Is CYA-bound chlorine measured as FC?

[kouroubel]

New to the forum and just beginning to digest this CYA-dependent FC dosage.

I have a couple of questions.

1. Does CYA affect the FC measurement in any way? Will the FC that is bound to CYA still be measured by pool testing kits? I am using Lamotte's ColorQ Pro7.
2. Since I am the manager of a small private pool (20-30 swimmers/day) I am somewhat reluctant to operate the pool at higher than 3 ppm FC levels (I currently have a CYA reading of 75 ppm). Although I understand that at the end of the day it is the CYA/FC ratio that determines the effect of chlorine on swimmers and garments, are there any official publications that I could use in order to convince common people that is is safe to swim in an 8 ppm FC pool?

 

[JohnT]

Welcome to TFP!

No, the CYA doesn't affect the measurement of the chlorine level.

Your second question is harder to answer without knowing local rules. The bottom line is that nobody will know the chlorine level unless you tell them. A pool maintained at the proper chlorine level has virtually no chlorine smell, even at 8ppm. It's combined chlorine that creates the objectionable smell, so in reality the smell is worse at low FC levels.

 

[kouroubel]

Thank you for the fast reply.

Local legislation is based on 40-year old law that dictates that chlorine residual level should be between 0.4-0.7 ppm!!!

However, I could easily justify going up to 3 ppm of FC because WHO says it is safe¹. 8 ppm, on the other hand, is not that easy.....

Also, we have the water analyzed (FC, TC, pH and microbial load) twice a month so in theory people could find out .

¹ The World Health Association- Guidelines for safe recreational water environments VOLUME 2 SWIMMING POOLS AND SIMILAR ENVIRONMENTS.

 

[duraleigh]

You should always do what you are most comfortable with.

On another subject, I will mention that the Color-Q you are using is not particularly accurate. Particularly with CYA, we have found enough error in the results that we don't recommend it.

 

[JoyfulNoise]

Thank you for the fast reply.

Local legislation is based on 40-year old law that dictates that chlorine residual level should be between 0.4-0.7 ppm!!!

However, I could easily justify going up to 3 ppm of FC because WHO says it is safe¹. 8 ppm, on the other hand, is not that easy.....

Also, we have the water analyzed (FC, TC, pH and microbial load) twice a month so in theory people could find out .

¹ The World Health Association- Guidelines for safe recreational water environments VOLUME 2 SWIMMING POOLS AND SIMILAR ENVIRONMENTS.

The 0.4-0.7ppm level only makes sense if the CYA is 0. At that point, then the active chlorine (hypochlorous acid) level is high enough to kill pathogens. If any CYA is present, then the hypochlorous acid level will drop significantly as the CYA will weakly bond with chlorine atoms buffering them.

Does Greece have to follow EU commercial/public pool standards? If so, I was under the impression that the use of CYA in public pool was banned in the EU.

Chem geek knows more about European standards than anyone else. Hopefully he can respond to your query.


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[pabeader]

Isn't the residual FC in the range 0.4ppm-0.7ppm when CYA is 80 and FC is 8ppm?

It isn't something easily measured, but I seem to remember a number of Chem Geeks posts that essentially said something like that.

 

[kouroubel]

On another subject, I will mention that the Color-Q you are using is not particularly accurate. Particularly with CYA, we have found enough error in the results that we don't recommend it.

That is interesting. Are there any relevant threads in this forum that I could have a look? I was thinking about draining/refilling to target for 40 CYA and I want to be 100% certain I am doing the right thing ......

On a side note, the 0.23 ppm that I measured in tap water with the ColorQ is in very good agreement with the 0.27 ppm given by the water company (water analysis results published on the internet).

The 0.4-0.7ppm level only makes sense if the CYA is 0. At that point, then the active chlorine (hypochlorous acid) level is high enough to kill pathogens. If any CYA is present, then the hypochlorous acid level will drop significantly as the CYA will weakly bond with chlorine atoms buffering them.

Well....I guess chloroisocyanurate pucks were not invented back then so it does make sense. Unfortunately, the law is the same for forty+ years now (since 1973!!!).

Does Greece have to follow EU commercial/public pool standards? If so, I was under the impression that the use of CYA in public pool was banned in the EU.
Chem geek knows more about European standards than anyone else. Hopefully he can respond to your query.

Don't know about any EU standards but as I said the Greek legislation does not forbid CYA (I bet the lawmaker had never heard of CYA back then). What I can tell you is that this is the standard recipe here for private/semi-public pools (i.e, the one each and everyone local pool expert would suggest):

• trichloro tablets to the skimmer once or twice a week,
• shock treatment with 10 ppm FC once a week and
• algaecide addition also once a week.

Besides these products are all over the pool stores so I doubt there is any prohibition....

 

[JoyfulNoise]

Don't know about any EU standards but as I said the Greek legislation does not forbid CYA (I bet the lawmaker had never heard of CYA back then). What I can tell you is that this is the standard recipe here for private/semi-public pools (i.e, the one each and everyone local pool expert would suggest):

• trichloro tablets to the skimmer once or twice a week,
• shock treatment with 10 ppm FC once a week and
• algaecide addition also once a week.

Besides these products are all over the pool stores so I doubt there is any prohibition....

Well, that's about the same procedure used here in the U.S. by our pool "experts". The only way to run a pool using that recipe is to closely monitor both FC and CYA and then drain half the pool water whenever the CYA goes over 60-70ppm. If the algaecide is Cu-based, then you need to monitor Cu as well and drain anytime the Cu level is greater than 0.5ppm. Polyquat-60 algaecide would be the safest and easiest to use if it is available in Greece.


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[BuckeyeChris]

I think it's safe to say either way that keeping the pool between 0.75 and 3ppm FC with a CYA of 75 and good bather load (higher than residential) will eventually lead to algae or even sickness. You should get the CYA level down while you stay at 3ppm. How are you currently chlorinating, and are there any secondary systems like UV or Ozone?

- - - Updated - - -

edit: this is the paper chem geek references as the basis for the CYA relationship, though the ratios for keeping algae at bay were resolved through observation and user feedback.

Equilibria in Aqueous Solutions of Chlorinated Isocyanurate

 

[Isaac-1]

We have had a number of threads over the years on the ColorQ, and the problem seems to be that it is great when it works, but it also seems that when it fails to work, it is consistently wrong. Take a look at this thread ColorQ vs TF100

 

[kouroubel]

I think it's safe to say either way that keeping the pool between 0.75 and 3ppm FC with a CYA of 75 and good bather load (higher than residential) will eventually lead to algae or even sickness.

You have a point of course but to be fair there is also a weekly addition of liquid algaecide....

You should get the CYA level down while you stay at 3ppm. How are you currently chlorinating, and are there any secondary systems like UV or Ozone?

Well, as I said above, getting the CYA level down is the plan but first I need to make sure that my ColorQ readings can be trusted. Tomorrow, I am gonna do some repeatability tests at least to be sure about THAT...

Until I found out about this forum a week ago, I used to chlorinate with trichlor tablets to the skimmers or trichlor powder directly to the water. The past 6 days however, I have switched to 12-14% sodium hypochlorite. I am targeting at a 3 ppm FC dose every night. There are no secondary disinfection systems.....

edit: this is the paper chem geek references as the basis for the CYA relationship, though the ratios for keeping algae at bay were resolved through observation and user feedback.

Equilibria in Aqueous Solutions of Chlorinated Isocyanurate

Thanks a lot. I will read it carefully when I have the chance.....

 

[kouroubel]

We have had a number of threads over the years on the ColorQ, and the problem seems to be that it is great when it works, but it also seems that when it fails to work, it is consistently wrong. Take a look at this thread ColorQ vs TF100

Thanks for the link. I read the entire thread and it seems to me that, almost in every case, the discrepancies between ColorQ and TF100 can be justified by the tolerances provided by Lamotte:

These are the ColorQ tolerances, per LaMotte. Each value is a 'plus or minus', so if you get 7.5 on your pH, it could be anywhere from 7.3 to 7.7.

pH 0.2
Alk 15
cH 25
CYA 15
Chlorine is graded, with it being less accurate at higher concentrations
from 0-1ppm 0.1
from 1-4ppm 0.2
from 4-6ppm 0.5
from 6+ppm 1.0

And then somebody mentioned that there is also the factor of reagent life and, of course, condition of test tubes (which makes sense because, after all, this is a colorimetric method, i.e., based on light absorbance)....

 

[chem geek]

The ColorQ does much worse at higher CH than their tolerances indicate. It's OK to 300 ppm, but if you've got 500 ppm CH the error can be far greater than just 25 ppm -- people saw 100 ppm or more error. If you have to use an electronic instrument, then it's better than others (except for high-end much more expensive equipment). See the following posts:

LaMotte ColorQ Water Tester
ColorQ...sad faces
Test Kits and Accuracy - Whom to Believe
LaMotte Colorimeter - TesTabs vs Liquid Drops

and for some people the unit as a whole isn't good (i.e. not just CH, but also pH and other readings):
ColorQ...sad faces

As for the chlorine/CYA relationship, the O'Brien paper is the starting point and this post describes why the FC/CYA ratio is a decent proxy for the equivalent FC with no CYA. As for high FC levels, you can look at this post with some U.S. state limits of 10 ppm FC in Florida, 8 ppm FC in Texas, and California with no limit.

As for Europe, the DIN 19643 swimming pool standard does not use CYA because it has an activated carbon filter in the circulation path that removes all chlorine as well as chloramines so that chlorine needs to be added again afterwards. It will also remove some CYA but more importantly it would waste a lot of chlorine if you removed it all. This is part of the process of keeping chloramines (especially organic chloramines) to a minimum which is very important in commercial/public pools with their very high bather loads.

If the FC/CYA ratio is roughly 10% then this is the same active chlorine (hypochlorous acid) concentration as in a pool with around 0.1 ppm FC and no CYA so lower than the numbers you are quoting as the "standard". See also the "Chlorine/CYA Relationship" section in this post that shows that it is the active chlorine level, not the FC level, that correlates with disinfection and pathogen kill times and with oxidation rates.

 

[kouroubel]

Thank you for all the references chem geek!!!!

One question though. Can you please explain how 10 ppm of my 90% available chlorine tablets will increase CYA by 6 ppm? I mean, assuming 90% is by weight and even if the other 10% is pure CYA, I would expect the added FC/CYA ratio to be 9/1 and not 10/6.

What's wrong with the above thinking?

EDIT. Another question after reading the Chlorine/CYA Relationship section:

Am I right in thinking that the chlorine dosage to achieve a certain ORP value (e.g. 700mV) will depend on the CYA level?

 

[chem geek]

It isn't 90% weight of chlorine atoms. It's 90% Available Chlorine where BY DEFINITION 100% is for chlorine gas (Cl₂) where only one of the two chlorine atoms becomes hypochlorous acid:

0 0 .................. +1 .... -1
Cl₂ + H₂O ---> HOCl + HCl

So 100% Available Chlorine in chlorine gas is only 50% chlorine atoms that actually become chlorine (hypochlorous acid) in water (the other chlorine atom becomes chloride). In Trichlor, all three chlorine atoms become hypochlorous acid so the 90% Available Chlorine is 45% chlorine atoms by weight.

........................................... +1
Trichlor + 3H₂O ---> CYA + 3HOCl

Note that (CYA weight)/(chlorine weight) = 55/45 = 1.22 and not 0.61 because chlorine is measured in weight units of chlorine gas which weighs twice as much as a single chlorine atom. 55/(2*45) = 0.6111. Doing this more accurately with Trichlor molecular weight of 232.41 gives the percentage of chlorine atoms of 3*35.453/232.41 = 45.76% with "CYA" without hydrogen atoms of 54.24% so 54.24/(2*45.76) = 0.5927. This is close to correct but off slightly because the 55% (or 54.24%) doesn't include hydrogen atoms in CYA. The molecular weight of CYA is 129.07 g/mole while the molecular weight of chlorine gas is 70.906 g/mole. Since there are 3 hypochlorous acid produced with one Trichlor, the CYA/FC is 129.07/(3*70.906) = 0.6068.

 

[kouroubel]

OK. I think I get it. So basically you are saying that trichlor effectively achieves the same FC concentration as chlorine gas with only half the chlorine atoms, is that correct?

What about 55% dichlor, though?

Clearly, CYA/FC is 129.07/(2*70.906) = 0.91 so this is in agreement with your number.

But how can I calculate the % by weight percentage of chlorine atoms in 55% dichlor? If it was 55/2 = 27.5 %, then (CYA weight)/(chlorine weight) = 72.5 / (2*27.5) =1.31, which is way off.

In order to have (CYA weight)/(chlorine weight) equal to 0.91, the % by weight percentage of chlorine atoms should be 35.4%.

What am I missing here?

EDIT.
The MW of Trichloroisocyanuric acid is 232.41 g/mol and the one of Cl is 35.453. Therefore, the % percentage of chlorine atoms in trichlor should be 3*35.453 / 232.41 = 45.76%. This is close to 90/2 = 45%.
The MW of Dichloroisocyanuric acid is 197.96 g/mol. Therefore, the % percentage of chlorine atoms in dichlor should be 2*35.453 / 197.96 = 35.81%. This is way of from 55/2 = 27.5%.

So maybe the fact that the calculation of the % percentage of chlorine atoms in 90% trichlor as 90/2 = 45% is close to the real value 45.76% is just coincidental?

 

[chem geek]

Basically yes. You can think of ALL chlorine sources except for chlorine gas as being twice as efficient as chlorine gas in terms of their chlorine atoms. So not only for Trichlor, but for Dichlor, Cal-Hypo, lithium hypochlorite, and sodium hypochlorite (chlorinating liquid or bleach) you need to divide the % Available Chlorine by 2 to get the actual weight percent of chlorine atoms.

55% Available Chlorine for Dichlor is because the Dichlor is a dihydrate (has 2 water as part of the solid molecule). Dichlor dihydrate has a molecular weight of 255.98 g/mole and there are 2 chlorine atoms in it so the ratio of CYA to chlorine (as Cl₂ weight) is 129.07/(2*70.906) = 0.9101 as you wrote.

There are two chlorine atoms in Dichlor so the weight % is (for chlorine atom, not Cl₂) (2*35.453)/255.98 = 27.70% which is half of 55.4% which is the % Available Chlorine of PURE Dichlor dihydrate (actual product is usually closer to 99% pure which would be 54.8% Available Chlorine). You are missing the fact that it is dihydrate so has two water that shouldn't count in the percentage calculation of "CYA" AND the fact that the molecular weight includes sodium as well because it is sodium dichloroisocyanurate dihydrate. So your 72.5% for "CYA" is way off because it includes water and sodium.

The water percentage of Dichlor dihydrate is (2*18.01528)/255.98 = 14.0755% while the sodium percentage of what is really sodium dichloroisocyanurate dihydrate is 22.9898/255.98 = 8.98% so factoring that out of the ratio analysis and using the factor of 2 for chlorine gives an approximate amount (because of the missing hydrogen atoms for CYA). Out of 100% there are 27.70% chlorine atoms, 14.08% water, 8.98% sodium atom and 100%-27.70%-14.08%-8.98% = 49.24% "CYA" but missing hydrogen atoms. So the proper ratio with the factor of 2 for chlorine Cl₂ units is 49.24%/(2*27.70) = 0.8888. This is a lot closer to the correct 0.9101 than you were getting with this technique.

 

[kouroubel]

Thanks a lot chem geek. It is all clear to me now!!!!

Now to wrap it all up, could you please also comment on my second question in post #14?

Am I right in thinking that the chlorine dosage to achieve a certain ORP value (e.g. 700mV) will depend on the CYA level?

 

[chem geek]

Yes, ORP is very roughly measuring the hypochlorous acid concentration so roughly the FC/CYA ratio so the ORP will depend on both the FC and the CYA level. However, it's much, much, worse than that because of how flaky ORP is due to higher CYA levels directly fouling the electrode, due to hydrogen gas bubbles from SWG systems interfering with the ORP reading, due to membranes getting contaminated with other organics in the pool water, due to many other contamination, noise, and variations both over time for an individual sensor and across sensors both from the same manufacturer as well as across manufacturers.

See the second graph in this post for how noisy/variable ORP readings get as the CYA level gets higher and at lower active chlorine levels. With that study of 620 samples from 194 real commercial/public pools, there were 130 of such pools with built-in ORP controllers that could be compared with the portable ORP device used for taking measurements (and yes, the device was used correctly waiting 30 minutes or more for equilibration). 23% of these pools with two ORP sensors measuring the SAME water had differences of 100 mV or more. That is HUGE because it represents roughly 4 doublings of active chlorine level. 4 doublings is a factor of 16!!! That's the difference between 1 ppm FC and 16 ppm FC with both at the same CYA level. Do you see why ORP is practically useless except for very short-term process control? This is one reason why state regulations in the U.S. requires MANUAL chlorine testing several times per day and that ORP cannot be used as a substitute and instead has a "setpoint" determined by such manual testing. ORP can be used for short-term process control, but MUST be validated by standard and MUCH more accurate test kits, such as the FAS-DPD chlorine test kit. Of course, the chlorine test kits are checking FC and CYA separately, but nevertheless, trying to use an absolute ORP value for that purpose is not very accurate.

 

[kouroubel]

I hear you loud and clear but (since an ORP/pH dosing system is on its way to our pool) I bet it still could work provided:

a) electrodes are protected by filters and are well-maintained
b) pool operates at a constant CYA level (i.e., sodium hypochlorite is used as chlorination agent)
c) ORP control is combined with pH control
d) the control objective (setpoint) is to maintain a given FC concentration (or CYA/FC ratio). This is going to be underpinned by frequent manual chlorine measurements.
And most importantly...
e) one knows what he is doing

Anything else I forget?