced, at that time does the CYA release more FC to maintain the 0.7 ppm FC? So basically releasing FC from the “reservoir” (the FC that is combined with CYA) as it is used up to maintain 0.7 ppm FC until its all gone? Does the 20 ppm CYA always keep a certain balance of chlorine in the water? When I hear “reservoir” I almost in vision a inventory that is restocked to the same level as it is sold.

I am sorry for not being better at typing my thoughts so you can understand …

Karl

[quantumchromodynamics]

When there is cyanuric acid in the water, most of the chlorine will be bound to the cyanuric acid. Only a fraction of the chlorine is not bound to cyanuric acid.

The chlorine that is bound to the cyanuric acid is not active. Only the unbound chlorine is active. Since the active, unbound chlorine maintains a consistent ratio with the inactive, bound chlorine, the chlorine is released from the cyanuric acid as the unbound chlorine is used up.

C₃H₂ClN₃O₃ + H₂O <-> C₃H₂N₃O₃^- + HOCl

As the HOCl (hypochlorous acid) gets used up, the equilibrium shifts and some of the chlorinated isocyanurate becomes hypochlorous acid. The chlorine in C₃H₂ClN₃O₃ is what is in reserve.

The ratios are based on the equilibrium constants for hypochlorous acid and cyanuric acid. Richard can better explain the actual chemical species that will be present under various conditions.

_________________________________________________________________

Notes:

I'm not sure if the C₃H₂ClN₃O₃ should be C₃H₂ClN₃O₃ or C₃HClN₃O₃. At normal pool and spa pH, most of the cyanuric acid will be in the form of C₃H₂N₃O₃. I'm not sure if the chlorine replaces a hydrogen or just combines at the empty hydrogen site, or both.

Cyanuric acid pKa1 = 6.88, pKa2 = 11.4, pKa3 = 13.5

Cyanuric acid. C₃N₃(OH)₃ <--> C₃H₃N₃O₃.

Monochloro-s-triazinetrione acid C₃H₂ClN₃O₃

1-chloro-1,3,5-triazinane- 2,4,6-trione

Dichloro-s-triazinetrione. C₃HCl₂N₃O₃

Trichloroisocyanuric acid. C₃Cl₃N₃O₃

[chem geek]

Karl,

Your description is essentially correct except that at a pH of 7.5 only about half of the equivalent FC with no CYA is the "active" form, hypochlorous acid, while the other half is hypochlorite ion that is far less effective. At any rate, you are correct that the active form is what kills pathogens quickly and is also what oxidizes bather waste (though hypochlorite ion can participate in that, depending on the reaction). As this active form is used up, more is replenished from the reservoir since there is an equilibrium between the chlorine bound to CYA and the chlorine that is unbound. The rates of chemical reactions, however, are based on the instantaneous concentration of the active species involved and have nothing to do with the reservoir amount.

QCD,

At a pH of 7.5, the primary form of Cyanuric Acid (CYA) is the cyanurate ion that I usually designate H₂CY^-. Also, the dominant chlorinated cyanurate species is HClCY^- which is a cyanurate ion with one of the hydrogen replaced by chlorine.

Richard