Degradation of Cyanuric Acid (CYA)

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Richh said:
I know Cl- is a pretty harsh oxidant, so would the chlorine be oxidizing the ammonia present from the bacteria into nitrite, then to nitrate, which would elevate these levels? Is there currently a way to remove nitrate from the pool without doing a drain/refill? I know there are nitrate removers for fish tanks, but they use polycaprolactone as a carbon source for the denitrifying bacteria in the tank which wouldn't (shouldn't) exist in the pool.
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Cl- is chloride ion and is not a harsh oxidant so did you mean HOCl which is hypochlorous acid? The latter is an oxidizer, as is OCl- hypochlorite ion. These are selective oxidizers and which one is stronger depends on the chemical species being oxidized. In the case of ammonia, it is HOCl that combines with ammonia (NH3) to form monochloramine and does further substitutions to form dichloramine or nitrogen trichloride, but these are substitution reactions (Cl+ replacing H+) and not oxidation. HOCl can then oxidize some of these to form nitrogen gas or nitrate. Mostly though, the largest final product is nitrogen gas. The proportion of nitrate from the reactions is usually around 10-20%. This spreadsheet can be used to calculate what happens using the Jafvert & Valentine and other models. See Chloramines and FC/CYA and you might also be interested in Oxidation of Urea as well.

There is no easy way to remove nitrate from pools other than by water dilution. However, nitrate is not a problem in pools (why do you think that it is?). It is an essential nutrient for algae just as with phosphates, but that is irrelevant if one maintains the appropriate FC/CYA ratio which has chlorine kill algae faster than it can grow regardless of algae nutrient level. Algae is ultimately limited in its growth rate by sunlight and temperature.
 
My Cyanuric Acid in my pool also drops to zero after each winter when I reopen the pool. I have another theory: I am vacuuming it out. I read somewhere that Cyanuric Acid is heavier than water and will sink to the bottom of the pool. The first thing I do when I open the pool is I attach the vacuum and suck all the water out from the bottom of the pool while adding new water to the top. I vacuum to waste and do so for an hour or more cleaning up my typically very dirty pool. Does anyone think my theory could be true?
 
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Question for the chemists: is there a mechanism to break down CYA with potassium monopersulfate, which is, as you know, the strong oxidizer in ”chlorine free shock”? I recently saw CYA loss of maybe around 20-30 ppm over the span of around 3 weeks. This seemed high given discussion here, which is quite interesting by the way. I did have elevated FC at times during this period, but one thing I did during this time was experiment a little with the usefulness of chlorine free shock.

I did some research online but didn‘t find much. One gets overwhelmed with marketing search results noting that chlorine free shock won’t raise CYA levels.

Could it be people who are reporting anomalously high CYA loss are also using chlorine free shock occasionally? Seems unlikely because if it were true this would be promoted as a mechanism to lower CYA levels when drain and refill is not so easy, but I thought I’d ask anyway.
 
chem geek said:
We hear about this more rapid loss in very sunny areas like Florida and Arizona. Not only will the water be warm, but the sun is pretty intense and on the pool for long periods of time. I wonder if sunlight can act as a catalyst for CYA breakdown. It does breakdown chlorine (hypochlorous acid and especially hypochlorite ion) so perhaps the intermediate free radicals that are formed (•OH •O- •Cl and subsequently •OCl •OH) may oxidize the CYA a bit faster. This is pure speculation on my part, though is based on some reactions described in this PDF file.

The question about Calcium Hardness (CH) is very relevant since it could indicate whether dilution is a factor (that's why I tracked that carefully in my own pool).
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Guest said:
Precipitation rate is slower but solubility decreases in cold water so if you let it develp to completion it will read high since more will precipitate out. Likewise, in warm water less will precipitate since solubility increases. The speed of the precipitation has nothing to do with the solubility of the precipitate. The reaction will procede faster in warm water than cold water. LaMotte specifies a temerpature range and time range for the test because that is how their meter is calibrated. This time and temp range does vary smewhat with the different meters they offer.
There is no discrepancy.
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My pool has been running really hot lately. When pulling from the skimmers, the heater (ironically, the easiest temp sensor to read) says 96F. If I pull from the main drain (which is usually closed), I get 94F.
A good reason why (aside from over a week of 90+ days) is my dark grey autocover. We leave the pool covered for most of the day to keep FOD, critters, and children out, and then open it in the late afternoon to swim. The cover clearly heats the water (which is usually very convenient).
In an effort to cool the pool, I decided I'd leave the cover open more. I haven't brought myself to leave it open at night because it has been so dry that I fear I'll find critters floating in it in the morning, so I opened it early in the morning and leave it open during the day.
I knew I probably needed to increase CYA since I've been running it on the low end -- somewhere between 50-60. I have been hesitant to raise it back up since I just finished a partial drain to get my (inherited) water down below 140ppm.
I had a jug of liquid CYA, so yesterday I shook it up and added half a gallon, which should've bumped me around 10ppm.
This morning, I tested and got a hair over 30ppm!
Where did it go?
Did the heat break it down? Was the sample too warm? The test water was definitely 90F, but the reagent was closer to 70 (basement).

I immediately decided to empty the bottle into the pool... and that's when I noticed that even though I shook it, there was quite a lot of gunk still in the bottle. So several fills and shakes later, I decided it was actually empty.
So it is quite possible that I didn't add 10ppm yesterday, but it certainly seems I decreased from 50-60 down to 30.

I was also watching some CL loss during the day, so I dropped the floatie in with about 1.5 lbs of trichlor (figuring I could use the CYA anyway).

About 2 hours after adding the rest of the liquid CYA, I tested CYA again and got around 50, so if I had added the whole gallon (meaning I didn't get much actual CYA out of the bottle yesterday), this could make sense.

When I can get out of my meetings, I will retest the water I collected earlier which should be closer to 70F now and see if I see a meaningful difference to when the water was 93F.
 
Elevated pool water temperature does increase the degradation rate of CYA. I see up to 20% loss in the summer months here in the desert. I test CYA monthly and add as necessary, using either CYA granules or a puck or two.

The test is not effected by the higher water temperature.
 
mknauss said:
Elevated pool water temperature does increase the degradation rate of CYA. I see up to 20% loss in the summer months here in the desert. I test CYA monthly and add as necessary, using either CYA granules or a puck or two.

The test is not effected by the higher water temperature.
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Of course, now it is cloudy so there goes my repeatability :p
I still got between 50-60. Maybe closer to 60 than before.

"a puck or two"? I think adding a 10 of these little 1" pucks only raises my CYA by a few ppm... but I have a much larger pool I guess.
 
mknauss said:
The test is not effected by the higher water temperature.
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There seem suggestions to the contrary. This PDF says:

"When testing for cyanuric acid with a melamine-based turbidity test, the most significant interference is water temperature. High temperatures, above 90 °F, can result in readings as much as 15 ppm low. Low temperatures, below 60 °F, can result in readings that are 15 ppm high. The ideal temperature is about 75 °F."

I believe the Taylor test a melamine-based turbidity test? I don't know whether this statement is correct, but it seems pretty easy to perform an experiment given that CYA is probably very stable in a sample pulled from a pool and either refrigerated or warmed in something like water bath.
 

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