chem geek
01-13-2007, 04:03 PM
This thread is a (vain) attempt to understand why Cyanuric Acid (CYA) levels drop precipitously over the winter for many pool owners. Before starting to figure this out, take a look at this thread (http://www.poolforum.com/pf2/showthread.php?t=5762) that talks about the anaerobic bacteria (i.e. that prefer oxygen-poor environments) that can consume CYA and break it down.
I recently received information (from a document found in this post (http://www.poolforum.com/pf2/showpost.php?p=41537&postcount=12)) about Patent 4,075,094 which is in this link (http://patft.uspto.gov/netacgi/nph-Parser?u=%2Fnetahtml%2Fsrchnum.htm&Sect1=PTO1&Sect2=HITOFF&p=1&r=1&l=50&f=G&d=PALL&s1=4075094.PN.&OS=PN/4075094&RS=PN/4075094) 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) and at a pH of 9-12 (with optimal reaction conditions at a pH of 9-10).
Now in a normal pool environment, the amount of CYA far exceeds the FC so the chlorine is mostly combined with CYA to form chlorinated isocyanurates and is unlikely to breakdown the CYA, at least not rapidly. So while this may not explain the wintertime reduction of CYA, it did lead me to another thought. Namely, that the regular addition of chlorinating liquid or bleach may, during part of its dilution, create conditions conducive to the breakdown of CYA. The following table shows the theoretical amounts of FC, CYA and pH at various stages of dilution (mixing) of chlorine with pool water. The thought was that at some point the pH will be in the optimal range and the stoichiometry will be such that the overwhelming amount of chlorine could break down some amount of CYA. I assume standard initial pool conditions of 100 ppm TA, 30 ppm CYA, 7.5 pH, 3 ppm FC (initial). The "Dilution" refers to the added chlorine, which is 12.5% chlorinating liquid, so 100:1 means 100 parts of pool water with 1 part of added chlorine. The "max. CYA" refers to the largest amount of CYA that could be completely oxidized by the amount of chlorine (FC).
Dilution FC pH max. CYA (FC/2.6)
5000:1 28 8.50 11
2000:1 65 8.78 25
1000:1 128 8.95 49
500:1 253 9.12 97
250:1 503 9.31 193
100:1 1253 9.65 482
50:1 2503 10.03 963
So during dilution of added chlorine into the pool water, the pH and FC to CYA amounts are favorable for destruction of CYA at dilutions of 1000:1 or higher. If any of this destruction were to occur, then one would see that the amount of FC rise in their pool after adding an alkaline/basic source of chlorine (such as hypochlorite) would be less than theoretically should be achieved and this difference would be the amount of CYA that is destroyed (with the amount of CYA destroyed being 1/2.6 the amount of FC that is consumed).
The major flaw in this speculative hypothesis is that the patent indicates that contact time for this degradation to occur in substantial quantities is on the order of hours, while the amount of time that the dilution is in some optimal range is probably on the order of a few minutes at the most. So the amount of degradation of CYA from added hypochlorite sources of chlorine is probably very small, but is possibly more than from acidic sources of chlorine such as from an SWG (which produces chlorine gas that then hydrolyses to hypochlorous acid plus hydrochloric acid).
So the point of that exercise was really just to eliminate a possibility. The anaerobic bacteria explanation still seems plausible, though I can't explain how such conditions are favorable unless such bacteria are resistant to chlorine levels normally found in pools and that dissolved oxygen levels somehow get lowered to favor such bacteria.
The other possibility is that CYA degrades on its own (or at normal pool conditions in the presence of chlorine), albeit slowly, and is only noticed over the winter since it is unexpected, while during the summer one expects some loss due to splash-out and backwash (and we may add more CYA to make up for that). I am currently monitoring the loss of CYA in my own pool over this winter and will continue to do so during the summer. I use a cartridge filter so never backwash and I will monitor CYA and CH levels since the CH level is a decent indicator of any dilution that might occur from the small amount of splash-out.
For anyone doing the CYA test, keep in mind that it is temperature sensitive, so at colder temperatures it takes longer for the precipitate to form while at higher temperatures the precipitate is more soluable. Let your pool water come closer to room temperature before performing the CYA test.
Richard
I recently received information (from a document found in this post (http://www.poolforum.com/pf2/showpost.php?p=41537&postcount=12)) about Patent 4,075,094 which is in this link (http://patft.uspto.gov/netacgi/nph-Parser?u=%2Fnetahtml%2Fsrchnum.htm&Sect1=PTO1&Sect2=HITOFF&p=1&r=1&l=50&f=G&d=PALL&s1=4075094.PN.&OS=PN/4075094&RS=PN/4075094) 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) and at a pH of 9-12 (with optimal reaction conditions at a pH of 9-10).
Now in a normal pool environment, the amount of CYA far exceeds the FC so the chlorine is mostly combined with CYA to form chlorinated isocyanurates and is unlikely to breakdown the CYA, at least not rapidly. So while this may not explain the wintertime reduction of CYA, it did lead me to another thought. Namely, that the regular addition of chlorinating liquid or bleach may, during part of its dilution, create conditions conducive to the breakdown of CYA. The following table shows the theoretical amounts of FC, CYA and pH at various stages of dilution (mixing) of chlorine with pool water. The thought was that at some point the pH will be in the optimal range and the stoichiometry will be such that the overwhelming amount of chlorine could break down some amount of CYA. I assume standard initial pool conditions of 100 ppm TA, 30 ppm CYA, 7.5 pH, 3 ppm FC (initial). The "Dilution" refers to the added chlorine, which is 12.5% chlorinating liquid, so 100:1 means 100 parts of pool water with 1 part of added chlorine. The "max. CYA" refers to the largest amount of CYA that could be completely oxidized by the amount of chlorine (FC).
Dilution FC pH max. CYA (FC/2.6)
5000:1 28 8.50 11
2000:1 65 8.78 25
1000:1 128 8.95 49
500:1 253 9.12 97
250:1 503 9.31 193
100:1 1253 9.65 482
50:1 2503 10.03 963
So during dilution of added chlorine into the pool water, the pH and FC to CYA amounts are favorable for destruction of CYA at dilutions of 1000:1 or higher. If any of this destruction were to occur, then one would see that the amount of FC rise in their pool after adding an alkaline/basic source of chlorine (such as hypochlorite) would be less than theoretically should be achieved and this difference would be the amount of CYA that is destroyed (with the amount of CYA destroyed being 1/2.6 the amount of FC that is consumed).
The major flaw in this speculative hypothesis is that the patent indicates that contact time for this degradation to occur in substantial quantities is on the order of hours, while the amount of time that the dilution is in some optimal range is probably on the order of a few minutes at the most. So the amount of degradation of CYA from added hypochlorite sources of chlorine is probably very small, but is possibly more than from acidic sources of chlorine such as from an SWG (which produces chlorine gas that then hydrolyses to hypochlorous acid plus hydrochloric acid).
So the point of that exercise was really just to eliminate a possibility. The anaerobic bacteria explanation still seems plausible, though I can't explain how such conditions are favorable unless such bacteria are resistant to chlorine levels normally found in pools and that dissolved oxygen levels somehow get lowered to favor such bacteria.
The other possibility is that CYA degrades on its own (or at normal pool conditions in the presence of chlorine), albeit slowly, and is only noticed over the winter since it is unexpected, while during the summer one expects some loss due to splash-out and backwash (and we may add more CYA to make up for that). I am currently monitoring the loss of CYA in my own pool over this winter and will continue to do so during the summer. I use a cartridge filter so never backwash and I will monitor CYA and CH levels since the CH level is a decent indicator of any dilution that might occur from the small amount of splash-out.
For anyone doing the CYA test, keep in mind that it is temperature sensitive, so at colder temperatures it takes longer for the precipitate to form while at higher temperatures the precipitate is more soluable. Let your pool water come closer to room temperature before performing the CYA test.
Richard