Degradation of Cyanuric Acid (CYA)

[chem geek]

This thread explores some of the ways that Cyanuric Acid (CYA) can degrade since many users report seeing CYA drop under various conditions.

CYA Degradation by Bacteria

Some pool users find that their CYA levels drop over the winter when their pool is let go (i.e. no chlorine added). Many of these pools have algae, but some do not. The degradation pathway for CYA by some bacteria and fungi (normally under anaerobic conditions) is well understood and described here (an early article describing the degradation but incorrectly attributing urea as an intermediate is in this paper while the correct pathway is described in this paper). In summary, it is the following:

CYA + 2H₂O --> Biuret + H⁺ + HCO₃^-
Biuret + H₂O --> Allophanate^- + NH₃ + H⁺
Allophanate^- + H⁺ + H₂O --> 2NH₃ + 2CO₂

so the net result is:

CYA + 4H₂O --> H⁺ + HCO₃^- + 3NH₃ + 2CO₂
Cyanuric Acid + Water --> Hydrogen Ion + Bicarbonate Ion + Ammonia + Carbon Dioxide

For every mole of CYA, 3 moles of ammonia are produced. This is equivalent to 10 ppm CYA producing 3.26 ppm ammonia (measured as ppm Nitrogen). Sometimes the ammonia will dissipate (probably outgas or get consumed by algae) over time, but any leftover ammonia would appear as unusual chlorine demand upon opening and take a LOT of chlorine to get rid of. For every 10 ppm CYA that is degraded and produces around 3 ppm ammonia, it would take nearly 30 ppm cumulative FC to get rid of (see this thread and this thread). One can buy an inexpensive ammonia test kit from a pet/fish/aquarium store.

Because ammonia has an equilibrium with ammonium cation (NH₄⁺) with a pKa of around 9.2 in pool water (accounting for ionic strength), the net result is that the pH rises from the above reaction of CYA degradation as shown below (and the pH actually goes up even more than shown since the primary starting species is cyanurate ion):

CYA + 6H₂O --> 2OH^- + HCO₃^- + 3NH₄⁺ + 2CO₂
Cyanuric Acid + Water --> Hydroxyl Ion + Bicarbonate Ion + Ammonium Ion + Carbon Dioxide

Some of the carbon dioxide may dissolve in the water to form carbonic acid so if that occurred completely then the following would be the result:

CYA + 6H₂O --> 3HCO₃^- + 3NH₄⁺
Cyanuric Acid + Water --> Bicarbonate Ion + Ammonium Ion

In the above situation, the pH does not change very much, perhaps rising some since the starting species is actually cyanurate ion, not CYA as shown. The TA rises as well.

H₂CY^- + 7H₂O --> 3HCO₃^- + 3NH₄⁺ + OH^-
Cyanurate Ion + Water --> Bicarbonate Ion + Ammonium Ion + Hydroxyl Ion

[EDIT]
This paper and this paper describe some of the anaerobic conditions and rates for oxidation of CYA into carbon dioxide and ammonia. Under certain conditions, there are two mechanisms by which bacteria can further oxidize ammonia or ammonium ion. The first is nitrification which is an aerobic process (i.e. requiring oxygen) with two types of bacteria with the following examples shown to produce nitrate (the Nitrosomonas bacteria in the first reaction can also use ammonium ion):

Nitrosomonas Bacteria: 2NH₃ + 3O₂ --> 2NO₂^- + 2H⁺ + 2H₂O
Nitrobacter Bacteria: 2NO₂^- + O₂ --> 2NO₃^-

The second is an oxygen-limited autotrophic nitrification-denitrification (OLAND) which combines the nitrification of nitrosomonas bacteria (above) or other sources of nitrite or nitrate with subsequent denitrification under anaerobic conditions. Nitrate and nitrite are used in place of oxygen for metabolic processes. Three conditions are required for significant denitrification: low oxygen, high nitrate concentration, and a supply of organic carbon. Pseudomonas aeruginosa is an example of denitrifying bacteria. The following are half-reactions where the other half is the oxidation of organic material.

NO₃^- + 2H⁺ + 2e^- --> NO₂^- + H₂O
NO₂^- + 2H⁺ + e^- --> NO + H₂O
2NO + 2H⁺ + 2e^- --> N₂O + H₂O
N₂O + 2H⁺ + 2e^- --> N₂ + H₂O

There is also anaerobic ammonium oxidation (annamox) to produce nitrogen gas.

Annamox Bacteria: NH₄⁺ + NO₂^- --> N₂(g) + 2H₂O

So each stage of bacterial degradation after the initial one to create ammonia lowers the chlorine demand as follows.
10 ppm CYA --> 3.07 ppm Ammonia nitrogen --> 24-30 ppm FC chlorine demand left to oxidize ammonia
3.07 ppm Ammonia nitrogen --> 3.07 ppm Nitrite nitrogen --> 16 ppm FC chlorine demand left to oxidize nitrite
3.07 ppm Nitrite nitrogen --> 3.07 ppm Nitrate nitrogen --> 0 ppm FC chlorine demand left
3.07 ppm Nitrite or Nitrate nitrogen --> Nitrogen gas --> 0 ppm FC chlorine demand left

So while the Nitrosomonas bacteria somewhat reduce the chlorine demand, it is the combination of that bacteria with Nitrobacter or with denitrifying bacteria or annamox bacteria that reduce the chlorine demand to zero. Of course, there will still be some chlorine demand from needing to oxidize the bacteria itself.
[END-EDIT]

CYA Degradation by Oxidation from Chlorine

Though the above can explain a loss of CYA when bacteria are allowed to grow, such as when chlorine is not added over the winter, it does not explain why some pools also experience a loss of CYA over the summer or during other times when chlorine is present and bacteria should not be growing. There is oxidation reaction of Cyanuric Acid by Hypochlorite Ion described by John A. Wojtowicz in the Journal of the Swimming Pool and Spa Industry (JSPSI) Volume 4, Number 2, pp. 23-28 (2001) reprinted in "The Chemistry and Treatment of Swimming Pool and Spa Water" in Chapter 5.3 "Oxidation of Cyanuric Acid with Hypochlorite". [EDIT] This is now available online here. [END-EDIT]

2(HNCO)₃ + 9ClO^- ---> 3N₂ + 6CO₂ + 9Cl^- + 3H₂O
Cyanuric Acid + Hypochlorite Ion --> Nitrogen Gas + Carbon Dioxide + Chloride Ion + Water

[EDIT] On a molar basis, 4.5 chlorine oxidize 1 CYA. On a ppm basis this is 2.47 ppm FC for every 1 ppm CYA. [END-EDIT]

The primary step is the cleavage of the triazine ring and it is proposed by Wojtowicz that this primarily involves a fully chlorinated isocyanurate species Cl₂CY^-. The decomposition was first order with respect to average chlorine and increased with pH. The decomposition rate was a decrease in Free Chlorine at a rate of 0.0147 per hour or 1-EXP(-0.0147*24) = 30% per day while the calculated rate of loss of CYA at 4 ppm FC was 0.87 ppm per day [EDIT] (I get 4*0.30/2.47 = 0.49 ppm CYA per day) [END-EDIT], but that was at FC/CYA ratios (in ppm units) of 0.34 (close to shock levels) whereas the more typical ratio in our pools is around 0.1. His experiments at lower FC/CYA ratios of 0.029 with 4 ppm FC and 138 ppm CYA showed a lower CYA decomposition rate of 0.24 ppm/day which is a factor of 3.6 lower. At 4 ppm FC and 11.76 ppm CYA (a ppm ratio of 0.34), the concentration of Cl₂CY^- (at temp 85F) is a factor of 9.9 higher while HClCY^- is a factor of 6.9 higher than the concentrations at 4 ppm FC and 138 ppm CYA which might mean that HClCY^- is the rate-critical species that degrades (this is speculation on my part). In another paper "Effect of Cyanuric Acid on Swimming Pool Maintenance" (in the same JSPSI collection), Wojtowicz describes a chlorine loss rate due to oxidation of CYA at 12.5% per day at 85F which is roughly consistent with a 30%/3.6 = 8.3% rate especially since his 12.5% number came from FC levels starting higher (5.4 ppm for indoor pools, 7.6 to 9.2 ppm for outdoor pools). So, assuming a CYA loss rate of around 0.4 ppm/day in our pools this comes to 12 ppm per month which is clearly enough to be noticeable as the months pass during a swim season. If one shocks the pool, then the rate of loss could be about 2-3 times faster.

Wojtowicz also shows a strong temperature dependence on the chlorine oxidation of cynauric acid where every 10F increase in temperature results in roughly doubling the rate of degradation. So his data was with pools at 85F so pools at 90F could have degradation rates about 1.4 times higher.

The thing is that some of what Wojtowicz has seen does not seem to be consistent with some of what we have seen in our own pools, especially with regard to chlorine loss rates. Wojtowicz implies that there is little breakdown from sunlight of the chlorinated isocyanrates -- that only hypochlorous acid and especially hypochlorite ion are affected. Yet the experiments mas985 (Mark) made showed that higher levels of CYA did protect chlorine better in a non-linear way in sunlight and he did not see losses overnight (that is, without sunlight) which Wojtowicz saw in indoor pools. My own pool is at 86-90F and is exposed to sunlight 1-2 hours most days (it has an opaque electric safety cover on most of the time) and about a 1 ppm FC per day loss which includes use of the pool (1 person bather load most days). The rate of chlorine loss from oxidation of CYA is, in ppm units, about 2.5 times higher so even if I assume 1 ppm FC loss per day all oxidizing CYA, that would be 0.4 ppm CYA loss per day or 12 ppm per month. I should be able to measure that as I started with 30 ppm CYA when I opened and added more CYA around April/May. [EDIT] I just measured my pool's CYA level and it's a little above 25 ppm so even attributing a 5 ppm CYA drop over 3 months, that's pretty low and could be explained by the error tolerance of the test or some by splash-out (I have an oversized cartridge filter that is only cleaned once a year so the only dilution is from splash-out). It's certainly not near 12 ppm per month of loss. [END-EDIT]

So I can see that it is possible for CYA to degrade slowly over time in pools that are at higher temperature or at higher FC/CYA ratios such as extensive periods of shocking. The inconsistency is in how much degradation is occurring from sunlight vs. oxidation from chlorine.

CYA Degradation by Hydroxyl Radicals From Chlorine Breakdown in Sunlight

As described in this post, when chlorine (hypochlorous acid or hypochlorite ion) breaks down in sunlight, hydroxyl radicals are produced. This paper describes why CYA is not broken down quickly by hydroxyl radicals, but this paper demonstrates that CYA is broken down by hydroxyl radicals (though not via titanium dioxide because adsorption is required to be close to such radical generation). So it is possible that CYA is broken down slowly in pools exposed to sunlight. Some report rates of around 10 ppm CYA per month which is higher than the roughly 2-3 ppm CYA per month seen in pools not exposed to sunlight (or the 5 ppm CYA per month seen in hot spas) which would be the degradation rate from chlorine oxidation of CYA.

Richard

 

[Swimgirl]

I didn't understand most of this, but I guess the upshot of it is, since my water is warm and I use sodium hypochlorite for daily chlorination, and calcium hypochlorite for shocking, I can expect some CYA degradation? So I guess something workable would be something like an occasional puck in floater or a one time shock of dichlor, if the CYA test indicates the CYA is getting low?

 

[chem geek]

We're still sorting out how real or sizable this effect is. The above post is in The Deep End because it is technical and because conclusions are still tentative. Fortunately, one can test their CYA level every few months to see if more is needed and if so, then yes the use of Trichlor pucks/tabs in a floating feeder (or inline chlorinator) is one option as is use of Dichlor or adding pure CYA.

 

[Guest]

The main thing we can all take from this (whether you understand it or not) is the importance of regular water testing and BELIEVEING your test results instead of questioning them. If your CYA drops then replace it!

I had a customer come in the other day who said that she thought her test strips were bad so she brought the water in to be tested. She said she couldn't get a reading on pH and some of the the other parameters. She uses trichlor. I tested her water and found that both here pH and TA had crashed from continued trichlor use but instead of believing her test strips, which told her as much, she assumed they had gone bad and ignored them for a month.

 

[PoolOwnerNumber9]

I think that one of the interesting things about many of the stories about people experiencing sudden loss of Cyanuric Acid is that many of them have been out of town and that a neighbor, friend or relative was watching the pool. I suspect that at least some of the cases involve the addition of substantial amounts of fresh water.

Perhaps the neighbor was adding water and forgot and left the water running; this is quite common. Or, they may have been backwashing and they were distracted and when they finally realized, half of the water was gone. I would recommend that anyone who has lost Cyanuric check their water bill for any unexplained spikes in their water bill, and that you just ask the temporary caretaker if that happened.

 

[PoolOwnerNumber9]

In the article by John A. Wojtowicz of Chemcon
"Oxidation of Cyanuric Acid with Hypochlorite" He concludes:
"The oxidation of cyanuric acid with available chlorine was studied in order to determine if excessive levels of cyanuric acid could be reduced by treatment with hypochlorite. This study showed that prohibitively high concentrations of available chlorine would be required to achieve significant reductions in cyanuric acid levels in a practical time. The study also indicates that the cyanuric acid loss rate under typical swimming pool conditions is probably not significant."

Here is another chemical equation for the breakdown of Cyanuric Acid.
C3H3N3O3 + 3H20 = 3CO2 + 3NH3.
Cyanuric Acid + 3Water = 3 Carbon Dioxide + 3 Ammonia

I also found a source (http://www.pubmedcentral.nih.gov/articl ... tid=186872) that noted that degradation of cyanuric acid also proceeds in 3.5% sodium chloride solution. This is approximately 10 times higher than in a salt water pool at 3,500 ppm. I don't know if a salt concentration of 0.35% (3,500 ppm) is significant or not.

 

[chem geek]

The article you quote is the same one I was looking at since I bought the book of his articles (but what you quoted was on the web here). He is basically saying that super-chlorinating isn't an effective way of reducing CYA because it takes too long. Nevertheless, the article does point out a drop in CYA, just that it's not fast. That was my point, to see if over many days to months could it be enough to explain what people are seeing. I think it might be a minor factor unless the chlorine levels are high for an extended period of time.

Carbon Dioxide in water is in equilibrium with carbonic acid which is in equilibrium with bicarbonate ion and hydrogen ion. However, the link I gave on the degradation pathway is the more definitive process step by step. It is unlikely for all the bicarbonate that is produced to go to carbon dioxide quickly -- this slow outgassing goes on in pools independent of any CYA degradation. At the pH of pools near 7.5, the main component is bicarbonate ion in concentration 16 times higher than that of dissolved carbon dioxide.

CO₂(aq) + H₂O <--> H₂CO₃ <--> H⁺ + HCO₃^- <--> 2H⁺ + CO₃^2-
Carbon Dioxide + Water <--> Carbonic Acid <--> Hydrogen Ion + Bicarbonate Ion <--> Hydrogen Ion + Carbonate Ion

At pH 7.5, the relative amounts of the species relative to carbonic acid (at 1) are: 650 : 1: 10677 : 23

It's possible that dilution occurred when on vacation, but in many cases when a pool is let go and the chlorine drops, the bacteria can grow and degrade the CYA to ammonia (especially under anaerobic conditions; that is when dissolved oxygen is low). I think we've got that part figured out, but still need to figure out the case when chlorine doesn't go away so bacteria should not be growing and yet CYA is seen to drop, sometimes over a longer period of time. I just don't see anything consistent for this latter case (yet).

 

[PoolOwnerNumber9]

Correction:
Related to my reference to the degradation of Cyanuric Acid in a 3.5% sodium chloride solution. The article did not say that the solution caused the Cyanuric Acid to breakdown; it said that it did not prevent it. I misunderstood.

 

[chem geek]

This season, I carefully kept track of my pool's water chemistry to see if the CYA drops and how much would be from dilution. I used the Calcium Hardness (CH) measurement in my pool as a proxy for water dilution since my fill water has a CH of 50 ppm. I took measurements near the end of May and again yesterday. The CH only dropped from 240 to 230 so even with 50 ppm CH fill water this is less than a 5% dilution. The pool cover is only open for 1-2 hours per day so the increase in CH from evaporation is very low -- assuming even 0.25" per day (double the daily rate based on annual pan evaporation) that's 6 gallons per hour so about 9 gallons per day (cover off 1.5 hours/day) or an increase in CH of 0.8 ppm per month or 4 ppm over 5 months. I have a cartridge filter so never backwash (and I cleaned the filter prior to the May measurement). There is some splash-out, but it is pretty minimal. 5% of 16,000 gallons would be about 5 gallons/day over the 5 months between tests and that sounds about right as a maximum splash-out.

My CYA went from 30 ppm to just a hair above 20 ppm, say 21 ppm (hard to be that precise, as you know) over 5 months. A 5% dilution would have had the CYA drop to 28.5 ppm so there was an actual degradation loss of around 7-8 ppm. My pool's water temperature was normally from 85F to 89F during this period of time. The FC level varied from 2 to 6 ppm (usually 3 to 5). So assuming that the loss is from oxidation of CYA by chlorine, then over 5 months this is a loss rate of 0.2% per day. This is far, far less than what Wojtowicz saw in his test pool, but it's more consistent with what most others have seen. Some have seen a much more rapid drop so perhaps there is something that can catalyze the reaction. It would make sense that higher levels of chlorine would make the reaction go faster and if the reaction is truly from hypochlorite ion, then higher pH would have a much higher concentration of hypochlorite ion. Going from a pH of 7.5 to 7.85 doubles the concentration of hypochlorite ion (the concentration of Cl₂CY^-, on the other hand, is lower by about 14%).

The 0.2% CYA loss per day at the initial 30 ppm CYA is a loss of 0.06 ppm CYA per day. The loss in FC that corresponds to this for oxidation would be about 2.5 times higher or 0.15 ppm FC per day. This is less than the daily loss of FC I normally see when the pool is used daily (around 1 ppm FC per day) and less than when the pool is not used (somewhat less than 0.5 ppm FC) so the amount lost due to oxidation of CYA would be in the noise and not easily distinguishable.

Note that a pool with a higher CYA level should see a faster absolute drop if the chlorine level were the same. At 80 ppm CYA, the loss is almost 5 ppm CYA per month, but that's with an average FC similar to mine of 10-15% of the CYA level. With a more typical SWG pool with an FC that is 5% of the CYA level, the CYA loss would be about half or closer to 2 ppm CYA per month based on the lower hypochlorite ion concentration, but the Cl₂CY^- is lower by a factor of 1.9 so perhaps 1 ppm CYA per month loss. So the more rapid loss seen in some pools must be due to some other factors that catalyze the reaction (when chlorine is present; we know that with no chlorine that some bacteria can degrade CYA).

There is a strong dependency on the Cl₂CY^- concentration on the FC/CYA ratio where it appears that every doubling of this ratio results in an increased concentration by a factor of 3.5 whereas the chlorine concentration is roughly doubled (for HClCY^-, the increase is a factor of 1.9). So the product increases by about a factor of 7 (for HClCY^- the factor would be almost 4). This means that the loss of CYA should be seen to be substantially higher when the FC/CYA ratio is higher and the corresponding loss of FC would be higher as well. Since there is also a strong dependence on pH, shock levels of chlorine for extended periods of time may lower the CYA level more rapidly. Unfortunately, as Wojtowicz showed in his own pools, it's still not fast enough as a way of reducing CYA quickly, but over time it should become noticeable.

Richard

 

[spishex]

Richard,

I've been told by some ChemCo's that CYA often tests very low at the beginning of the season because of high chlorine demand, and have seen more than a few cases where CYA magically reappeared after an initial shock. Have you addressed this anywhere already?

Thanks,

Tim

 

[KCAZ]

Interesting thread. I have a pool biz in AZ and we often run into high CYA issues here due to extensive use of tri-chlor tabs and di-chlor shock by pool owners and many poolmen. Lots of sun and warm water compound the problem. I personally like to keep my CYA under 70ppm and find that this works well here.

I recently acquired some pools which were heavily overstabilized (CYA 150+). I am running into resistance from cash-strapped pool owners that do not want to spend the money to drain and re-fill. They think the new pool guy is just trying to take their money because the old guy would just throw tabs in and shock the heck out of the pool when the algae bloomed. We all know that this cycle of tab and shock cannot continue forever with CYA over 150.

This leads to my question about alternate ways to reduce the CYA. I have been told that heavy doses of ammonia (1-4 gallons) followed 24-36 hours later by heavy shock will knock out the CYA. I have searched the net far and wide and have not been able to confirm this old poolman trick.

Does anyone have any info as to the validity of this method?

Thanks in advance for the replies.

 

[chem geek]

Richard,

I've been told by some ChemCo's that CYA often tests very low at the beginning of the season because of high chlorine demand, and have seen more than a few cases where CYA magically reappeared after an initial shock. Have you addressed this anywhere already?

Thanks,

Tim

Tim,

Though we've had quite a few reports of CYA dropping, I can only think of one or perhaps two where the CYA seemed to increase without a logical explanation of a CYA source. Obviously, if the shock was with stabilized chlorine, such as Dichlor, then the CYA would increase substantially, but I presume you mean the CYA appeared after a shock with unstabilized chlorine. Haven't heard of that before and I don't have any explanation for it.

I don't really understand the concept that "CYA often tests very low at the beginning of the season because of high chlorine demand" unless that means that people intentionally start with lower CYA because they plan to use a lot of stabilized chlorine initially, say to clear a pool of algae upon spring opening.

Richard

 

[spishex]

Well, i've tried to find some good evidence but to no avail. I did find a reference to it, though, and it cites bioguard which is one of the manufacturers I heard it from.

Some may believe that the additional cyanuric acid is OK because the stabilizer level has been reduced to ZERO. The problem is that the chlorine demand will completely MASK the stabilizer or cyanuric acid level. DO NOT add stabilizer. Retest the cyanuric acid about 5 days AFTER the treatment.

http://www.pool-care.net/page/ExoticCar ... demand.htm

I wonder if this has anything to do with the Biolab CYA reagent? It's a solid tablet, or was when I worked with them. I would imagine it's the same chemical but maybe not?

I also stumbled across an article mentioning that CYA changes the mV potential of water, but I guess that's another thread...

 

[chem geek]

Interesting thread. I have a pool biz in AZ and we often run into high CYA issues here due to extensive use of tri-chlor tabs and di-chlor shock by pool owners and many poolmen. Lots of sun and warm water compound the problem. I personally like to keep my CYA under 70ppm and find that this works well here.

I recently acquired some pools which were heavily overstabilized (CYA 150+). I am running into resistance from cash-strapped pool owners that do not want to spend the money to drain and re-fill. They think the new pool guy is just trying to take their money because the old guy would just throw tabs in and shock the **** out of the pool when the algae bloomed. We all know that this cycle of tab and shock cannot continue forever with CYA over 150.

This leads to my question about alternate ways to reduce the CYA. I have been told that heavy doses of ammonia (1-4 gallons) followed 24-36 hours later by heavy shock will knock out the CYA. I have searched the net far and wide and have not been able to confirm this old poolman trick.

Does anyone have any info as to the validity of this method?

Thanks in advance for the replies.

KCAZ,

Welcome to TFP! As I'm sure you've surmised by looking at the Pool School and the Chlorine / CYA chart, the FC/CYA ratio roughly determines the amount of "active" chlorine that prevents algae (and kills bacteria, etc.). With very high CYA levels, one would need very high FC levels to prevent algae growth or would need to use a supplemental algaecide (such as PolyQuat 60 or a phosphate remover) at extra cost.

I have not heard about the heavy ammonia dose followed by shock to lower the CYA. Adding heavy doses of ammonia to the pool would raise the pH substantially and would also combine with any chlorine to form monochloramine. At heavy doses of ammonia, however, one mostly is left with ammonia. If the heavy shock that was later added was from unstabilized chlorine (i.e. a hypochlorite of some sort -- chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite), then this would form even more monochoramine lowering the pH some and any excess chlorine would raise the pH and FC. So it's possible that a combination of high pH with monochloramine has the effect of oxidizing CYA though I would expect hypochlorite to be better at that.

Though it's possible that adding lye or another base to raise the pH combined with high shock levels of chlorine could breakdown CYA quickly enough, the high pH would also tend to form scale in pools that are already near saturation with calcium carbonate (i.e. pools with a saturation index already near zero). I had earlier reported on The Pool Forum (here) about Patent 4,075,094 which is in this link that indicates that Sodium Hypochlorite can break down Cyanuric Acid (CYA) in a molar ratio of 4.5:1 (which is 2.6:1 by weight and where optimal reaction conditions have a molar ratio of 6:1 to 8:1 [EDIT] or a chlorine to CYA ppm ratio of 3.3 to 4.4 [END-EDIT]) and at a pH of 9-12 (with optimal reaction conditions at a pH of 9-10) with reaction time of hours. Shock levels of chlorine do not approach these ratios or pH, but nevertheless it is still possible for the reaction to occur slowly over a period of weeks and months. I also speculated in the post on short-term reaction exposure from manual chlorine addition, but the exposure time is quite short (minutes).

Anyway, if you or anyone does come up with a reliable method to reduce CYA other than by dilution that we already know works, that would be helpful. There are some expensive products on the market that reduce CYA, but they appear to be melamine which would not only precipitate CYA, but would tend to make the water cloudy, just as in a CYA test.

Richard

 

[chem geek]

Some may believe that the additional cyanuric acid is OK because the stabilizer level has been reduced to ZERO. The problem is that the chlorine demand will completely MASK the stabilizer or cyanuric acid level. DO NOT add stabilizer. Retest the cyanuric acid about 5 days AFTER the treatment.

:
:
I wonder if this has anything to do with the Biolab CYA reagent? It's a solid tablet, or was when I worked with them. I would imagine it's the same chemical but maybe not?

I also stumbled across an article mentioning that CYA changes the mV potential of water, but I guess that's another thread...

It sounds like the chlorine demand they are referring to is what is seen after a winter closing when the CYA seems to drop a lot (sometimes to zero) and there is an insatiable chlorine demand. This is well understood if the chlorine level went to zero during the winter since bacteria can biodegrade CYA into ammonia (see the first post in this thread for more info on this). It does take an extraordinary amount of chlorine to get rid of the ammonia, but I am surprised that their "chlorine demand" test station doesn't just simply test for ammonia since that is far simpler. Testing for chlorine demand can take hours since the breakpoint reaction is not fast, especially if there is CYA remaining in the water.

The statement that the chlorine demand will completely mask the stabilizer level doesn't make sense unless ammonia interferes with the CYA test -- there is nothing in the Taylor instructions about such interference. If Bioguard uses a CYA test that does not have a strong acid buffer in it, then the melamine-CYA complex may not form if the pH is high which it may very well be after the biodegradation of CYA. In fact, at a pH of 7, the melamine-CYA complex is soluble up to 20 ppm. The Taylor test has an acid buffer in its melamine reagent to maximize precipitation in the CYA test giving accurate results (otherwise, they'd never be able to measure down to 20 ppm). So I think you hit the nail on the head in thinking that the Bioguard CYA test tablet is a poor test reagent that is inaccurate under conditions of a pH out of normal range (especially high pH). Nevertheless, the advice in the link of using unstabilized chlorine for shocking makes sense regardless.

CYA by itself does not change the ORP (mV) of water if there is no chlorine present, but it will combine with chlorine (hypochlorous acid) such that the remaining amount of hypochlorous acid (and hypochlorite ion) is much lower in concentration and THAT will lower the ORP substantially compared to the same chlorine level without CYA. See the graphs in this post for a real-world example from hundreds of samples in a couple of hundred pools and spas that shows this effect and how it is related to hypochlorous acid concentration and not to FC alone. This post explains why the FC/CYA ratio is a decent approximation proportional to the "active" chlorine (hypochlorous acid) concentration.

Richard

 

[JasonLion]

I've been told by some ChemCo's that CYA often tests very low at the beginning of the season

I know of two effects that could create this appearance. One is that the CYA test doesn't always work the way a novice might expect when the water is very cold. When the water is cold it takes more time for the precipitate to fully form and most people don't know that and end up taking the reading early and getting a lower than actual result.

The other is that the CYA level seems to test differently when the pump has not been on in a long time. I have gotten different results on spring opening day and the day after that appear to suggest that CYA has either "settled" to the lower water or that CYA has crystalized somewhere. After 24 hours with the pump running the results return to more plausible levels.

 

[spishex]

The other is that the CYA level seems to test differently when the pump has not been on in a long time. I have gotten different results on spring opening day and the day after that appear to suggest that CYA has either "settled" to the lower water or that CYA has crystalized somewhere. After 24 hours with the pump running the results return to more plausible levels.

That seems very plausible. I'm going to try to contact somebody at bioguard about this and see if they've got a chemical explanation.

 

[Guest]

This leads to my question about alternate ways to reduce the CYA. I have been told that heavy doses of ammonia (1-4 gallons) followed 24-36 hours later by heavy shock will knock out the CYA. I have searched the net far and wide and have not been able to confirm this old poolman trick.

Does anyone have any info as to the validity of this method?

Thanks in advance for the replies.

This trick of forming monochloramine is used to kill algae and will temporarily 'knock out the CYA'. It's still in the water but it won't have any effect on the ability of the monochloramine to kill algae. This is how the ammonia based 'chorine enhancers' for killing algae work. Sodium bromide works much in the same way since neither monochloramine nor hypobromous acid is affected by CYA so they are not inactivated by high levels of stabilzer. This will help kill an algae bloom but once the monochloramine or bromine is oxidized by additional chlorine the problem of an overstabilized pool and subsequent algae blooms will return. It is not a fix but a 'band- aid' for the problem.
Hope this is helpful.

 

[chem geek]

This trick of forming monochloramine is used to kill algae and will temporarily 'knock out the CYA'. It's still in the water but it won't have any effect on the ability of the monochloramine to kill algae.

Thanks waterbear. I takes things too literally like "knock out" as in literally reducing the CYA level rather than what really goes on as in "working around" by removing the attached chlorine from the CYA to form monochloramine. It won't actually reduce the CYA level itself. We still don't have a good way, other than dilution, for intentionally reducing a high CYA level.

 

[dschlic1]

On my pool I am seeing about a 10 ppm loss per month of CYA. CYA went from 70 to 60 ppm with FC between 3 and 4 ppm. Water temp is 90 F. During the month I did not pump out any water, and the only water added was from rain. My chlorine demand is about 1 ppm per day.