Cyanuric Acid Drop Over Summer with Chlorine

[chem geek]

Since some people have reported drops in Cyanuric Acid levels even when there was Free Chlorine (FC) present, I thought I would track more carefully my CYA level this summer along with CH and other parameters that could act as a proxy for water dilution. My fill water has a CH of around 50 ppm and I have very little evaporation since the pool cover is on the pool most of the time. I've monitored my splash-out (dilution) level from the CH and it was minimal at far less than 10% over the entire summer.

And yet, my Cyanuric Acid level fell from 25 down to somewhere guessing at around 10 ppm. I noticed that the drop "seemed" faster around the time I needed to do some shocking because the water looked a little dull (I had let the chlorine drop too low, forgetting to add chlorine often enough) and was also faster at the lower CYA levels because I was still maintaining the same FC level so the FC/CYA ratio was higher than usual.

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) 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).

So I would guess that there is a slow reduction in CYA over time, probably due to the oxidation of CYA by chlorine, and that higher chlorine-to-CYA levels, such as found during shocking, may somewhat speed this process up. The drop I saw was higher than I expected, but it seems quite real and without an alternative explanation.

While I wouldn't say it's a great way to reduce CYA, perhaps super-shocking a pool (to at least an FC of 60% of the CYA level -- similar to yellow/mustard algae shocking) and raising it's pH could work to reduce high CYA levels, but would probably need to be kept that way for days, perhaps a week. It would just take a LOT of chlorine. Doesn't sound practical though drain/refill isn't exactly simple or cheap either.

Richard

 

[chem geek]

Another interesting effect that my wife noticed was that at the lower CYA level, but same FC level (so net higher disinfecting chlorine level) there seemed to be more chlorine smell (my wife's got a sensitive nose and indicates that it's a faint clean bleach smell, not a bad smell like chloramine). After I've added some Trichlor tabs to get the CYA up, she commented on how the smell has noticeably gone away -- the FC being about the same but the CYA being higher.

I would estimate that at 4 ppm FC and 10 ppm CYA the disinfecting chlorine was around 0.25 ppm while at 4 ppm FC and 20 ppm CYA it's around 0.10 ppm so she noticed the smell when it was around 2.5 times higher.

Could be just coincidence, but she never noticed the smell before except when the pool was being shocked (and typical shock level is around 0.3 ppm disinfecting chlorine).

Richard

 

[Shelley N]

So what you're saying is that CYA protects the "nose" as well as the chlorine?

 

[Ohm_Boy]

Any chance that the CC levels weren't consistent? Like, say... if someone might have peed in the pool?

 

[chem geek]

Very funny. Actually, I did check for CCs just in case, but there were none, as is usually the case. And yes, the CYA can be thought of as protecting the nose, at least at the same FC level. It makes sense. The "clean" chlorine bleach smell comes from the hypochlorous acid, which I usually call disinfecting chlorine, and doesn't come from the chlorine that is bound to CYA and is not volatile.

 

[mas985]

Something odd happened to me this week. The last time I measured CYA, about 6 weeks ago, it was 50 ppm. I just added 5 lbs of CYA via skimmer sock and leaf rake method on Sunday, to bring the CYA up to 80 ppm. I saw all of it dissolve into the water as a white milky cloud. On Friday, I measured the CYA and it was still about 50 ppm. So either 30 ppm of CYA disappeared in 5 weeks or the CYA that I thought dissolved really didn't although it passed through the sock. However, I did not change my SWG settings and my free chlorine jumped from 2.8 ppm to 4.4 ppm so the water is behaving as though there was an increase in CYA. Reagent is only a few months old too.

I am going to wait another week to see if there is any change in the CYA level. Also, can the CYA test be influenced by anything else in the water or perhaps temperature? The water is in the high 80s.

One more thing, I thought about a water leak but then my salt level would have gone down which it hasn't.

 

[JasonLion]

There have been several reports this season of stores selling containers labeled stabilizer that either didn't in fact have any stabilizer in them or had less than the usual amount. No one has figured out what they actually contained.

 

[JCJR]

My CYA level has fluctuated as well. New pool in April, CYA level up to 45. End of May the rain over flowed my pool twice and I had to drain a total of 4 inches of water. I calculated less than 7% of my pool water but over those two months my CYA level went down to below 30, I added 4 lbs (i think) of CYA and got it back up to 40 and now it is down to 35 the past couple of weeks. I had no CC at all since I have had my pool, Chlorine levels from .5 to 5ppm ( let it go low a couple of times but no more than 2 days w/o checking CL levels). My ph has gone above 8 a couple of times and my pool uses a lot of muratic acid (2-3 gals per week) I am fighting scale I believe and have kept my pool below 7.4 the last 3 weeks and that has helped. I use my water features maybe 2 hrs per week at most and have kept my TA between 70-80.

I couldn't explain the CYA levels fluctuating so did not bother to ask, but from this thread it seems that others have CYA losses. I will monitor it more but I have gone through a lot of reagent and check it 2x a month.

 

[mas985]

Something odd happened to me this week. The last time I measured CYA, about 6 weeks ago, it was 50 ppm. I just added 5 lbs of CYA via skimmer sock and leaf rake method on Sunday, to bring the CYA up to 80 ppm. I saw all of it dissolve into the water as a white milky cloud. On Friday, I measured the CYA and it was still about 50 ppm. So either 30 ppm of CYA disappeared in 5 weeks or the CYA that I thought dissolved really didn't although it passed through the sock. However, I did not change my SWG settings and my free chlorine jumped from 2.8 ppm to 4.4 ppm so the water is behaving as though there was an increase in CYA. Reagent is only a few months old too.

After a week, I tested the water again and it is still 50 ppm CYA. Added a CYA grain from the container to my test vial and it is definitely CYA so my only conclusion is that I lost 30 ppm of CYA over 6 weeks some how. No rain and no leaks (did a bucket test) so I have no other explaination. During that period I used about 4 gallons of 10% liquid chlorine in four doses for pool parties but that should of only raised the chlorine level by 5 ppm each time.

I wonder if anything else besides chlorine and algae (haven't had that) will break down CYA.